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A RING TEST METHOD FOR CRACK RISK EVALUATION OF CONCRETE UNDER
RESTRAINED SHRINKAGE
Conference Paper · June 2014
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A RING TEST METHOD FOR CRACK RISK EVALUATION OF
CONCRETE UNDER RESTRAINED SHRINKAGE
STEFANO CANGIANO and SARA SGOBBA
CTG -Italcementi Group
ABSTRACT
Concrete under restrained shrinkage conditions may suffer of early-age cracking that, even if the structural
integrity may not be compromised, can represent a serious risk of aggressive substances ingress.
At present time, the only standardized method, is the so-called "ring test" (ASTM C1581). This test method,
however, provides information limited to the time of crack onset and it is applicable only on a few real
applicative cases since it allows to develop very low degree of restraint. Indeed, a useful tool for service life
design should be more properly based on the knowledge of parameters that could account also for crack
opening displacement at the onset crack and crack opening rate, based on more realistic restrained shrinkage
situations.
In the present work, a modified ring-based test method (called MRT), able to provide these parameters for
identification of post-cracking behavior, is described. Experimental results obtained on different concrete
mixtures, including ultra high performance concrete (UHPC) and fiber reinforced concrete (FRC), also
containing SRA, confirm the robustness of this method.
Key-words: Degree of restraint, FRC, Restrained shrinkage, Ring test method, UHPC.
INTRODUCTION AND BIBLIOGRAPHIC REVIEW
In recent years there is increasing awareness that the phenomenon of shrinkage of casting concrete is of the
utmost importance and cannot be neglected in structural design. The presence of physical restraints or
thermo-hygrometric gradients in concrete can in fact induce the onset of stress states that can exceed the
tensile strength of the material, thus generating cracking of significant extent. These technical problems also
imply a considerable economic importance, in particular, for embodiments where, as an effect of the high
surface/volume ratio, the phenomenon of drying shrinkage is particularly accentuated (for example in
industrial flooring).
Currently, most of the national (UNI 11307:2008) and international standards (ISO 1920-8:2009, DIN 1045)
only provide the measure of the total shrinkage of concrete in the absence of external restraints (free
shrinkage). This measurement, while allowing to discriminate the behavior of concretes of different quality,
however, does not provide enough information to evaluate materials in real restrained conditions to which
they are normally subject. Moreover, testing material under restrained shrinkage is of fundamental
importance, not only from a structural point of view, but also for durability considerations. It is known
indeed, that limited value of crack opening are accepted for service life prevision of concrete structures
(Eurocode 2). It is well known that such a phenomena appears more pronounced in the case of high and
ultra-high performance concrete (HPC/UHPC), which normally provide excellent durability performance,
but that, due to their relatively high cement paste content, may be subject to early age cracking due to
restrained shrinkage. This behavior is the result of a combined effect of higher free shrinkage, lower specific
creep, and higher modulus of elasticity that result in tensile stresses that can overcome the higher tensile
strength of these materials. More in general, also fiber reinforced concrete (FRC) need to be experimentally
characterized in a more suitable manner to assess the real shrinkage cracking potential by means of
appropriate test method that could account for post-cracking effect (in which fiber reinforcement plays the
main role).
In this framework, the experimental assessment of concrete under restrained shrinkage has been the subject
of many studies; the most common method is the so-called "ring test" (Swamy and Stavrides [1],
Grzybowski and Shah [2,3], Shah et al [4,5,6], Kovler et al [7], Wiegrink et al [8], Weiss et al [9,10,11],
Hossain et al [12,13], See et al [14], Voigt et al [15], He et al [16], Moon et al [17], Turcry et al [18], Kwon
et al [19,20] and Hwang et al [21], Yoo et al., 2013 [22], Pour-Ghaz et al., [23]), which was initially framed
in a specific AASHTO (PP 34-99). In ring test, the concrete is cast around a steel toroid which is able to
counteract concrete shrinkage; this leads arising in tensile strains in concrete. In this way, it is possible to
simulate, by neglecting frictional effects, the restrained shrinkage effects in concrete. Then, a standardized
version of the "ring test" method has been accepted by ASTM C1581 - 04. The ASTM method is based
essentially on the importance of time of onset cracking and the tensile stress in the concrete specimen is
calculated by starting from the strain measurement at the inner steel ring instrumented with strain gauges.
