-A
Environmental Durability of FRP Bond to Concrete
Subjected to Freeze-Thaw Action
by
Pavel DohnAlek
Bachelor of Science in Civil Engineering
Northeastern University 2004
SUBMITTED TO THE DEPARTMENT OF CIVIL & ENVIRONMENTAL ENGINEERING IN
PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF SCIENCE IN CIVIL AND ENVIRONMENTAL ENGINEERING
AT THE
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
OF TECHNOLOGY
FEBRUARY 2006
HAR
©2006 Pavel Dohndlek. All rights reserved.
0 9 2006
LIBRARIES
The author hereby grants to MIT permission to reproduce
and to distribute publicly paper and electronic
copies of this thesis document in whole or in part
/411 1///
Signature of Author
Certified by
Dipartment of CilA and Environmental Engineering
November 21, 2005
-4-
Eduardo Kausel
Professor of Civil and Environmental Engineering
Thesis Supervisor
i;
!
Accepted by
Andrew Whittle
Chairman, Departmental Committee for Graduate Students
BARKER
Environmental Durability of FRP Bond to Concrete
Subjected to Freeze-Thaw Action
by
Pavel DohnAlek
Submitted to the Department of Civil and Environmental Engineering
on November 21, 2005 in Partial Fulfillment of the Requirements
for the Degree of Master of Science in Civil and Environmental Engineering
ABSTRACT
An experimental study was performed to determine the environmental durability of the
adhesive bond between fiber-reinforced plastic (FRP) and concrete. The study
specifically focused on freeze-thaw cycling exposure of such bonds and their ultimate
strength prior and after the environmental exposure. To investigate the bond strength 84
single lap shear specimens were manufactured utilizing two different types of carbon
FRP pultrued strips and three different structural adhesives for total of three
FRP/adhesive combinations. Two types of concrete substrate were used: regular high
strength and air-entrained concrete. The specimens were freeze-thaw cycled for three
different numbers of cycles using two different freeze-thaw procedures. First freeze-thaw
procedure used chloride solution (3% NaCl) as its medium; the second procedure
utilized tap water. This main program was complemented by the same freeze-thaw
cycling of pull-off specimens of the adhesively bonded system and dumbbell tension
specimens of the three adhesives. Coefficients of thermal expansion of the three
structural adhesives were also experimentally measured.
Results show that the ultimate strength of the adhesive bond between FRP and concrete
deteriorates measurably during freeze-thaw cycling in chloride solution. This must be put
into perspective as the concrete itself severely deteriorates during this type of freezethaw cycling. Therefore, the durability of the adhesive bond between FRP and concrete
is dependent on the durability of the concrete. This is also supported by the results of
testing of the adhesive tensile specimens that did not show decrease in strength only an
increase in ultimate strain. The freeze-thaw cycling in water did not result in any
deterioration of the specimens' strength, most specimens actually acquired higher
strength, due to moist curing of the concrete during the freeze-thaw cycling.
Thesis Supervisor: Eduardo Kausel
Title: Professor of Civil and Environmental Engineering
* )'/,j
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ACKNOWLEDGEMENTS
The author would like to acknowledge the help and support of Ing. Zdenek Vdvra
and Stephen Rudolph in preparation and testing of the specimens. The author would
like to acknowledge the help and support of Professor Kausel who worked on this thesis
as both thesis supervisor and academic advisor. The author would like to also
acknowledge help of Professor Oral Buyukozturk with preparation of the plan of the
experimental program and Professor George Wyner of Boston University for help with
finilizing this document.
4
TABLE OF CONTENTS
CHAPTER 1
INTRODUCTION
6
CHAPTER 2
LITERATURE REVIEW
8
CHAPTER 3 MATERIAL CHARACTERIZATION & TESTING 14
3.1
3.2
3.3
3.4
3.5
3.6
CHAPTER 4
4.1
4.2
4.3
4.4
EXPERIMENTAL PROGRAM
Shear test results
Pull-off test results
Adhesive tensile test results
Adhesive coefficient of thermal expansion results
CHAPTER 5
5.1
5.2
5.3
5.4
14
16
18
20
21
22
Freeze-thaw cycling
Material properties
Single lap shear specimens
Pull-off specimens
Adhesive tensile specimens
Thermal expansion specimens
DISCUSSION & CONCLUSION
Discussion of current results
Comparison to Previous Research
Conclusion
Further Work
23
25
31
35
38
40
40
42
44
44
BIBLIOGRAPHY
46
APPENDIX A - Overview of FRP Strengthening
50
APPENDIX B - Review of Previous Research
84
APPENDIX C - Material Characterization & Testing
110
APPENDIX D - Experimental Program
126
5
CHAPTER 1
INTRODUCTION
Today, many countries are faced with the fast deterioration of their infrastructure,
which adversely affects their prosperity and sustained economic growth. A substantial
part of that infrastructure is composed of reinforced concrete structures. These
structures are often subjected to combined effects of aggressive environments, such as
deicing salts or seawater ingress. At the same time live loads on many of these
structures, such as bridges, are increasing significantly. Therefore, there is a great need
to develop a durable and cost effective method to repair and also strengthen these
reinforced concrete structures and to prolong their useful life.
One method that might help solve this problem is the adhesive bonding of fiberreinforced polymers (FRP). Detailed overview of this method is presented in Appendix A.
This area of research has been the focus of numerous studies in the past 15 years.
These high performance materials seem to be the appropriate solution for this problem
because they have high specific strength, high specific modulus and at the same time
they are noncorrosive and chemically resistant. The up-to-date research has focused
mostly on the initial mechanical properties of such strengthened beams, columns or
other types of structural members. Another very important factor that needs to be
thoroughly investigated to ensure the safety of this method is an evaluation of the longterm environmental durability of the adhesively bonded system. To date, there have
been numerous studies on the environmental durability of the adhesive bond between
6
FRP and concrete, but given the vast number of combinations of environmental
exposure, concrete type, adhesive type, FRP type and member geometry, many aspects
of durability are not yet well understood. In particular, freeze-thaw cycling of the
adhesive bond between FRP and concrete has received little attention (Karbhari et. al
2003, CERF Report 2001, Bonacci et. al. 2000).
A large number of reinforced concrete structures in need of repair are located in
regions that regularly experience freeze-thaw cycles and further, some of these
structures may also be exposed to other environmental conditions, such as deicing salts
or seawater. Therefore, the long-term durability of both the repair materials individually
and of the system as a whole is crucial to ensure the safety of FRP-reinforced structures
in cold climate regions, such as in the United States, Canada and Europe.
The objective of the research described herein is to investigate experimentally the
durability of the adhesive bond between FRP and concrete. The durability will be
assessed in terms of the effect of freeze-thaw cycling on shear strength and pull-off
strength of adhesive bond between carbon FRP (CFRP) and concrete.
7
CHAPTER 2
LITERATURE REVIEW
This section reviews earlier studies on the freeze-thaw durability of FRP bond to
concrete in flexural and shear applications, discusses and compares their findings, and
identifies the unresolved issues which motivate the current study. A more detailed
literature review is presented in Appendix B. The state of research on freeze-thaw
cycling of adhesive bond between FRP and concrete was assessed by a review of
earlier studies. In chronological order, the earlier works on the subject reviewed here
are: Chajes, Thomson, Farschman and Farschman (1995, University of Delaware),
Karbhari and Engineer (1996, University of California San Diego), Karbhari, Engineer
and Eckel (1997, University of California San Diego), Green, Soudki and Johnson (1997,
Queen's University and University of Waterloo), Mukhopadhyaya, Swamy and Lynsdale
(1998, University of Sheffield), Green, Bisby, Beaudoin and Labossiere (2000, Queen's
University and University of Sherbrooke), Myers, Murthy and Micelli (2001, University of
Missouri and University of Lecce). These various researchers used a wide variety of
FRP materials, specimen setups, freeze-thaw cycling procedures and so on.
A common denominator of these studies is that they tested the adhesive bond
between FRP and concrete in flexural, shear or peel loading, using three general types
of specimens: single-lap shear or peel, double-lap shear and small-scale beam
specimens. The small-scale beam specimens were typically strengthened on the tension
8
surface and loaded in a four-point bending mode. A wide variety of FRP materials were
used, including CFRP strips, aramid FRP (AFRP), glass FRP (GFRP) and CFRP
fabrics, combined with various types of adhesives or resins. In addition, the concrete
substrates differed significantly. Each of the studies used a different type of freeze-thaw
cycling procedure repeated for a different number of cycles. In general, the procedures
were either dry-freeze water-thaw or constant chloride solution immersion freeze-thaw.
Only some of these procedures were standardized procedures. The particulars of each
of the studies are presented in Tables 2.1 and 2.2.
Table 2.1. Experimental program particulars of reviewed studies
Type of FRP
Type of specimens
Study
small-scale beams AFRP, GFRP, CFRP
Chajes et. al.
GFRP, CFRP
Karbhari and Engineer small-scale beams
GFRP, CFRP
single lap
Karbhari et. al.
GFRP, CFRP
small-scale beams
Green et. al. (1997)
GFRP
double lap
Mukhopadhyaya et. al.
small-scale beams,
CFRP strips
single lap
Green et. al. (2000)
small-scale beams AFRP, GFRP, CFRP
Myers et. al.
Loading
four-point bending
four-point bending
peel
four-point bending
shear
four-point bending,
shear
four-point bending
Table 2.2. Freeze-thaw cycling particulars of reviewed studies
Number of Cycles
Freeze-Thaw Exposure
Study
ASTM C672 constant immersion in 4%
50, 100
CaCl solution
Chajes et. al.
30
Karbhari and Engineer non-standard dry-freeze, water-thaw
30
non-standard dry-freeze, water-thaw
Karbhari et. al.
50
non-standard dry-freeze, water-thaw
Green et. al. (1997)
non-standard immersion in 5% NaCl
252
solution followed by F-T cycling
Mukhopadhyaya
50, 150, 300
ASTM C310 dry-freeze, water-thaw
Green et. al. (2000)
mixed exposure
non-standard mixed exposure
Myers et. al.
9
Given the numerous differences among these studies, it is difficult to draw
general conclusions from them, but some common findings are evident, and are
summarized in Table 2.3. The main conclusions inferred from the various freeze-thaw
studies are as follows:
" CFRP has a much higher environmental durability than either GFRP or
AFRP. The ultimate strength of the adhesive bond between FRP and
concrete deteriorates measurably, but not excessively, during freeze-thaw
cycling in chloride solution (Chajes et. al. 1995).
" On the other hand, the bond strength is either unchanged (Green et. al.
1997) or even improved (Green et. al. 2000) by the freeze-thaw cycling
procedures that employ dry-freeze water-thaw procedure and small
number of freeze-thaw cycles. This increase in bond strength is explained
by the wet curing of the concrete substrate during the freeze-thaw
procedure, which causes the strength to increase.
" The most pronounced degradation of the bond properties was recorded
by Chajes et. al., where small-scale CFRP strengthened beams lost 9% of
the ultimate strength after 100 freeze-thaw cycles in a 4% CaCI solution.
This result shows that the combined action of freeze-thaw cycling and
chlorides is possibly the most damaging type of freeze-thaw cycling.
" There were also significant changes in failure modes recorded in two of
the presented studies (Mukhopadhyaya et. al., Green et. al. 2000).
Karbhari and
Engineer 1996 also recorded negative changes in
mechanical parameters such as toughness and deflection at failure.
10
Table 2.3. Major findings of reviewed studies
Major Findings
Study
aI
t
Q
C'hajes . .
GFRP and AFRP strengthened beams lost about 50% of their
strength gain after freeze-thaw cycling, whereas CFRP
strengthened beams lost only 9% of the ultimate strength
Karbhari and Engineer Significant decrease in strength accompanied by an increase
in flexural stiffness, both changes were more pronounced in
GFRP strengthened beams
Karbhari et. al.
Significant increases in both mode I and mode II interfacial
fracture energies, with higher increases exhibited in the GFRP
specimens
Green et. al. (1997)
Strengthened beams not damaged by freeze-thaw exposure
Ultimate strength of the bond between FRP and concrete was
Mukhopadhyaya
not affected, differential movements (between FRP and
concrete) during tests increased after f-t exposure, failure
mode changed from concrete shear failure to adhesive failure
after f-t cycling
Green et. al. (2000)
Single lap shear specimens exhibited ultimate strength
increases of up to 54% after the freeze-thaw exposure and
failure mode progressively changed from concrete shear
failure to adhesive failure. Ultimate strength of small-scale
beam specimens increased during the f-t exposure, similarly
mid-span deflection increased after f-t exposure and failure
mode changed from concrete shear failure to adhesive failure.
Myers et. al.
Combined environmental exposure produced decrease of
ultimate strength and flexural stiffness in the specimens,
specimens loaded with sustained load shown larger
decreases in bond strength, contribution of f-t exposure on the
overall bond degradation cannot be determined
Table 2.4. Availability of data in the research area
Availability of Data (*)
Small-scale Single (double)
beam
lap shear
Available (2) Available (1)
Dry-freeze water-thaw
Chloride solution immersion freeze-thaw
Available (1)
cycling
Available 1
Standardized dry-freeze water-thaw
Standardized chloride solution immersion
Available (1)
freeze-thaw cycling
Direct comparison between dry-freeze, waterUnavailable
thaw and chloride solution immersion f-t
* Number of independent studies
11
Available 1
Available 1
Unavailable
Unavailable
Table 2.4 presents a summary of experimental data for various specimen types.
The number of independent experimental programs for each specified research topic is
shown in parentheses. From a review of Table 2.4, it is obvious that information on
freeze-thaw durability of shear or flexural strengthening with FRP is very limited. For
example, no study has directly compared the effects of dry-freeze water-thaw procedure
with the effects of chloride solution freeze-thaw procedure. In other cases, the data
available is limited only to one or two independent experimental programs and it is not
sufficient to draw general conclusions. Based on this analysis of previous research a
number of significant issues are highlighted:
1.
There is a need for a direct comparison between the effects of dry-freeze waterthaw procedure with the effects of chloride solution freeze-thaw procedure.
Making this comparison should be a next step towards better understanding of
the freeze-thaw durability of FRP strengthening in flexural or shear applications.
2. Another important aspect is that most of the reviewed studies did not use a
standardized freeze-thaw procedure that could be easily repeated by other
researchers. Therefore, it is important to utilize an existing standardized freezethaw procedure, for which experimental equipment is widely available and thus
the results can be independently verified.
3. It is also apparent that there have not been a large number of experimental
studies in the area of freeze-thaw durability of adhesive bond between FRP and
concrete. In addition, a great number of possible combinations of materials,
freeze-thaw procedures, specimen setups must be examined. Relative to the
12
number of possibilities, there is very limited data available on the behavior of the
adhesive bond of FRP to concrete.
Therefore, the experimental study described herein is meant to increase the
knowledge and understanding in this area by making a direct comparison between dryfreeze water-thaw and constant chloride solution immersion freeze-thaw procedures.
Both of the procedures used in the experimental study described herein were
standardized freeze-thaw procedures.
13
CHAPTER 3
EXPERIMENTAL PROGRAM
An experimental program was setup to investigate the durability of adhesive bond
between CFRP strips and concrete and to compare two types of freeze-thaw cycling.
Key factors varied in this study include: freeze-thaw procedures, number of freeze-thaw
cycles, material properties of the specimens and type of specimens (single lap shear,
pull-off, dumbbell). Each of these is discussed in detail below. A more detailed
presentation of the topics of this chapter is provided in Appendix C.
3.1
Freeze-thaw cycling
The freeze-thaw cycling of specimens was carried out according to two Czech
national standards. Standardized freeze-thaw cycles were used, because they are well
established and therefore can be replicated with the experimental equipment available.
The following two freeze-thaw standards were used:
CSN
73 1322 and CSN 73 1326.
These procedures were selected as representative of the two approaches employed in
previous research: freeze-thaw cycling with dry-freeze water-thaw and chloride solution
freeze-thaw cycling.
Chloride Solution Freeze-Thaw Cycling (CSN 73 1326)
The standard
CSN
73 1326 specifies freezing and thawing of specimens
constantly submerged in 5 mm of 3% NaCl solution in a specially built testing chamber
14
KD-20-S1.1. This type of exposure simulates real world applications subjected to deicing
chemicals or to seawater. The specimens lie in the 3% NaCI solution with the CFRP
strip on the bottom with approximately 3.5 mm of concrete being submerged. The
freeze-thaw cycle is as follows: start at +200C, decrease temperature to -150C in 45 to
50 minutes, hold -150C temperature for 15 minutes, increase temperature to +200C in 45
to 50 minutes and hold +200C for 15 minutes, this completes 1 cycle. Groups of
specimens were subjected to 25, 50 and 75 cycles.
Dry-Freeze Water-Thaw Freeze-Thaw Cycling (C SN 73 1322)
The standard CSN 73 1322 specifies freezing in cold air at -1 80C for 4 hours and
thawing in water bath at +200C for 2 hours. In the present study, the standard cycles
were modified to accommodate the experimental equipment available. The actual
freeze-thaw cycle used in this study consisted of freezing in cold air at -10 C for 4 hours
and thawing in +170C water bath for 1 hour. The temperature of the specimens was
monitored using a calibrated digital thermometer throughout the freeze-thaw cycling.
Groups of specimens were subjected to 25, 50 and 100 cycles.
Control Specimens
Specimens not subjected to freeze-thaw cycling were stored at room temperature
and ambient humidity.
15
3.2
Material properties
The testing program utilized specimens constructed from three components:
uniaxial pultrued CFRP strip, thixothropic epoxy adhesive and concrete mix. The
specifications of these components were varied as described below.
CFRP strips were selected because of their inherent corrosion resistance and
because they are widely used in actual field applications. The corrosion resistance of the
CFRP is important to minimize the influence of degradation of the FRP material on the
degradation of the bond. This would be the case if, for example, GFRP fabrics were
used. Also the CFRP strips are factory produced and therefore have very consistent
properties, which might not be the case with FRP made using the wet lay-up method.
The two types of CFRP used were SIKA Carbodur S512 and Fyfe Tyfo UC, which have
comparable mechanical properties (Table 3.1).
The manufacturers of each of the CFRP strips specify their own thixotropic
adhesive for overhead applications. Therefore, adhesives SIKA Sikadur 30 and Fyfe
Tyfo TC were used with their respective type of CFRP strip. Additionally, the Fyfe CFRP
strips were used in combination with Betolit 0-1 DC TH epoxy-based thixotropic
adhesive manufactured by Betosan, Czech Republic. Therefore three FRP/adhesive
combinations were used. The manufacturer-specified properties of the SIKA adhesive
are presented in Table 3.2. Detailed specifications for the other two adhesives are
unavailable, because the Fyfe Company does not publish this data and the Betosan
adhesive is only in a development stage.
The CFRP strips and adhesives were combined with either regular high strength
concrete or an air-entrained concrete mix. The properties of the two concrete mixes are
presented in Table 3.3. The air-entrained concrete was used to simulate real world
16
applications, whereas the non air entrained "regular" mix represented concrete with low
freeze-thaw resistance. Specimens of both concrete mixes were freeze-thaw tested
according to CSN 73 1326 in tap water and in 3% NaCI solution, to asses the relative
difference in degradation between the two freeze-thaw media. The specimens, with their
original surfaces mechanically abraded in preparation for adhesion of the FRP, were
also freeze-thaw cycled. This was done so as to establish the influence of the
mechanical abrasion procedure on the freeze-thaw resistance of the resulting surface.
Table 3.1. Properties of CFRP strips
SIKA
Carbodur
S512
Property
68
(%)
content
Fiber volumetric
1600
Apparent density (kg/m3)
Modulus of elasticity (MPa) >165 000
>2 800
Tensile strength (MPa)
>1.7
Elongation at break (%)
1.2
Thickness (mm)
50
Width (mm)
Fyfe
Tyfo UC
68
1810
155 000
1800
1.3
1.4
50.8
Table 3.2. Manufacturer specified adhesive properties
Sikadur 30
Property (23 0C and 50% R.H.)
1.77
Density (kg/L)
(ASTM D 638 @ 7 days)
24.8
Tensile Strength (MPa)
1
Strain (elongation) at break (%)
4500
Modulus of elasticity (MPa)
9
C)
0-/
(1
expansion
thermal
of
Coefficient
Table 3.3. Properties of concrete mix
Tensile
strength
28-day
compressive (3-point
bending)
strength
(MPa)
(MPa)
Mix Type
7.7
42
Air-entrained
9.5
66
Normal
Air
entrainment Slump
(mm)
(%)
70
9.5
3
3.5
17
3.3
Single lap shear specimens
The single lap shear specimens were selected as the main specimens for this
study, because of their relative geometrical, manufacturing and testing simplicity. For the
single lap shear tests, a total of 84 specimens were used, of which 12 specimens were
left in the laboratory under ambient conditions as control specimens. The remaining 72
specimens were subdivided into three groups of 24 specimens, each using a different
FRP/adhesive combination. Each specimen group was then further subdivided into
halves (twelve specimens each) by the type of concrete (regular or air-entrained). The
twelve specimens were further subdivided into two groups (six specimens), according to
type of freeze-thaw cycling procedure (CSN 73 1322 or CSN 73 1326). Finally the six
specimens were subdivided into pairs according to number of freeze-thaw cycles. Each
pair of the single lap shear specimens created one data point by averaging the results.
The concrete blocks for single lap shear specimens were manufactured in
standard small-scale beam forms (100 x 100 x 400 mm), split in half using a stainless
steel insert. The blocks were removed from the forms after 24 hours and wet cured for
seven days. After the 28-day curing period the specimens were cut to their final size of
50 x 100 x 200 mm. The regular concrete mix had a 28-day strength of 66 MPa and the
air-entrained mix had a 28-day strength of 42 MPa (150 x 300 mm cylinder specimens).
The surface preparation was done using a Hilti demolition hammer with a bushing tool
chisel. Following the mechanical abrasion, the surface was vacuum cleaned. The FRP
was also cleaned of all dust and chemically cleaned using SIKA Colma Reiniger. The
FRP was then adhered to the specified region on the concrete block, using one of the
three adhesive types. A bond length of 160 mm was used, based on the overall
geometry of the concrete blocks (length 200 mm). To prevent any possible edge effects,
18
20 mm in the main
the distance from the bonded region to the free edges was kept at
FRP) or 50.8 mm
direction of the FRP. The width of the bond was either 50 mm (SIKA
which the freeze-thaw
(Fyfe FRP). The specimens were then cured for 7 days, after
the single lap shear
cycling commenced. The overall shape and dimensions of
specimen are shown in Figure 3.1.
Following the environmental exposure, the
of their different
specimens were tested using two different testing frames because
computer controlled
availability. The control specimens group was tested on a 100 kN
on an older 400 kN
MTS 810 frame whereas the rest of the specimens was tested
manually controlled testing frame shown in Figures 3.2 and 3.3.
Fig.3.1. Single lap shear specimen
-4E
'Don
A
19
Fig.3.2. Shear test setup
3.4
Fig.3.3. Shear test detail
Pull-off specimens
The pull-off specimens utilized the same concrete blocks as the single lap shear
specimens. In this case, two pull-off tests (ASTM D4541 type I) were performed on each
of the blocks. Each pull-off specimen consisted of 50 x 50 mm square of FRP strip
adhesively bonded to the concrete block. After being freeze-thaw cycled in the same
manner as the single lap shear specimens, aluminum dollies (50 x 50 mm) were
adhered using SIKA Sikadur 30. The specimens were then tested in direct tension to
failure using a Dyna Z15 pull-off tester (Figure 3.4). The geometry of the specimens with
the aluminum dollies adhered is shown in Figure 3.5. The pull-off specimens were used
to discern if there is any correlation between the simple pull-off test that can be easily
performed in field conditions and the shear strength of the FRP bond to concrete. Such
20
correlation would be very beneficial, as it could relate the results of a very simple field
test to the shear strength of the bond that can be only measured using laboratory
equipment. This relation could be utilized both for quality assurance and for the design
of the FRP strengthening.
