Dimensional Compatibility Between Patch Repair Materials and Concrete Under Short-term Compressive...
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Proceedings of the First Makassar International Conference on Civil
Engineering (MICCE2010), March 9-10, 2010, ISB" 978-602-95227-0-9
DIME�SIO�AL COMPATIBILITY BETWEE� PATCH REPAIR MATERIALS
CO�TAI�I�G TYRE FIBERS A�D CO�CRETE U�DER SHORT-TERM
COMPRESSIVE STRESS
S.A. Kristiawan 1
ABSTRACT: Degradation of concrete will reduce service life of structural concrete. Patch repair may be ulitised to
recover its appearence, dimension, load bearing capacity and durability of damaged concrete and thus extends its
service life. Dimensional compatibility between patch repair material and the existing substrate is one of the
performance criteria which determine the success of application of this material. Under compressive stress, the repaired
concrete should perform as a composite system that will work together in its function as load bearing. This research is
aimed to evaluate the dimensional compatibility of repair materials containing various tyre fiber content under short-
term compressive stress. The stress-strain relations of the repaired concrete (composite) under compressive stress were
measured both on the existing concrete and on the patch repair material. A model is developed to predict the stress
distribution in composite system by considering the different in strains measured on concrete substrate and repair
material. The stress in the patch repair material calculated by this model is less than that based on iso-strain asumption.
Keywords: compatibility, dimensional, repair material, stress distribution, tyre fiber.
1 Senior Lectures, Sebelas Maret University, Solo 57126, INDONESIA
INTRODUCTION
Concrete has been utilized in the construction of
many types of structures for ages. Nowdays, many of
these structural concrete show signs of degradation due
to a variety factors. Corrosion of reinforcement is a
major cause of deterioration which disrupts the cover
zone of reinforced concrete (Mangat and O’Flaherty
2000). It is unfortunate that not only old structural
concrete that has experience this kind of problem, but it
can also be found in newly built structural concrete.
Under severe enviroments where chloride concentration
is very high such that in seashore area, steel
reinforcement corrosion can occur just in few years. In
many instances, this problem could be worst when there
are combined with lack of good design and construction
practices.
Degradation of reinforced concrete will certainly
shorthen the service life of the structural concrete. There
are several choices of materials and methods that could
be used to extend service life of this deteriorated
concrete. In many cases and dependent on the extent of
deterioration, patch repair may be the most cost-effective
solution to recover the size and appearence of
deteriorated concrete occured in cover zone. This
method involves the removal of the deteriorated parts
and reinstatement with a fresh repair mortar (Hassan et
al. 2001).
Maintenance and repair or rehabilitation is becoming
increasingly important part of design and construction
industry. Much of the repair work undertaken in the first
half of the century was relatively simple from a materials
engineering perspective, as it primarily involved
replacement of damaged or deteriorated concrete with
conventional Portland cement based materials. While the
conventional Portland cement based repair materials
have served the construction industry well for many
decades, there are many reported in the literature where
performance has been less than satisfactory (Morgan
1996).
New enhanced concrete repair and materials and
systems have been introduced and found increasing
utilization. Some of these repair materials have been
developed specially for patch repair application and
claimed to have a better performance compared to
conventional mortar. Propetiary repair materials are
usually protected by patents. Manufacturers are
understandably reluctant to provide complete details of
their materials. The information offered to potential
users is in many cases indequate. Engineers and
specifiers faced with a wide choice of materials and little
guidance on their properties, may opt for materials
having properties as close as possible to those of the base
40
concrete . Since disclosure of the composition of repair
materials is not realistically possible there is need to
establish a set of requirements which should be based on
performance related properties. Cabrera and Al-Hasan
(1997) suggested these properties include dimensional
stability, compatibility, strength and protection provided
to the reinforcement. In term of dimensional stability,
Robery and Shaw (1997) and Baluch et al (2002) noted
the important to address restrained shrinkage effect,
where repair product is bonded onto concrete. For a
restrained situation, other properties of the repair
materials such as creep and modulus of elasticity are as
important as free shrinkage. All these properties ensure
that the applied repair materials will be resistance to
cracking due to restrained shrinkage effect. Banthia et al
(2006) point out the role of fibers to mitigate such
cracking. Currently, there are a wide range of fibers to be
used to enhance the performance of repair materials
including tyre fiber.
Structural reinforced concrete repair requires a
material that can to some extent strengthen the
deteriorated concrete. The effectiveness of this
strengthening influenced by compatibility of the
combined system (concrete-repair). Compatibility in a
repair system is the combination of properties between
the repair material and the existing concrete substrate
which ensures that the combined system withstands the
applied stress and maintains its structural integrity
(Hassan et al. 2001).
