Seminar Rubber is Ed Concrete

Strength, Modulus of Elasticity and Brittleness Index of Rubberized Concrete

ACKNOWLEDGEMENT

First and foremost i would like to thank the almighty for the successful completion of this seminar.

I am thankful for the support of Prof. Jayakumari V, The Head, Civil Engineering Department, Govt.

Engineering College, Thrissur.

I am also extremely grateful to the guide of my seminar Smt.Asha, Lecturer, Civil Engineering

Department, for her timely advice and unceasing motivation.

I am also thankful to the faculty members of The Department Of Civil Engineering, without whom

this would have been a distant reality.

Last but not the least i am also thankful to our friends for their help rendered.

Dept. Of Civil Engineering, Govt. Engineering College Trichur


CONTENTS

1. ABSTRACT

2.INTRODUCTION

3.RUBBERIZED CONCRETE

4.PRINCIPLES OF ANALYSIS



ABSTRACT

This paper presents a study of rubberized concretes designed by replacing coarse aggregate in

normal concrete with ground and crushed scrap tyre rubber in various volume ratios. The

objective of the study was to investigate the effect of rubber types and rubber content on

strength and deformation properties. The compressive strength, static, and dynamic modulus

of elasticity of rubberized concrete were tested and studied. The stress-strain hysteresis loops

were obtained by loading, unloading, and reloading on specimens.

General introduction of rubberised concrete and its properties were also discussed.

Brittleness index values were calculated based on the hysteretic loops. The experiments

revealed that strength and modulus elasticity of rubberized concrete decreased with the

increasing amount of rubber content. Brittleness index values of rubberized concrete were

lower than that of normal concrete, which means that rubberized concrete had higher ductility

performance.



1.INTRODUCTION

Rubberised concrete is relatively a new concept as far as civil engineering is concerned. The

coarse aggregate normally used in concrete is graded rock materials. But in rubberised

concrete the coarse aggregate used is ground and crushed scrap tyre rubber. Substitutes for

fine aggregates and coarse aggregates are been thought of now days by stressing concern on

the effect of this concept on environment and its economy. Rubber is an elastomer that was

originally derived from latex, a milky colloid produced by some plants. Rubber produces

unique physical and chemical properties.

Solid waste disposal is a major environmental issue for cities around the

world. American motorists discard approximately 290 million tires each

year, which is approximately one tyre for every person in the United

States. Around 16 million of these tires are retreated or reused, leaving

roughly 274 million scrap tires to be managed annually. In 2003, the total

numbers of scrap tires going to market was about 233 million. About 40

million scrap tyres are estimated to be disposed of in landfills. In addition,

275 million tires that have accumulated over the years are currently

stockpiled throughout the United States Rubber Manufacturers Association

2004. These stockpiles are dangerous not only due to potential

environmental threat but also from fire hazards and they provide breeding

grounds for rats, mice, and mosquitoes. Because of rapid depletion of

available sites for waste disposal, disposing of waste tires in landfills is

becoming unacceptable. Over the years, disposal of waste tyres has

become one of the serious problems for the environment. Innovative

solutions to solve the tyre disposal problem have long been in

development. Cement-based concrete is a brittle material in general and

is of high rigidity. In some applications such as foundation pads and

traffic barriers, it is desirable for concrete to have high toughness and

good impact resistance. Although concrete is the most commonly used

construction material, it does not always fulfill these requirements. It has

been observed from previous research that the properties of concrete



would change when used automobile tyre chips are added into concrete.

From the studies conducted by scientists Eldin and Clark it is said that

adding rubber to traditional concrete will enhance the deformability or

ductility of rubberised concrete members.

Because of the unique elasticity properties of rubber material, the

rubberized concrete showed minimizing vibration and impact effect.

Although more and more reports gave evidence of the use of tyre rubber

in the past decade (1990s), the study on rubberized concrete is still

incomplete. Further research is still needed to optimize the percentage of

rubber and particle-size distribution to achieve the desired properties of

rubberized concrete.