FI .3.4. Pull-off test setu
3.5
Fig.3.5. Pull-off specimen
Adhesive tensile specimens
To investigate the changes in bulk properties of the structural adhesives during
freeze-thaw cycling, a set of ASTM D638 dumbell specimens was utilized. The
D638
specimens were manufactured using Teflon forms of the specified shape (ASTM
at
Type I). The specimens were removed from the forms after 24 hours and left to cure
room temperature and ambient humidity. These specimens were freeze-thaw cycled in
the same manner as the single lap shear and pull-off specimens. Following the
exposure, the specimens were tested in tension in a 100 kN Instron frame and their
elongation was measured with an extensometer (Figure 3.6). Two dumbbell specimens
21
of the same material and exposed to the same freeze-thaw cycling created one data
point by averaging the results.
Fig.3.6. Adhesive testing setup
3.6
Thermal expansion specimens
The coefficient of thermal expansion (CTE) of the structural adhesives is very
important, because it differs significantly from that of concrete and the CFRP strip. This
mismatch might play an important role in the deterioration of the adhesive bond
properties during temperature cycling. To measure the CTE of each of the adhesives, 10
x 10 x 120 mm small-scale prisms were manufactured. Two prisms were manufactured
for each of the adhesive types. The thermal expansion properties were then determined
by precise measurement of the overall length of the prisms after 24-hour exposure to a
given temperature. At temperatures at or above 300C the specimens were warmed up by
an electronically controlled water bath. At temperatures below 250C the specimens were
cooled or frozen in air, while a calibrated thermometer determined their exact
temperature.
22
CHAPTER 4
EXPERIMENTAL RESULTS
The following sections present the results of the experimental program. A more
detailed presentation of these results is provided in Appendix D. The results are in each
case divided into two halves according to the two freeze-thaw cycling procedures used
(CSN 73 1326 and CSN 73 1322). This is due to the very different effect of the two
freeze-thaw procedures on the properties of the bonded system. This difference is
clearly apparent in the freeze-thaw cycling of concrete specimens according to CSN 73
1326 both in tap water and 3% NaCl solution, the results of which are summarized in
Table 4.1. The results quote grams of oven-dried debonded material per square meter
after 75 cycles, as prescribed by the standard. The limiting value for freeze-thaw
resistant concrete is set to 1000 g/m
2
by the standard. The addition of the chloride
solution increases the damage (weight of delaminated material) to non air-entrained
concrete by an order of magnitude and the damage to air-entrained concrete by a factor
of 2.5.
The preceding results also explain the significant difference between shear
strength trends of specimens cycled according to the two different freeze-thaw
standards. In freeze-thaw cycling in water (CSN 73 1322) the concrete is actually wet
cured while the freeze-thaw damage is minimal. Therefore, the concrete attains higher
strength during the freeze-thaw cycling than the control specimens left in dry storage.
On the other hand, in freeze-thaw cycling in chloride solution (CSN 73 1326) the
23
combined action of chlorides and freeze-thaw cycling is 10 times more damaging on
regular concrete and 2.5 more damaging on air-entrained concrete. Therefore, the
concrete gradually loses its strength. This damage was obvious in both air-entrained
and regular concrete specimens after only 50 cycles (Figure 4.1), at which point the
edges and exposed surfaces started to disintegrate. At 75 cycles the damage to the
concrete was even more pronounced, with a larger volume of material coming off the
edges and the exposed surface of the specimens. The total of 75 cycles prescribed by
the tSN 73 1326 seems to be an adequate number for the chloride solution freeze-thaw
cycling as a larger number of cycles would likely result in a complete disintegration of
the concrete substrate submerged in the chloride solution. Finally, the data in Table 4.1
also shows that the mechanical surface preparation procedure appears to have a
minimal impact on the freeze-thaw resistance of concrete in this type of freeze-thaw
cycling.
Table 4.1. Freeze-thaw properties of concrete substrate
Total
delaminated
concrete after 75
Testing cycles (CSN 73
Concrete Type
Medium 1326) (g/m 2)
Air-entrained concrete
H2 0
305
Air-entrained concrete
3% NaCl
765
Air-entrained surface-prepared concrete 3% NaCl
851
Normal concrete
H2 0
191
Normal concrete
3% NaCl
1812
Normal surface-prepared concrete
3% NaCl
1762
24
Fig.4.1. - Deteriorated pull-off specimen
4.1 Shear test results
Chloride Solution Freeze-Thaw Cycling (CSN 73 1326)
The results of shear testing of single lap specimens freeze-thaw cycled according
to SN 73 1326 are presented in Table 4.2. A tested single lap specimen is shown in
Figure 4.4. The relative changes in the shear strength between the control specimens
and the freeze-thaw cycled specimens are of the greatest importance, as opposed to the
absolute values of the shear strength, which are determined by the particular type of
concrete, the surface preparation procedure and the specimen geometry. The
SIKA/SIKA adhesive/FRP combination exhibited 8% loss of shear strength in the airentrained concrete and 10% loss in regular concrete specimens. The Fyfe/Fyfe
combination showed 4% loss of shear strength in the air-entrained concrete and 24%
loss in regular concrete specimens. Finally, the Fyfe/Betosan combination experienced
a 23% loss of shear strength in the air-entrained concrete and a 21% loss in regular
concrete specimens.
25
An initial increase of the shear strength followed by a decrease after additional
cycles is evident in all these results (Figures 4.2 and 4.3): Wet curing of the concrete
produced the initial increase in strength. At about 25 cycles the freeze-thaw damage
started to dominate, because the positive effect of wet curing was much smaller in
comparison. At this point the shear strength of the bond started to decrease gradually.
Dy-Freeze Water-Thaw Freeze-Thaw Cycling ( SN 73 1322)
The results of the shear tests of specimens freeze-thaw cycled according to CSN
73 1322 are presented in Table 4.3. The SIKA/SIKA adhesive/FRP combination
exhibited a 52% increase in shear strength in the air-entrained concrete and a 3%
decrease in shear strength in the regular concrete specimens. The Fyfe/Fyfe
combination showed an 8% increase of shear strength in air-entrained concrete
specimens and was not changed by freeze-thaw cycling in regular concrete specimens.
Finally, the shear strength of the Fyfe/Betosan combination was not affected in the airentrained concrete specimens, but experienced a 14% loss in regular concrete,
compared to the control specimens. The results for specimens using regular concrete
show
a
decreasing
trend
for
common
the
SIKA/SIKA
and
Fyfe/Betosan
adhesive/combinations. On the other hand, the Fyfe/Fyfe combination was unaffected in
the regular concrete specimens. This difference was probably caused by the Fyfe
adhesive being rather liquid, which caused the adhesive to spread to a much larger area
during the adhesion procedure. This would lead to some of the Fyfe/Fyfe specimens
being protected by a layer of adhesive with surface area larger than in the other two
types of adhesives. This protective layer would lengthen the diffusion path of the water
into the bondline, thus delaying the deterioration.
26
Failure Modes
The failure modes were also surveyed after the shear testing. Failure occurred in
three modes as follows: 1.) Concrete cohesive failure, 2.) Adhesive failure between
concrete and the adhesive, 3.) Adhesive failure between the adhesive and the FRP strip.
These modes usually appeared in combination in the failure plane. The area of each of
the failure modes was measured and is presented below as a percentage of the overall
failure plane area. In general, there were no obvious trends observed in the failure
modes after the freeze-thaw cycling. It appears that the failure mode was determined
solely by the type of adhesive and the type of concrete substrate, as opposed to the
freeze-thaw exposure. This contrasts with the results of Mukhopadhyaya et. al. (1998)
and Green et. al. (2000), which found clear change in the failure modes. The failure
mode in these studies changed from cohesive failure of concrete in the unexposed
specimens, to adhesive failure between concrete and the adhesive after the freeze-thaw
exposure.
The following section presents the area percentage of each failure mode on the
overall failure plane area in specimens of all material combinations. The SIKA bonded
specimens had cohesive failure of concrete in approximately 80% of area in specimens
using air-entrained concrete with remaining 20% of failure plane area exhibiting
adhesive failure between concrete and the adhesive. The SIKA bonded specimens
using regular concrete had around 40% of bond area attributed cohesive failure of
concrete, regardless of the freeze-thaw cycling procedure or the number of cycles. The
Fyfe adhesive bonded specimens had approximately 75% of failure plane in concrete in
air-entrained concrete specimens and around 50% of failure plane in concrete in
specimens using regular concrete. Finally, the specimens bonded with the Betosan
27
adhesive had approximately 60% of failure plane in concrete in specimens using airentrained concrete and around 40% of failure plane in concrete in specimens using
regular concrete. In most cases the remaining failure plane area was attributed to the
second type of failure mode (adhesive failure between the adhesive and the concrete).
The Betosan adhesive also exhibited up to 15% of area debonding between the
adhesive and the CFRP strip. This undesirable failure mode is probably related to the
adhesive being only in its development stage. The properties of this adhesive should be
improved for the final product, as this type of debonding is undesirable. Overall, the
failure mode was determined by the relative mechanical properties of the concrete and
the adhesive and was not influenced by either of the freeze-thaw cycling procedures.
Ultimate Displacement
The ultimate (at break) displacement of the CFRP strip relative to the concrete
block was also recorded and the data surveyed for trends. The shear test data did not
show any trends in the Fyfe and Betosan adhesives. On the other hand, the SIKA
adhesive exhibited increasing ultimate displacement in both of the freeze-thaw
procedures and in both types of concrete substrates. This might be explained by the
adhesive plasticization after the exposure to water during freeze-thaw cycling, and is
consistent with the tensile specimen results of the SIKA adhesive, which showed larger
ultimate strains after both types of freeze-thaw cycling, as documented in Table 4.6 in
section 4.
28
Table 4.2. Single lap shear results of specimens cycled according to
CSN 73 1326
Ultimate Displacement (mm)
Shear Stength (MPa)
Number of
Betosan
SIKA Fyfe
Betosan
SIKA Fyfe
Cycles
Air-entrained concrete
N/A
N/A
N/A
3.0
2.6
2.5
0
2.5
2.2
2.6
3.2
2.9
2.9
25
2.2
2.1
2.7
3.1
2.7
3.0
50
2.3
3.0
2.7
2.3
2.5
2.3
75
Normal concrete
N/A
N/A
N/A
2.9
2.9
3.1
0
2.1
1.8
2.8
2.5
3.1
3.0
25
3.2
2.7
3.0
2.4
2.5
3.3
50
2.7
4.2
4.1
2.3
2.8
2.2
75
Table 4.3. Single lap shear results of specimens cycled according to
CSN 73 1322
Ultimate Displacement (mm)
Shear Stength (MPa)
Number of
Betosan
SIKA Fyfe
Betosan
Fyfe
SIKA
Cycles
Air-entrained concrete
N/A
N/A
N/A
3.0
2.6
2.5
0
2.3
2.2
3.3
2.9
3.2
2.8
25
2.2
3.6
3.7
3.3
2.7
2.8
50
2.1
2.4
3.6
3.0
2.8
3.8
100
Normal concrete
N/A
N/A
N/A
2.9
2.9
3.1
0
2.9
2.5
2.6
3.2
3.1
3.3
25
2.7
2.9
2.4
2.7
3.4
3.1
50
2.3
1.7
4.9
2.5
3.0
3.0
100
29
Fig.4.2. Shear strength during freeze-thaw exposure according to tSN 73 1326
(air-entrained concrete)
.~~
..
-...-.
4.0
--
-.
3.5
3.0
2.5
2.0
.0
1.5 -
V
) 1.0 0.5
0.0
0
50
25
75
Number of Cycles
-+Fyfe/Fyfe
SIKA/SIKA
W, Fyfe/Betosan
Fig.4.3. Shear strength during freeze-thaw exposure according to CSN 73 1326
(normal concrete)
4.0
3.5
3.0
e 2.5
2.0
V)
L1.5
S1.0
0.5
0.0
0
50
25
Number of Cycles
SSIKA -$.
Fyfe/Fyfe -0- Fyfe/Betoan
30
75
Fig.4.4. Typical tested single lap shear specimen
4.2
Pull-off test results
As described in section 3.6, a large number of pull-off specimens were subjected
during
to the same freeze-thaw cycling procedures as the single lap shear specimens,
which the peak load and the failure mode were recorded.
Chloride Solution Freeze-Thaw Cycling (CSN 73 1326)
The ultimate strength attained in the pull-off tests of specimens cycled according
in
to CSN 73 1326 is presented in Table 4.4, but more important are the changes
ultimate strength during the freeze-thaw cycling. The SIKA/SIKA FRP/adhesive
concrete
combination experienced a 21% increase in pull-off strength in air-entrained
Fyfe/Fyfe
specimens and a 7% increase in regular concrete specimens. The
a 48%
combination experienced a 5% increase in strength in air-entrained concrete and
loss of strength in regular concrete specimens. Finally, the Fyfe/Betosan combination
35% of
was unaffected by the freeze-thaw cycling in air-entrained concrete, but lost
strength in regular concrete specimens.
31
DQy-Freeze Water-Thaw Freeze-Thaw Cycling (CSN 73 1322)
The ultimate strength of the pull-off specimens cycled according to CSN 73 1322
is presented in Table 4.5. The SIKA/SIKA FRP/adhesive combination experienced a
14% increase in pull-off strength in air-entrained concrete and a slight decrease of 4% in
regular concrete specimens. The Fyfe/Fyfe combination experienced an 11 % increase
in strength in air-entrained concrete and a 33% gain in strength in regular concrete
specimens. Finally, the Fyfe/Betosan combination experienced a 58% strength increase
in air-entrained concrete, but lost 9% of strength in regular concrete specimens.
Discussion of Shear Strength Results
The complex behavior of the shear strength results can be explained by three
important factors, which together affect overall bond strength during freeze-thaw cycling.
The first factor is the wet curing of concrete substrate during the freeze-thaw cycling.
Control specimens kept in dry storage are not wet cured. The second factor is the actual
damage to the concrete substrate, caused by the freeze-thaw action of the liquid within
the pore structure (this damage is negligible in air-entrained concrete). Finally, there is
the ability of the adhesive to bond to the concrete surface, which is dependent on the
chemical composition of the adhesive and the type and size of fillers.
As is apparent from all the results of this study, the overall bond strength is
dominated by the strength of the concrete. This is due to the concrete being the weakest
of the three materials used in the bonded system. Therefore, the response of the
concrete to freeze-thaw cycling is of utmost importance. For example, air-entrained
concrete is unaffected by the freeze-thaw damage in the pore structure, and because it
is wet cured, its strength increases. On the other hand, regular concrete is significantly
32
damaged by the freeze-thaw action and loses its strength to a degree that outweighs the
benefits of wet curing. The varying degree of strength increase or loss of a specific
bonded system is then given by the composition and bonding properties of the particular
adhesive. This explains the rather significant differences between the three adhesives in
the amount of strength loss or gain during the freeze-thaw cycling.
Failure Modes
In both types of freeze-thaw cycling, there were only two modes of failure
observed: cohesive failure in concrete and adhesive failure between the adhesive and
the concrete. The failure modes did not change with freeze-thaw cycling and were only
dependent on the type of concrete substrate. Air-entrained concrete specimens
exhibited 100% cohesive failure of concrete, as this substrate is weaker than the
adhesive. The regular concrete specimens exhibited about 90% cohesive failure of
concrete. This 10% difference results from the relatively higher tensile strength of
regular concrete.
Correlation Between Single Lap Shear Strength and Pull-off Strength
A correlation between the single lap shear strength and the pull-off strength was
anticipated to some degree, as both are directly proportional to the tensile strength of
the concrete. This expected correlation was not corroborated by the experimental data.
This was probably caused by the different response of each type of specimen to freezethaw exposure. In most cases, the pull-off specimens were much more affected than the
single lap shear specimens. This can be attributed to the much smaller bond area (50 x
50 mm) of the pull-off specimens, which allowed for rapid liquid ingress, whereas the 50
33
x 160 mm bond area of the single lap shear specimens required much longer for liquid
to penetrate, so the subsequent freeze-thaw damage was delayed. The much larger
changes of pull-off strength, as compared to shear strength, most likely result from the
fact that the pull-off test is more directly proportional to the strength (tensile) of concrete.
In the case of the shear test, part of the shear strength is also contributed by the
mechanical interlocking mechanism, not present in the pull-off test.
Table 4.4. Pull-off test results of
specimens
cycled according to (SN 73 1326
Pull-off Stength (MPa)
Number of
SIKA Fyfe Betosan
Cycles
Air-entrained concrete
1.9
1.9
2.8
0
2.2
2.1
3.1
25
0.9
3.2
2.1
50
1.9
2.0
3.4
75
Normal concrete
3.4
2.1
2.7
0
2.6
2.1
2.6
25
2.3
1.9
1.9
50
2.2
1.1
2.9
75
Table 4.5. Pull-off test results of
specimens
cycled according to CSN 73 1322
Pull-off Stength (MPa)
Number of
SIKA Fyfe Betosan
Cycles
Air-entrained concrete
1.9
1.9
2.8
0
1.7
1.8
3.4
25
2.3
1.7
3.3
50
3.0
2.1
3.2
100
Normal concrete
3.4
2.1
2.7
0
3.1
2.5
3.5
25
3.3
2.9
3.6
50
3.1
2.8
2.6
100
34
Fig.4.5. Typical tested pull-off specimen
4.3
Adhesive tensile test results
As described in section 3.7, dumbell specimens (ASTM D638 Type I) of the three
adhesives were used to investigate changes of their bulk properties in tension. The
initial properties of the three adhesives significantly differ in ultimate strength, modulus,
and ultimate strain. Thus, it was necessary to assess the possible deterioration of these
properties when subjected to the two freeze-thaw cycling procedures. The results of
these experiments are presented in Table 4.6.
Chloride Solution Freeze-Thaw Cycling (CSN 73 1326)
The ultimate strength of the SIKA adhesive (32.5 MPa) was unaffected by freezethaw cycling according to
SN 73 1326, but was about 31%
higher than the
manufacturer specified strength of 24.8 MPa. The ultimate strength decreased by 10%
for the Fyfe adhesive and by 6% for the Betosan adhesive. The SIKA adhesive had an
initial tangent modulus of approximately 11500 MPa, which is significantly (+155%)
higher than manufacturer specified modulus of 4500 MPa (Table 3.2), obtained from the
same test. After the freeze-thaw cycling (tSN 73 1326), some of the SIKA specimens
35
showed an increase in the modulus to about 12500 MPa (+9%), whereas some
specimen's modulus did not change. The modulus of the Fyfe adhesive (1816 MPa) did
not change significantly during the freeze-thaw cycling in any of the specimens. On the
other hand, the initial tangent modulus of the Betosan adhesive of about 4500 MPa
decreased to approximately 3900 MPa (-13%). A review of the ultimate strain (at break)
did not show any trends and overall the ultimate strain changed by no more than 7%.
Dry-Freeze Water-Thaw Freeze-Thaw Cycling (CSN 73 1322)
The ultimate strength of the SIKA adhesive increased by 15% after the freezethaw cycling according to CSN 73 1322. The ultimate strength of the Fyfe and Betosan
adhesives was unaffected by freeze-thaw cycling of this type, as the recorded changes
were statistically insignificant. The SIKA adhesive had an initial tangent modulus of
approximately 11500 MPa. After this type of freeze-thaw cycling the modulus increased
to about 12350 MPa (+7%). On the other hand, the modulus of the Fyfe adhesive (1816
MPa) did not change significantly after the freeze-thaw cycling (CSN 73 1322). The
modulus of the Betosan adhesive decreased from its initial 4500 MPa to approximately
4100 MPa (-9%). Review of the ultimate strain in this case does show an increasing
trend in the SIKA (22%) and Fyfe (27%) adhesives. The ultimate strain of the Betosan
adhesive was unaffected by this type of freeze-thaw cycling.
Discussion of Adhesive Testing Results
The results of the adhesive tensile specimens cycled according to both the CSN
73 1322 and
CSN
73 1326 norms show significant difference between the response of
the SIKA adhesive to freeze-thaw cycling when compared to the Fyfe and Betosan
36
adhesives. The properties of these latter two adhesives deteriorated considerably with
the freeze-thaw cycling (CSN 73 1326). Overall, the most dramatic changes in
properties were the increase in ultimate strain of the SIKA (22%) and Fyfe (27%)
adhesives during the SN 73 1322 cycling. In case of the SIKA adhesive, the ultimate
strain increase in the adhesive testing correlates with the significant increase in the
ultimate displacement during the shear test, as described in section 4.1.
The
plasticization of the two adhesives can be explained by the ingress of water into the
adhesive during freeze-thaw cycling, resulting in a loss of cross-link density. The
reduced density of the cross-link network resulted in significantly altered mechanical
properties of the adhesives.
Table 4.6. Measured adhesive properties (ASTM D638)
Ultimate Strain (%)
Pull-off Stength (MPa)
Number of
Fyfe_ Betosan
SIKA
Betosan
Fyfe
SIKA
Cycles
iSN 73 1326
1.23
0.32 2.83
25.81
32.3 34.2
0
1.01
0.38 2.48
26.6
36.2 30.9
25
0.99
0.30 2.86
25.0
32.5 29.4
50
1.31
0.30 2.75
24.2
32.7 30.9
75
CSN 73 1322
1.23
0.32 2.83
25.8
32.3 34.2
0
1.24
0.40 3.23
28.9
36.1 33.8
25
1.35
2.47
0.29
26.0
32.8
29.6
50
1.12
0.37 3.23
25.1
33.6 32.1
75
1.22
3.58
0.39
24.7
33.3
37.1
100
37
Fig.4.6. Stress-strain diagram for typical adhesive specimens
------ ------~ ------
40
___
_
30
25
(n
20
15
10
5
0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Strain (%)
-
4.4
SIKA -
Fyf e -
Betos an
Adhesive coefficient of thermal expansion results
The CTE measurements for the adhesives used in this study are presented in
Table 4.7. The results show significant differences in CTE among the three adhesives.
The SIKA adhesive has the smallest CTE of the group, at around 2 x 10-5 per OC in most
of the temperature range. On the other hand, Fyfe and Betosan have a highly variable
CTE, dependent on the temperature range. Their CTE is around 4 x 10-5 per OC in the
low temperature range (-32 0C to +25 0C) and much higher and gradually increasing CTE
in the high temperature range (+30 OC to +60 OC). These values must then be compared
with the CTE of concrete (around 1
10-5
x
10-5 per OC) and the CTE of the CFRP strip (0.1 x
per OC for SIKA Carbodur S512). This comparison shows that in some temperature
ranges, there is an order of magnitude difference between the CTE of concrete and the
CTE of the adhesive. Also, there is a difference of two orders of magnitude between the
CTE of the CFRP strip and the CTE of the adhesive. This mismatch in the CTE of the
three materials is very significant and it should be further investigated to assess its
38
contribution to the deterioration of bond strength during long-term exposure to
temperature cycles.
Table 4.7. Coefficient of thermal expansion of
the adhesives
Coefficient of thermal expansion (10-5/o(
Betosan
SIKA Fyfe
Interval (OC)
3.8
4.2
2.0
-32 -- 16
3.8
3.8
1.8
-16-+1
4.4
1.4
-0.5
+1 -+25
10.8
1.1
8.7
30-40
10.1
13.1
40-50
2.6
13.0
16.0
4.8
50-60
39
CHAPTER 5
DISCUSSION & CONCLUSION
5.1
Discussion of current results
This section presents a summary and discussion of the most significant findings
in this thesis.
The most important finding of this study is that the strength of the adhesive bond
of FRP to concrete degrades noticeably during freeze-thaw cycling in chloride solution.
This is caused mostly by degradation of the properties of concrete with severe
deterioration after only 50 cycles. Continuing beyond the standard 75 cycles would
result in complete disintegration of the concrete substrate submerged in the chloride
solution, without significantly reducing the mechanical properties of either the adhesives
or the CFRP.
It can thus be stated that if CFRP strengthening is applied to high quality, freezethaw resistant concrete, the durability of the strengthening is good. On the other hand,
bond properties might degrade significantly in "not quite" freeze-thaw resistant concrete.
Such concrete substrates must be properly monitored and measures must be taken to
prevent bond deterioration. The changes in mechanical properties of the epoxy
adhesives due to freeze-thaw cycling are relatively small and do not contribute
significantly to the bond strength deterioration. The changes of the mechanical
properties of the CFRP strips were assumed to be negligible in this type of
environmental exposure as the CFRP strips are highly durable.