The ASTM method provides in addition the tensile stress rate in concrete ring calculated by strain
measurement.
Recently in the literature more sophisticated instrumental techniques have been adopted [23-6-10-13-17] in
order to overcome the limited information provided by the ASTM standard and then to provide quantitative
indication of the distribution and the stress levels in the ring of concrete during incipient cracking. However,
key issues such as those related to post-cracking response of the material in terms of crack opening and
corresponding kinetics of growth, particularly important in case of FRC characterizations, are currently
estimated only through indirect measures. Some more recent effort in terms of ASTM test method
modification has been dedicated by Plizzari and Reggia [24], however, the assumed degree of restraint is the
same adopted in ASTM so it could be not representative of some real conditions.
In many papers, in fact, the speculations regarding the validity of the ring geometry in assessing concrete
with larger aggregates or fibers effect have been well documented (Moon, et al. [6]; Shah, et al. [11]). The
ASTM test method, indeed became applicable to concrete mix with a maximum nominal aggregate size less
than 13 mm, being the concrete circular crown thickness equal to 372 mm, (as known, the concrete thickness
is permitted to be three times the maximum aggregate size, at least) or to a limited range of real restrained
conditions. For example, the degree of restraint that is developed by the ASTM ring may be too low to
represent the case of new high performance mortar/concrete that was used as reinforcing layer for repair or
strengthening of existing concrete. According to the consolidated literature, a change in the ring mould size
thus is needed and imply the adoption of a different thickness of the steel ring that could have the effect to
change the degree of restraint as well as the stress values [25].
In this framework, the present work suggests the development of an alternative ring test method based on a
ring mould having different dimensions in comparison to the ASTM one and that enable a direct detection of
the cracking opening and of some main mechanical parameters describing the onset cracking and postcracking behavior of mortar/concrete in restrained conditions.
The assumption of a concrete circular specimen different in size, in comparison to the ASTM one, consists in
a more suitable degree of restraint (called “DOR” in the following). Thus, alternative test geometry was
developed. In detail, a thicker steel ring was used and a pre-formed notch was made in order to directly
detect the time of cracking and the increase of the amplitude of the notch opening with time.
The development of the method aims also to develop a sufficiently "robust" methodology, (i.e. able to
appreciate significant change in concrete composition, and from which it is reasonable to expect significantly
different physical-mechanical behavior). More specifically, the method should provide the advantage to
assess more suitably the concrete cracking potential at early-age and the evaluation of restrained behavior of
HPC/UHPC.
Resuming, the proposed modified ring test aims to qualify and rank concrete mixes with different
compositions and performance in terms of cracking resistance to restrained shrinkage, on the basis of the
following parameters:
- Cracking time (TC): time of cracking onset detected as the time at which a sudden change in the
CMOD/CTOD pattern vs time occur;
- Critical CMOD (CMODc): crack mouth opening at the crack onset;
-
Instantaneous crack amplitude (ΔCMOD): crack opening recorded at the cracking time;
Crack opening rate (vc): rate of the CMOD development after the sudden change until a predefined
opening value. The knowledge of the cracking development is very important from a durability point
of view. This last parameter indeed, may represent a useful tool in service lifetime prevision.
TECHNICAL SIGNIFICANCE OF THE DOR CHOICE
As before described, a simple parameter that summarizes the complex interaction that occur between the
concrete and the restraint is defined as “degree of restraint” (DOR).
