De Lorenzis, L., D. Tinazzi, A. Nanni, A., “Near Surface Mounted FRP Rods for Masonry
Strengthening: Bond and Flexural Testing,” Symposium, “Meccanica delle Strutture in Muratura
Rinforzate con FRP Materials,” Venezia, Italy, December 7-8, 2000.
NEAR-SURFACE MOUNTED FRP RODS FOR MASONRY
STRENGTHENING: BOND AND FLEXURAL TESTING
Laura De Lorenzis
PhD Student, Dipartimento di Ingegneria dell’Innovazione
Via per Monteroni, 73100 Lecce
laura.delorenzis@unile.it
Davide Tinazzi
Civil Engineer
Via Comboni 15, 37060 Buttapietra (VR) Italy
dtinazzi@tiscalinet.it
Antonio Nanni
V.&M. Jones Professor of Civil Engineering, University of Missouri – Rolla
Rolla, MO 65409 (USA)
nanni@umr.edu
ABSTRACT
In unreinforced masonry walls, failure due to out -of-plane bending causes the majority of
the material damages and loss of human life. Therefore, the development of effective
strengthening techniques needs to be addressed. Near-surface mounted (NSM) FRP rods
are now emerging as a promising technique for increasing flexural and shear strength of
structural members. The aim of this research program was a first evaluation of the
effectiveness of NSM FRP rods as a strengthening system for masonry elements subjected
to out-of-plane loads. A first series of tests focused on the bond behavior of NSM rods
embedded in concrete masonry units (CMU). Successively, three flexural tests we re
performed on CMU walls. Test results are of encouragement to further investigate the
potential of FRP rods in the field of strengthening and retrofitting infill and load-bearing
walls.
1
De Lorenzis, L., D. Tinazzi, A. Nanni, A., “Near Surface Mounted FRP Rods for Masonry
Strengthening: Bond and Flexural Testing,” Symposium, “Meccanica delle Strutture in Muratura
Rinforzate con FRP Materials,” Venezia, Italy, December 7-8, 2000.
SOMMARIO
L’ambito in cui si inserisce il presente lavoro è la ricerca di soluzioni efficaci per il rinforzo
esterno di elementi strutturali in muratura non armata soggetti a sollecitazioni di flessione
fuori piano. Fra le tecniche di rinforzo strutturale basate sui materiali compositi (FRP), la
più recente acquisizione è l’uso di barre in FRP inserite in scanalature praticate sulla
superficie degli elementi strutturali da rinforzare e ivi fissate per mezzo di resine
epossidiche. E’ stata condotta una prima valutazione dell’efficacia di questa tecnica nel
caso di elementi in muratura soggetti a flessione fuori piano. Una prima serie di prove
sperimentali è stata incentrata sull’aderenza tra barre in FRP montate superficialmente e
blocchi in muratura artificiale. Successivamente sono state condotte tre prove di flessione
su muretti costruiti con lo stesso tipo di blocchi. I risultati sono descritti e commentati nel
presente articolo.
INTRODUCTION
Due to seismic events, excessive overloads, design inadequacies and changed load conditions,
both load-bearing and infill walls are often submitted to extraordinary actions which may have
detrimental results to the member or even to the overall building. Less well-known is the fact
that also ordinary long-term actions, even if not close to limit levels, also involve creep
phenomena that can cause sudden failure of the structural function of masonry assemblages [1].
Unreinforced masonry (URM) walls can be either load-bearing or infill walls. Due to weak
anchorage to adjacent concrete members (load-bearing walls), or due to the absence of anchorage
(infill walls), these walls may fail and collapse under out-of-plane loads generated by seismic
forces. In URM walls, failure due to out-of-plane bending causes the majority of the material
damages and loss of human life [2]. Therefore, the development of effective strengthening
techniques needs to be addressed.
While most of the research conducted on the use of Fiber Reinforced Polymer (FRP) composites
has focused on the retrofitting and repair of reinforced concrete (RC), current literature on
masonry indicates that causes of distress can be prevented and lessened by using FRP laminates.