40
Freeze-thaw cycling in water (CSN 73 1322) did not produce any significant
damage to the bond strength. The bond strength actually went up in most specimens as
the concrete was saturated throughout the exposure and thus wet cured. The negative
effect of freeze-thaw deterioration without the presence of chlorides was much smaller
than the positive effect of wet curing. If dry-freeze water-thaw freeze-thaw procedure is
utilized, a much greater number of cycles is needed to generate the same bond/concrete
deterioration as in freeze-thaw cycling with 3% NaCl solution. The actual ratio of the
number of cycles needed for equivalent freeze-thaw damage, based on the measured
deterioration of the concrete, was found to be about 10 times for regular concrete and
2.5 times for air-entrained concrete.
In all specimens, the adhesive surrounding the bonded area played an important
role by protecting the bondline. This superfluous adhesive created a much longer
diffusion path into the bondline and held the concrete surface together when the
surrounding areas were quickly disintegrating. Therefore, it might be a good practice to
apply adhesive outside of the bond area in order to minimize deterioration of the
bondline. Waterproof coatings can be also applied to perform the same function.
There is no clear correlation between the pull-off test (ASTM D4541 type I) and
the shear test. Therefore, the pull-off test should not be used as a measure of the shear
strength of the bond. The pull-off test should be used only as an approximate guide. A
minimal requirement for the pull-off test can be specified to make sure that the bond
strength is in an acceptable range.
The coefficients of thermal expansion (CTE) of the adhesives measured in this
study were found to be significantly different than that of concrete or CFRP. The CTE of
the SIKA adhesive was found to be lower than specified by the manufacturer, about
41
twice that of concrete. The Fyfe and Betosan adhesive showed a much higher CTE,
which was dependent on temperature range, about an order of magnitude higher that
that of concrete. The CTE of CFRP is an order of magnitude lower than that of concrete
and the contribution of this CTE mismatch to overall deterioration of the bond properties
during temperature cycling should be further investigated.
5.2
Comparison to Previous Research
Comparison of the current findings to the studies referenced in the literature
review shows that while there is good agreement in the results in many respects there
are significant differences. Unfortunately, there exist only a few comparable studies in
the area of freeze-thaw cycling of shear loaded adhesive bond between FRP and
concrete. It is important to note that the studies referenced have used different types of
materials, most of them using laboratory manufactured composites prepared using the
wet lay-up method, as opposed to the factory made CFRP strips utilized in this study.
Also, the specimens were different, mostly concrete beams strengthened with FRP and
loaded in four-point bending. Only one of the referenced studies (Green et. al., 2000)
used single lap shear specimens as in this study. All of these differences must be taken
into account when comparing the results and findings.
Overall, this study has found that the ultimate strength of the adhesive bond of
FRP to concrete decreases measurably in the chloride solution freeze-thaw cycling, but
that it increases in water freeze-thaw cycling. This fully agrees with the findings of the
referenced studies, which showed improvements of ultimate strength in water freezethaw cycling (Mukhopadhyaya et. al. 1998), but a decrease in strength in the chloride
solution freeze-thaw cycling (Chajes et. al. 1995). The most pronounced degradation
42
was recorded by Chajes et. al., where small scale FRP-strengthened beams lost 9% of
their ultimate strength after 100 freeze-thaw cycles in a 4% CaCl solution
Mukhopadhyaya et. al. (1998) and Green et. al. (2000) have recorded significant
changes in the failure modes prior to and following the freeze-thaw cycling. This was not
the case in this study, where the failure modes of both single lap shear and pull-off
specimens did not change appreciably during freeze-thaw cycling. The failure modes in
this study were determined solely by the type of concrete substrate and type of adhesive
used. This difference is explained by the Mukhopadhyaya et. al. study using 450 freezethaw cycles in 5% NaCl solution and Green et. al. (2000) study using 300 cycles in
water. Both studies used many more cycles, which suggests that the pronounced
change in failure modes occurs at a higher number of freeze-thaw cycles than used in
this study. It is also important to take into account the possible differences in durability
properties of both the adhesives and concrete used in the different studies.
Changes in other parameters like toughness and deflection are also very
important signs of bond deterioration. For example, Karbhari and Engineer (1996)
recorded significant changes in toughness and ultimate deflection in their small-scale
beam specimens. In the current study, the relative ultimate displacement in the SIKA
single lap shear specimens significantly increased, because of adhesive plasticization
after exposure to water during freeze-thaw cycling. A similar trend was also found in the
SIKA adhesive tensile specimens. This finding is supported by the results obtained by
Bowditch (1996) and Knox and Cowling (1999), which showed significant plasticization
of the adhesives following absorption of water into the adhesive.
As in this thesis, the Green et. al. (2000) utilized single lap shear specimens in a
dry-freeze water-thaw freeze-thaw procedure. Like the current study, the Green study
43
found that specimens attained significantly higher ultimate strengths with increases up
to 54% over the dry stored control specimens, after up to 300 freeze-thaw cycles in
water. This study recorded increases in shear strength of up to 52% after 100 freezethaw cycles in water. This similarity in strength increase, despite a very different number
of freeze-thaw cycles, is explained by the limited maximum strength of concrete that can
be achieved through wet curing. The Green study also found pronounced shifts in the
failure mode from a concrete cohesive failure in the control specimens and specimens
subjected to 50 cycles, to mostly adhesive failure at 150 freeze-thaw cycles. In contrast,
the current study did not find any significant changes in failure modes due to the freezethaw cycling, which is explained by use of a smaller number of freeze-thaw cycles.
5.3
Conclusion
This study has found that the freeze-thaw durability of the adhesive bond
between CFRP and concrete is good and depends primarily on the freeze-thaw
durability of the concrete. Significant deterioration of the bond strength was recorded
after freeze-thaw cycling in chloride solution, but this was caused by severe freeze-thaw
damage of the concrete. Slight changes to the bulk properties of adhesive specimens
were also observed, but they are deemed to be insignificant compared to the severe
deterioration of the concrete.
5.4
Further work
The experimental results presented herein represent only a first step in a more
thorough investigation of freeze-thaw durability of adhesive bond of FRP to concrete.
Similar tests using more types of concrete with different freeze-thaw resistances should
44
be performed to establish the relation between concrete freeze-thaw durability and the
rate of degradation of the bond. Another important area for investigation is the freezethaw cycling of specimens with sustained load, which would be a better approximation to
the real world applications. A finite element analysis of the temperature cycling behavior
of the three materials in this bonded system (concrete, adhesive, FRP) with different
CTE should be undertaken to better understand the stresses in the adhesive bond due
to the CTE mismatch and to assess its contribution to the deterioration of bond strength.
45
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49
APPENDIX A
OVERVIEW OF FRP STRENGTHENING
A.
Advantages of using Fiber-Reinforced Polymer Composites in
Concrete Strengthening
Strengthening of reinforced concrete structures using FRP composites remains a
relatively new technique with great potential as proven by many examples of already
strengthened concrete structures in the United States, Europe and Japan. This method
builds mainly upon concrete and steel strengthening techniques used traditionally in the
construction industry.
In strengthening by application of concrete, the cross-sectional area of the
original structural member is simply increased by application of shotcrete (concrete
sprayed onto actual member) or by pouring concrete into a formwork around an existing
member. Therefore, the structural member is simply enlarged to produce another
member with a higher load bearing capacity. Unfortunately addition of concrete also
produces a member with much increased dead weight, since concrete has a unit weight
of around 2400 kg/M 3 . Steel is used commonly as externally bonded plates for
strengthening of concrete members. Unfortunately this method has
numerous
drawbacks, such as deterioration of its adhesively bonded interface due to corrosion of
the steel plate or twist of the steel plate. Another way of using steel for strengthening is
post-tensioning. Similarly to strengthening with concrete, strengthening with steel adds
50
significant amount of dead weight to the structural member, since the unit weight of steel
is 7850 kg/M 3.
There are many advantages of the use of the FRP composites over the traditional
materials. The FRP composites have very high specific strength and stiffness, compared
to the traditional construction materials, allowing for small addition to dead weight of the
structure and also easy installation. They also have better environmental durability than
traditional materials, for example steel rusts, which not only degrades its visual
appearance, but also negatively influences the steel adhesive interface (Hollaway and
Leeming 1999).
B.
Application of FRP composites
Since the FRP strips or sheets are very light, no heavy transportation or
installation equipment is needed for the application, making the process very fast and
cost efficient. Also, in this method the FRP composites can be installed from mobile
platforms, since epoxy is fast and easy to apply
Fig. 2.1. Application of FRP strips
onto underside of a concrete slab
(TGP 2003)
with the ability to hold the weight of the FRP
strips at any moment as shown in Figure A.1,
obviating the need for permanent scaffolding.
These are additional benefits of the strengthening
with FRP over traditional strengthening materials
such as steel or concrete, which always require
heavy equipment,
permanent scaffolding and
much longer time for application. This might be a
very significant if not deciding factor in structures where loss of use is very expensive,
51
such as offices, highway construction or industrial construction. A critical point for the
installation is the surface preparation of a member to be strengthened. The surface
needs to be made perfectly flat, composed of only sound material and clean in order to
ensure proper long-term adhesion of the FRP under all conditions (Hollaway and
Leeming 1999).
C.
Schemes for Strengthening of Concrete
Using FRP Composites
In reinforced concrete strengthening practice the FRP composites are most
commonly used as strips or sheets externally bonded with structural adhesive onto the
tension side of a structural member, although many other application schemes, such as
beam shear reinforcement, columns wraps, internal rods, prestressing tendons are also
possible. Table A.1 on following page (Hollaway and Leeming 1999) lists these various
possible applications of the FRP composites. Advantages of strengthening of reinforced
concrete with FRP are also comprehensively summarized in Uomoto et. al (2002).
52
Table A.1. Applications for [Fiber-Reinforced Polymer] Composite Plate Bonding
(Hollaway and Leeming 1999)
Comments
Structural need/deficiency FRP composite plate
bonding solution
Damaged concrete
Corrosion of reinforcement in Replacement of lost
reinforcement by plates of
must be replaced
reinforced concrete
without impairing
equivalent effect
behavior of plates
Inadequate flexural capacity Design FRP composite plate Extent of
bonding solution to add tensile strengthening limited
of reinforced concrete
elements
by capacity of
concrete in
compression. Plates
anchored by bond or
mechanically at their
ends
Need to ensure no
Replace prestress that has
Lost prestress due to
been lost with stressed
overstress of concrete
corrosion in prestressed
composites
in the short term
concrete
Safety net to cover uncertain Add plates, either stressed or Method may be
unstressed, to ensure safet y. particularly
durability of prestressed
Particularly appropriate if
appropriate with
concrete
corrosion unlikely but possi ble segmental
construction. May be
combined with
monitoring system
Add external prestress by
Inadequate stiffness or
means of a stressed
serviceability of cracked
reinforced concrete structure composite plate
Analyze stresses due to
Potential overstress due to
required structural alteration alteration, and design
composite reinforcement
before removing load-beari
members
Particularly
Increase in structural capacity Increase in stiffness and
ultimate capacity by plate
appropriate with
of timber structures
bonding
historic structures
Enhanced by external bonding Web reinforcement
Enhancement of shear
of stressed plates, or by web techniques little
capacity
reinforcement
researched
Horizontal structural members like beams, slabs and girders are usually
strengthened in flexure by application of FRP strips (Figure A.2 and A.3) on tension
53
face. Shear strengthening of reinforced concrete can be done by applying FRP
composites along vertical sides of strengthened beams (Figure A.2 and A.5) or by
wrapping the whole member in FRP sheets. In the shear loading case the concrete
develops shear cracks that the FRP will bridge and prevent the member from failure.
Fig. A.2 & A.3. Strengthening of horizontal structural members with
the FRP composites (MDA 2003)
Vertical structural members are strengthened with FRP composites by wrapping
the whole cross-section with FRP sheets, as shown in Figure A.4. In this application, the
FRP strengthens in two ways; first there is confinement effect of transverse fiber sheets,
which leads to an increase in the axial compressive strength of the confined concrete,
resulting subsequently in an increase in the contribution of concrete to the load-carrying
capacity of the column. Second, possible application of longitudinal fiber sheets can
contribute directly to the load-carrying capacity of the column (Tan 2002). The ductility of
members strengthened in such a way also increases.
54
Fig. A.4 & A.5. Strengthening of vertical structural members with
the FRP composites (MDA 2003)
r
A.2
re
Material Systems
The fiber-reinforced polymers (FRP) are high strength and stiffness materials that
are extremely lightweight compared to other strengthening materials such as steel or
concrete. These materials are nowadays widely used in retrofitting and strengthening of
older reinforced concrete structures, especially in North America, Europe and Japan, as
the use of FRP as retrofitting and strengthening material for concrete has been steadily
increasing during 1990s.
Fortunately today there is a large amount of experimental data on a short-term
behavior of the FRP strengthened concrete systems summarized into construction
specifications that have been developed in the United States under the ACI-440 and the
NCHRP 10-59 initiatives. These specifications allow for gradual increase of use of the
FRP composites as method of strengthening for concrete, since the designers now have
specific design guidelines based on validated experimental data.
55
In contrast, the long-term durability of FRP strengthened concrete members is an
area that is not yet fully explored and understood. Although the FRP composites
themselves are very durable in various environmental conditions, as proven by their
development and long-term use in areas such as aerospace, military and automotive, in
civil engineering applications the durability FRP concrete systems depends on the
performance of the adhesive bond of FRP to the concrete.
Even though there have been numerous studies into the effect of environmental
factors on FRP bond to concrete, there are still many areas in need of further research.
This is mainly given by vast number of combinations of environmental factors that can
affect differently various types of FRP, structural adhesives, concrete substrates and
their resulting bond. The effects of environmental conditioning can also present
themselves differently in various experiments on different types of specimens.
The specific environmental effect areas lacking research data are clearly shown
in comprehensive detailed analyses of up-to-date durability research data of FRP
strengthening of concrete presented in Karbhari et. al (2003) , CERF Report (2001) and
Bonacci et. al. (2000). One of areas in need of more experimental data highlighted in the
above reports is the effect of freeze-thaw cycling on the behavior of the adhesive bond
of FRP to concrete.
Given the above information the intention of this chapter is to provide state-of-theart review of FRP composites and structural adhesives in light of their use as a
strengthening technique for reinforced concrete. In the second part of this chapter will
focus on detailed review of up-to-date research articles concerned with freeze-thaw
cycling effect on the behavior of the FRP to concrete bond.
56
A.2.1 FRP Composites
A.
Overview
The fiber-reinforced polymers are composite materials (composed of two distinct
phases) consisting of long oriented fibers bonded together by a polymer resin. The goal
of this combination of two materials is to produce another material with properties
enhanced beyond simple addition of properties of the constituents. In the particular case
of FRP, the goal is to take advantage of specific properties of the fibers and the resin in
a way that maximizes the resulting properties of the FRP.
Another benefit of FRP composites is that the overall properties can be varied in
many ways to achieve properties required by a specific application. The variables in
FRP design are numerous, for example; type of fibers, orientation of fibers, fraction of
fibers in the cross-section, type of resin, type of hardener, manufacturing process and
many more. These can be varied depending on design conditions such as design load,
chemical exposure, temperature range, fire exposure, cost, etc.
It is important to note that the engineering (short-term) properties of the
composite are mostly dominated by the excellent tensile properties of the fiber
reinforcement; the resin properties dominate the long-term (durability) properties. The
role of the fibers is to support the tensile loads imposed onto the FRP. Structurally, these
lightweight small diameter fibers are very efficient, since they have extraordinary
strength and stiffness. These properties are achieved by the fibers' highly oriented and
defect-free microstructures. The fibers can be oriented in one direction or in two
directions in the form of woven fabric.
The role of the resin matrix is to support and protect the fibers. The matrix
supports the fibers under compressive loads and prevents them from buckling. The
57
matrix also ties the fibers together through adhesion and cohesion to develop composite
action and provides shear strength necessary to resist delamination. The resin also
protects the fibers from mechanical and chemical damage and micro cracking, and thus
provides for an excellent durability of the composite.
Manufacturing
The FRP composites used in civil engineering applications are manufactured or
applied by three different techniques know as pultrusion, wet lay-up and prepreg.
Pultrusion
is
a very good
method
for
consistency and economy of manufacturing
Fig A.6. Example of typical pultrued
FRP strip (Vector Group 2003)
large numbers of identical FRP members,
since it is an automated continuous process.
Dry fabric sheets of reinforcing fibers are
soaked with the resin and forced through a
heated die that cures the resin and makes the
composite solid. This way the structural shapes like sheets, strips or rods can be
manufactured with great quality and efficiency and then bonded to concrete on site
using structural adhesive. A typical pultrued CFRP strip is shown in Figure A.6.
The FRP composites can be also applied by the so-called wet lay-up method.
This method manufactures the FRP on site by hand-laying the dry fiber sheets on their
place of application and then saturating them with the resin. The fibers can also be
saturated with the resin on site prior to final placement, sometimes using mechanical
saturator machines for efficiency and consistency. The resin in this case can be both
ambient or high temperature curing. This allows for custom shapes with specific
58
properties to be built. Similar to the wet lay-up method is the so-called prepreg method.
In this method the fiber strips or fabrics are pre-impregnated with a resin in a tacky
consistency by the manufacturer. They are then adhered to a concrete member and
heat or pressure cured (ACI-440 2001, Barbero 1999, Peters 1998).
Fiber Reinforcement
B.
Today in civil engineering applications three types of fibers are commonly used to
manufacture the FRP composites; glass fibers, aramid fibers and carbon fibers. These
fibers differ both in their initial mechanical properties (Table A.2) and their long-term
durability characteristics; therefore the most advantageous type of fiber is dependent on
requirements of specific project. Following section includes brief description of each fiber
type.
Table A.2. Physical properties of three commonly used types fibers and their
comparison to steel
Type of Fiber
Modulus of Elasticity (GPa)
Tensile Strength (MPa)
Density (kg/M3)
Glass
72 - 85
3500 4600
2700
Aramid
8 - 100
600~
3000
1440
Carbon
120 - 800
2200~
5600
2000
Steel
170 - 210
2400~
3800
7850
Glass Fibers
In civil engineering applications there are commonly used two types of glass
fibers, so-called "Structural"
abbreviated as S-glass, and so-called "Electrical"
abbreviated as E-glass. S-glass, as it is obvious from its structural designation, has
better mechanical properties and chemical resistance, whereas E-glass is a standard
reinforcement and an excellent electrical insulator. E-glass has a strength in the range of
59
3500 MPa and a tensile modulus of elasticity around 72 GPa, S-glass has a strength of
around 4600 MPa and a modulus of about 85 GPa. Their unit weight is significantly
higher than that of aramid or carbon fibers, which is balanced by their relatively low cost.
Although the glass fibers have good initial characteristics, their main problem is the
durability. The glass fibers are prone to moisture-induced degradation if exposed to
moisture while also being subjected to stresses above certain critical values. Therefore,
if the glass fibers are not protected from the moisture attack, their physical properties
degrade significantly, which can lead to premature failure. It is also important to note
that the glass fibers are inorganic and they do not support combustion (Hollaway and
Leeming 1999, Peters 1998, Kelly 1989).
Aramid Fibers
Aramid fibers are organic fibers known under the commercial names of Kevlar
and Nomex. Their chemical formulation is aromatic polyamide and they divide into two
types; the para-aramids (Kevlar) and meta-aramids (Nomex). In general, the paraaramids have much higher strength and modulus than meta-aramids. The para-aramids
have a strength around 3000 MPa and a tensile modulus of elasticity around 100 GPa,
whereas meta-aramids have a strength in the range of 600-800 MPa and a modulus in
the range of 8-12 GPa. Aramid fibers also posses a very high fracture toughness. The
problematic areas of aramid fibers include high moisture absorption, lower adhesion to
matrix resin, very weak bending and compressive strength (fibers buckle or kink under
compression). It is important to note that the density of aramid fibers is the lowest of the
other fiber types and therefore they have very high strength-to-weight ratio. They are
also very resistant to organic solvents and electrical insulators.
60
The aramid fibers have very good fire resistance compared to both glass and
carbon fibers, given by their highly ordered and aromatic structure. Even though they
can be ignited by direct fire, they start decomposing at around 4500C but stop burning
upon removal of the source of fire. Another important fire resistance related property of
aramid fibers is that the fibers do not readily conduct heat into the matrix and thus limit
the heat exposure (Hollaway 1999, Peters 1998, Kelly 1989).
Carbon Fibers
Carbon fibers have the highest strength and stiffness of the three types of fibers
used in civil engineering applications. There are two distinct groups of carbon fibers, the
polyacrylonitrile (PAN) based fibers and pitch-based fibers, both of which are produced
by thermal decomposition of the constituents. In general, the PAN-based carbon fibers
are most commonly used carbon fibers, since they can achieve higher strengths (27005600 MPa) than pitch-based carbon fibers (2200-3500 MPa). On the other hand,,, the
pitch-based carbon fibers have a higher tensile elastic modulus (500-800 GPa) than
PAN-based carbon fibers (120-440 GPa). The high strength of the best PAN-based
carbon fibers is unfortunately accompanied by similarly high price that significantly limits
the use of the high strength fibers in construction industry. The pitch-based carbon fibers
have lower cost according to the lower cost of its constituents. The unit weight of carbon
fibers is directly in the middle between glass fibers at the higher end of range and
aramid fibers at the lower end. The carbon fibers show excellent fatigue resistance,
while they are generally more brittle than the other fiber types (Hollaway and Leeming
1999, Peters 1998, Kelly 1989).
61
Fiber Reinforcement Forms
The reinforcing fibers can be manufactured and used in various different forms.
Usually they are in form of continuous fibers called filaments, but they can also be in
form of discontinuous short fibers called staples. Continuous fibers are commonly
processed further into tows, yarns and fabric cloths.
Tow, also called strand or end, is an untwisted bundle of continuous fibers used
as one unit. Similarly a yarn is a twisted tow and a roving is a group of parallel
continuous fibers. These continuous fiber arrangements are used in manufacturing of
unidirectional FRP composites. They maximize the use of excellent tensile properties of
the fibers, since the fibers are straight, unlike in fabric cloths.
Fabric cloths are two-dimensional reinforcements manufactured by knitting,
weaving or simply adhering the individual strand or yarns. Their patterns are very similar
to those of textile fabrics. Since the fibers in fabrics are not straight, but rather wavy they
have lower tensile strength than unidirectional reinforcements. The decrease in tensile
strength is caused by a tendency of the fibers to straighten under tensile load leading to
local stress concentrations in the polymer matrix. Also, it is believed that in woven
fabrics the fibers sustain more damage during processing, due to additional handling of
the fibers.
Staples are very short fibers used to manufacture composite materials with
isotropic properties; they are placed in random directions, so that overall mechanical
properties are almost the same in every direction. In similar fashion the staples can be
directly added into concrete during mixing, in both concrete and polymer matrix they
help prevent development of micro cracking under loading (Barbero 1999, Peters 1998).
62
A. C.
Resin Matrix
There are two distinct categories of matrix resins used in civil engineering
applications; the two categories are thermosets and thermoplastics. The thermosets are
polymers that form permanent three-dimensional cross-linked network upon addition of
curing (cross-linking) agent. They are widely used in the construction industry due to
their excellent mechanical properties, such as strength and stiffness and also for their
limited curing shrinkage. The thermosets used in civil engineering are usually modified
in many ways to achieve specific properties. On the other hand, the thermoplastic resin
can be liquefied by heating and hardened by cooling infinite number of times without
damage to the hardened polymer properties. The resins most widely used in civil
engineering applications are thermosets, such as epoxy, polyester, vinyl ester and
phenolic resins, of which the first two will be introduced in more detail below (Barbero
1999, Hollaway and Leeming 1999, Peters 1998).
Epoxy Resin
Two-part epoxy is widely used in construction industry due to its good overall
properties. Epoxy has very low curing shrinkage, outstanding adhesion to various types
of fibers, due to its polar nature, and excellent resistance to hostile environments, given
by the cross-linked structure. It is also important to note that epoxy during curing does
not release any volatile byproduct that could otherwise cause void formation. The epoxy
used today can be both ambient or elevated temperature curing. The other advantage of
the epoxy that it is very versatile; therefore it can be formulated to meet a wide variety of
performance and processing requirements. The drawback of epoxy is that it readily
absorbs moisture both in cured and uncured condition, which has durability implications.