From a mechanical point of view, the DOR index can vary between two extreme conditions: DOR equal to 0
corresponds to free shrinkage without any restraints. This condition however is not interesting from an
engineering point of view since it does not produce tensile stresses and then cracking phenomena, by
neglecting strains due to internal moisture gradients. DOR equal to 1 corresponds instead to the condition of
full restrain provided by an internal steel ring ideally undeformable. By expliciting the above relationship, as
function of the elastic properties, it is possible to obtain the following relationship [6]:
Ψ = 1−
EC
ES
1
R
1 −  IS
EC
 ROS
−
ES
R
1 −  OC
 ROS

 (1 + υ ) ROC

C 
 ROS
 
2
 
R
 (1 + υ S ) IS
R
 
 OS
2
2


 + (1 − υ C )


2


 + (1 − υ S )


Equation (1)
EC - Elastic modulus of concrete;
ES - Elastic modulus of steel;
ROS - External radius of steel ring;
ROC - External radius of concrete ring;
RIS - Internal radius of steel ring;
υC - Poisson coefficient of concrete;
υ S - Poisson coefficient of steel;
On the basis of this formula, it was quantified the DOR provided by the ring test configuration of ASTM
specifications. It was seen that such value of DOR is too low and this implies long time of testing and not
realistic estimation of cracking time. Thus, in order to investigate cracking development in better-restrained
conditions, it was studied the DOR developed in real situations such as in UHPFRC application. Thus, an
example of real case study of concrete for repair/strengthening was investigated. The DOR developed in a
new UHPC overlay with different thickness on a circular pillar made of ordinary concrete (assumed elastic
modulus of 25.000 MPa) and with radius equal to 150 mm, was calculated by using Eq. 2.
In Figure 1, it can be seen the trend of DOR with varying thickness (up to 100 mm) and elastic modulus (up
to 50.000 MPa) of the UHPFRC overlay. It can be deduced that the DOR tends to sensibly increase as the
thickness of the overlay decreases and elastic modulus of the reinforcement layer increases, resulting in
detrimental effects on the risk of cracking of the new concrete.
Even if, this case study is referred to a strengthening application, the necessity of testing material under
higher DOR has more general validity. Higher DOR is developed for example in the most diffused cases of
beam restrained at ends of pillars or in pavements. A research study carried out by Samaris [26], has
demonstrated that the most cast-in-place application of high performance concrete for rehabilitation
applications of existing structures are subject to very high DOR (70%÷90%). This is the case for example of
overlay on multiple beam bridge or cast-in-place kerbs.
1
E=35.000MPa
E=40.000MPa
E=45.000MPa
E=50.000MPa
0.95
0.9
DOR [%]
0.85
0.8
0.75
0.7
DOR - Ring ASTM
0.65
0
20
40
60
80
Thickness of reinforcement layer [mm]
100
Figure 1 -Trend of DOR vs. thickness of the high performance concrete overlay with different elastic moduli.
The colored range is representative of the most common overlay thickness in rehabilitation application.
In fact, from Figure 1, it can be observed that for a range of thickness of the overlay 25÷50 mm (quite
common values for repair applications) and elastic modulus in the range 35.000÷50.000 MPa, the
corresponding DOR varies between 73% and 88%. Then, an average value of DOR equal to 80% can be
assumed as representative of this type of concrete application. Comparing this value with DOR developed by
the ASTM standard (about 65%), it can be observed that in this latter case, the test method is representative
of very thick layer of the new concrete (> 90 mm), thus it can be easily deduced that such test method is not
suitable to characterize the performance of repair materials in real conditions. The choice of such value of
DOR in ASTM ring test method, is probable justified by the need of inducing detectable strains at the inner
steel ring with the commercially available strain gauges.
The method proposed in the present work (in the following it will be called “Modified Ring Test” - MRT) is
thus different for dimensions (Table 1) in comparison to the ASTM one in order to provide an increased
value of DOR (about 80%). In this way it is also possible to enhance and accelerate the tendency for
cracking and thus to assess differences in concrete properties more readily.