Previous works on URM masonry walls strengthened with FRP laminates have shown
remarkable increases in capacity and ductility. Variables such as loading
configurations/mechanisms, strengthening schemes, and anchorage systems have been
investigated ([3]-[6]).
Near Surface Mounted (NSM) FRP rods are now emerging as a promising technique, in addition
to externally bonded FRP laminates, for increasing flexural and shear strength of structural
members [7]. Particular aspects, such as aesthetic requirements and local/global anchoring
issues, represent a limit for externally bonded laminates and stimulate the use of hybrid or rodbased systems.
The aim of this research program was a first evaluation of the effectiveness of NSM FRP rods as
a strengthening system for masonry elements subjected to out-of-plane loads. A first series of
tests focused on the bond behavior of NSM rods embedded in concrete masonry units (CMU).
Successively, three flexural tests were performed on CMU walls.
2
De Lorenzis, L., D. Tinazzi, A. Nanni, A., “Near Surface Mounted FRP Rods for Masonry
Strengthening: Bond and Flexural Testing,” Symposium, “Meccanica delle Strutture in Muratura
Rinforzate con FRP Materials,” Venezia, Italy, December 7-8, 2000.
MATERIAL CHARACTERIZATION
In order to represent in the test program some of the masonry units most common in the MidAmerica, standard concrete blocks were selected. In particular, hollow two-cell blocks with 50%
solid were used, with a net compressive strength of 22.4 MPa.
Mortar used is available in bags in a dry premixed composition of masonry cement and sand, and
is classified as Type N according to ASTM C270. Standard tests on mortar samples revealed a
compressive strength of 5.3 MPa and a tensile strength at the mortar-brick interface of 0.56 MPa.
Stock prism and RILEM compressive tests were also performed on the masonry assemblage,
obtaining a compressive strength of 6.22 MPa and a modulus of elasticity of 12.4 GPa.
The NSM FRP strengthening system consisted of the paste used to embed the FRP rods, and of
the rods themselves. Epoxy mortar, obtained from a mixture of epoxy paste and pure quartz sand,
was selected as the workable material with strong bond properties suitable to anchor FRP rods.
The mechanical properties of the epoxy, as specified by the manufacturer, were: 14 MPa tensile
strength, 4% elongation at break, 56 MPa compressive yield strength and 2.8 GPa compressive
modulus. Glass FRP (GFRP) ribbed and Carbon FRP (CFRP) sandblasted No. 3 rods (nominal
diameter 9.5 mm) were considered for the bond testing, while only the GFRP ones were used for
the flexural program. The manufacturer specified for the GFRP No. 3 rods a tensile strength of
847 MPa and a Young’s modulus of 42 GPa. Properties of the CFRP No. 3 sandblasted rods
were: 1575 MPa tensile strength and 167.3 GPa Young’s modulus.
BOND TESTING
Specimens. Each specimen consisted of two standard hollow concrete blocks. One NSM FRP
rod was applied to each face of the blocks in the longitudinal direction, connecting the two
blocks together. Therefore, each of the specimens had two grooves saw-cut on the two sides and
oriented along the longitudinal axis, where the FRP rods had to be mounted. The grooves were
air blasted to remove loose particles produced by the cutting process, then the epoxy paste was
prepared by mixing the two components (resin and hardener) in 2:1 proportion by volume with a
power mixer. The groove was filled half- way with the paste, the rod was then placed in the
groove and lightly pressed, thus forcing the paste to flow around the bar and fill completely
between the bar and the sides of the groove. Finally, the groove was filled with more paste and
the surface was leveled.
Only one block was the test region, with the NSM FRP rod having a limited bonded length and
being unbonded in the remaining part. Length and position of the bonded part were the same for
both faces of the test block. The rod was fully bonded on the other block, to cause bond failure to
occur in the test region.
The size of the groove was chosen as 19 mm taking into account the dimensions of the standard
concrete blocks. In fact, a groove size of 19 mm was believed to be the largest that can be
possibly adopted without creating excessive damage in the blocks. For the same reason, it was
decided to use only No. 3 rods. For each type of FRP rod, specimens with three different values
of the bonded length were tested, equal to 6, 12 and 18 times the rod diameter.