63
Epoxy can be effectively used only in structures with small deformations, since it's
elongation-to-failure is relatively low (Barbero 1999, Peters 1998).
Polyester Resin
Although polyester resins have very good mechanical properties, UV resistance,
and environmental durability and properly formulated can be highly chemical resistant,
there are two main problems with their use in FRP composites that favor the use of the
epoxy. First, there is relatively large curing shrinkage in polyesters; second, the
adhesion of polyester resins to the carbon and aramid fibers is not adequate. Therefore,
polyester resins are only combined commonly with glass fibers, where adequate
adhesion is achieved. The above drawbacks are in part balanced by lower cost of
polyester resin compared to epoxy resin. As with the epoxy, the polyesters can be both
ambient or elevated temperature curing (Barbero 1999, Peters 1998).
A. Mechanical Properties
As it is obvious from the above list of possible constituents, the FRP composites
can be manufactured or "tailored" in a way to suit specific application. This can be
achieved by combination of different resins with various types fibers in one of the
fabrication processes. Because of that they have very wide range of both mechanical
and physical properties. As with other composite materials the mechanical properties
are weighted average of those of the fibers and the resin, which are differing according
to angle between fiber reinforcement and the loading.
64
Strength
High tensile strength in the direction of reinforcing fibers is the main advantage of
FRP composites. In most civil engineering applications the high tensile strength is
utilized. This strength is given mainly by the contribution of the fibrous reinforcement
rather than the matrix. The tensile strength in the direction of the fibers for FRP
composites is in the range of 900 MPa for E-glass polyester combination to 2700 MPa
for carbon epoxy combination. Although these values are similar to one's achieved in
steel, what gives the FRP the advantage is the low density and resulting high specific
tensile strength.
On the other hand,,, the compressive strength of FRP composites is a fraction of
the tensile strength. This is the result of the buckling failure of the fibers under
compressive stress, where only the resin matrix carries the whole load. The
compressive strength for FRP composites ranges from 350 MPa to 1700 MPa.
Shear strength of the FRP composites is not great, since it is developed mainly
by the shear strength of the resin without contribution of the fibers. To achieve
reasonable shear strength, the fiber reinforcement of the composite has to be arranged
in multiple directions, preferably with one of these being the direction of maximum shear
stress. In this case the shear strength is excellent, since it is mainly developed by the
fibers instead of the resin matrix (Barbero 1999, Hollaway and Leeming 1999, Peters
1998).
Stiffness
Most noteworthy feature of FRP stiffness is the fact that the stress-strain
relationship is linear or nearly linear up to the point of failure, unlike that of steel or
65
concrete. Hence, the stiffness is constant with increasing load. The value of the modulus
of elasticity of the FRP composites in the direction of the fibers ranges from 38 GPa for
E-glass polyester combination to 155 GPa for carbon epoxy combination. These are on
the same order of magnitude as the elastic modulus of steel (210 GPa) (Barbero 1999,
Peters 1998).
Failure modes
There are three major failure modes specific to fiber-reinforced composites;
matrix micro cracking, delamination and fiber fracture. Matrix micro cracking is the most
common failure mode referring to creation of cracks under increasing load that extend
through thickness of a ply. This crack formation releases energy from the material and
also changes its stiffness.
The second failure mode is delamination of the composite plies. Delamination is
caused by differing response of each ply to local loading; therefore energy is released
from the material if the plies separate. Delamination is usually initiated at material
boundary such as free edge or cutout where inter-laminar regular stresses attain their
maximums. Delamination can be prevented by increasing the inter-laminar regular
strength in the FRP composite. This can be achieved using a toughened resin matrix,
edge caps or interleaved layers of adhesive at the free edges.
66
The third failure mode of fiber-reinforced composites is fiber fracture. Fiber
fracture is a very important failure mode since FRP composites have fiber dominated
mechanical properties. Individual fiber failure can be initiated by matrix cracks in an
adjacent ply and can similarly cause the fracture of neighboring fibers. To avoid this
fiber fracture propagation the fibers in some FRP composites are preventively coated
with material decreasing this propagation (Peters 1998, Reddy and Murty 1992).
Impact Resistance
As with other materials, impact resistance is dependent on impact velocity,
geometry as well as the size of the area impacted. The impact resistance of FRP
composites depends on fiber content, weave pattern and weave density. In multi-layer
laminates, the inter-laminar shear strength is also an important factor. In general,
impact-loading results in delamination in and around impacted area combined with
peeling in the direction of fibers. These local failures can significantly degrade the
mechanical properties of the composite. It is important to note that the damage caused
by impact loading is usually not visible to its full extent as it might be hidden inside the
composite, in the form of broken fibers or cracked matrix. Therefore, impact damage
should be properly assessed using nondestructive testing techniques (Peters 1998).
B. Physical Properties
Density
Density is one of the main advantages of FRP composites over traditional
construction materials such as steel and concrete. The density range of FRP composites
is from about 1400 kg/M 3 for aramid combined with epoxy to about 2100 kg/M 3 for E67
3
glass combined with epoxy, compared to 7850 kg/M for steel. The low density
combined with high strength and stiffness gives rise to excellent specific strength and
stiffness. The main benefit is that the strengthening can be accomplished with much
smaller addition to dead load and in much smaller profile than with concrete or steel.
The Above characteristics not only decrease the overall cost of the material
transportation, but also decrease the cost and time of application on site, since no
permanent scaffolding is required and the material need not be held in place by
temporary supports before the adhesive sets (Barbero 1999, Peters 1998).
Coefficient of Thermal Expansion
The coefficient of thermal expansion (CTE) of FRP materials is worth noting,
because unidirectional aramid and carbon FRP composites under temperature increase
either shrink or expand negligibly, which is contrary to the properties of traditional civil
engineering materials, such as concrete. This is a very important fact in the study of
adhesive bond of FRP to the concrete, where the FRP does not expand but the concrete
expands under temperature increase, thus developing residual stress in the bonded
interface. The CTE of the glass FRP (GFRP) is positive as with the other engineering
materials. The behavior of aramid FRP (AFRP) and carbon (CFRP) with negative, or
near zero CTE is given by the fibers with negative CTE resisting the expansion of the
epoxy matrix that has positive CTE (Barbero 1999, Peters 1998).
Fire Resistance
Fire resistance is a serious concern with FRP composites. Polymers such as
polyester or epoxy are organic materials and therefore will readily burn when exposed to
68
flames. For use in civil engineering applications, it is desirable that both flame and
smoke characteristics be rated as low as possible. To improve the fire resistance of a
polymer matrix, a fire retardant additive can be included, but unfortunately these are
generally toxic chemicals and will result in degraded smoke toxicity rating. Also,
inclusion of fire retardant additives will adversely impact the mechanical and/or chemical
resistance properties of the composite. Increasing the amount of inorganic filler will also
improve the fire rating (Barbero 1999, Peters 1998, Clarke 1996).
Alkaline Environment Resistance
Alkaline environment resistance is a very important factor in the use of FRP
composites as strengthening system for concrete. In general, carbon and aramid fibers
are resistant to alkali environment, whereas glass FRP has durability issues in an alkali
environment. Therefore, only high-alkali-resistant glass fibers can be used in concrete
strengthening applications, as regular E-glass fibers are susceptible to alkali attack and
degradation.
C. Durability
As was noted in the introduction, nowadays there exists a large knowledge base
on the mechanical and physical properties of FRP composites both in general and civil
engineering applications. Although much durability data is available for FRP applications
in the aerospace or automotive industries, the durability data in civil engineering
applications is not very comprehensive and is in need of further research. There are two
main reasons for this need, one is that the FRP composites have been used in civil
engineering applications only in past fifteen years, second is the fact that expected
69
service life for FRP composites in civil engineering applications is much longer than in
any of its other applications, such as aerospace, automotive or military. The following is
a summary of main durability issues, some of which are in need of further investigation
(Karbhari et. al. 2003, Bonacci et. al. 2000).
Fatigue
In general, FRP composites have good fatigue resistance to tension-tension cycling,
although there are significant differences between composites reinforced with different types
of fibers. Their fatigue resistance also depends on moisture and temperature, degrading with
increases in both; the GFRP is especially sensitive to the presence of humidity. The FRP has
four different fatigue failure modes; matrix cracking, delamination, fiber fracture, and interface
debonding (Cardon 1996). The failure mode is dependent on material properties, such as
fiber fraction, stacking of layers, angle of fibers to the loading and so on.The CFRP with its
stiff carbon fibers results in relatively low strains and, therefore, high ability to bridge cracks
developed in the matrix and reduce stress intensity at the crack tip. Hence, CFRP with high
fiber content almost never fatigues. The fatigue resistance of CFRP is viewed as better than
that of steel, especially since FRP in general fails in progressive fashion whereas steel fails
suddenly upon propagation of a single crack (Hollaway and Leeming 1999, Chung 1994).
On the other hand,,, the fatigue resistance of GFRP is much lower, given by less stiff glass
fibers and low resin/fiber interface strength. Therefore, GFRP does not have good ability to
bridge cracks in the matrix and reduce stress intensity at the crack tip and therefore GFRP
fatigues much more rapidly (Cardon 1996).
Creep
70
Unidirectional FRP composites under tensile load have very good resistance to
long-term creep. Since polymers are viscoelastic, they deflect continuously under static
load. More creep will be observed at elevated temperatures or at high moisture contents
in the resin, since these two factors lead to damage of internal bonds and softening of
the resin, decreasing the creep resistance of the composite. There are two distinct creep
mechanisms described in the literature. At ambient temperatures the creep behavior is
attributed to the time-dependent growth of fiber/matrix debonding and cracks. At
elevated temperatures close to the glass transition temperature, the creep observed
would be mainly caused by viscoelastic flow of the matrix (Peters 1998, U.S.Army 1997,
Clarke 1996).
The CFRP composites have the best creep resistance in the fiber direction that is
much higher than that of standard steel and comparable to that of low relaxation steel
(Hollaway and Leeming 1999). Also, thermosetting resins are more creep resistant than
thermoplastic resins, due to their rigid three-dimensional cross-linked structure.
Moisture
Although FRP composites are in general environmentally stable and resistant to
many chemicals, moisture absorption is very detrimental to their mechanical properties.
In FRP composite water acts as a resin plasticizer, softening the material and thus
changing its mechanical properties. The water absorption also decreases glass
transition temperature of the composite.
In general, moisture absorption is dependent on many variables such as void
content, amount micro cracks, fiber type, resin type, temperature and applied stress.
Also, various FRP composites will react differently to certain moisture content. For
71
example epoxy resin matrix will better resist moisture attack than thermoplastic resins.
Also, various fiber types behave differently, carbon fibers do not exhibit any degradation
due to moisture, whereas glass fibers are prone to moisture attack especially when
under stress. Although aramid fibers can absorb large amount of water, they are
protectively coated during fabrication to prevent moisture absorption (Hollaway and
Leeming 1999, Peters 1998, U.S.Army 1997).
UV Radiation
Ultraviolet radiation from sunlight can discolor and harden surface layer of the
polymer resin through a molecular weight change and cross-linking degradation.
Although this discolored layer acts as a self-screen that protects the rest of the resin
from UV radiation effects, coating the FRP preventively with UV-resistant coating is
common practice. This UV-resistant coating effectively prevents any UV caused damage
from occurring. To simplify the FRP application, the UV protection chemicals can be
mixed into the matrix resin as additives, removing the need for separate UV coating step
during application.
72
The UV-resistant coatings are composed of UV screeners, absorbers, energy
quenchers and light-stabilizing antioxidants, each of which has particular mechanism of
protection (Calbo 1987). UV screeners absorb the light and increase the photo-stability
of the polymers; examples are carbon black and titanium oxide. UV absorbers transform
the light energy in UV range into heat energy. They are colorless additives such as 2hydroxy-benzophenone
or
2-cyano-3,3-diphenylacrylic
acid.
Light-stabilizing
antioxidants are a new group of UV stabilizers called usually hindered amine light
stabilizer (HALS). Their protection mechanism is different from classic UV stabilizers,
since they do not absorb UV radiation, but they terminate free radicals formed in the
polymer from photolytic degradation in combination with photooxidative mechanisms.
The combination of UV screeners or UV absorbers with HALS has been observed in
experiments as the most effective in providing UV protection. The UV protection
experiments are usually performed using so-called "Florida exposure" test (U.S. Army
1997, Wypych 1995, Calbo 1987).
Thermal Effects
Constant low temperature, even down to cryogenic levels, does not appear to
have a significant effect on the FRP composites. Effects of extremely low temperatures
were tested for FRP use in aerospace, where temperatures can reach much lower than
in common civil engineering applications (Peters 1998). This is contradicted by findings
of experiments at the Cold Regions Research Laboratory of the Corps of Engineers
(CRREL) that found that axial tensile strength of unidirectional FRP of various types
tends to decrease when subjected to extreme
73
-50 0C conditions (U.S. Army 1997).
The effect of high temperatures on FRP composites is much more significant,
even more so when moisture is present. In this mode the matrix may undergo softening
depending also on the fiber lay-up. Experiments show that unidirectional composites are
much more susceptible to heat softening than multidirectional composites. Similarly to
the low temperature effects, the high temperature effects were thoroughly tested for FRP
use in aerospace applications, where temperatures can reach significantly above the
range of civil engineering applications (Peters 1998).
Thermal cycling effects on the FRP composites from cycles such as freeze-thaw
in range of -200C to +300C is usually marginal. But under more extreme or prolonged
thermal cycling micro-crack development was observed. This leads to decrease in
stiffness as well as to degradation of other matrix-dominated properties (U.S. Army
1997). On the other hand, the effect can be positive if higher than ambient temperature
is used for the high end of the thermal cycling range. The higher than ambient maximum
cycling temperature allows the matrix material to post-cure and, therefore, improve its
strength and stiffness. This is especially true for carbon/epoxy FRP that can increase its
tensile strength appreciably (Hollaway and Leeming 1999). Also, freeze-thaw cycling
can have a very negative effect on FRP composites with significant percentage of
interconnected voids that are filled with water. In this case the thermal cycling can
ultimately cause cyclic thermal fatigue (U.S. Army 1997).
G.
Review Summary
In general, there are significant differences between the three commonly used
types of fibers and resulting FRP composites (CFRP, AFRP, GFRP) based on abovedescribed factors. Table A.3 compares between the three FRP types, based on the most
74
differentiating factors. We can see clearly from review of the table that the CFRP has the
highest mechanical and durability properties, while being the most expensive. On the
other hand,,, the GFRP is relatively inexpensive, but its mechanical and durability
properties are inferior, and thus unsuitable for applications in aggressive environments.
Table A.3. Relative comparisons between the three most commonly used FRP types
Glass FRP
Aramid FRP
Carbon FRP
Factor
low
middle
high
Cost
low
middle
high
Strength
low
middle
high
Modulus
high
low
middle
Density
positive
zero
near
negative
zero
near
negative
Coefficient of thermal expansion
low
high
high
Alkaline environment resistance
low
middle
high
Fatigue resistance
Low
Jhh
high
Moisture attack resistance
2.2.2 Structural Adhesives
Structural adhesives used today in adhesion of FRP composites to concrete
substrate are monomer compositions that harden by polymerization creating an effective
stress transfer mechanism between the two adherents. The main challenges faced by
adhesives in civil engineering are the application and curing in field conditions that can
be controlled only to a certain degree. The conditions that need to be within specified
limits in order to produce durable high quality bond in field conditions are surface
preparation on both concrete substrate and FRP composite, application temperature,
exclusion of air bubbles from the bond line, thickness of the bond line and etc. (Hollaway
and Leeming 1999, Mays and Hutchinson 1992).
Other major challenge posed on structural adhesives in civil engineering
applications is the requirement for much longer life cycle than in other engineering fields.
75
This fact is also combined with highly variable long-term environmental effects that the
adhesives must withstand in field conditions. Fortunately today there are many
companies producing adhesives specifically designed for adhesion of FRP composites
to concrete with very good initial properties, the long-term durability of which under
various environmental conditions has to be yet proven by continuing research in this
field (Hollaway and Leeming 1999, Mays and Hutchinson 1992). The following section
will introduce the structural adhesives used commonly in civil engineering today and
present information about our knowledge of their adhesion mechanisms and durability.
A. Adhesive Classification
Structural adhesives are usually organic and can be divided into two distinct
groups; thermosets and thermoplastics. The adhesives do not have to be of the same
group as the resin matrix in the case of pultrued shapes. In the case of wet lay-up or
prepreg applications the function of the adhesive is performed by the matrix resin that
functions both as matrix and as adhesive to the concrete substrate.
Thermosetting adhesives, as inferred by their name, permanently set upon
curing, since permanent cross-linking takes place. When subjected to heat the
thermosets loose a significant amount of strength and stiffness, since they become
viscous. There are many thermosetting adhesives available based on a variety of
chemical groups, but in civil engineering applications the most widely used are epoxy
and unsaturated polyester.
The epoxy is especially popular thermosetting adhesive in civil engineering,
because it can be tailored in a wide variety of ways to suit specific requirements. The
epoxy can be modified by addition of fillers to reduce creep, improve fire and corrosion
76
resistance and decrease cost. Toughening can be achieved by addition of rubber like
particles to avoid premature fracture failure. Other modifications include use of antioxidants, thinners, flexibilisers, surfactants or adhesion promoters. The epoxy can be
both ambient or elevated temperature curing.
The Thermoplastic adhesives soften when subjected to heat and solidify
upon cooling without any permanent damage to their properties. Thermoplastic
adhesives are usually derivatives of vinyl, polyester, nylon or cyanoacrylate. In civil
engineering the thermoplastic adhesives are commonly
used as non-structural
adhesives for example for finishes (Mays and Hutchinson 1992).
B. B.
Adhesion Mechanisms
In general, there are five different factors affecting adhesion, which are believed
to act either separately or in various combinations. The factors are as follows;
mechanical interlocking, adsorption, chemical bonding, diffusion and electrostatic
attraction. It is thought that adsorption combined with mechanical interlocking is the
principal mechanism. Following is brief description of each of the adhesion models:
The mechanical interlocking model proposes that the adhesive flows into
irregularities or pores of the adherent surface and upon hardening is mechanically
locked inside the adherent. This theory is supported by the fact that rough surfaces with
pores will have higher adhesion areas and thus better adhesion. On the other hand,,, it
is hard to justify this mechanism in bonding of smooth surfaces such as glass, where
excellent adhesion can be also achieved. In the case of concrete substrate, this theory is
justifiable as even smooth concrete surfaces include irregularities and voids that allow
for mechanical interlock. The effectiveness of this mechanism should be considered
77
direction dependent contributing much more in shear loading than in peel loading (Mays
and Hutchinson 1992, Kinloch 1982).
Adsorption, as previously mentioned, is believed to be the main adhesion
mechanism. In essence, this theory proposes that secondary bonds of the adhesive to
the adherent surface are the main contributors to adhesion. Since the adhesive
molecules are polar in nature and in intimate contact with the adherent, they are able to
form secondary molecular bonds to the surface, such as van der Waals' forces. The
measure of the ability of the adhesive molecules to come into close contact with the
adherent surface is called wetting. Wetting can be observed macroscopically as the
ability of the adhesive to spread spontaneously on the substrate. This property is used to
classify the adhesion of various adhesives (Kinloch 1982).
Primary chemical bonds can be formed across adhesive interface between
adhesive primer or coupling agent and the adherent. The primary bonds are
theoretically unnecessary to achieve high joint strengths, but are beneficial for
environmentally stable adhesion.
Diffusion can only occur when both the adhesive and the adherent are polymeric,
as in the case of adhesive and FRP. In polymers with high cross-link density, such as
epoxy, diffusion is very limited.
The electrostatic adhesion theory originated in Russia. It proposes that adhesion
is a product of balance of electrostatic forces arising from the transfer of electrons
between the adhesive and the substrate, resulting in the formation of a double layer of
electrical charge at the interface.
In the case of FRP strengthening of concrete there are two different bond planes,
one between adhesive the concrete substrate and second between the adhesive and
78
the FRP plate. At the adhesive/concrete substrate bond interface the adsorption and
mechanical interlocking can be attributed for the adhesion. At the FRP plate/adhesive
bond interface the adsorption, primary chemical bonding and diffusion are the
mechanisms producing adhesion (Mays and Hutchinson 1992).
C. C.
Mechanical properties
The mechanical properties of the adhesive can be obtained from either in bulk
tests or bond tests. Use of both test methods is favored as bulk tests provide better
information about the properties of the adhesive itself, but it might not be very relevant in
cases with small bond line thickness. The most interesting properties that can be
obtained from the bulk tests are the modulus of elasticity, shear modulus, toughness,
creep, fatigue properties, coefficient of thermal expansion, glass transition temperature
and moisture resistance. Bond tests can. On the other hand,,, realistically simulate
actual field applications, and thus obtain values specific for certain application (Mays
and Hutchinson 1992).
D. D.
Durability
As with initial mechanical properties, the durability can be found from both bulk
and bond tests. The bulk tests can provide durability of the adhesive separate from
influence of the adherent, whereas the bond tests provide durability data for a system of
adhesive and two adherents. Combination of these two methodologies will yield data for
best understanding of the behavior of the adhesive as well as the whole bonded system.
Unfortunately there is no clear understanding or standard methodology on how to
connect these two datasets and gain deeper understanding from their combined result.
79
Another
issue concerning the durability of adhesives is the relatively limited
experimental data, given the vast number of combinations of possible environmental
parameters and their combined effects (Mays and Hutchinson 1992).
Moisture has been identified as the single most important factor in the durability
of adhesives and bonded joints. This is given by the hydrophilic nature of structural
adhesives resulting from their chemical structure based on polar groups. Unfortunately
humidity or water is ever present in civil engineering applications and, therefore, the
effect of moisture on the adhesive has to be fully taken into account. The actual effect of
moisture is specific to each adhesive dependent on its chemical composition. Moisture
can be present in the adhesive via two mechanisms: absorption and adsorption.
Absorption means that water is simply diffusing into voids in the resin. Adsorption is that
water molecules react chemically with the polymer through hydrogen bonding. The
polymer chains are disrupted and the effective cross-link density decreased, resulting in
reduced mechanical properties. Fortunately most of the loss in mechanical properties
can be recovered upon returning the adhesive to its original dry state.
Another important impact of moisture on the adhesive is the decrease in the glass
transition temperature. This is a problem especially in ambient temperature cured
adhesives that do not have very high glass transition temperature even in their dry state.
Upon absorption of water, their glass transition temperature can actually decrease into
the range of service temperatures. This could lead to possible viscoelastic failure of the
bonded joint under regular service temperature (Mays and Hutchinson 1992, Kinloch
1982).
80
E. E.
Epoxy
As previously stated epoxy is a very widely used adhesive in civil engineering
today. The epoxy is also widely used in adhering FRP composites to a concrete
substrate. Because of these two facts, the following sections will focus on the epoxy in
detail. In general, epoxy has excellent mechanical and physical properties given by
tough three-dimensional polymer network created by addition of the hardener (crosslinking agent). The epoxy base is usually based on one of the following chemical groups
DGEBA (diglycidyl ether of bisphenol A) or DGEBF (diglycidyl ether of bisphenol F). As
mentioned in the previous section, one of the main advantages of epoxy is that it can be
modified in many ways to suit specific requirements. This can be achieved by the use of
different hardeners and other additives as described below (Mays and Hutchinson 1992,
Kinloch 1982).
Epoxy Hardeners
There are three main groups of epoxy hardeners used commonly today; aromatic
amines, aliphatic polyamines and cycloaliphatic amines.
Use of aromatic amine hardeners results in epoxy adhesive that is relatively low
strength and brittle, which is given by high cross-link density, which creates a brittle
structure. Another drawback of this type of hardeners is that they do not cure well at
room temperature and thus must be cured at elevated temperatures. The advantages
are good chemical and heat resistance.
For ambient temperature curing epoxy, the aliphatic polyamine hardeners are
most widely used. The resulting epoxy has very good mechanical properties and is
resistant to chemicals and moisture. One possible disadvantage is that some aliphatic
81
amine hardeners react with air moisture and carbon dioxide, creating a surface layer
that is not suitable for bonding of additional layers.
