39

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

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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.

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