Table 1: DOR and dimensions of the compared rings: MRT and ASTM (dimensions in mm) [27]
Ring Test Method
MRT
ASTM
RIS
ROS
ROC
DOR
92.5
127.5
Notched Section hn=30 mm
187.5
81%
88%
152
165
202.5
65%
Figure 2 - View of the CTG modified notched ring
As can be noted in the Table 1, in order to develop about 80% of DOR, it was assumed higher radius also of
the concrete specimens. This allowed to test concrete with maximum nominal aggregate size less than 10
mm, which is about the same allowed by the ASTM ring, despite the presence of a notch. Moreover, the
concrete specimen in the MRT has a higher surface-to-volume ratio (more then 20%), which better simulates
real applicative conditions. The height of the ring was also reduced in order to minimize the effects of nonuniform hygrometric gradients.
An important investigated aspect has concerned the instrumentation chosen to detect cracking. The adoption
of a thicker steel ring, indeed, makes much harder to measure the stress/strain development with the use of
strain gauges located at the internal surface of the steel ring due to the higher degree of restraint and then it
allows no measurable deformation to take place as the concrete shrinks. For this reason, a pre-formed notch
was introduced in the concrete ring in order to control and force the cracking process. Tip and mouth
opening displacement of the notch (called CTOD/CMOD, respectively1) were monitored by means of a clip
gauge. The use of clip gauge has also the advantage to detect the cracking time more precisely because the
transducers are applied directly on the cracking zone and allows a continuous monitoring of the notch
opening. Another difference with the ASTM concrete ring is that the MRT was not sealed immediately after
demoulding (24 h after casting) but leave free to dry in all directions (top/bottom/circumferential surface).
Thus the MRT method improves the evaluation criteria, since the concrete dries from all surfaces rather than
just one. Moreover, the application of a sealing waterproof layer may be a cause of variability because it is
often not known the vapor permeance. This assumption about drying is compatible with the adopted
relationship (Equation 1) for DOR calculation, indeed, DOR is not significantly influenced by drying
direction, as demonstrated by Moon. et al. [6].
EXPERIMENTAL PROGRAM
Test procedure and set-up
The steel ring moulds were prepared by pre-forming a notch to force the cracking position by introducing a
steel made triangular shaped wedge with a sharp edge at 90° for a depth of 30 mm. The wedge was then
removed together with the cast outer confining ring at 24 hours. The base of the mould was made of a Teflon
sheet, in order to minimize eventual friction effects. As before described, the specimens were not sealed to
encourage the natural process of water evaporation in the concrete ring in concrete on the top/bottom surface
ad also along the circumferential surface in order to more realistically simulate free evaporation conditions.
After mixing, concrete was poured into the ring mould and immediately covered with a polythene sheet.
Then until the apparatus was stored into the lab for 1 day until dismoulding (the outer ring for the cast
confinement of concrete was removed) so that it was possible to glue the transducers holders to the
specimen. As far as quickly possible, the ring specimen was moved into a conditioned room with controlled
temperature and humidity (20° ± 2° C, RH 50% ± 5%). The specimens were instrumented at the tip and
mouth of the pre-formed notch for monitoring respectively the CMOD and the CTOD, instrumented with a
resistive strain gauge in full bridge configuration (TML UB-5A). The measurement range is ±5 mm,
1
C.M.O.D. - Crack-Mouth-Opening-Displacement; C.T.O.D. - Crack-Tip-Opening-Displacement
sensitivity 2,85 mV/V and as power supply voltage it was chosen a value of 2 V. A special attention was put
into the glued materials by choosing a fast setting and relatively high elastic modulus in order to avoid
(especially in the first time of test) creep components into displacement measurements.
Mix proportions
The experimental program was designed in order to evaluate the robustness of the proposed test method and
repeatability according to ISO 9725. In Table 2 are described the main composition mix parameters.
The mix were also evaluated on the basis of the standard characterization tests of concrete at hardened state
(Table 3). The adopted criteria at the basis of the experimental program was that of investigating two main
“concrete family”, that is the “Normal Concrete” (NC) characterized by ordinary mechanical performance
and the “Ultra High Performance Concrete” (UHPC). Starting from these two main families, various mixes
were made by varying the content of some constituent or by adding metal fibers (steel fibers) to assess the
effects that these constituents have on restrained shrinkage behavior. All the mixes contain aggregates with
maximum diameter of 10 mm. The cement type is CEM II A 42.5R for NC mixes and CEM I 52.5R for
UHPC mixes.