The cycloaliphatic amine hardeners produce epoxy of similar mechanical and
chemical resistance properties to the aliphatic polyamine hardeners. The advantage of
cycloaliphatic amine hardener epoxy is their increased environmental resistance and,
therefore, they are used in applications where high humidity or low temperature is an
issue (Hollaway and Leeming 1999, Mays and Hutchinson 1992, Kinloch 1982).
Epoxy Additives
Tougheners and fillers are the most commonly used epoxy additives, although
there are many other epoxy additives such as anti-oxidants, thinners, flexibilisers,
surfactants or adhesion promoters. Tougheners such as CTBN (carboxyl-terminated
butadiene acrylonitrile) are introduced into the epoxy to increase its overall toughness.
This is important for improving the resistance to peel failure of the adhesive system at
the end of the FRP strip or sheet common in FRP strengthening of flexural members. In
such FRP applications there is natural tendency towards this type of failure, given by the
different elastic properties of the FRP composite and the concrete substrate. The
tougheners introduce fine rubber particles into the adhesive that provide an additional
energy absorption mechanism increasing the overall toughness of the material.
Fillers, On the other hand,,, serve many purposes in the epoxy adhesive. They
are introduced to reduce cost, improve fire resistance, reduce creep, reduce exothermal
reaction and inhibit corrosion. One drawback of fillers is their negative impact on
moisture and chemical resistance of the epoxy. Fillers can be also used to produce
thixotrophic adhesives that can be used in overhead applications without dripping. The
82
fillers are usually inert materials such as silica fume or very fine sand (Hollaway and
Leeming 1999, Mays and Hutchinson 1992, Kinloch 1982).
2.2.3 Concrete
Concrete is a very common civil engineering material used in construction
industry for more than hundred years. Therefore, mechanical, physical and durability
properties of concrete are well researched and widely known. Vast amount of
experimental data is also available in countless publications. Because of these facts this
review will not further discuss general properties of concrete.
83
APPENDIX B
REVIEW OF PREVIOUS RESEARCH
1. A. Effect of Freeze-Thaw Cycling on FRP Bond to Concrete in
Flexural and Shear Applications
Chajes, Thomson, Farschman and Farschman (1995, University of Delaware)
Objectives
This publication presents the second half of a research project at the University of
Delaware that studied the flexural strengthening of concrete beams by external
application of FRP composites. In this second part of the research project the focus was
on environmental durability of the strengthening system. The environmental conditions
included chloride exposure combined with both wet/dry and freeze-thaw cycling.
Experimental Program Particulars
A total of 60 reinforced concrete beams (38.1 mm x 28.6 mm x 330 mm) were
produced for this experimental program. A concrete mix having water cement ratio of 0.5
was used. The mix was composed of water, Type I Portland cement and aggregate
having maximum size of 3.175 mm. The beams were reinforced with one 2.38 mm
threaded rod with tested yield strength of 658.5 MPa. Total of 45 of the specimens were
strengthened by application of aramid, E-glass or carbon (15 specimens each)
composite prepreg fabrics to their tension surface using epoxy. Fifteen specimens were
left without strengthening for comparison. Twelve specimens of each fabric type were
84
subjected to two different degrees of environmental exposure (50 or 100 cycles). There
were two types of environmental exposure used in the program. The first type of cycles
was the wet/dry cycles, where the specimens were submerged in 4% CaCI solution for
16 hours and then dried in laboratory conditions for 8 hours. The second type of cycles
was freeze-thaw cycles according to ASTM C672-84, where the specimens were also
submerged in the 4% CaCl solution. The remaining specimens left in the laboratory
environment were used as control specimens. Following the environmental exposure,
the beams were loaded to failure in a four-point bending test and mid-span displacement
was measured using a dial gage.
Findings and Conclusions
As expected, the study proved that the environmental conditioning is detrimental
to the ultimate strength of both strengthened and un-strengthened beams. The
experiments
show very significant initial increase in ultimate strength
of the
strengthened beams due to application of the FRP, AFRP 191%, GFRP 88% and CFRP
139% as expected. A trend of decreasing ultimate strength was observed from the
experimental results as the specimens underwent more environmental cycles under
both types of conditioning. The only exception were the AFRP strengthened beams
under freeze-thaw condition, that did not show any decrease in ultimate strength, which
was explained by higher concrete strength of the particular batch of concrete. After
environmental exposure to both conditions, AFRP and GFRP strengthened beams lost
about 50% of their initial gain from strengthening. On the other hand, CFRP
strengthened beams that displayed initial gain of 139% decreased only to 127% after
100 freeze-thaw cycles and to 120% after 100 wet/dry cycles. From this and other data it
85
was concluded that of the two environmental conditions the wet/dry cycling in solution of
calcium chloride is more detrimental to the FRP strengthening. It was also concluded
that CFRP is best suited for applications involving exposure to deicing chemicals, where
repeated wetting and drying and/or freezing and thawing is expected to occur.
The environmental exposure also changed the observed failure modes from
shear failure of the concrete substrate in the control specimens, to the adhesion failure
in the cycled specimens. The researchers observed that the exposed CFRP and AFRP
strengthened beams had begun to deteriorate the concrete-epoxy-fabric bond and the
adherence of concrete to the FRP was greatly reduced.
Karbhari and Engineer (1996, University of California San Diego)
Objectives
This experimental program studied the effect of five different environmental
conditions on the performance of FRP-plated beams from the point of view of material
selection and durability. Specifically, the program focused on the evaluation of shortterm environmental exposure effects on the bond between FRP and concrete, with
emphasis on relative changes in performance, determined through tests on small-scale
mortar beam specimens.
Experimental Program Particulars
In this experimental program a number of small-scale (25.4 x 50.8 x 330.3 mm)
mortar beams were used. The beams were manufactured using a 1:3 (cement:sand)
mortar with a water cement ratio of 0.45. The beams were allowed to cure for 28 days in
water. The average compressive strength from cylinder tests was 25.91 MPa with
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modulus of 21.53 GPa. Two types of reinforcing fabrics were used; glass and carbon,
combined with two different resin epoxies (Tonen Corporation and Shell). Therefore, a
total of four combinations of fibers and resins were used in the experimental program.
The strengthening was manufactured by the wet lay-up method, using three layers of
fabric on the tension face of each mortar beam. The composite was cured under
ambient temperature. The total length of the reinforcement was 193 mm.
Specimens were then subjected to environmental exposure in five different
conditions for a period of 60 days. This exposure was chosen to allow for sufficient time
for moisture saturation of the composite, predetermined in previous tests. The conditions
were the following: ambient, water, synthetic saltwater (ASTM D1 141), frozen (-15.50C)
and freeze-thaw cycling (-15.50C for 24hr and 200C for 24 hr). Following the
environmental exposure, the specimens were tested to failure in four-point bending with
a span length of 203.2 mm and displacement rate of 1.27 mm/min. Deflection was
measured using an LVDT at midpoint. A Minimum of four specimens were tested for
each exposure condition.
Findings and Conclusions
The study concluded that freeze-thaw cycling caused both plasticization and
matrix stiffening in the test specimens. A determination of the cause and effect is
difficult, because the system behavior is dependent on changes in the concrete
substrate, the FRP composite and the structural adhesive. Therefore, the study focused
on an examination of the effect of other environmental conditions in the experiments.
Besides the freeze-thaw testing, the researchers concluded that the most significant loss
of strength was exhibited by specimens exposed to fresh water and seawater
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immersion, while the steady freezing condition resulted in the least degradation of
strength.
The experiments found that after 30 freeze-thaw cycles, the ultimate load
decreased by 1 0%(CFRP) and 18%(GFRP) for the Tonen epoxy composites and did not
change for the Shell epoxy composites. The deflection at failure decreased by 15%
(CFRP) and 35%(GFRP) for the Tonen epoxy composites and 7%(CFRP) and 27%
(GFRP) for the Shell epoxy composites. Finally, the flexural stiffness increased by 5%
(CFRP) and 30%(GFRP) for the Tonen epoxy composites and 12%(CFRP) and 27%
(GFRP) for Shell epoxy composites. These results demonstrate significant changes in
mechanical properties of FRP strengthened mortar beams caused by only 30 freezethaw cycles. The results also confirm superior durability properties of carbon fibers over
glass fibers and highlight not only the importance of fiber selection, but also resin
selection as there are very significant differences between the two epoxy resins from
different manufacturers.
Karbhari, Engineer and Eckel (1997, University of California San Diego)
Objectives
This research paper presents second part of experimental program started in the
preceding research article (Karbhari and Engineer 1996). This second half of the
experimental program focused on relative changes in peel force and interfacial fracture
energies resulting from five different environmental exposure regimes and hence the
overall durability of FRP strengthening of concrete.
88
Experimental Program Particulars
In this experimental program a number of concrete blocks (25.4 mm x 152.4 mm
x 228.6 mm) was cast using 1:3 cement sand ratio, with a water cement ratio of 0.45.
The resulting concrete had a 28-day strength of 25.91 MPa and a modulus of 21.53
GPa. Two types of fibrous reinforcement were used; E-glass and carbon. These were
combined with two different epoxy resins, for total of four fiber-resin combinations. The
FRP peel strips had a length of 304.8 mm and width of 25.4 mm and were made up of
two plies of unidirectional fiber sheet. They were manufactured by a wet lay-up method
and cured for one week in ambient temperature.
Thereafter, the specimens were exposed to five different environmental
conditions for a period of 60 days. This exposure was chosen to allow for sufficient time
for moisture saturation of the composite, predetermined in previous tests. The conditions
were as follows: ambient, water, synthetic saltwater (ASTM D1141), frozen (-15.50C)
and freeze-thaw cycling (-15.50C for 24hr and 200C for 24 hr). After the environmental
exposure the specimens were tested to failure in a peel test with constant crosshead
movement of 5.08 mm/min. Therefore, the peel rate was determined by the peel angle
used for the specimens. The apparatus is shown in Figure A.7. The load was recorded
from a load cell and the actuator movement was measured using a LVDT. Correction
was made for the frictional resistance of the slider.
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Fig. A.7. Details of the peel test apparatus
Direction of force
Grip
I
Composite peel strip
RollerConcrete plate
Slider (moving part)
Slie
Slider (fixed part)
moement
Base plate
Findings and Conclusions
study concluded
As in the previous research program by the same authors, the
matrix stiffening and
that freeze-thaw cycling results in both plasticization and
system behavior is
determination of the cause and effect is difficult, because the
and the structural
dependent on changes in the concrete substrate, the FRP composite
the effect of the other
adhesive. The study therefore focused on a discussion of
environmental conditions in the experiments.
relative changes
The freeze-thaw cycling conditioning resulted in significant
II interfacial fracture
(compared to control specimens) in both mode I and mode
70% for the GFRP and
energies. The mode I interfacial fracture energy increased up to
negligible change. The
by 28% for the CFRP, one of the CFRP composites exhibited
for the GFRP and by
mode I interfacial fracture energy increased by 50% and 110%
cycling exposure
15% and 30% for the CFRP. Therefore, in general the freeze-thaw
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caused an increase in force and interfacial energy over the control specimens. This was
attributed to a stable brittle crack growth, resulting in an increase in toughness of the
system due to a brittle/ductile transition of the epoxy resin. Overall, the carbon fiber
based composites were found to have superior durability properties under all
environmental conditions. The authors also found that there were significant differences
in the durability properties of not only the fiber reinforcement, but also the two different
epoxy resins.
Green, Soudki and Johnson (1997, Queen's University and University of Waterloo)
Objectives
This study focused on freeze-thaw cycling behavior and durability of FRP
strengthened reinforced concrete beams.
Experimental Program Particulars
This experimental program used 21 small-scale (100 x 150 x 1220 mm) steel
reinforced concrete beams. The beams were made from ready-mix concrete of 30 MPa
strength, with air content of 5% to 7%. The beams were reinforced with two #5 bars of
grade 400 reinforcing steel. The beams were strengthened on their tensile face with a
single layer of either Replark CFRP sheets or Forca tow sheets. Some of the beams
were also strengthened in shear with additional FRP wrap. The FRP sheets were
ambient-cured for seven days. Consequently, approximately half of the beams were
subjected to 50 freeze-thaw cycles. The freeze-thaw cycles consisted of 16 hours of
freezing in -180C cold room and 8 hours of thawing in +150C water bath.
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After completion of the freeze-thaw cycling the beams were tested to failure in
four-point bending. The beams were instrumented with LVDTs at midspan and at the
supports. The strains were measured using strain gages attached to the FRP sheet at
mid span.
Findings and Conclusions
The study concluded that CFRP strengthened concrete beams were not
damaged by an exposure to 50 freeze-thaw cycles as they failed at slightly higher
ultimate loads than unexposed specimens. The dominant failure mode observed was
peel of the CFRP sheets from the concrete substrate. The strains at mid span were
observed to be significantly higher for the exposed beams. This large difference was
explained as being the result of either slipping of the CFRP sheets due to bond failure in
the unexposed control specimens or stress concentration, possibly due to flexural crack,
near the strain gages in the exposed specimens.
Mukhopadhyaya, Swamy and Lynsdale (1998, University of Sheffield)
Objectives
This research paper presents results of a study on the effects of aggressive
environmental exposure conditions on behavior of adhesive bonds between pultruded
GFRP composites and concrete substrate. The experimental program was conducted
on small-scale double lap shear specimens exposed to three different environmental
conditions.
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Experimental Program Particulars
The experimental program consisted of 24 concrete beams (100 x 100 x 300
mm). The concrete was air-entrained with approximately 7% air-entrainment to prevent
damage of the concrete itself during environmental exposure. Two different concrete
mixes were used; mix A with water cement ratio of 0.57 and mix B with water cement
ratio of 0.32, both using Type 1 Portland cement. Mix A was designed to have 28-day
strength of 35 MPa and Mix B was designed to have 28-day strength of 50 MPa. Each of
the beams was bonded with two pultruded GFRP plates (90 mm x 3.5 mm x 470 mm) on
the two opposite faces. The plates included both random continuous filament mat and
unidirectional rovings and were used in the maximum strength direction. Structural
adhesive was selected to be a two-part thixotropic epoxy, with a tensile strength of 24
MPa, a flexural strength of 55 MPa and a bulk shear strength of 21 MPa.
The specimens were then exposed to three different environmental conditions:
alternate wet/dry cycles in 5% NaCl solution, freeze-thaw cycling and combination of the
previous two exposures (wet/dry cycles in 5% NaCl followed by freeze-thaw cycling).
The wet dry cycling consisted of 1-week salt-water immersion followed by 1-week of
drying in air. The specimens underwent a total of 18 wet/dry cycles. The freeze-thaw
cycling was done in 12hr intervals between -17.80C
and 200C. The specimens
underwent a total of 450 freeze-thaw cycles. The dual exposure specimens underwent
total of 18-weeks of salt-water immersion mixed with total of 252 freeze-thaw cycles.
During the environmental exposure period the strains in both GFRP plates and concrete
were continuously monitored.
After the end of the environmental exposure the specimens were tested to failure
in double lap shear in a purpose built setup. Each of the specimens was instrumented
93
with ten strain gages and four LVDTs. The test were conducted in load controlled mode
with a rate of 2.5 kN/min which was halved after initiation of cracking.
Findings and Conclusions
The study concluded that the environmental exposure regimes increased the
GFRP slip, increased the bond transfer length and increased the magnitude of the shear
stress distribution. Although the duration of the exposure was not found to be long
enough to affect the ultimate strength of the joints, the increase in the above stated
values demonstrated that prolonged exposure to these environmental conditions would
lead to loss of bond capacity and premature failure.
The combined wet/dry and freeze-thaw exposure regime was found to be most
detrimental to the GFRP bond to concrete with highest differential movements that might
lead in the long-term to significant damage of properties of the bond. This regime also
produced the most significant change in the failure mode, exhibiting large areas of
adhesion failure between the concrete and the adhesive, whereas control specimens
failed in concrete shearing.
Green, Bisby, Beaudoin and Labossiere (2000, Queen's University and University
of Sherbrooke)
Objectives
The objective of this experimental program was to assess the durability of FRP
strengthening of concrete in cold regions. The durability study was based on the effects
of freeze-thaw cycling on the bond between the FRP composite and concrete substrate.
The investigation used both single lap shear and flexural beam specimens with CFRP
strips.
94
Experimental Program Particulars
The experimental program was divided into two phases. First phase consisted of
testing CFRP bond to concrete using single lap shear specimens and second phase
used modified flexural specimens. This was to test the strengthening both in shear and
in flexure to determine the bond durability for these two respective concrete
strengthening schemes.
For the single lap shear tests a total of twelve concrete blocks (150 mm x 150
mm x 400 mm) with 28-day strength of 35 MPa were manufactured. For the second part
of the program a total of nine small-scale steel-reinforced beams (100 mm x 150 mm x
1220 mm) were manufactured. This study used uniaxial pultruded CFRP strips (SIKA
Carbodur) with strength of 2400 MPa, modulus of 155 000 MPa and a thermal
expansion coefficient of 1.0 x 10-6/0C. The structural adhesive was selected to be twopart epoxy resin (SIKA). It is important to note that the CFRP composite was bonded to
the flexural beam specimens only in 300 mm long regions starting from each end of the
FRP strip. The central portion of the FRP strip (390 mm) was not bonded to the
concrete. All the specimens were subjected to freeze-thaw cycles in accordance with
ASTM C310 (1971) at a rate of one cycle per day. The cycles consisted of 16 hours of
freezing in cold air at -180C followed by 8 hours of thawing in water bath at +150C. The
specimens underwent 0, 50, 150, 300 freeze-thaw cycles.
After completion of prescribed number of freeze-thaw cycles the specimens were
tested to failure. The single lap shear specimens were instrumented with strain gages
and LVDTs to monitor slip and displacement; load was monitored from a load cell. The
flexural beam specimens were tested to failure in four-point bending, instrumented with
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LVDTs to measure vertical displacement and strain gages to measure strain in the FRP
reinforcement; load was recorded from a load cell.
Findings and Conclusions
The single lap shear specimens exhibited ultimate strength increases of 36%,
53% and 54% after 50, 150, 300 freeze-thaw cycles. This is explained by the authors as
being the result of enhanced curing of the concrete substrate in the specimens exposed
to thawing in a water bath, whereas control specimens were left in dry conditions. As
expected, for the control specimens, the failure mode was concrete shear failure, for the
exposed specimens the failure mode progressively changed from concrete shear failure
to FRP-adhesive interface failure. This was explained as being the result of a slight
reduction of shear modulus of the adhesive with freeze-thaw cycling, which reduced the
magnitude of the stress concentrations in the concrete. Both load-deformation curves
and strain profiles were examined and did not show any significant changes with freezethaw cycling. The authors concluded that FRP-concrete bond did not appear to be
damaged by freeze-thaw cycling.
The experiments on the FRP strengthened beams indicated increasing ultimate
load and mid-span deflection with increasing length of environmental exposure. Again,
the load deflection curves did not appear to be influenced by the exposure. As in the
shear specimens, the control specimens' failure mode was concrete shear failure, for the
exposed specimens the failure mode progressively changed from concrete shear failure
to FRP-adhesive interface failure. The authors concluded that the freeze-thaw cycling
does not degrade the load carrying capacity of the FRP bond to concrete, although they
noted that the adhesive is affected and therefore the failure mode changes.
96
Myers, Murthy and Micelli (2001, University of Missouri and University of Lecce)
Objectives
The focus of this experimental program was to study the durability of bond
between various FRP composite sheets (CFRP, GFRP, AFRP) and the concrete
substrate. The study utilized pre-cracked beams that were then strengthened with FRP.
The beams were subjected to a combination of four environmental conditions including
freeze-thaw cycling. Some beams were conditioned under sustained load.
Experimental Program Particulars
The experimental program used 48 steel-reinforced concrete beams (152 x 152 x
610 mm) manufactured from concrete mix with 28-day strength of 34.5 MPa and
modulus of 27.1 GPa. All the beams were pre-cracked under three-point bending after
28-day cure period. Sixteen beams were strengthened on their tension face using 38
mm wide CFRP strips with ultimate strength of 3800 MPa and modulus of 227.5 GPa.
Seventeen beams were strengthened using 38 mm AFRP strips with ultimate strength of
2000 MPa and modulus of 117.2 GPa and fifteen using 38 mm GFRP strips with
ultimate strength of 1500 MPa and modulus of 72.4 GPa. All of the strengthened beams
were also applied with end U-wraps of 51 mm width.
The environmental exposure of all the beams was a specific combination of
freeze-thaw cycles, extreme temperature cycles, relative humidity cycles and ultra-violet
radiation exposure. The Freeze-thaw cycle consisted of 50 min at -17.80C, a 30 min
transition period and 50 min at 4.40C. Extreme temperature cycles ensued consisting of
25 min at 26.70C, 20 min of transition period and 25 min at 490C. Relative humidity
cycles consisted of changes between 60% and 100% each maintained for 20 min, with a
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30 min transition period. The overall exposure was as follows: 50 freeze-thaw cycles, 20
RH cycles at 15.50C, 40 cycles of extreme temperature, 20 RH cycles at 26.70C, 40
cycles of extreme temperature, 20 RH cycles at 15.50C and 40 extreme temperature
cycles. Specimens were subjected to either three or five such environmental exposure
cycles. Some of the beams were conditioned under a sustained load with magnitude
equal to 25% and 40% of the ultimate load. After completing the environmental
exposure the longitudinal steel reinforcement was cut. Strain gages were attached to
monitor the strain in the FRP strips; an extensometer was used to monitor growth of the
pre-crack. The beams were then tested in four-point bending to failure.
Findings and Conclusions
The authors concluded that a combined environmental exposure has detrimental
effects on the bond between the FRP strip and the concrete substrate. The degradation
of the bond resulted in decreased flexural stiffness of the system. This effect was even
more pronounced when the specimens were conditioned under sustained loading. The
bond degradation showed that the exposed specimens exhibited peeling of the FRP
from the concrete with much smoother concrete surface after failure than in the control
specimens. Overall, the GFRP specimens degraded more than AFRP and CFRP
specimens.
Comment
Although this is a very significant study towards overall environmental durability
of FRP strengthening, the combined exposure cycle prevents us from distinguishing
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separate degradation contributions of each of the environmental exposures like in our
case freeze-thaw cycling.
B.
Effect of Freeze-Thaw Cycling on FRP Composites
Lord and Dutta (1988, Michigan Technological University and U.S. Army Cold
Regions Research and Engineering Laboratory)
This study focused on low temperature hygrothermal effects influencing short and
long-term properties of FRP materials, through review of preceding research studies and
experimental programs. This was one of earliest studies into this area that established
four major points of concern in response of FRP composites to a low temperature
environment. The points of concern are 1) hygrothermal residual stresses, 2) material
degradation due to low temperature cycling 3) moisture effects on freeze-thaw cycling 4)
long-term combined effects of loading and environmental exposure on durability.
The study concluded that there are significant residual stresses in FRP materials,
due to manufacturing which result from the mismatch of the coefficients of thermal
expansion between the fibers and the matrix. Moisture and temperature can significantly
change these residual stresses and therefore they must be taken into account. The
freeze-thaw cycling changes the residual stresses and leads to matrix microcracking,
degrading the matrix-dominated properties, such as chemical resistance. As the
microcracks coalesce into macrocracks, water can collect in these cracks and continued
freeze-thaw cycling leads to further degradation of the FRP composite.
99
Dutta (1988, U.S. Army Cold Regions Research and Engineering Laboratory)
This experimental program focused on extreme temperature cycling effects on
the mechanical properties of unidirectional GFRP composites. Coupons of four types of
GFRP composites were tested in tension at both 230C and -400C after being subjected
to thermal cycling between -600C and 600C. The study concluded that the unidirectional
tensile strength of GFRP composites decreases when tested at lower temperatures. The
researchers explain this phenomenon as resulting from micro buckling of the fibers
during curing, caused by the shrinking of the matrix. When tested at ambient
temperatures the matrix is able to yield and the fibers straighten, but at low
temperatures the matrix is hardened and the fibers are not allowed to straighten leading
non-uniform load over the fiber length. The experiments have shown that the transverse
strength increases at low temperatures, due to matrix hardening. On the other hand,,,
extreme thermal cycling was shown to degrade transverse strength and stiffness due to
development of matrix microcracking.