Mangat and O’Flaherty (2000) suggested that the
relative stiffness of repair material and concrete substrate
is the primary parameter for the design of efficient repair.
They noted that repair applied with relatively stiff
materials (higher elastic modulus compared to that of
concrete substrate) will display effictive stress transfer to
the substrate. Using polymeric and polymer –modified
concrete repair materials, Shambira and Nounu (2000)
indicate that in the short-term both repair materials assist
the repaired concrete column to carry load. Meanwhile,
Hassan et al. (2001) proposed a simple model describing
the modulus behaviour of combined repair and concrete
materials and presented an experimental programme on
the application of this model. Basically, the model is
developed on the assumption that strain is constant over
any cross section (iso-strain). In reality, this may not be
tha case.
Evaluation of compatibility between repair materials
containing tyre fibers and concrete substrate by
considering the differential strains (or elastic modulus) is
the main focus of this research. The stress-strain
relations of the repaired concrete (composite) under
compressive stress were measured both on the existing
concrete and on the patch repair materials. The
difference of strains observed on existing concrete and
patch repair materials are used to evaluate their
dimensional compatibility. A new model that takes into
account those differential strains is proposed to describe
elastic modulus behaviour of the composite system. This
model is than used to estimate the stress distribution
across section of patch repaired concrete. Comparison is
made to asses the difference of stress distribution
estimated by this model with that based on iso-strain.
MODEL THEORY
Elastic modulus behaviour of combined repair
material and parent concrete based on assumption of iso-
strain has been suggested by Hassan et al. (2001). Fig. 1
ilustrates the combined system subjected to external
stress (σo), have modulus of elasticity Eo and Poisson’s
ratio υo . The corresponding properties of the two phases
are shown in Fig. 1 with substrate concrete indicated by
“c” and repair mortar by “m”.
Fig.1. Stress and strains of composite system
The proposed model to compute elastic modulus of
the combined system from the known elastic modulus of
two phases is as follow:
(1)
According to Emberson and Mays (1996) the
Poiaaon’s ratio has only second –order effect on the
stress distribution is patch repair. If the differences in
Poisson’s ratios are neglected, i.e. υ0=υc=υm, then Eq.1
becomes:
(2)
For the case of non-iso strain, the model is developed
as follows: Let the strain over cross section of the
combined system is shown as in Fig. 2. The observed
σc
Ec
υc
σm
Em
υm
0.5 0.5
σo
−+
−−=
m
m
c
c
oo
EEE
υυυ
2121)21(5.0
)(5.0 mco EEE +=
41
strain due to applied external stress σo on the surface of
concrete and repair material is εcs and εms , respectively
while the strain in the interface or transition of repair
material and concrete is εt . The average strain in the
concrete εc and mortar εm may be calculated using Eq. 3
and 4.
.
(3)
(4)
For equilibrium of forces in the combined system:
(5)
or
(6)
Substituting Eqs. 3 and 4 into Eq. 6:
(7)
Fig.2. Asummed strain over cross section of combined
specimen
MATERIALS AND METHODS
This research utilizes mortar containing various level
of tyre fibers as patch repair materials. The properties of
mortar is enhanced by the use of two admixtures i.e.
superplasticizer and accelerator to modify its workability
and hardening rate, respectively. Since the repair
material should have comparable strength with that of
parent concrete, the water/cement ratio has to be kept at
a level that will produce mortar having strength in the
range of 20-30 MPa. This range of strength is assumed
to be that of normal concrete use in common old
structural concrete. Trial investigation showed that this
could be achieved when mortar was proportioned at
1:2.5 by weight of cement:sand with water/cement ratio
of 0.50. The proportion of superplasticizer is determined
at 2% by weight of cement. At this composition, 12 % of
tyre fiber by volume of mortar is found to be the
maximum level of fiber that can be added into mix to
maintain workability of fresh repair material still suitable
to be mixed, handled and applied manually. Fig. 3 and
Table 1 ilustrate physical properties of tyre fiber used in
this research. It is noted that tyre fiber used in this
research is that of passing grading size of 4.75 mm.
Meanwhile, the use of accelerator is necessary to
increase hardening rate of repair material since in
practice the repair material should adhere to the parent
concrete and work as composite system as fast as
possible. Table 2 summarises the proportions of the
materials. A part from the repair materials, concrete
having target strength of 30 MPa was also proportioned
to represent substrate concrete which will eventually be
repaired.
Fig.3. Tyre fiber used in this research
Table 1. Tyre fiber grading
Grading
size
Physical properties
of fiber
Cumulative
Passing Length
(mm)
Diameter
(mm)
4.75 - - 66.6%
2.36 21.5 1.8 56.5
1.18 9.2 1.2 29.14
0.85 2.35 0.8 7.8
Table 2. Composition of repair materials
(*by weight of cement)
Cylindrical specimens having size 15 mm diameter
and 300 mm height were cast for the purpose of
determining elastic modulus. These cylindrical
specimens include that of parent concrete specimens.