Table 2: Mix proportions
Concrete
Family
Code
NC_n
NC_f1
Normal
Concrete
NC_f2
NC_f3
UHPC_sa1
UHPC_sa2
Ultra High
UHPC_f1
Performance
Concrete
UHPFRC_f2
UHPFRC_f3
w/c
ratio
0.50
0.45
0.45
0.45
0.34
0.34
0.34
0.34
0.34
Aggregate Pozzolanic
Plasticizer/
(vol. ratio
addition Superplasticizer
on
(vol. ratio (dry on cement
binder)
on binder)
weight)
3.6
3.9
0.34%
3.8
0.49%
3.8
0.45%
1.7
2.6%
0.62%
1.6
2.6%
0.61%
1.6
2.6%
0.67%
1.6
2.6%
0.66%
1.6
2.6%
0.70%
SRA
(on
cement
weight)
1%
2%
1.5%
1.5%
1.5%
Steel
Fibers
(on
volume)
0.5%
1%
0.5%
1%
Table 3: Main mix performance
Compressive
Strength at 28
days (MPa)
Standard
NC_n
NC_f1
NC_f2
NC_f3
UHPC_sa1
UHPC_sa2
UHPC_f1
UHPFRC_f2
UHPFRC_f3
EN 12390-3
33.0
54.6
57.4
55.0
135.2
137.3
138.5
135.3
124.5
Elastic
Dynamic
Modulus at
28 days
(GPa)
ASTM C215
34.5
35.8
35.0
45.9
45.9
39.6
39.6
39.5
Indirect
tensile
strength at 1
day (MPa)
1.55
2.10
2.65
2.00
3.60
3.65
4.20
4.15
7.30
Indirect tensile
strength at 28 day
(MPa)
EN 12390-6
3.10
4.25
6.25
4.45
8.05
9.65
7.00
7.05
10.25
Free
shrinkage at
28 days
(μm/m)
UNI 11307
406
410
349
358
346
311
287
278
267
RESULTS AND DISCUSSION
In the following graphs the most salient results referred to the two concrete families are shown. The graphs
relate to the crack opening in terms of CMOD development with time. For sick of simplicity and clearity, it
was decided to exclude CTOD representation also because CMOD and CTOD showed a good relationship.
Moreover, all the results have been confirmed on almost other two twin rings.
All the curves have been studied with respect to the parameters already defined (TC, CMODc, ΔCMOD, vc). In
Figure 3, a comparison between the two examined concrete families NC and UHPC is shown. The curves
refer to two ring specimens coming from the same UHPC batch (mix UHPC_sa1) and three ring specimens
from the NC batch (mix NC_f1). Both the mix did not contain addition of fibers.
In order to analyze better the observed trend, the restrained shrinkage parameters of MRT method have been
extrapolated for the investigated concrete specimens. The main results are collected in Table 4.
Table 4: Main MRT-based restrained shrinkage parameters of curves in Fig. 3
TC (h)
CMODc (µm)
∆CMOD (µm)
NC - Average
166.28
8
95
UHPC - Average
86.50
42
219
vc (%)
0.3
0.6
Tempo
(hours)
Time
(hours)
24,00
0
44,00
64,00
84,00
104,00
124,00
144,00
164,00
184,00
204,00
-0,3
cracking NC_1
-0,25
cracking NC_2
-0,2
cracking NC_3
-0,15
cracking UHPC_1
CMOD (mm)
-0,1
cracking UHPC_2
-0,05
-0,35
-0,4
-0,45
-0,5
NC_1
NC_2
NC_3
UHPC_1
UHPC_2
Figure 3 - Comparison between NC and UHPC concrete families.