Verghese, Morrell, Horne, Lesko and Haramis (2000, Virginia Polytechnic Institute
and State University)
This experimental program focused on the effects of freeze-thaw cycling on the
durability of the GFRP composite. The composite in question was a combination of Eglass fibers with commercial vinyl ester resin. The environmental exposure program
consisted of two distinct parts: the first part combined a single extreme temperature
cycle with subsequent freeze-thaw cycling and simultaneous fatigue testing. The second
part was a freeze-thaw testing of the GFRP composite according to ASTM C666 for 100
cycles, either in ambient humidity or submerged in water, in both dry and saturated
100
state. Only this second aspect will be further discussed as the first one combines
environmental exposure with fatigue testing. The ASTM C666 cycle consists of
temperature cycling between -17.80C
and 4.40C. After the end of environmental
exposure the GFRP specimens were tested to failure in tension.
The authors conclude that the effects of freeze-thaw cycling were shown in the
study, but believe that 100 cycles is too low a number to generate degradation in the
unloaded material. On the other hand,,, their belief is that simultaneous loading and
environmental exposure would lead to faster degradation, as the loading would open
existing microcracks, opening a path for the movement of water into the composite that
in a subsequent freeze causes further damage by its expansion. This is supported by
the tensile strength results that show that saturated specimens degraded after freezethaw cycling in ambient humidity to about 28 Ksi (control 50 Ksi), whereas dry
specimens after freeze-thaw cycling in ambient humidity degraded only by 2 Ksi to 48
Ksi.
Rivera and Karbhari (2002, University of California San Diego)
This experimental program focused on environmental durability of thin ambient
cured carbon/vinyl ester specimens prepared using a wet lay-up procedure. The CFRP
specimens were subjected to five different environmental exposures: a) ambient b)
-100C c) freeze-thaw at regular humidity level d) freeze-thaw immersed in deionized
water e) freeze-thaw immersed in chloride solution (5% NaCl). The freeze-thaw cycle
varied temperature between -100C and 22.50C every 24 hours; the specimens were
subjected to total of 30 or 100 such cycles. The testing program included gravimetric
measurements of moisture absorption, tensile tests according to ASTM D3039,
101
compression tests using the modified Wyoming compression fixture and protocol, and a
split-D ring test according to ASTM D2290.
The study concluded that exposure to a constant temperature of -10 OC increases
slightly the tensile strength of the composite, and that this can be explained by matrix
hardening. On the other, hand all three types of freeze-thaw exposure lead to a
decrease in both tensile strength and modulus, elicited by matrix microcracking and
fiber-matrix debonding. As expected, the dry freeze-thaw cycling resulted in the
relatively smallest decrease, whereas cycling in chloride solution resulted in the largest
drop in CFRP properties, almost double that of cycling in deionized water. This larger
decrease of mechanical properties in chloride solution cycling is explained by the
researchers as a result of NaCl ions moving more easily through resin along cracks and
fiber-matrix debonds. Overall, the tensile strength of the CFRP composite dropped by
9.75% and the modulus decreased by 6.93% after 100 cycles in deionized water. In the
chloride solution, the tensile strength dropped by 16.1% and the modulus decreased by
6.93% after 100 cycles.
C.
Effect of Freeze-Thaw Cycling on Structural Adhesives
Lopez-Anido, Michael and Sanford (2004, University of Maine and University of
Florida)
This study evaluated the freeze-thaw durability of underwater curing epoxy
adhesive to be used underwater in adhesion of cylindrical E-glass/vinyl ester FRP
jackets in repair of wooden piles. The FRP composite specimens were manufactured
similar to the actual FRP jackets used in the field, using the vacuum assisted resin
transfer molding (VARTM) process. The FRP composites were bonded together using
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underwater curing epoxy adhesive to create single lap shear specimens. The bonded
specimens were then cured in tap water bath at 380C for period of 21 days. Thereafter,
the specimens were exposed to 20 freeze-thaw cycles according to standards
developed by the International Conference of Building Officials (ICBO 2001) that was
slightly modified given the experimental equipment available. The actual freeze-thaw
cycle consisted of 8 hours of freezing at -180C and 16 hours of immersion in 380C water
bath. After the exposure the specimens were tested according ASTM D5868 - single lap
shear adhesion for FRP composite bonding.
The study found that the shear strength of the structural adhesive tested was
sensitive to the freeze-thaw cycling. The mean shear strength of the control specimens
was 16.2 MPa, whereas the mean shear strength of the exposed specimens was 9.2
MPa. The researchers explain this behavior as resulting from the presence of voids in
the bond line that facilitates water ingress and the subsequent freeze-thaw damage,
which can be avoided by application of clamping pressure during curing. The cycling
also leads to a change in the failure modes: from mostly adhesive type failure in control
specimens, to adhesive/cohesive failure in the exposed specimens.
Effect of wet environmental exposure on adhesively bonded joints
Bowditch (1996, Structural Materials Centre, Defense Research Agency, UK)
This research paper summarizes knowledge on the effects of water on adhesive
joints gained through research conducted at the Defense Research Agency (UK). The
paper discusses changes in both mechanical and physical properties of the adhesives
as well strength of adhesively bonded joints. The paper discusses various bond
103
degradation issues such as the effect of adhesive plasticization and 'displacement' of
the adhesive by water in adherents with high surface free energy. For example, a study
on the effect of 500C water immersion on bulk properties of epoxy-based adhesive is
presented. The example illustrates degradation due to water absorption and subsequent
plasticization. The study further discusses the effect of high temperature, time and
applied stress on bonded joints. In the case of temperature the author warns against
using too high temperatures as means to accelerate aging of adhesives, as the behavior
of the adhesive might be totally different at lower, more realistic temperatures. The
author presents data that show gradual degradation of bond strengths with increasing
time. The author also notes that failure modes might change indeterminately with
passing time. With respect to testing of specimens loaded with sustained stress during
environmental exposure, the author presents the widely recognized fact that applied
stress increases the rate of degradation of bonded joints in wet environment. This more
rapid degradation is caused by opening of matrix cracks by the applied stress, which
allows for easier water ingress. The paper goes on to review the concept of critical water
content in the adhesive, below which plasticization does not occur.
Knox and Cowling (1999, University of Glasgow)
This research program investigated durability aspects of adhesively bonded joints
in wet and marine applications. The experiments included tests on thick-adherent steel
lap joints and the bulk adhesive. The adhesive tested was Ciba Polymers AV1 19 onepart, hot-cure, toughened epoxy paste. The curing of bulk adhesive specimens was
done at 1200C for 2 hours, whereas curing of bonded joints was done at 1800C for 20
minutes. A Total of 48 lap shear specimens were involved. The specimens were
104
subjected to accelerated aging in hot-wet environment at 300C and 100% RH. Selfstressing fixtures loaded half of the lap joint specimens during the environmental
exposure with 15% of initial failure load. Every two weeks after the beginning of the
exposure two specimens were removed and tested in tension with crosshead speed of
0.5 mm/min. The testing of the bulk adhesive was done in similar fashion, but with 20%
and 50% initial failure load applied during the exposure.
The study found that the bond strength decreased in all specimens. The
combined application of preload and hot-wet exposure was shown to be most
detrimental to the durability of the thick adherent lap joints. The strength of the lap joints
decreased to 65% of the non-exposed specimens after 12 weeks of environmental
exposure. The failure surfaces have also shown transition from surfaces with numerous
ragged edges in the control specimens to smooth in the aged specimens, which is a
sign of poor adhesion and/or adhesive plasticization. The bulk adhesive was found to
undergo plasticization, resulting in reduction of strength as a result of the hot-wet
exposure at 300C.
Wang, Huang, Xv, Liu (2004, Harbin Institute of Technology, Harbin Engineering
University, Heilongjiang Petrochemistry Institute)
This experimental program focused on the durability of adhesive/carbon-carbon
composite joints in saltwater. The adhesive tested consisted of thermosetting phenolic
resin
modified
by
polyphenylmethyl
siloxane,
asbestos
modified
by
polyphenylmethylsiloxane terminated by thermosetting phenolic resin, NBR and a curing
accelerator. The FRP materials to be adhered were unidirectional carbon-carbon
composites. These composites were pretreated with a silane coupling agent, heated for
105
1 hour at 1500C, bonded with the adhesive into a single lap shear specimens and cured
for 3 hours at 1300C at pressure of 0.15-0.3 MPa. The specimens were then conditioned
in a combined hot and salt spray (0.5% saltwater spray every 0.5 h) environment for
three different periods of time (1700 h at 350C, 900 h at 450C, 450 h at 550C). Separate
specimens were also exposed to 500 hours at 550C with 98-100% RH. After completing
the environmental exposure, the specimens were tested in single lap shear according to
ASTM D1002. IR spectra and EDX spectra were recorded for the exposed specimens to
increase understanding of the damage mechanisms.
The study found that the saltwater diffusion speed in the carbon-carbon
composite was faster than in the adhesive. The rate of saltwater diffusion in the
adhesive is greater than diffusion rate of the fresh water, due to acceleration of the
diffusion by the Na+, Cl- ions. The initial shear strength of the bonded joints was around
13 MPa. This initial strength stayed constant up to a certain threshold given by timetemperature equivalence, i.e. the lower the temperature the longer to reach the
threshold. Beyond this threshold the shear strength decreased linearly up to a saturation
threshold after which the decrease continued at a lower rate. The behavior of all
specimens proved the time-temperature equivalence for the hot-wet exposure. The
100% humidity exposure at 550C resulted in total degradation of the bond after 500
hours, whereas salt spray conditioning at 550C resulted in total degradation of the bond
in only 450 hours. This is explained as being the result of the higher diffusion rate of
saltwater into the adhesive.
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D.
Effect of Freeze-Thaw Cycling on Concrete
Mohamed, Rens and Stalkaner (2000, University of Colorado at Denver)
This research investigated the effects of different cement types, air-entrainment
and air-entrainment methods on the durability of concrete under freeze-thaw cycling
conditions. The study consisted of a field inspection of parapet wall panels of the Green
Mountain Dam (Colorado). The panels, installed in 1943, were made from 28 different
cement types in a program initiated by the Portland Cement Association (PCA). This
field inspection, performed in 1997 after 53 years of service life, showed substantial
signs of damage of the panels due to freeze-thaw action. The field inspection was also
accompanied by a review of benchmark freeze-thaw testing on different concrete types
at the beginning of a long-term program in 1946. The initial freeze-thaw durability was
assessed for each concrete type, using 76 mm x 152 mm cylinders, as the number of
freeze-thaw cycles needed to achieve either 0.3% or 0.6% length change. The results of
these tests showed superior freeze-thaw durability of air-entrained concrete as most of
the specimens did not reach the specified limiting extension during the test program.
These experiments have also shown that concrete air-entrained by intergrinding the airentrainment agent expanded much less than concrete air-entrained by air-entrainment
added in solution form. The program also included current day compression tests on
core samples from the panels, therefore the researchers were able to correlate these
results with the results of visual inspection and the initial benchmark tests.
The study concluded that ASTM type Ill cement has a low freeze-thaw resistance
compared to the other cement types. The air-entrainment was found to increase freezethaw durability of the concrete, however it did not prevent freeze-thaw deterioration from
occurring in the concrete panels after 53 years of service life.
107
Sun, Mu, Miao, Luo and Sun (2002, Southeast University, China)
This experimental program studied the effect of freeze-thaw cycling in fresh water
and saltwater (3.5% NaCI) on concrete. The program used a number of small-scale
concrete beam specimens (40 mm x 40 mm x 160 mm) made of three different
strengths of non-air-entrained concrete that were environmentally conditioned according
to ASTM C666A. The specimens underwent the environmental exposure both
unstressed and stressed at different stress ratios (0.1, 0.25, 0.5 of ultimate strength),
produced by a specially designed loading frame applying a flexural load. After the
completion of the environmental exposure, the specimens were tested to failure in threepoint bending. The losses of dynamic modulus of elasticity and weight of the specimens
were measured. The ultimate number of freeze-thaw cycles to was recorded.
The results show that higher strength concrete is able to withstand more freezethaw cycles before failure. Also, as the 3.5% NaCl solution has lower freezing point
(-2.030C) than fresh water and the concrete specimens were able to withstand about
20% more cycles in NaCl solution than in fresh water. The specimens conditioned in the
NaCl solution sustained severe surface scaling, with double the weight losses than in
fresh water. On the other hand,, the dynamic modulus of elasticity decreased at lower
rate during freeze-thaw cycling in NaCI solution due to the lower freezing point of the
solution. The study found that the externally applied sustained flexural load accelerated
the freeze-thaw deterioration of the concrete, the higher the sustained stress the faster
deterioration. Similarly the dynamic modulus of elasticity also dropped at a higher rate in
specimens with higher sustained load.
108
Soroushian and Elzafraney (2004, Michigan State University)
This research program investigated changes in concrete properties as a result of
various damaging conditions. The conditions studied were compression, impact, fatigue
and freeze-thaw cycles. This section will only focus on information pertaining to the
freeze-thaw cycling in this study. The study used two concrete mixes of standard and
high strength manufactured using type I Portland cement. For purposes of the freezethaw cycling, small-scale beams (76 mm x 76 mm x 305 mm) were manufactured. The
specimens were exposed to 300 freeze-thaw cycles in water according to ASTM C666,
either right after curing or after being subjected to compressive loading to peak
compressive stress. After the environmental exposure the beam specimens were tested
in four-point flexure test according to (ASTM C78). To assess the microstructural
manifestation of the damaging effects the researchers then used environmental
scanning electron microscopy (ESEM) on sections of the concrete beams perpendicular
to the loading direction.
The study found that both freeze-thaw cycling and compression combined with
freeze-thaw cycling had a statistically significant damaging effect on the flexural strength
of the concrete. The researchers also found the effect of freeze-thaw cycling and the
effect of compression and consequent freeze-thaw cycling on the flexural strength to be
very similar in magnitude. The ESEM showed that the freeze-thaw cycling produced less
tortuous microcrack growth paths, when compared to other damaging effects. These
less tortuous microcrack paths had a relatively significant effect on the permeability of
the concrete.
109
APPENDIX C
MATERIAL CHARACTERIZATION
& TESTING
Experimental program of this study was setup to investigate the durability of
adhesive bond of CFRP strips to concrete substrate in freeze-thaw cycling conditions.
The experimental program utilized single lap shear and pull-off specimens as the main
specimens that would be freeze-thaw cycled. As a complement, a set of adhesive tensile
specimens was also freeze-thaw cycled to assess the deterioration of the bulk properties
of the adhesives. A Total of three FRP/adhesive combinations were used, applied on
either regular or air-entrained concrete and freeze-thaw cycled according to two Czech
national freeze-thaw standards C SN 73 1322 and CSN 73 1326 selected for comparison
based on findings of the literature review. One of the freeze-thaw standard used
immersion in 3% NaCI solution and the second used thawing in tap water. The overall
bond properties were investigated using both the single lap shear specimens and pull-off
specimens. The bulk properties of the adhesives used were also investigated using the
tensile dumbell specimens
110
C.1
Material Selection
The study utilized the following materials: 2 types of uniaxial pultrued CFRP
strips, 3 types of two-part thixothropic epoxy adhesives and 2 types of concrete mix. The
CFRP strips were selected because of their inherent corrosion resistance, because they
are relatively widely used in actual field applications and because they are industrially
manufactured and therefore have very consistent properties. The corrosion resistance
was emphasized in order to minimize the influence of the degradation of the FRP itself
during freeze-thaw cycling on the overall degradation of the bond properties, as might
have been the case with for example GFRP fabrics. The thixothropic type of adhesives
was selected, because FRP strips are commonly used in overhead applications, such as
the soffit of beams or bridges that require the adhesive not to flow freely, but also to hold
the FRP strip in place while the adhesive cures. The thixothropic adhesives were also
selected because the thixothropic properties are attained by addition of fillers like silica
fume and very fine sand. These inclusions significantly change the mechanical,
durability and thermal expansion properties of the adhesives. Finally, the two concrete
mixes were designed to be either freeze-thaw resistant or not freeze-thaw resistant
according to C SN 73 1326 to asses the contribution of concrete deterioration to the
overall bond deterioration. The air-entrained concrete mix was also designed to
represent concrete used in actual field applications.
C.2
Material Description
The study used two types of industrially made CFRP pultrued strips. The two
types used were SIKA Carbodur S512 and Fyfe Tyfo UC with comparable mechanical
properties, as shown in Table C.1. These types were selected not only because of their
111
comparable properties, but also because they are the most widely utilized types of the
pultrued CFRP strips of the two manufacturers.
Table C.1. Manufacturer Provided Properties
SIKA
Carbodur
Property
S512
Fiber volumetric content (%)
68
3
Apparent density (kg/M )
1600
Modulus of elasticity (MPa)
>165 000
Tensile strength (MPa)
>2 800
Elongation at break (%)
>1.7
Thickness (mm)
1.2
Width (mm)
50
of CFRP strips
Fyfe Tyfo UC
68
1810
155 000
1800
1.3
1.4
50.8
The Manufacturers of each of the CFRP strips specify their own thixotropic twopart adhesives for overhead applications. Therefore, adhesives SIKA Sikadur 30 and
Fyfe Tyfo TC were used with their respective type of CFRP strip. Additionally, the Fyfe
CFRP strips were used in combination with Betolit 0-1 DC TH epoxy-based thixotropic
adhesive manufactured by Betosan, Czech Republic, currently in a development stage.
This resulted in a total of three FRP/adhesive combinations. The manufacturer specified
properties of only the SIKA adhesive as presented in Table C.2, as Fyfe Company does
not publish this type of data and the Betosan adhesive data was not yet available.
Table C.2. Manufacturer specified adhesive properties
Property (230C and 50% R.H.)
Sikadur 30
Density (kg/L)
1.77
(ASTM D 638 @ 7 days)
Tensile Strength (MPa)
24.8
1
Strain (elongation) at break (%)
Modulus of elasticity (MPa)
4500
9
Coefficient of thermal expansion (1 0-5/OC)
112
Finally, the CFRP strips and adhesives were combined with either regular high
strength concrete or with an air-entrained concrete mix, the properties of which are
presented in Table C.3. The air-entrained concrete was used not only to simulate real
world applications, but also to minimize the direct degradation of the concrete itself
during freeze-thaw cycling, thus simulating the best-case scenario when concrete is
highly freeze-thaw resistant. Specimens of both types of concrete were also freeze-thaw
tested according to CSN 73 1326 both in water and 3% NaCl solution to asses the
relative difference in degradation from the addition of the chloride solution to freeze-thaw
cycling. Similarly, the surface prepared specimens were freeze-thaw cycled to establish
the influence of the mechanical abrasion surface preparation procedure on the freezethaw resistance of the resulting surface. This data is presented in detail in section C.3.1
D.
C.3
Material Testing
The following sections present the material testing program preceding the actual
freeze-thaw testing program of single lap shear, pull-off and adhesive specimens. The
sections therefore introduce the manufacturing of concrete specimens together with
fresh concrete testing, compression testing of concrete cylinder specimens, 3-point
bending of small-scale concrete prisms specimens, freeze-thaw testing of the two types
of concrete substrate and the thermal expansion measurements of the epoxy adhesive.
The first three sections present quantifications of the properties of materials used, while
the remaining sections present the main experimental results of the study.
113
C.3.1 Concrete Testing
A.
Specimen Manufacturing & Fresh Concrete Tests
The 50 x 100 x 200 mm concrete blocks used in this study for both the single lap
shear and pull-off specimens were manufactured in standard small-scale beam forms of
dimensions 100 x 100 x 400 mm split into halves using a stainless steel insert as shown
in Figure C.2. After curing they were cut to their final dimensions 50 x 100 x 200 mm. An
industrially manufactured concrete dry premix, with water cement ratio of 0.42, was used
for ease of manufacturing and for better consistency between concrete specimens. The
dry mix was placed in batches of 50 kg in the rotary mixer and 4.7 liters of water was
poured in. In case of the air-entrained concrete the appropriate ratio (0.005%) of airentraining agent (Silipon) was carefully worked into the dry mix so as not to loose any
part of it, which might be the case when added during the running time of the mixer. The
mixer was then started for 7 minutes. Both the volume of entrapped air and the slump
were measured directly after mixing (Figure C.3 & C.4). The concrete was then carefully
placed in the forms and compacted by 20 strokes of a rectangular compacting tool with
base dimensions 20 x 200 mm at both half and full depth. The concrete was also shortly
vibrated (so as not to loose any of the air-entrainment) at both half and full depth by
placing the form on a mechanical vibration table. This procedure resulted in a high
strength concrete lacking large voids and having high quality surfaces. The blocks were
removed from the forms after 24 hours and 7 days wet-cured in a water bath. The blocks
were placed in laboratory conditions after the 7 day wet curing period. After the 28-day
curing period the specimens were cut to their final size of 50 x 100 x 200 mm and
surface preparation and adhesion of the CFRP strips was commenced.
114
Prior to placement of the wet concrete mix to the forms both air content and
slump was measured. The air content was measured according to CSN 72 2444 using a
pressure method apparatus as shown in Figure C.4. The average results of both of
these tests for each of the concrete mix types are presented in Table C.3.
Table C.3. Properties of the two
28-day
compressive
strength (MPa)
Mix Type
42
Air-entrained
66
Regular
concrete mixes
Tensile strength Air
(3-point bending) entrainment Slump
(mm)
(%)
(MPa)
70
9.5
7.7
33
3.5
9.5
Fig. C.1 & C.2. Concrete mixer and concrete forms divided by a stainless steel insert
Fig. C.3 & G.4. Slump test
measurement of entrapped air
and
115
B.
Compression Testing
To measure the compressive strength of the concrete a set of 150 mm x 300 mm
cylinders was manufactured. Three of the cylinders were of regular concrete and three
were air-entrained (Figure C.5). The specimens were removed from forms after 24 hours
and wet cured for 7 days and tested to failure at 28 days of age. The specimens were
tested according to CSN 73 1317 on a 200 kN Instron frame using 10 kN/sec loading
rate. The regular concrete mix had average 28-day strength of 66 MPa and the airentrained mix had average 28-day strength of 42 MPa. All the results are shown in
Table C.4.
Table C.4. 28-day concrete strength measured on 150 mm x 300 mm cylinders
Average Average
Diameter Length
Weight (g)
(mm)
(mm)
Specimen Material
11744
Non Air-entrained 149.49 285.07
1
11969
Non Air-entrained 149.65 287.45
2
11989
Non Air-entrained 149.33 289.13
3
AVERAGE
4
5
6
Air-entrained
Air-entrained
Air-entrained
150.38
150.22
149.77
287.50
289.91
288.13
11126
11198
11050
AVERAGE
116
Specific
Weight
(kg/m 3)
2347.24
2367.34
2367.59
2360.72
Ultimate
Ultimate
Compressive Compressive
Stress (MPa)
Force (kN)
67.74
1189
65.61
1154
65.49
1147
AVERAGE
66.28
787
2178.87
709
2179.52
735
2176.88
2178.42 AVERAGE
44.31
40.01
41.72
42.01
Fig. C.5. Compressive strength of concrete specimens
C.
3-Point Bending
The tensile strength of the concrete substrate was measured using 3-point
bending setup (CSN 73 1302). For this test six 40 x 40 x 160 mm specimens of airentrained mix and nine of regular concrete were manufactured and tested. The
specimens were manufactured in the same manner as the concrete blocks for the single
lap shear tests. They were removed from the forms after 24 hours, were cured for 7
days in a water bath and then tested at 28 days. Following the three point bending test,
the halves of the specimens were also tested in compression, as prescribed by the
standard procedure. The results are presented in Tables C.5 and C.6 and testing setup
is shown in Figures C.6 and C.7. The compression test setup for the prism halves is
presented in Figures C.8 and C.9 and results are shown together with 3-point bending
data in Tables C.5 and C.6. As expected these smaller specimens attained much higher
average compressive strengths than the 150 mm x 300 mm cylinder specimens.