3
De Lorenzis, L., D. Tinazzi, A. Nanni, A., “Near Surface Mounted FRP Rods for Masonry
Strengthening: Bond and Flexural Testing,” Symposium, “Meccanica delle Strutture in Muratura
Rinforzate con FRP Materials,” Venezia, Italy, December 7-8, 2000.
Procedure. The epoxy paste was allowed to cure for 15 days (full cure time at room
temperature) prior to testing of the specimens. The test bed was a steel plate on which five steel
angles were bolted to delimitate the position where the concrete blocks had to be placed. The
purpose of the plate was to ensure the proper positioning of the specimens during preparation
and testing. A plastic sheet was placed between the plate and the bottom surface of the blocks, in
order to minimize friction between the two surfaces during testing.
Load was applied by means of a 12-ton hydraulic jack placed horizontally between the two
blocks (Figure 1) and connected to an hydraulic pump. A pressure transducer connected to the
hydraulic jack was used to record the load. Strain gages with 12- mm gage length were applied on
the surface of the rods prior to their application, to monitor the strain distribution along the rod
during the test. Three strain gages were placed within the bonded length and an additio nal one
was applied in the unbonded region. No strain gages were applied to the specimens with the
shortest bonded length, where their presence would have significantly influenced the bond
behavior, especially in the case of ribbed rods where the superficial properties had to be modified
prior to their application. Finally, slip at the end of the FRP rods in the test region was measured
using two LVDTs.
Figure 1. Bond Test Setup
Results. Test results in terms of ultimate load applied to each rod (half of the maximum load
applied to the specimen), average bond strength and failure mode are summarized in Table 1.
The ultimate load increased, as expected, with the bonded length of the rod, but the average bond
strength was found to decrease when the bonded length increased, as a result of the non uniform
distribution of the bond stresses.
Two different failure modes were observed. In the specimens with ribbed rods, failure occurred
by splitting of the epoxy paste in which the NSM rods were embedded, accompanied by cracking
of the concrete surrounding the groove. During testing, a typical crackling noise revealed the
progressive cracking of the epoxy paste as a result of the radial component of the bond stresses,
until the epoxy cover was completely split and the load suddenly dropped. The concrete material
surrounding the groove was also damaged, as shown in Figure 2a.
Specimens with CFRP sandblasted rods all failed at the interface between epoxy and CFRP rod.
This failure mode has been referred to as “pull-out” in Table 2. No sign of damage was visible in
the test side of these specimens after failure (Figure 2b). In these cases, the degree of microdeformation on the surface did not provide sufficient mechanical interlocking and the rod was
pulled out as soon as adhesion was lost.
4
De Lorenzis, L., D. Tinazzi, A. Nanni, A., “Near Surface Mounted FRP Rods for Masonry
Strengthening: Bond and Flexural Testing,” Symposium, “Meccanica delle Strutture in Muratura
Rinforzate con FRP Materials,” Venezia, Italy, December 7-8, 2000.
No free-end slip prior to failure was recorded for any of the specimens, with the only exception
of GD6. For this specimen, the average bond stress vs. free-end slip curves obtained by the two
LDVTs are reported in Figure 3.
Table 1. Test Results
Ultimate
Percentage
Bonded
Spec. Type of
Surface
Pull-Out
of Ult.
Length
Code FRP Rod Config.
Load
Tensile
(No. of db )
(kN)
Load (%)
CS6
6
13.14
12
CS12 Carbon
Sandbl.
12
14.69
13
CS18
18
18.16
16
GD6
6
15.53
26
GD12
Glass
Ribbed
12
16.72
28
GD18
18
26.99
46
PO = Pull-Out; SOE = Splitting of Epoxy; C = Concrete Cracking.
(a)
Average
Bond
Strength
(MPa)
7.72
4.32
3.56
9.13
4.91
5.29
(b)
Figure 2. Specimens after Failure by (a) Splitting; (b) Pull-out
5
Failure
Mode
PO
PO
PO
SOE+C
SOE+C
SOE+C
De Lorenzis, L., D. Tinazzi, A. Nanni, A., “Near Surface Mounted FRP Rods for Masonry
Strengthening: Bond and Flexural Testing,” Symposium, “Meccanica delle Strutture in Muratura
Rinforzate con FRP Materials,” Venezia, Italy, December 7-8, 2000.