The specimens were demolded after 24 hours and stored
in the laboratory environment (27-32oC and 65-70% RH)
before testing. All the specimens were tested under
compressive stress (Fig.4). The stress was applied
Repair
Material
Fiber
Composition
M-0% 0% All repair materials have proportion:
cemen: sand = 1: 2.5, w/c ratio =
0.5, superplasticizer = 2%*,
accelerator = 0.4%*
M-4% 4%
M-8% 8%
M-12% 12%
εms εm εf εc εcs
)(5.0 tcsc εεε +=
)(5.0 tmsm εεε +=
5.05.01 xxx mco σσσ +=
)(5.0 mmcco EE εεσ +=
[ ])()(25.0 tmsmtcsco EE εεεεσ +++=
42
incrementally until reaching 30% of the ultimate strength
of the specimen. The stress and the coressponding strain
were recorded. For concrete specimens, testing was
performed at the age of 28 days. Meanwhile, for repair
materials testing was carried out at the age of 1 day.
Fig.4. Testing of elastic modulus under compressive
stress
Fig.5. Casting a half of combined cylinder
Combined cylindrical specimens were cast as follow;
First a half of cyllinder concrete specimens was cast (Fig.
5). After 24 hours, they are demolded and stored in the
laboratory environment until 28 days. At this age, the
half cylinder concrete specimens were placed again in
their original moulds. The other halves of the moulds
were cast with the different repair materials to produce
combined specimens (Table 3). The specimens were
compacted and kept in the mould for 24 hours before
demoulding. The combined specimens were then tested
under compressive stress similar to those of uncombined
specimens. The stress and the coressponding strains on
the surface of concrete substrate and repair materials
were recorded.
Table 3. Combined specimens identification
Sample
identification
Combined specimen
C-M0% Combined specimen made of a half
concrete (C) and a half of repair
materials (M) with respective % of
fiber content
C-M4%
C-M8%
C-M12%
RESULTS AND DISCUSSION
The stress and corresponding strain of concrete
substrate and all repair materials tested under
compressive stress up to 30% of their respective ultimate
strength are given in Fig.6. Each curve represents
average of three specimens. Generally, it can be
reckoned that concrete substrate is stiffer than those of
repair materials. This is not surprising since stress-strain
relationship of concrete substrate is determined at the
age of 28 days while the corresponding repair materials
determined at the age of 1 day. This difference in age of
testing is intended to signify the difference in the age of
damaged structural concrete and repair materials used to
restore the damage. Furthermore, it is expected that
repair materials should immediately contribute to carry
load 24 hours after application. The results of this
investigation imply that even though repair materials has
been proportioned to have similar strength to that of
concrete substrate, but at early age it has not yet
developed its full potential properties and in turn, when
it carries external compressive stress it will show higher
deformation compared to that of concrete substrate. The
difference in elastic deformation between concrete
substrate and repair materials raise concern over it
dimensional compatibility.
From Fig.6, elastic modulus of the materials may be
obtained from the gradient of these curves. The elastic
moduli of materials determined in this way are presented
43
in Table 4. These values of elastic modulus may further
be analyzed to investigate the effect of fiber content. The
result is presented in Fig.7. It is indicated from Table 4
that the elastic modulus of repair materials is in the range
of 32-64% of the concrete substrate. Increasing fiber
content tends to reduce the elastic modulus. From Fig. 6
it is clear that each addition of 4% fiber into repair
material will reduce elastic modulus by about 17%. The
reduction in elastic modulus by addition of fiber could
be related to the fact that this material is softer compared
to sand. At a given volume, repair material with higher
fiber content will have lower fraction of sand. Hence, a
higher fraction of softer ingredient in the repair materials
causes these materials deform more under compressive
stresss and vice versa.
Fig.6. Stress-strain relationships of concrete and repair
materials
Table 4. Elastic modulus of materials
Specimen Elastic Modulus
C (concrete substrate) 24.6 GPa
M-0% 15.6 GPa
M-4% 13.5 GPa
M-8% 9.4 GPa
M-12% 7.9 GPa
Fig.7. Effect of fiber content on elastic modulus of repair
materials
Fig. 8. Stress and corresponding strains on the concrete and repair materials phase of combined specimens
0
1
2
3
4
5
6
7
8
9
0 50 100 150 200 250 300 350 400 450
Strain (10-6)
Stress (MPa)
Concrete substrate
M-0%
M-4%
M-8%
M-12%
44
Fig.8 shows the stress-strain relationships of
combined specimens under compresive stress. All
specimens confirm the different in strains observed on
the surface of concrete substrate and repair materials.