117
Table C.5. Tensile testing of air-entrained concrete prisms
Spec. Width
1
2
3
4
5
[mm]
41.8
42.7
42.0
42.6
42.9
6
40.8
Height
[mm]
40.6
40.1
40.1
40.1
40.7
40.1
Length
[mm]
160.1
160.6
161.0
160.4
160.1
159.5
Weight
[g]
601.6
620.1
598.2
577.7
592.5
548.0
AVG
3-Point
Spec.
Bending
weight
[kg/M 3] [kN] [MPa]
7.22
2214.2 2.21
2255.0 2.81
9.21
2206.1 2.62
8.73
2108.4 2.05 6.73
2119.6 2.24 7.09
2100.0 2.13 7.30
2167.2 AVG 7.71
[kN]
188.5
200.2
192.4
141.8
146.0
135.50
Compression
[MPa]
[kN]
72.15
185.8
75.02
155.8
73.30
196.0
140.8
53.26
154.7 54.45
128.50 53.14
AVG
[MPa]
71.12
58.38
74.67
52.88
57.70
50.39
62.20
Table C.6. Tensile testing of regular concrete prisms
Spec.
1
2
3
4
5
6
7
8
9
Width
[mm]
40.1
40.6
40.5
40.9
40.6
40.1
41.2
41.2
41.9
Height
[mm]
39.7
39.8
39.8
40.0
40.1
40.2
40.1
39.5
39.7
Length
[mm]
159.7
159.7
159.8
159.8
160.0
159.8
161.1
161.0
161.1
3-Point
Bending
Weight
Spec.
weight
[g]
607.6
617.4
613.0
619.4
618.6
608.4
631.8
626.8
633.6
[kg/m3 ]
2389.9
2392.5
2379.8
2369.3
2374.8
2361.8
2373.8
2392.3
2364.4
[kN]
2.15
2.90
2.90
2.64
2.74
2.98
2.83
2.91
2.62
[MPa]
7.65
10.15
10.17
9.08
9.44
10.35
9.61
10.19
8.93
AVG
2377.6
AVG
9.51
Fig. C.6 & C.7. 3-point bending setup
118
Compression
[kN]
209.3
209.3
207.8
217.5
222.8
206.7
201.7
210.8
215.3
[kN]
219.7
220.8
207.9
219.5
215.2
211.7
203.9
197.0
217.6
[MPa]
83.51
82.48
82.09
85.09
87.80
82.47
78.33
81.86
82.21
[MPa]
87.66
87.01
82.13
85.87
84.81
84.47
79.18
76.50
83.09
AVG
83.14
Fig. C.8 & C.9. Compressive testing of the prism specimens
D.
Freeze-thaw Resistance
This section presents the results of experiments testing the freeze-thaw durability
of the concrete substrate itself. These experiments were undertaken not only to measure
the freeze-thaw durability of the concrete substrates used in this study, but also to asses
the difference between the freeze-thaw cycling of concrete in water and in chloride
solution (3% NaCl), that were previously found to have very different influence on the
bonded system, but their effect was not directly compared in a single study. The two
freeze-thaw cycling procedures selected for this study were CSN 73 1322 and CSN 73
1326. CSN 73 1322 is a dry freeze, water thaw procedure, whereas CSN 73 1326 is a
freeze-thaw procedure where specimens are constantly submerged in 5 mm of 3% NaCl
solution. Both procedures are explained in great detail in section D.3. For comparison
purposes the CSN 73 1326 procedure was selected and a 3% NaCl solution or tap
water was used as the testing medium. The results of these experiments are presented
in Table C.7. The results quote grams of oven dried debonded material per square
meter after 75 cycles as prescribed by the standard. The actual procedure is as follows;
119
the specimens are cycled in a pan of known weight submerged in 5 mm of testing
medium (chloride solution or tap water) and after 75 cycles the specimen is removed
and all delaminated material is left in the pan. The pan is then carefully drained (in order
not to loose any material) and dried at 1000C for 48 hours. The pan together with the
debonded materials is weighed and the difference in weight is calculated. The maximum
value for freeze-thaw resistant concrete is set to 1000 g/m 2 by the standard.
The addition of the chloride solution increases the damage to non air-entrained
concrete by almost an order of magnitude as the weight of debonded material increases
from 191 g/m
2
in cycling with tap water to 1812 g/m 2 in cycling with chloride solution.
The deterioration to the air-entrained concrete increases approximately by 2.5 times,
from 305 g/m 2 in tap water to 765 g/m 2 in chloride solution. Table C.7 also presents the
results for the testing of specimens that already underwent a surface preparation for
bonding of the FRP strips. This was a very important to measure, because there might
have been a significant difference between the freeze-thaw resistance of the original
surface and the freeze-thaw resistance of the mechanically abraded surface. According
to the results there is no significant difference between the original and surface prepared
concrete, for air-entrained concrete the comparison yields 765 g/m 2 to 851 g/m 2, and for
regular concrete, 1812 g/m 2 to 1762 g/m 2. These results are fully consistent with longterm (20 years) experience of the Czech civil engineering community with these two
freeze-thaw procedures, where comparisons between them were conducted many times
with similar findings.
Since the surface deterioration due to freeze-thaw cycling in the shear and pull-off
specimens cannot be measured directly, Figures C.10 and C.11 illustrate the severity of
the surface and edge deterioration after only 50 cycles in 3% NaCI solution. Specifically
120
Figure C.11 shows very well the deteriorated corner and edge of the specimen; also
notice the volume of debris surrounding each of the specimens. Figures C.12 and C.13
show the freeze-thaw cycled specimens of the two concrete mix types. Figure C.12
presents regular concrete specimen freeze-thaw cycled for 50 cycles in water not
exhibiting any significant damage besides erosion of the edges of its largest pores.
Figure C.13 shows a regular concrete specimen freeze-thaw cycled for 50 cycles in 3%
NaCl solution. The specimen experienced severe deterioration of its surface with even
some larger aggregates showing. Similarly, Figures C.14 and C.15 present regular
concrete specimens after 75 cycles in water and in chloride solution. The damage is
proportionally more pronounced than after the 50 cycles, especially the water cycled
specimen only now begins to show signs of surface damage. Similar tests performed on
the air-entrained concrete show that the surface of the specimens is damaged much
less, as documented by the numerical results, but the damage in visual inspection is
always much smaller in water cycled specimens than in chloride cycled specimens.
Table C.7 Freeze-thaw properties of the concrete substrate
Total
delaminated
concrete after 75
cycles (CSN 73
Testing
Medium
1326) (g/m 2)
Concrete Type
305
H2 0
Air-entrained concrete
765
NaCl
3%
concrete
Air-entrained
851
Air-entrained surface-prepared concrete 3% NaCI
191
H2 0
Regular concrete
1812
NaCl
3%
Regular concrete
1762
3% NaCl
Regular surface-prepared concrete
121
C.10 & C.11. Deteriorated
>ecimens after 50 cycles
C.12. Regular concrete specimen after 50 cycles in water (CSN 73 1326)
Fi. C.13. Regular concrete specimen after 50 cycles in 3% NaCl solution
(CSN 73 1326)
122
C.14. Regular concrete specimen after 75 cycles in water (CSN 73 1326)
Fig. C.15. Regular concrete specimen after 75 cycles in 3% NaCl solution
(CSN 73 1326
C.3.2 Adhesive Thermal Expansion
To measure the coefficient of thermal expansion (CTE) of each of the adhesives,
10 mm x 10 mm x 120 mm small-scale prisms were manufactured. The CTE of the
adhesives is very important, since it is significantly different from that of concrete and
the CFRP strip, which might play an important role in the deterioration of the adhesive
bond properties. For each of the adhesive types, two such prisms were manufactured
123
and the thermal expansion properties were determined by precise measurement of their
overall length with attached metal bearing points (Figure C.17) after 24-hour exposure to
given temperature. At temperatures at and above 300C, the specimens were heated by a
precise electronically controlled water bath with continuous pump driven circulation of
the water. At temperatures below 250C, the specimens were cooled or frozen in air while
a calibrated thermometer determined their exact temperature. The room temperature
was 250C at the time of measurement.
The results of measurement of CTE of the adhesives used in this study are
presented in Table C.8. The results show significant differences amongst the three
adhesives, with SIKA having the smallest CTE of the group, namely around 2 x 10-5 per
0C
in most of the temperature range. On the other hand,,, Fyfe and Betosan have highly
varying CTE dependent on the temperature range; around 4 x 10-5 per 0C in the low
temperature range (-320C to +250C) and much higher and gradually increasing CTE in
the high temperature range (+30
0C
to +60 C). The values must then be compared with
CTE of concrete (around 1 x 10-5 per OC) and CTE of the CFRP strip of 0.1 x 10-5 per 0C
(SIKA Carbodur S512). This comparison shows that, in some temperature ranges, there
is an order of magnitude difference between the CTE concrete and the CTE of the
adhesive. Also, there exists two orders of magnitude difference between the CTE of the
CFRP strip and the CTE of the adhesive in the case of Fyfe and Betosan adhesives.
This mismatch in the CTE of the three materials results in differential stresses at the
bonded interface that could lead to gradual damage of the bond. This phenomenon
should be thoroughly investigated, probably using finite element analysis, to assess its
contribution to deterioration of the bond properties during long-term exposure to
temperature cycles.
124
Table C.8. Coefficient of thermal expansion of
the adhesives
Coefficient of thermal expansion (1 0-5/OC)
Betosan
SIKA Fyfe
Interval (SC)
3.8
4.2
2.0
-32 - -16
3.8
3.8
1.8
-16-+1
4.4
1.4
-0.5
+1 -+25
10.8
8.7
1.1
30-40
13.1
10.1
2.6
40-50
13.0
16.0
4.8
50-60
Fig. C.18 & C.19. Thermal expansion specimens and electronically controlled water
bath
125
APPENDIX D
EXPERIMENTAL PROGRAM
D.1
Selection of Specimen Configuration
The single lap shear specimens were selected as the main specimens for this
study, because of their relative geometrical, manufacturing and testing simplicity and
also the availability of experimental data from previous studies, for example Green et. al.
(2000) and Au (2004). There are many other types of specimens and experimental
setups, but most of them are much more complicated while providing the same
information as the single lap shear specimens. Also, the plain shear case is a limit case
of behavior experienced by the flexural member during loading when a crack is created
and the FRP is peeled with a force at an angle (figure D.1). The other limit case is the
peel case, which might use the same specimen, but was not utilized in this study,
because of focus is on the single lap shear case in two different environmental
conditions.
126
Fig. D.1. Differential crack mouth movement causing debonding (Kaiser 1989)
~IN\
pel
Imnate
The second type of specimens used in this study was the pull-off specimen. The
pull-off test is a widely used in tests that can readily be performed in the field and is
commonly used as a measure of adhesion quality of coatings or other layered systems.
The test is standardized in ASTM D4541. A tester Dyna Z1 5 (ASTM D4541 Type 1)was
used in this study. The motivation for the use of pull-off specimens was to ascertain if
there is a relationship between the pull-off strength as measured in this simple field test
and with the shear test that requires laboratory setup. If such correlation could be
established it would certainly help the quality control of this type of bonded system. This
experimental program was thought to be ideal way to find if there is such correlation,
because the experiments created many data points of shear and pull-off specimens
exposed to the same freeze-thaw cycling.
Another important part of the overall bonded system is the adhesive. The
specimens of the thixothropic epoxy based adhesives were freeze-thaw cycled in order
to ascertain their possible degradation due to the freeze-thaw cycling. For this goal a set
of ASTM D638 Type I dumbbell specimens was manufactured and freeze-thaw cycled in
similar fashion as the shear and pull-off specimens and loaded in tension to failure.
127
D.2
Specimens
D.2.1 Specimen Dimensions
Single Lap Shear Specimens
The single lap shear specimens, shown in Figure D.2, consisted of a 50 x 100 x
200 mm concrete block and a 460 mm long CFRP strip. The CFRP strip overlaps the
concrete block on 180 mm of the length while 160 mm are actually bonded. A region of
20 mm from the free edge of the concrete block was not bonded in order to prevent any
edge effects from influencing the performance of the bonded area during the shear test.
To prevent the adhesion in this region, a clear plastic tape was used to cover this 20 mm
long region prior to the bonding procedure. Because this type of plastic tape had a very
weak adhesion to the concrete, this region was not bonded, even though the adhesive
was bonded to the plastic tape from the top side. The bond width was either 50 mm
(SIKA) or 50.8 mm (Fyfe) according to the width of the CFRP strip. There was a 280 mm
section of the FRP cantilevering from the concrete block. The actual bond width was
variable, because the adhesive spread during the adhesion procedure past the lines of
the specified bond area. Hence, the width of the fracture plane was always measured
after the test.
128
Fig. D.2. Single lap shear specimen
POOO
Pull-off Specimens
The pull-off specimens utilized the same blocks as the single lap shear
specimens (50 mm x 100 mm x 200 mm). In this case two 50 mm x 50 mm squares of
the CFRP strip were bonded to each block. Two such pull-off blocks created one pull-off
data point. Aluminum testing dollies were adhered to the FRP surface prior to the testing
with SIKA Sikadur 30 adhesive and allowed to cure for one day. The overall dimensions
of the concrete block with the testing dollies are shown in Figure D.3.
Fig. D.3. Pull-off specimen configuration
200
129
Adhesive Tensile Specimens
The third type of specimens was the adhesive tensile specimens of ASTM D638
type I dumbell shape. The specimens were manufactured in two-layer Teflon forms
compressed together by thick plastic compression plates. The specimens were removed
from forms after 24 hours and let cure in room conditions for 7 days prior to
commencement of the testing. The exact dimensions of the Type I dumbbell specimens
are shown in Figure D.4.
Fig. D.4. ASTM D638 Type I dumbbell specimen, dimensions in mm [in]
dt,
R7O-
"afrahi
ar
n]
iettion3
tpe
Fig. D.5 & D.6. Teflon forms between compression plates and scraper used to finish the
surface of the specimens
130
D-7- fltnil of finished snecimen
D.2.2 Number of Specimens
For the single lap shear tests, a total of 84 specimens were used. A total of 12
specimens (3 types of FRP/adhesive combinations, 2 types of concrete and 2
specimens per data point) were left in laboratory conditions as control specimens. The
remaining specimens were divided into three groups (24 specimens each) of each
FRP/adhesive combination and then divided into subgroups (12 specimens each) by the
type of concrete (regular or air-entrained). The 12 specimen subgroups were then
divided into two groups (6 specimens each) according to type of freeze-thaw cycling
procedure (CSN 73 1322 or tSN 73 1326). Two of the specimens created each data
point.
The pull-off specimens were set to create the same data points as the shear
specimens, but provided two data points per one concrete block. Therefore, 42 concrete
blocks with 84 pull-off squares of FRP were utilized in the study.
131
The number of adhesive tensile specimens was exactly the same as the number
of shear specimens. The specimens were also freeze-thaw cycled in the same fashion
as the shear and pull-off specimens.
D.2.3 Concrete Surface Preparation and Adhesion of FRP
The surface preparation was done using a Hilti demolition hammer TE-505 with a
bushing tool chisel TE-Y-SKHM. Similarly, a needle scaler could have been used in this
application. The resulting surface was flat with minor depressions from debonded
aggregates. Thereafter, the surface of concrete was carefully vacuum cleaned to
remove the fine dust created by the mechanical abrasion. Similarly the FRP was
cleaned of all dust and chemically cleaned using SIKA Colma Reiniger, a product
prescribed by SIKA for this application. The FRP was then adhered to the specified
region, pre-drawn on the concrete surface to precisely locate the bond, using one of the
three adhesive types. The two-part adhesives were hand mixed for 5 minutes in
disposable plastic bowls. The adhesive was always first worked into the concrete
surface by a spatula and then applied onto the FRP in bowed out shape to prevent
creation of any air pockets in the bond line. The FRP was then firmly pressed against
the concrete block as the adhesive overflowed from the edges of the bonded area. A
bond length of 160 mm was selected, with respect to the total length of the blocks (200
mm). To prevent any possible edge effects from affecting the results of the test, the
distance to free edges was kept at 20 mm from both front and back of the specimen as
shown in Figure D.2. The width of the bond was either 50 mm (SIKA FRP) or 50.8 mm
(Fyfe FRP). The specimens were then cured for 7 days after which the freeze-thaw
cycling was commenced. The overall shape and dimensions of the single lap shear
specimen are shown in Figure D.2.
132
Fig. D.8 & D.9. Demolition hammer and bushing tool chisel used for surface preparation
of the concrete blocks
Fiq. D.10. Concrete block after surface preparation
D.3
Description of Freeze-Thaw Cycling
For freeze-thaw cycling the specimens of all three types (shear, pull-off, adhesive
tensile) were divided into two equal halves and cycled according to one of the following
freeze-thaw standards described below. The two standards were selected to compare
between freeze-thaw cycling in water and in chloride solution. The idea was to use
standard freeze-thaw cycles described in the Czech national standards, used for many
years now, since they can be readily repeated given the availability of the experimental
133
equipment. The following two freeze-thaw standards were used; CSN 73 1322 and cSN
73 1326. These procedures were also selected with respect to findings of the reviewed
freeze-thaw studies that have found very different results based on the type of cycling.
Therefore, the two procedures were selected to directly compare between freeze-thaw
cycling with dry freeze and water thaw commonly used in freeze-thaw cycling of
concrete, for example ASTM C666, and freeze-thaw cycling with constant exposition to
chloride solution, which was previously shown to be much more damaging to the
bonded system as well as concrete itself, as presented by the literature review.
A.
CSN 73 1326 Standard
The standard CSN 73 1326 specifies freezing and thawing of specimens
constantly submerged in 5 mm of 3% NaCl solution in a purpose built testing chamber
KD-20-S1.1 (Figure D.11). This type of exposure simulates real world applications
subjected to deicing chemicals or seawater. The procedure was originally developed in
early 1980s as a standard for testing freeze-thaw durability of concrete highway
surfaces. Therefore,, the damaging effect of this procedure is limited to only 5 mm of
depth of any specimen, damaging mostly its submerged surface. Similarly the purpose
built testing chamber is unable to freeze and thaw the testing compartment in required
time if more than 5 mm of liquid medium is present. This fact results in inability of
freezing whole concrete specimens to determine their post freeze-thaw bulk properties.
In this study the specimens were submerged in 5 mm of 3% NaCl solution with
the FRP strip on the bottom, with approximately 2.6 mm of concrete being submerged
(Figure D.14). The freeze-thaw cycle is as follows; start at +200C decrease temperature
to -150C in 45 to 50 minutes, hold -150C for 15 minutes, increase temperature to +200C
134
-i-haall-a.i..a.
.bl.
in. NM.ww.
Who.,
n EM.RLEli.el.Em
. -a.JIdu ,'
*M."
'.
in 45 to 50 minutes and hold +200C for 15 minutes, which completes 1 cycle. This
procedure was completely automated, controlled by a digital control unit that has two
independent temperature sensors inside the testing medium (Figure D.12). Groups of
specimens were subjected to 25, 50 and 75 cycles. 75 is the number of cycles specified
in this standard. Specimens not subjected to freeze-thaw cycling were stored at room
temperature and humidity.
Fig. D.11 & D.12. Freeze-thaw chamber KD-20 and its inner compartment with
s ecimens and cable runnin to temperature sensors
Fig. D.13 & D.14. Detail of the adhesive specimens in the testing compartment and
schematic of solution submersion
3% NckCl
SOLuY iOn
135
9
B.
CSN 73 1322 Standard
The
CSN
73 1322 standard is a universally used procedure using dry-freeze and
water-thaw, similar to ASTM C666. The
CSN
73 1322 procedure specifies freezing in
cold air at -1 80C for 4 hours and thawing in water bath at +200C for 2 hours, in this case
the standard cycles were modified based upon experimental equipment available for the
experimental program. The actual freeze-thaw cycle used in this study consisted of
freezing in cold air at -100C for 4 hours (Figure D.15 & D.17) and thawing in +170C
water bath for 1 hour (Figure D.16). Unfortunately, the limited power of the cooling
system of the freezer used in the study was unable to fully freeze thick concrete
specimens during the time specified, therefore the bulk properties of the concrete could
not be measured following the freeze-thaw exposure. To verify that the specimens were
freezing and thawing at the temperatures and times utilized, the actual temperature of
the specimens, was monitored throughout the freeze-thaw cycling by a calibrated pin
thermometer. The thermometer was always touching the FRP/adhesive interface that
was changing its temperature the slowest, because of the low thermal conductivity
properties of the FRP (Figure D.18). Groups of specimens were subjected to 25, 50 and
100 of these cycles.
136
N 73 1322) and water thawir
Fig. D.17 & D.18. Specimens inside the freezer and detail of temperature
measurement of the adhesive bond
D.4
Testing Program
D.4.1 Single Lap Shear Test Setup
The single lap shear specimens were tested using two different testing frames
because of their different availability. The control specimens group was tested on a 100
137
kN computer controlled MTS 810 frame shown in Figure D.19. On the other hand,,, the
rest of the specimens were tested on an older 400 kN manually controlled testing frame
shown in Figure D.23. Because the test setup of each of the tests was slightly different,
the two setups will be described separately in the following paragraphs.
The setup of the control specimen testing used a fixed upper crosshead with
hanging steel fixture for the concrete block to rest on (Figure D.20). In this case the test
relied on perfect perpendicularity between the axis of the FRP strip and the front side of
the concrete block, which if not perfect might lead to introduction of partial peeling load
into the test. The FRP strip was then gripped by a lower crosshead with 50 kN load cell
and tested at load rate 0.2 kN/sec. The data was acquired using a computerized data
acquisition system. Besides the load, the strain was measured at the beginning of the
FRP bond to concrete on top of the FRP plate using an extensometer (Figure D.21) and
displacement was measured at the end of the FRP plate furthermost from the loading
grip. The general loading scheme is presented schematically in Figure D.22.
138
i. D.19 & D.20. MTS testinq fran
test setup
Fig. D.21 & D.22. Detail of extensometer attachment and overall loading scheme
A
Re'trr
L'D
The second setup (Figure D.23) used for the freeze-thaw cycled specimens
utilized a fixed upper crosshead that in this case gripped the CFRP strip. The lower
moving crosshead gripped a purpose made steel fixture (Figure D.24) designed to firmly
hold the concrete block and thus provide a reaction force. This fixture also utilized a
semicircular bearing joint that was introduced into the system in order to prevent any
139
peel action from occurring during the shear test. The block of the specimen was firmly
held in place by a plastic wedge and a couple of side tie bolts. The main focus in placing
the concrete block in the fixture was always to firmly press the front of the concrete to
the top of the fixture (the reaction plate) to prevent any movement during the running of
the test. Since the testing frame was manually controlled, it was always set to its slowest
possible running speed and the actual load rate was back calculated from the
displacement. The position of the crosshead was recorded using a digital displacement
gage (Mitutoyo) with a printer output, shown in Figure D.23. Upon completion of each
test the failure surface was surveyed for the type of failure mode.
Fig. D.23 & D.24. 2 nd single lap shear test setup and its detail with reaction plate and
attached di tal readout of displacement measurements
*AK
140
/
D.4.2 Pull-off Test Setup
The pull-off test utilized the ASTM D4541 Type I pull-off tester Dyna Z15 made by
Proceq (Figure D.25). In order to provide enough reaction area around the pull-off dolly
for the tester's three legs to bear against during the test, the specimen was always
placed underneath a structural C profile with a cut out (Figure D.26 and D.27). The
specimen was always first firmly pulled against the reaction plate by turning the center
knob and then slowly loaded to failure using the crank. The maximum pull-off strength
was recorded and failure surface was surveyed for type of failure. Typical tested
specimen with the respective pull-off dolly is shown in Figure D.28. The general loading
scheme of the pull-off specimens is shown schematically in Figure D.29.
& D.26. Dvna Z15 pull-off tester and the pull-off sett
141
Fig. D.27 & D.28. Detail of the test setup with ball joint connection and tested specimen
with the pull-off dolly attached material showing
Fig. D.29. Schematic of loading of the pull-off specimens
4.