1400
Average Bond Stress (psi)
1200
1000
800
LVDT1
600
LVDT2
400
200
0
0
2
4
6
8
10
12
14
Free-End Slip (milli-in.)
Figure 3. Average Bond Stress vs. Free-End Slip for Specimen GD6
(1 psi = 6.89 kPa; 1 in. = 25.4 mm)
ANALYSIS OF BOND TEST RESULTS
Analysis of Strain Data. The data from the strain gages was us ed to plot strain vs. location
graphs. In these graphs, the strain in the rod along the bonded length is plotted for different
values of the pull-out load. All points were obtained from the readings of the strain gages, except
for the strain at the end of the bonded length, which was assumed to be equal to zero.
From the strain- location data many useful information can be drawn. Equilibrium of a piece of
rod of length dx (Figure 4), along with the assumption of linearly elastic behavior of the rod,
leads to the following:
d
dε ( x )
τ ( x) = b ⋅ Eb ⋅ b
(1)
4
dx
where db , Eb , εb are diameter, Young’s modulus and strain of the rod, respectively, while τ is the
bond stress and x is the coordinate along the bonded length. Given that strain measurements are
available at discrete points along the bonded length, and indicating with ε bi the strain reading at
the location expressed by the coordinate x i, (1) may be approximated as follows:
x + xj
ε − ε bi
d
τ( i
) = b ⋅ Eb ⋅ bj
(2)
2
4
x j − xi
Recalling the definition of slip as the relative displacement between reinforcement and parent
material, and recalling also that:
du
du
εb = b
and
εe = e ,
(3)
dx
dx
where ub and ue are the displacements of the FRP reinforcement and of the epoxy, respectively, it
follows that:
ds
= εb − εe ≅ εb
(4)
dx
6
De Lorenzis, L., D. Tinazzi, A. Nanni, A., “Near Surface Mounted FRP Rods for Masonry
Strengthening: Bond and Flexural Testing,” Symposium, “Meccanica delle Strutture in Muratura
Rinforzate con FRP Materials,” Venezia, Italy, December 7-8, 2000.
where the epoxy strain, ε e, is considered negligible when compared to the FRP strain, ε b . Thus:
x
s( x) = s (0) + ∫ ε b ( x )dx
(5)
0
or, in the case of discrete strain readings,
s( xi ) = s( 0) + ∑ ε bj ⋅ (x j − x j −1 )
i
(6)
j =1
τ
σb
σ b + dσ b
dx
Figure 4. Free-body Diagram of Bar with Length dx
Figures 5 through 7 illustrate an example of strain vs. location, slip vs. location and bond stress
vs. location curves obtained as explained above from the strain readings of specimen GD12. In
all figures, each curve is relative to a certain load level indicated as percentage of the ultimate
load. This was done to allow comparison between different specimens in spite of the different
failure load. The left end (location equal to zero) and the right end of the x axis correspond to the
free end and to the loaded end of the bonded length, respectively.
The strain distribution along the bonded length, highly non- linear at moderate load levels, tends
to approach an almost linear shape as the applied load increases. This means that, as load
increases, bond stresses become more evenly distributed along the bonded length as a result of
changes in the nature of bond. When ribbed reinforcement is used, the primary bond mechanism
changes from chemical adhesion to mechanical interlocking as soon as the ribs are brought into
bearing. The radial components of the bond stresses produce internal microcracks in the epoxy
resin that have been visually observed in all specimens after failure. Microcracking and the
consequent slip between reinforcement and parent material tend to make the bond stress more
evenly distributed. Figure 7 shows that, while at low loads the bond stresses at the rod’s free end
are close to zero, as the load increases the peak of the bond stresses gradually shifts towards the
free end and the whole bonded length contributes to resist the pulling force. The radial stresses
are balanced by circumferential tensile stresses in the epoxy cover which may lead to the
formation of longitudinal splitting cracks. The concrete material surrounding the groove is also
subjected to tensile stresses along inclined planes, and the tensile strength of the material may be
eventually reached causing fracture along these planes. Whether failure in the concrete occurs
before or after the formation of splitting cracks in the epoxy or even the complete fracture of the
epoxy cover, depends on the groove size and the tensile strength of the two materials.