By observing the graph, it can be noted, that the trend of the curves reflect the very different intrinsic nature
of the materials. UHPC indeed is well known to be a very brittle composite material and this aspect leads to a
larger “jump” of the related curves with respect to the NC specimens. Moreover, UHPC are usually subject
to high level of autogenous shrinkage due to the very low water/cement ratio and this characteristic
determines a earlier cracking in comparison to the NC sample. An interesting observation is related to the
initial shrinkage trend that is similar in both the examined concrete family. Results demonstrate that the
method is therefore clearly able to distinguish very well the performance of the two concrete families.
The post-cracking rate vc in UHPC mix has a coefficient of variation on the two concrete specimens that is
more than 45%, so it is not reliable to determine graphically this parameter in high performance concrete.
Indeed, for UHPC mix the cracking propagation after the onset is very rapid and growth with a non linear
law, then tends to stabilize quickly more linearly. Other repeatability tests have been carried out on NC mix
(on 6 rings) and the final results indicate that the most stable parameters are the cracking time (COV 9%) and
the cracking rate (COV 10%), while the most dispersing ones are the CMODc (COV 27%) and the cracking
amplitude (COV 16%).
From Table 2, it was seen that all the UHPC/UHPFRC mix design include a certain amount of SRA in order
to reduce free total shrinkage that can be larger than that recorded on normal concrete due to the high level of
binder volume fraction. So, another investigation concerned the effect of SRA on restrained shrinkage and
the possibility to appreciate this effect by means of the MRT method.
26,00
-1E-15
46,00
66,00
Time (hours)
86,00
106,00
126,00
146,00
cracking UHPC_sa2
cracking UHPC_sa1
CTOD/CMOD (mm)
-0,1
-0,2
-0,3
-0,4
-0,5
-0,6
UHPC_sa1_CMOD
UHPC_sa1_CTOD
UHPC_sa2_CMOD
UHPC_sa2_CTOD
Figure 4 - Comparison between UHPC with different SRA content (UHPC_sa1 – 1% SRA; UHPC_sa2 – 2%
SRA).
A comparison between two UHPC mixes with different SRA content was examined. Figure 4 clearly show
that SRA dosage has an important effect in terms of cracking delay. The mix UHPC_sa2 with double SRA
content (2%) indeed cracks after about 43 hours later than the same mix UHPC_sa1 with lower SRA content
(1%). However, the SRA content seems to have any effect on the crack amplitude and on the cracking rate.
Table 5: Main MRT-based restrained shrinkage parameters of curves in Fig. 4
CMODc (μm)
∆CMOD (μm)
TC (h)
79.50
43
215
UHPC_sa1
122.80
40
193
UHPC_sa2
vc (%)
0.4
0.3
In Figure 5, a sensitivity analysis to the fiber content of three different NC mix is shown. Mix NC_f1 does
not contain any fiber, while NC_f2 and NC_f3 contain hook-ended steel fibers (aspect ratio 70), respectively
equal to 0,5% and 1% on volume. It can be seen that the observed trend is not influenced by the fiber content
in terms of cracking time (the three mixes have all a TC of about 164 hours). However, the instantaneous
crack amplitude ∆CMOD, visible as a “jump” in the curves, that reduces with increasing of the fiber content.
Also the rate of cracking propagation seems to influenced by the fiber content. This would confirm the
mechanical action of fibers (sewing action) which do not influence the elastic behavior but only the postcracking stage by reducing the crack opening and the cracking development. It should be noted that the
irregularities noted up to 100 hours were not related to development of visible cracking (as checked by visual
inspection) but probably due to some pre-cracking process, moreover confirmed on other rings, that should
be investigated further in the future.
A similar approach was adopted to study the robustness of the MRT method in terms of sensitivity to the
fiber content in UHPC concrete family. Three batches were compared: one of reference without any fiber,
the other two ones with waved steel fibers having aspect ratio 72 and increasing fiber content (0,5% and 1%
by concrete volume). From Figure 6, it can be observed that all the mixes containing steel fibers do not show
any visible cracking nor as CMOD curve discontinuity nor at a visual check. This trend is different from that
already observed in the NC family, indeed, in the case of UHPC, probably a very thin crack onset, however it
is so fine to result invisible at a visual inspection and so it is not reflected by an evident drop of the curve.