4t
D.4.3 Adhesive Tension Testing Setup
The adhesive tensile dumbbell specimens were tested upon completion of the
freeze-thaw cycling in tension. A 100 kN Instron 1331 testing frame (Figure D.30) with
10 kN load cell was used to test the specimens with a loading rate of 2.5 mm/min. This
rate was selected because the ASTM D638 prescribed 5 mm/min produced a break in
the SIKA adhesive specimens before 30 seconds of running time as also specified in the
standard as a minimal running time of the test. Figure D.31 shows the dumbbell
specimen in the grips. An extensometer with a 25 mm gage length was used to record
the strain during the test as shown in Figure D.32. During the test crosshead position,
142
load and strain were recorded using a computerized data acquisition system (Figure
D.33).
Fig. D.30 & D.31. Instron testing frame used in the tensile tests of the adhesive
specimens and detail of ri s with dumbbell specimen inserted
Fig. D.32 & D.33. Detail of extensometer attachment and computerized data acquisition
svstem
143
D.5 Experimental Results
The following sections present the main experimental results of this study. The
results of each of the tests are subdivided according to the two freeze-thaw cycling
procedures used (CSN 73 1326 and CSN 73 1322). This is due to very different effect of
the two freeze-thaw procedures on the mechanical properties of the bonded system.
This first section presents the results of freeze-thaw cycling of concrete
specimens according to tSN 73 1326 both in tap water and 3% NaCl summarized in
Table D.1. This study was performed to compare the effect of freeze-thaw cycling in the
two different mediums. The results quote grams of oven dried debonded material per
square meter after 75 cycles as prescribed by the standard. The maximum value for
freeze-thaw resistant concrete is set to 1000 g/m 2 by the standard. The addition of the
chloride solution increases the damage to non air-entrained concrete by approximately
an order of magnitude and damage to air-entrained concrete by 2.5 times, as discussed
in greater detail in section 3.3.1.D. This difference in damage of concrete itself also
explains the significant difference between the results of shear and pull-off tests of
specimens subjected to the two different freeze-thaw standards.
In freeze-thaw cycling in water (tSN 73 1322) the concrete is actually wet cured
while the freeze-thaw damage is minimal and therefore the concrete attains higher
strength during the freeze-thaw cycling than control specimens left in dry storage.
Therefore, the shear and pull-off specimens can attain higher ultimate strength as it is
mostly dependent on the strength of the concrete. On the other hand,, in freeze-thaw
cycling in chloride solution (CSN 73 1326) the combined action of chlorides and freezethaw cycling is much more damaging as described above. The shear and pull-off
strength of the specimens have therefore an initial upward trend as the concrete is
144
initially wet cured and the freeze-thaw damage is not yet very pronounced due to time
necessary for the chloride solution to penetrate the whole bond. When the specimens
are subjected to larger number of cycles, the solution has time to gradually penetrate the
bond and cause damage. Hence, after initial upward trend the ultimate strength curve of
the specimens levels off and starts declining.
The freeze-thaw damage was obvious in both air-entrained and regular concrete
specimens after only 50 cycles (Figure D.34) where the edges and exposed surface
started to disintegrate. At 75 cycles the damage was similar, only more pronounced with
larger volume of material debonding off from the edges and exposed surface of the
specimens. A total of 75 cycles prescribed by the iSN 73 1326 seems to be appropriate
as a larger number of cycles would result in complete disintegration of the concrete
material submerged in the chloride solution even in the air-entrained concrete. Based on
the visual inspection this is estimated to occur at around 125 cycles for the air-entrained
concrete. Finally, the data of Table D.1 also show that the surface preparation
procedure appears to have a minimal impact on the freeze-thaw resistance of concrete
surface in this type of freeze-thaw cycling as proven by the two such treated and tested
specimens.
Table D.1. Freeze-thaw properties of concrete substrate
Total delaminated
concrete after 75
cycles (tSN 73
Testing
1326) (g/m 2)
Medium
Concrete Type
305
H2 0
Air-entrained concrete
765
3% NaCI
Air-entrained concrete
851
NaCl
3%
concrete
Air-entrained surface-preparated
191
H2 0
Regular concrete
1812
3% NaCl
Regular concrete
1762
NaCI
3%
Regular surface-preparated concrete
145
Fig. D.34. Severely deteriorated pull-off specimen
D.5.1 Shear Testing Results
The following section will present the results of the single lap shear testing
program separately for the two freeze-thaw procedures. First, the ultimate strengths are
discussed followed by presentation of failure modes and ultimate displacements.
The results of the single lap shear tests of specimens freeze-thaw cycled
according to CSN 73 1326 are presented in Table D.2 and graphically in Figures D.35
and D.36. The absolute values of the shear strength are not important, because they are
dependent on the particular type of concrete, surface preparation procedure and
specimen geometry. On the other hand,,, the most important results are the relative
changes in bond strength between the control specimens and the freeze-thaw cycled
specimens. The SIKA/SIKA adhesive/FRP combination has exhibited 8% loss of shear
strength on the air-entrained concrete and 10% loss on regular concrete. The Fyfe/Fyfe
combination has shown 4% loss of shear strength on the air-entrained concrete and
146
24% loss on regular concrete. Finally, the Fyfe/Betosan combination has experienced
23% loss of shear strength on the air-entrained concrete and 21% loss on regular
concrete. Specifically the last result shows that with only two specimens per data point a
high statistical accuracy cannot be achieved, since the Fyfe/Betosan combination
experienced higher deterioration on air-entrained concrete. This result is not supported
by the results of the other two material combinations and it is likely that the deterioration
was approximately same for the air-entrained and regular concrete specimens of the
Fyfe/Betosan combination and the difference was caused only by statistical inaccuracy
of the results. On the other hand, it is important to note that the Betosan adhesive
exhibited consistently higher ultimate shear strengths when applied on the air-entrained
concrete. This fact might be explained by a better adhesion between this type of
adhesive and the air-entrained concrete, which has larger pore structure.
The results of the shear tests of specimens freeze-thaw cycled according to CSN
73 1322 are presented in Table D.3 and graphically in Figures D.37 and D.38. The
SIKA/SIKA adhesive/FRP combination exhibited a 52% increase of ultimate shear
strength in the air-entrained concrete specimens and was not affected in the regular
concrete specimens. The Fyfe/Fyfe combination has shown 8% increase of shear
strength in the air-entrained concrete specimens and was not influenced by freeze-thaw
cycling in regular concrete specimens. Finally, the shear strength of Fyfe/Betosan
combination was not changed in the air-entrained concrete specimens, but specimens
experienced a 14% loss of shear strength in regular concrete specimens compared to
the control specimens. The results show that some adhesives (SIKA) were able to fully
benefit from the increase of concrete strength caused by the moist curing of the
concrete. The significant increase of shear strength in the SIKA adhesive bonded
147
specimens (+52%) is probably due to the adhesive penetrating better the pore structure
of the concrete, therefore protecting it longer from freeze-thaw damage. In comparison,
the Fyfe adhesive bonded specimens must have experienced the same increase in
concrete strength as the other specimens, but the adhesive did not protect the concrete
from deterioration very well and the increase in strength of concrete was lost due to
freeze-thaw deterioration. The results of the Betosan adhesive bonded specimens can
be explained by the same mechanism and the slightly worse results are probably
caused by it being a development stage product.
The failure modes were also surveyed after the shear testing. A typical tested
specimen and its failure plane are shown in Figure D.39. The failure occurred in one of
the following modes: concrete cohesive failure, adhesive failure between concrete and
the adhesive and adhesive failure between the adhesive and the FRP strip. These
modes usually appeared in combination in the failure plane and a percentage of each of
the failure modes on the overall bond area was assessed. In general, there were no
trends observed in the failure modes after the freeze-thaw cycling. It appears that the
failure mode was determined only by the type of adhesive and by the type of concrete
substrate and not by the freeze-thaw exposure. This contrasts with studies like
Mukhopadhyaya et. al. (1998) and Green et. al. (2000) which have found a clear trend in
the failure mode changes from cohesive failure of concrete to adhesive failure between
concrete and the adhesive after the freeze-thaw exposure.
In general, the SIKA bonded specimens exhibited approximately 80% of break in
concrete in air-entrained concrete, and around 40% of break in concrete in regular
concrete, regardless of freeze-thaw cycling type or number of cycles. The Fyfe adhesive
bonded specimens experienced approximately 75% of break in concrete in air-entrained
148
concrete; around 50% of break in concrete in regular concrete. Finally, the specimens
bonded with the Betosan adhesive have shown approximately 60% of break in concrete
in air-entrained concrete, around 40% of break in concrete in regular concrete. This type
of adhesive exhibited up to 15% of area debonding between the adhesive and the FRP
strip. In the other two adhesives this failure mode was exhibited in no more than 5% of
the area of the bond. This is probably related to the adhesive being only in its
development stage and the properties should be corrected in the final product, as this
type of debonding is not desirable. Overall, the failure mode was determined by the
relative mechanical properties of the concrete and the adhesive and was not influenced
by either of the freeze-thaw cycling programs.
The survey of the ultimate (at break) displacements during the shear test from the
test data did not show any clear trends in the Fyfe and Betosan adhesives. On the other
hand,,, the SIKA adhesive exhibited increasing ultimate displacement in both freezethaw exposures and on both types of concrete substrates. This might be explained at
least partially by the adhesive plasticization after the exposure to freeze-thaw cycling.
This is supported by the tensile specimen results of the SIKA adhesive that have shown
significantly (+22% SIKA, +27% Fyfe) larger ultimate strains after CSN 73 1322 type of
freeze-thaw cycling as documented in Table D.6 in section D.5.3.
149
Table D.2. Single lap shear results of specimens cycled according to
CSN 73 1326
Ultimate Displacement (mm)
Shear Strength (MPa)
Number of
Betosan
Fyfe
SIKA
Betosan
SIKA Fyfe
Cycles
Air-entrained concrete
N/A
N/A
N/A
3.0
2.6
2.5
0
2.5
2.6
2.2
3.2
2.9
2.9
25
2.2
2.1
2.7
3.1
2.7
3.0
50
2.3
2.7
3.0
2.3
2.3
2.5
75
Regular concrete
N/A
N/A
N/A
2.9
2.9
3.1
0
2.1
1.8
2.8
2.5
3.0
3.1
25
3.2
2.7
3.0
2.4
2.5
3.3
50
2.7
4.2
4.1
2.3
2.2
2.8
75
Table D.3. Single lap shear results of specimens cycled according to
CSN 73 1322
Ultimate Displacement (mm)
Shear Strength (MPa)
Number of
Betosan
SIKA _Fyfe
Betosan
SIKA Fyfe
Cycles
Air-entrained concrete
N/A
N/A
N/A
3.0
2.6
2.5
0
2.3
2.2
3.3
2.9
2.8
3.2
25
2.2
3.6
3.7
3.3
2.7
2.8
50
2.1
2.4
3.6
3.0
2.8
3.8
100
Regular concrete
N/A
N/A
N/A
2.9
2.9
3.1
0
2.9
2.5
2.6
3.2
3.1
3.3
25
2.7
2.9
2.4
2.7
3.4
3.1
50
2.3
1.7
4.9
2.5
3.0
3.0
100
150
Fig. D.35. Shear strength during freeze-thaw exposure according to C SN 73 1326
(air-entrained concrete)
4.0
3.5
6. 3.0
I.
2.5
-
-
2.0
1.5 1.0
0.5 0.0
75
50
25
0
Number of Cycles
~S I K A'S IKA
- Fyfe/Fyfe
*
Fyfe/Betosan
Fig. D.36. Shear strength during freeze-thaw exposure according to CSN 73 1326
(regular concrete)
-
4.0
3.5
0.
0)
2.0
1.51.0
0.5
0.0
0
75
50
25
Number of Cycles
-*--SIKA/SIKA
- Fyfe/Fyfe
151
Fyfe/Betosan
Fig. D.37. Changes in shear strength during freeze-thaw exposure according to CSN 73
1322 of the three material combinations on air-entrained concrete
-~--
- -----
- -
4 .0
3.5
a3.0
2.5
w2.0
1.5
.c
1.0
0.5
0.0
100
75
50
25
0
Number of Cycles
----
- Fyfe/Fyfe
SIKA/SIKA -
4
Fyfe/Betosan
Fig. D.38. Changes in shear strength during freeze-thaw exposure according to CSN 73
1322 of the three material combinations on regular concrete
.......
.......
4 .0
3.5
a.3.0
2.5
2.0
1.5
.~1.0
0.5
0.0
0
50
25
75
Number of Cycles
---- SIKA/SIKA -
-
Fyfe/Fyfe
152
Fyfe/Betosan
100
Fig. D.39. Typical tested single lap shear specimen
D.5.2
Pull-off Testing Results
As described above in section D.1 there was a large number of pull-off
specimens subjected to the same freeze-thaw cycling procedures as the single lap
shear specimens. Peak load and failure mode were recorded in this test. Ultimate pulloff strength was calculated from peak load and bond area, which was measured after
the test. The results of this testing program are presented in this section.
The ultimate strength attained in the pull-off tests of specimens cycled according
to CSN 73 1326 is presented in Table D.4 and plotted in Figures D.40 and D.41.
Important results concern the relative changes during the freeze-thaw cycling between
the cycled and control specimens. The SIKA/SIKA FRP/adhesive combination
experienced 21% increase in pull-off strength on air-entrained concrete and 7% on
regular concrete. The Fyfe/Fyfe combination experienced 5% increase in strength on
air-entrained concrete and 48% loss of strength on regular concrete. Finally, the
153
Fyfe/Betosan combination was unaffected by the freeze-thaw cycling on air-entrained
concrete, but lost 35% of strength on regular concrete.
The ultimate strength of the pull-off specimens cycled according to CSN 73 1322
is presented in Table D.5 and graphically presented on plots in Figures D.42 and D.43.
The SIKA/SIKA FRP/adhesive combination experienced 14% increase in pull-off
strength on air-entrained concrete and slight decrease of 4% on regular concrete. The
Fyfe/Fyfe combination experienced 11 % increase in strength on air-entrained concrete
and 33% gain in strength
on regular concrete. the Fyfe/Betosan combination
experienced 58% strength increase on air-entrained concrete, but lost 9% of strength on
regular concrete.
This complicated behavior of the pull-off strength, or for that matter, even the
shear strength of the specimens can be explained by four important factors competing
during the freeze-thaw cycling that affect the overall bond strength. The first factor is the
wet curing of the concrete substrate during cycling, compared to dry storage for the
control specimens. The second factor is the actual damage to the concrete substrate,
caused by the freeze-thaw action of the liquid in the pore structure, either 3% NaCI
solution or water, with the chloride solution being much more damaging. The third factor
is the difference between the air-entrained concrete and regular concrete. The regular
concrete has much lower freeze-thaw resistance than air-entrained concrete as shown
in section 3.3.1.D. Finally, there is the ability of the adhesive to bond to the concrete
surface that is dependent on the chemical composition and the type and size of fillers in
the adhesive.
The specimens on air-entrained concrete were unaffected by the freeze-thaw
cycling, because they are resistant to freeze-thaw damage. Moreover, they are wet
154
cured during freeze-thaw cycling and their pull-off strength increases. On the other
hand,,, in the specimens made of regular concrete the substrate were significantly
damaged and the specimens lost their strength to a degree that the moist curing was
unable to compensate. The varying degree of strength increase or loss is then
dependent on the composition of the adhesive.
In both types of freeze-thaw cycling there were only two modes of failure
recorded: cohesive failure in concrete and adhesive failure between the adhesive and
the concrete substrate. The failure modes did not change with the freeze-thaw cycling
and were only dependent on the type of concrete substrate. Air-entrained concrete
specimens exhibited 100% cohesive failure of concrete, as this substrate is weaker
relative to the adhesive; whereas the regular concrete specimens exhibited about 90%
cohesive failure of concrete. A typical tested specimen with its respective pull-off dollies
is shown in Figure D.44.
Correlation between shear strength and pull-off strength
Correlation between the single lap shear strength and pull-off strength was
anticipated to some degree, as both are directly proportional to the tensile strength of
the concrete substrate. The comparison of the two data sets is shown graphically in
Figure D.45. Such correlation was not shown by the herein experimental data. This was
given by the different response of each type of the specimens to the freeze-thaw
exposure. In most cases the pull-off specimens were much more affected than the single
lap shear specimens. This is attributed to the larger bond area of the single lap shear
specimens protecting better the bonded area from invasion of liquid and consequent
freeze-thaw damage, whereas the 50 x 50 mm square of bond area for the pull-off
155
specimens is much more easily penetrated and consequently more rapidly damaged.
The higher increases of the pull-off strength results are probably caused by the test
being more directly proportional to the strength of concrete, since there is no interlocking
mechanism acting as in the case of the shear test. Also, the correlation between shear
and pull-off results was not proven even in comparison of only the control specimens.
This was probably caused by the pull-off test not being very accurate. Hence, to gain
statistical accuracy much larger number of specimens would have to be used.
Table D.4. Pull-off test results of specimens
cycled according to SN 73 1326
Pull-off Strength (MPa)
SIKA Fyfe Betosan
Number of Cycles
Air-entrained concrete
1.9
1.9
2.8
0
2.2
2.1
3.1
25
0.9
2.1
3.2
50
1.9
2.0
3.4
75
Regular concrete
3.4
2.1
2.7
0
2.6
2.1
2.6
25
2.3
1.9
1.9
50
2.2
1.1
2.9
75
Table D.5. Pull-off test results of specimens
cycled according to CSN 73 1322
Pull-off Strength (MPa)
SIKA Fyfe Betosan
of
Cycles
Number
Air-entrained concrete
1.9
2.8 1.9
0
1.7
1.8
3.4
25
2.3
1.7
3.3
50
3.0
3.2 2.1
100
Regular concrete
3.4
2.7 2.1
0
3.1
3.5 2.5
25
3.3
2.9
3.6
50
3.1
2.6 2.8
100
156
Fig. D.40. Changes in pull-off strength during freeze-thaw exposure according to CSN
73 1326 of the three material combinations on air-entrained concrete
---- - ----- -----
4 .0
-
-
3.5
3.0
E 2.5
2.0
t
1.5
0
a.
0.5
0.0
75
50
25
0
Number of Cycles
F
KA/SKA
-
-
Fyfe/Fyfe
*
Fyfe/Betosan
Fig. D.41. Changes in pull-off strength during freeze-thaw exposure according to CSN
73 1326 of the three material combinations on regular concrete
4.0
-
3.5
a-3.0
2.5
2.0
1.5
0
-
1.0
__
_
_
_
_
_
_
_
0.5
0.0
0
50
25
Number of Cycles
-
Fyfe/Fyfe
- 157A
157
#-Fyfe/Betoan
75
Fig. D.42. Changes in pull-off strength during freeze-thaw exposure according to CSN
73 1322 of the three material combinations on air-entrained concrete
4.0.3.5
3.0
5 2.5
2.0
-
V 1.5
0
= 1.0
0.5
0.0
75
50
25
0
100
Number of Cycles
-*-SIKA/SIKA
-
Fyfe/Fyfe
-
*
Fyfe/Betosan
Fig. D.43. Changes in pull-off strength during freeze-thaw exposure according to CSN
73 1322 of the three material combinations on regular concrete
-
-
4.0
-
- -
3.5
a 3.0
.
10 - -
-
.
..., 2.5
0)
2.0
1.5
1.0
0.5
0.0
0
50
25
75
Number of Cycles
-,--SIKA/SIKA -
-
Fyfe/Fyfe -4-Fyfe/Betosan
158
100
Fig. D.44. Typical tested pull-off specimen with the two pull-off dollies
Fig. D.45. Comparison of specimens' shear strength and pull-off strength
given same material and environmental exposure
4.0
3.0
ttA
*
-
a-
#4
*
*
4..
2.0
a'
y = P.1 424x + 2.5128 R2 = 0.0801
1.0 !
I
0.0
1
-J
0.0
2.0
1.0
Pull-off Strength (MPa)
159
3.0
4.0
D.5.3 Adhesive Tension Testing Results
As described in section D.1 dumbell specimens (ASTM D638 Type I) of the three
adhesives were used to investigate the changes in bulk properties in tension prior to and
after the freeze-thaw exposure. The initial properties of the three adhesives significantly
vary in ultimate strength, modulus and ultimate strain, but the focus of this study was to
assess the possible deterioration of their properties due to the two freeze-thaw cycling
procedures (CSN 73 1326 and
CSN
73 1322). The results are presented in Table D.6
with typical stress-strain curves for each of the adhesives being shown in Figure D.46.
The ultimate strength of the SIKA adhesive (32.5 MPa) was unaffected by the
freeze-thaw cycling according to CSN 73 1326, but was about 31% higher than the
manufacturer-specified value of 24.8 MPa. The ultimate strength decreased by 10% for
the Fyfe adhesive and by 6% for the Betosan adhesive. The SIKA adhesive had an
initial tangent modulus of approximately 11500 MPa, which does not compare well
(+155%) to manufacturer specified 4500 MPa (Table 3.2) obtained from the same test.
After the freeze-thaw cycling (CSN 73 1326) some of the SIKA specimens has shown an
increase in the modulus up to about 12 500 MPa (+9%), whereas some specimen's
modulus did not change. The modulus of the Fyfe adhesive (1816 MPa) did not change
significantly after the freeze-thaw cycling in any of the specimens. On the other hand,,
the initial tangent modulus of the Betosan adhesive of about 4500 MPa did decrease to
approximately 3900 MPa (-13%). Review of the ultimate (at break) strain does not show
any trends and overall the ultimate strain changed by no more than 7%.
After the freeze-thaw cycling according to CSN 73 1322 the ultimate strength of
the SIKA adhesive increased by 15%. The ultimate strength of the Fyfe and Betosan
adhesives seems to be unaffected by the freeze-thaw cycling of this type as the changes
160
are insignificant compared to the statistical accuracy of the method and number of
specimens. The SIKA adhesive had an initial tangent modulus of approximately 11500
MPa. After the freeze-thaw cycling the modulus has shown an increase to level of about
12 350 MPa (+7%). On the other hand,, the modulus of the Fyfe adhesive (1816 MPa)
did not change significantly after the freeze-thaw cycling. The modulus of the Betosan
adhesive has decreased from its initial 4500 MPa to approximately 4100 MPa (-9%).
Review of the ultimate strain in this case does show an increasing trend in the SIKA
(22%) and Fyfe (27%) adhesives. The ultimate strain of the Betosan adhesive seems to
be unaffected by this type of freeze-thaw cycling.
Both results of the adhesive tensile specimens cycled according to CSN 73 1322
and
SN 73 1326 show significant difference between the response of the SIKA
adhesive to freeze-thaw cycling and response of the Fyfe and Betosan adhesives,
properties of which deteriorated with the freeze-thaw cycling (CSN 73 1326). Overall,
the most dramatic changes in properties were the increase of ultimate strain of the SIKA
(22%) and Fyfe (27%) adhesives during the CSN 73 1322 cycling that correlate with the
significant increase in the ultimate displacement during the shear test in the SIKA
adhesive bonded specimens. This might be explained by the adhesive plasticization
after the exposure to freeze-thaw cycling.
161
Table D.6. Measured adhesive properties (ASTM D638)
Pull-off Strength (MPa)
Number of Cycles
CSN 73 1326
0
25
50
75
(SN 73 1322
0
25
50
75
100
Ultimate Strain (%)
Betosan
Fyfe
SIKA
SIKA
Fyfe
Betosan
32.3
36.2
32.5
32.7
34.2
30.9
29.4
30.9
25.8
26.6
25.0
24.2
0.32
0.38
0.30
0.30
2.83
2.48
2.86
2.75
1.23
1.01
0.99
1.31
32.3
36.1
29.6
33.6
37.1
34.2
33.8
32.8
32.1
33.3
25.8
28.9
26.0
25.1
24.7
0.32
0.40
0.29
0.37
0.39
2.83
3.23
2.47
3.23
3.58
1.23
1.24
1.35
1.12
1.22
Fig. D.46. Stress-strain diagram for typical adhesive specimens of the three types
40 -
35
-
----- - - - - - - --~-
30
0.
25
U)
20
a)
cn
15
In
I-.
.6.D
-
Ido*#
Of
10
-4
I'
5 Il
V
I
-g
I
I
______________________________________
_______________________________________
0 ____________________________________
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
____________________________________
_______________________________________
____________________________________
______________________________________
______________________________________
______________________________________
Strain (%)
SIKA - - - - Fyfe
162
Betosan
____________________________________
____________________________________