The behavior is different in the case of sandblasted reinforcement, where the degree of
microdeformation on the rod’s surface is not enough to provide sufficient mechanical
interlocking and bond is primarily guaranteed by chemical adhesion at very low levels and
friction after onset of slip. Although a certain amount of internal microcracking of the epoxy was
found also in the specimens with sanblasted rods, this was not sufficient to allow the bond
stresses to redistribute along the bonded length, as clearly visible in Figure 8.
7
De Lorenzis, L., D. Tinazzi, A. Nanni, A., “Near Surface Mounted FRP Rods for Masonry
Strengthening: Bond and Flexural Testing,” Symposium, “Meccanica delle Strutture in Muratura
Rinforzate con FRP Materials,” Venezia, Italy, December 7-8, 2000.
9000
Strain ( µε )
7000
6000
20%
40%
60%
80%
100%
Slip (mil in)
10%
30%
50%
70%
90%
8000
5000
4000
3000
20
18
16
14
12
10
8
6
2000
20%
40%
60%
80%
100%
4
2
0
1000
0
0
1
2
x (in.)
3
0
4
1
20%
40%
60%
80%
100%
2
x (in)
3
4
1,2
10%
30%
50%
70%
90%
1
Bond Stress (ksi)
10%
30%
50%
70%
90%
1,2
1
Figure 6. Slip vs. Location (GD12)
(1 in. = 25.4 mm)
Figure 5. Strain vs. Location (GD12)
(1 in. = 25.4 mm)
Bond Stress (ksi)
10%
30%
50%
70%
90%
0,8
0,6
0,4
0,2
0,8
20%
40%
60%
80%
100%
0,6
0,4
0,2
0
0
0
1
2
x (in)
3
4
5
0
Figure 7. Bond Stress vs Location (GD12)
(1 ksi = 6.89 MPa; 1 in. = 25.4 mm)
2
x (in)
4
6
Figure 8. Bond Stress vs. Location (CS18)
(1 ksi = 6.89 MPa; 1 in. = 25.4 mm)
Linear Analysis of the Bonded Joint. The analysis of the bonded joint in the linear elastic
range can be conducted by means of a simple shear lag approach. At moderate load levels, a
linear bond stress-slip behavior can be adopted:
τ = K ⋅s
(7)
where K is the so called slip modulus and s is the slip. This assumption yields the following
equation for the strain distribution along the FRP bar:
σ
sinh αx
ε b ( x) = appl ⋅
(8)
Eb sinh αl
where σappl is the tensile stress applied to the rod, l is the bonded length and:
4K
α=
(9)
Eb ⋅ d b
The slip modulus K has been evaluated by best fitting between the theoretical strain distribution
given by (8) and the experimental strain values. In other words, K was computed at each load
level by minimizing the percent error, evaluated as follows:
8
De Lorenzis, L., D. Tinazzi, A. Nanni, A., “Near Surface Mounted FRP Rods for Masonry
Strengthening: Bond and Flexural Testing,” Symposium, “Meccanica delle Strutture in Muratura
Rinforzate con FRP Materials,” Venezia, Italy, December 7-8, 2000.
N
Err =
∑ (ε
i =1
theor
bi
− ε bi exp ) 2
max ε biexp
⋅ 100
(10)
i
where ε bitheor and εbiexp are the theoretical and experimental strain, respectively, at the location of
the ith strain gage, and N is the total number of strain gages.
As an example, Figure 9 shows theoretical and experimental strain distributions at moderate load
levels for specimen GD12. Figure 10 illustrates, for the same specimen, K and Err at different
load levels between 10% and 50% of the ultimate. As expected, the slip modulus decreases as the
load increases, while the percent error increases as the linear model of the local bond stress-slip
relationship is more and more inaccurate. The theoretical curves of Figure 10 were plotted for the
value of K that minimizes the average of the errors at the 10% to 40% load levels, that is
K=151.6 MPa/mm. It can be seen that the agreement between experimental strains and
theoretical curves is reasonably good, the average Err for this value of K being 6.7%.