This behavior may be explained as the consequence of an higher bond between the cement matrix and the
fiber probably linked both to the fiber shape (waved) and to the presence of the pozzolanic addition that
reduce the quantity of Ca(OH)2 at the interfacial zone and thus increase the bond strength.
Tempo (hours)
24,00
-1E-15
44,00
64,00
84,00
104,00
124,00
144,00
164,00
184,00
204,00
224,00
1% on vol.
-0,1
cracking NC
CMOD (mm)
0.5% on vol.
-0,2
-0,3
-0,4
0% on vol.
-0,5
-0,6
NC_f1
NC_f2
NC_f3
Figure 5 - Comparison between NC with different fiber content (NC_f1 - 0%, NC_f2 – 0,5%, NC_f3 – 1%) .
Time (hours)
0
130
180
-0,2
0,01
Time (hours)
0
0
200
400
600
800
1000
280
330
-0,01
-0,03
-0,04
-0,05
-0,06
cracking 2HPC_f3_1
-0,4
-0,02
cracking 2HPC_f3_2
-0,3
CMOD/CTOD (mm)
CMOD (mm)
-0,1
230
cracking UHPC_f1_2
80
cracking UHPC_f1_1
30
UHPFRC_f3_2_CMOD
UHPFRC_f3_1_CMOD
UHPFRC_f3_2_CTOD
UHPFRC_f3_1_CTOD
-0,5
UHPC_f1_1
UHPC_f1_2
UHPFRC_f2_1
UHPFRC_f2_2
UHPFRC_f4_1
UHPFRC_f4_2
Figure 6 - Comparison between UHPC with fibers: UHPC_f1 – 0%, UHPFRC_f2 – 0,5%, UHPFRC_f3 –
1% .
Table 6: Main MRT-based restrained shrinkage parameters of curves in Fig. 5
∆CMOD (μm)
CMODc (μm)
TC (h)
162.83
37
2
NC_f1
166.87
131
36
NC_f2
168.67
349
94
NC_f3
vc (%)
0.07
0.05
0.04
However, looking more in detail the trend of the two UHPFRC_f3 mix both in terms of CTOD and CMOD,
it can be seen that, even if no visible cracking it can be appreciated, the monitoring system is enough
sensitive to detect some physical process related to cracking. From CMOD/CTOD vs. time curves, indeed, it
is possible to recognize a very small “jump” in the CMOD graph of ring specimen 2 at about 280 hours. Also
a change in the curve slope is visible at about 350 hours in ring specimen 1.
As main result, from Table 6, it can be noted that the addition of steel fibers to the UHPC matrix, affect
significantly the restrained shrinkage behavior. In detail, it plays the most important role in controlling the
cracking propagation, indeed the parameter vc is two order of magnitude lower in the mix with 1% on
volume of steel fibers (UHPFRC_f3) with respect to the same mix without fibers (UHPFRC_f1).
CONCLUSIONS
In this paper, the cracking sensitivity of ultra high-performance and normal concrete under restrained
shrinkage was investigated by means of a new ring-test-based experimental method (called MRT)
characterized by different dimensions with respect to the ASTM standardized ring and by the presence of a
preformed notch. The main purpose was to develop an experimental tool useful to detect some cracking
parameters that should help the concrete technologist to know better the behavior of concrete or mortar in
real site conditions for mix design of high performance materials also in terms of strength to restrained
shrinkage-induced cracking. On the basis of these experimental results, it can be concluded that the MRT
method, being able to develop a degree of restraint equal to about 80% in concrete specimens, is capable to
simulate more realistically the behavior of ordinary concrete (also admixtured with fibers) and UHPC even
when they are subjected so severe restraints (for example when they are used for rehabilitation applications).
Finally, it was also demonstrated that the proposed MRT is able to provide enough robustness and
repeatability.
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