As the elastic modulus of repair materials is lower than
that of concrete substrate, therefore when external
stress applied to the combined specimens the strains
observed on the repair materials tend to be higher than
that of concrete substrate. To determine whether this
difference in strain under short-term compressive stress
is still within acceptable level or not, it is necessary to
evaluate its effect on the stress distribution over cross
section of combined specimen. If the stress distribution
does not different with that calculated from the
assumption of constant strain over any cross section of
combined specimen (iso-strain), it is justified to state
that the repair materials and concrete substrate still
have dimensional compatibility.
For a case of iso-strain any external load applied to
combined specimen will produce constant strain over
any cross section which is equal to the applied stress
divided by the elastic modulus of combined specimen
determined from Eq.2. The resulting strain is then used
to calculate the stress distribution by considering the
elastic modulus of each phase. It can be deduced that
the external stress will produce ratio of stress
distributed on repair material and concrete phase equals
to the ratio of their respective elastic modulus. For the
case of non-isostrain, the stress distribution is
calculated as follows: First, the measured strain on
each surface of concrete and repair material is
determined from Fig. 8. Then Eq. 7 could be employed
to calculate the strain in the interface between concrete
substrate and repair material with elastic modulus data
taken from Table 4. With the strain in the interface has
been obtained, so the average of strain in concrete
substrate and repair material can be calculated using Eq.
3 and 4, respectively. The resulting average strains are
then used to compute the stress distribution by
multiplying of the strains with the corresponding
elastic modulus. The results of stress distribution
calculation both for cases of iso-strain and non iso-
strain are presented in Fig. 9. As can be seen from this
figure, the stress distributed in the repair material and
concrete depends on the ratio of elastic modulus of
repair material and concrete substrate. For a case of
iso-strain the ratio of stress distributed on repair
material and concrete substrate is exactly equal to the
ratio of their respective elastic modulus. In the case of
non iso-strain, a higher ratio of elastic modulus is
required for the same level of stress distribution
compared to that of iso-strain case. It means that for a
given repair materials, actual stress carried by the
repair materials is less than that calculated by the
asumption of iso-strain. This clearly confirms that
dimensional incompatibility between repair material
and concrete substrate will affect the capability of the
repair material in assisting to carry load. The effect is
to reduce its contribution to support external load, and
the magnitude of its contribution is less than that
expected from the iso-strain asumption.
Fig. 9. Relationship between ratio of elastic modulus of
repair material and concrete substrate and ratio of their
respective stress distribution.
Fig. 9 also suggests that to expect repair material
carrying external stress at 50% of that carrying by
concrete substrate, the required elastic modulus
elasticity must be at least 75% of the elastic modulus of
concrete substrate. This is higher than that expected for
the case of iso-strain which requires only 50%.
CONCLUSIONS
This research confirms that application of patch
repair material requires consideration of dimensional
compatibility. The difference of elastic modulus
between repair material and concrete substrate result in
uneven of strain across section of combined specimen.
Consequently, the stress distributed over the repair
material is less than that calculated based on iso-strain
assumption. For patch repair material used in this
research, inclusion of a higher fiber content reduces
the elastic modulus of repair material, which in turn,
lowering the contribution of the repair material in
carrying compressive stress.
y = x
y = 0,8841x + 34,713
0
20
40
60
80
100
0 10 20 30 40 50 60 70
Ratio stress in repair material and concrete (%)
Ratio of elastic m
odulus
of repair m
aterial and concrete (%)
iso-strain
non iso-strain
45
REFERENCES
Baluch, M.H., Rahman, M.K. and Al-Ghadib, A.H.
(2002). Risks of cracking and delamination in patch
repair. Journal of Material in Civil
Engineering.14(4):294-302.
Cabrera, J.G., Al-Hasan, A.S. (1997). Performance
properties of concrete repair materials. Construction
and Building Materials. 11(5-6):283-290.
Hassan, K.E., Brooks, J.J. and Al-Alawi, L. (2001).
Compatibility of repair mortars with concrete in a
hot-dry environment. Cement and Concrete
Composites. 23:93-101.
Mangat, P.S. and O’Flaherty, F.J. (2000). Influence of
elastic modulus on stress redistribution and
cracking in repair patches. Cement and Concrete
Research. 30:125-156.
Morgan, D.R. (1996). Compatibility of concrete repair
materials and system. Construction and Building
Materials. 10(1):57-67.
Robery, P. and Shaw, J. (1997). Materials for the repair
and protection of concrete. Construction and
Building Materials. 11(5-6):275-281.
Shambira, M.V. and Nounu, G. (2000). Construction
and Building Materials. 14:425-432.
46