Local Bond Stress – Slip Relationship. The local τ – slip relationship can be obtained by
combining the two curves τ(x) and s(x) expressed by equations (1) and (5). A τ vs. x curve and
an s vs. x curve can be obtained for each va lue of the applied load, therefore, a τ vs. slip diagram
corresponding to each value of applied load can be drawn. At a given load level, the τ vs. slip
diagram covers the range of slip between the free-end slip and the loaded-end slip at that load
level. The entire curve can be obtained as the envelope of the partial curves at all load levels
between zero and ultimate. It is obvious that, given the small number of strain gage readings and
the influence exerted on these readings by the unavoidable local effects, given also that
derivation performed on experimental data increases their degree of irregularity, these envelopes
are often very irregular curves.
An example of local bond stress vs. slip diagram is reported in Figure 11. This diagram was
obtained from the strain readings as previously explained, except that the curves, relative to
different percentages of the ultimate load, have not been enveloped. The average bond stress vs.
loaded-end slip diagram is also plotted for comparison. The lack of correspondence between the
two is obvious, the former diagram being stiffer than the latter and reaching higher values of
bond stress. This comparison emphasizes the need for a local bond stress - slip relationship and
the impossibility to approximate it with curves obtained by LVDT data, especially given the low
elastic modulus of FRP materials if compared to steel. It can be expected that the difference
between the two curves increases as the bonded length increases and the elastic modulus of the
reinforcement decreases.
9
De Lorenzis, L., D. Tinazzi, A. Nanni, A., “Near Surface Mounted FRP Rods for Masonry
Strengthening: Bond and Flexural Testing,” Symposium, “Meccanica delle Strutture in Muratura
Rinforzate con FRP Materials,” Venezia, Italy, December 7-8, 2000.
10%
20%
1500
30%
1000
40%
500
700
16
600
14
500
12
10
400
K
8
6
300
errore
200
4
100
2
0
0
0
1
2
3
0
0
4
% Error
Strain (µ ε )
2000
Slip Modulus (ksi/in)
2500
20
40
60
Load (% of Ultimate)
x (in.)
Figure 10. Slip Modulus and Error in its
Evaluation (GD12)
(1 in. = 25.4 mm)
Figure 9. Theoretical and Experimental
Strain Distribution at Moderate Loads
(GD12)
(1 in. = 25.4 mm)
1,2
10%
Bond Stress (ksi)
1
20%
30%
0,8
40%
Local Bond
Stress vs. Local
Slip
0,6
50%
60%
70%
0,4
Average Bond Stress
vs. Loaded-End Slip
80%
90%
0,2
100%
0
0
5
10
15
20
25
30
35
Slip (mil in)
Figure 11. Bond Stress vs. Slip (GD18)
(1 ksi = 6.89 MPa; 1 in. = 25.4 mm)
Once a local τ – slip relationship is obtained, the next step will be to find an analytical
expression capable to model such relationship. For steel and FRP reinforcing bars in concrete,
various equations have been previously proposed ([8], [9]). Each of them contains a certain
number of unknown parameters, which need to be calibrated by comparison with experimental
data. Provided that one of these laws is found to be representative of the bond behavior of NSM
FRP rods, the values of the unknown parameters may be determined by best fitting techniques
10
De Lorenzis, L., D. Tinazzi, A. Nanni, A., “Near Surface Mounted FRP Rods for Masonry
Strengthening: Bond and Flexural Testing,” Symposium, “Meccanica delle Strutture in Muratura
Rinforzate con FRP Materials,” Venezia, Italy, December 7-8, 2000.
using the experimental curves. Once the analytical τ – slip relationship is known, it can be used
to analytically solve all problems related to the bond behavior, particularly, to calculate the
development length of NSM FRP rods.
FLEXURAL TESTING
Three concrete block walls were tested. The nominal dimensions of the walls were 600 x 1200 x
190 mm, which resulted from a stack of six courses, one and a half block each course. One
specimen was maintained unreinforced as control wall, while the other two concrete block walls
were reinforced respectively with one and two GFRP No. 3 rods perpendicular to the bed joints.
The rods were positioned in the middle or on the thirds of the width respectively, embedded into
epoxy- filled grooves cut on the surface of the blocks as explained previously.
The block walls were subjected to a typical four-point bending test (Figure 12), ensuring that the
hinge supports did not provoke uncontrolled restraint of the rods. A reaction frame and an
hydraulic jack were used to apply the load. Slip at the end of the rods, strain in the rods at
various locations, concrete and mortar strain on the compressed side of the midspan cross-section
were recorded during the tests.
From the envelopes of the flexural tests, the dramatic increase of flexural capacity and ultimate
deformation is evident (Figure 13). The flexural capacity of the walls with one-rod and two-rods
strengthening was 7 times and 15.7 times the capacity of the unreinforced specimen,
respectively. Load cycles on the masonry assemblage revealed an elastic behavior until slip of
the reinforcement into the grooves started taking place. After this limit, energy dissipation was
developed by friction and flexural deformation increased.
Failure occurred in both strengthened specimens by splitting of the epoxy paste in which the
NSM rods were embedded, which confirmed results of the bond testing at the structural member
level. When the cracking moment was reached, mortar joints opened up and typical V-shaped
cracks due to the bond stresses formed in the resin on both sides across the joint. As the load
increased, longitudinal splitting cracks starting from each joint propagated along the resin
surface. During propagation of the cracks, a typical crackling noise and slight loss of load were
recorded, until final collapse by expulsion of the rods. In the wall reinforced with two FRP rods,
this mechanism was accelerated as the high load level caused a progressive shear sliding of a bed
joint close to one support, leading to the sudden expulsion of the reinforcement from the groove.
Hence, in this case, expulsion of the rods can be associated mostly to dowel action, even if the
splitting cracks were already developed. This “premature” shear failure may be attributed to the
limited surface of contact the mortar joints have in the block masonry typology.
11
De Lorenzis, L., D. Tinazzi, A. Nanni, A., “Near Surface Mounted FRP Rods for Masonry
Strengthening: Bond and Flexural Testing,” Symposium, “Meccanica delle Strutture in Muratura
Rinforzate con FRP Materials,” Venezia, Italy, December 7-8, 2000.
Figure 12. Flexural Test Setup
Load
(KN)
30
2 rods
Reinforcement
29.45 KN
20
1 rod (linearized)
13.39 KN
10
Unreinforced Wall1.87 KN
GFRP rods
0
0
1
2
3
4
5
6
7
Mid Span Deflection (mm)
Figure 13. Flexural Test Resuls
CONCLUSIONS
This research project represents the first attempt of a technical investigation on the use of FRP
rods for the reinforcement of masonry assemblages. The use of rods inserted into grooves close
to the surface has demonstrated to be a reliable system for masonry as well as for concrete.
Results of bond and flexural tests are coherent in indicating splitting of the epoxy cover as the
12
De Lorenzis, L., D. Tinazzi, A. Nanni, A., “Near Surface Mounted FRP Rods for Masonry
Strengthening: Bond and Flexural Testing,” Symposium, “Meccanica delle Strutture in Muratura
Rinforzate con FRP Materials,” Venezia, Italy, December 7-8, 2000.
critical failure mechanism when ribbed FRP rods are used as NSM reinforcement under out-ofplane load conditions. Flexural tests showed that a remarkable increase in the flexural capacity of
masonry block walls can be achieved by means of NSM FRP reinforcement, as the specimens
strengthened with one and two GFRP rods failed at 7 times and 15.7 times the load of the control
specimen, respectively.
Thus, the test results presented herein are of encouragement to further investigate the potential of
FRP rods in the field of strengthening and retrofitting infill and load-bearing walls.
ACKNOWLEDGEMENTS
This research study was sponsored by the National Science Foundation Industry/ University
Cooperative Research Center on Repair of Buildings and Bridges with Composites (RB2 C) at the
University of Missouri – Rolla.
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