For a material of high toughness, most of the total energy generated upon fracture is plastic,

while for a brittle material, most of the total energy generated upon fracture is elastic. The

energy that material absorbs until it is broken is plastic energy. It is desirable for concrete to

have more ductility. This study was undertaken to examine how replacing part of the

aggregate volume with rubber would improve the ductile property of concrete. In this study,

rubberized concrete was proportioned by replacing coarse aggregate in normal concrete with

scrap tire rubber in various volume ratios and by using two typical sizes of rubber particles.

Both cylinder compressive strength and cube compressive strength were tested. The

relationship between the two strengths was studied. Cylinder specimens were used to get the

static modulus of elasticity and the relationships between the static modulus of elasticity and

compressive strength were investigated. Cube specimens were used in this research to

calculate the dynamic modulus of elasticity of rubberized concrete with the pulse velocity

tests. Upon loading, unloading, and reloading on prepared cylinder specimens, the σ-ϵ

hysteresis loops were drawn and brittleness index values were calculated based on a typical

hysteresis loop. The variation of brittleness index values was evaluated for different types of

rubber particles and compressive strength.

The objectives of the research were to investigate: (1) The effect of rubber types and rubber

content on cylinder compressive strength, cube compressive strength, static modulus of



elasticity, and dynamic modulus of elasticity; (2) the relationship between modulus of

elasticity and compressive strength of rubberized concrete; and (3) the effect of rubber types

and rubber content on the brittleness index of rubberized concretes and the relationship

between the brittleness index and compressive strength.

2.RUBBERISED CONCRETE

Rubberised concrete is a new concept used in civil engineering. The coarse aggregate used is

tyre rubber flakes or ground and crushed scrap tyre pieces. The rubber aggregate can be

added in fixed proportion with respect to the total volume or weight. Various tests such as

compressive strength , brittleness index , modulus of elasticity and shrinkage tests were

carried out and from these tests the properties of rubberised concrete were revealed. Addition

of rubber can reduce the brittleness index and hence the ductility is enhanced. More over the

use of rubber prevents cracking of concrete. But the tests reveal that the bonding between

cement and rubber is poorer compared to the bonding between cement and aggregates. The

strength of concrete with rubber is low and the capability to withstand heavy loads is less.

3.PRINCIPLES OF ANALYSIS

3.1STATIC MODULUS OF ELASTICITY AND DYNAMIC MODULUS OF ELASTICITY

The modulus of elasticity is one of the most important elastic properties of concrete since it

impacts the serviceability and performance of concrete structures. The elastic modulus of

concrete is closely related to the property of the cement paste, the stiffness of the selected

aggregates, and also the method of determining the modulus. The static modulus of elasticity

was tested and calculated by a method similar to that in ASTM C469 (ASTM 2002b). The

modulus was determined based on the slope between the two linear points on a stress-strain

plot. The first point is when longitudinal strain is 50 millionths and the second point is that

the applied load is equal to 40% of the ultimate load.



To determine the dynamic modulus of elasticity, velocities (Vp) and (Vs) of two kinds of

ultrasonic wave, the longitudinal stress

wave (P wave) and the transverse wave (S wave), in rubberized concrete specimens were

measured in this study. The P wave (primary wave) is a wave in which particles vibrate

parallel to the direction of wave travel, while the S wave (secondary wave) is a wave in

which particles vibrate perpendicular to the direction of wave travel. The P wave travels with

a higher velocity while the S wave travels somewhat more slowly. With measured Vp and Vs,

the dynamic modulus of elasticity Ed can be calculated by Eq. (1)

3.2BRITTLENESS INDEX OF CONCRETE

Topcu (1997) used the stress-strain (σ−ϵ) hysteresis loops and the

corresponding envelope line to evaluate the toughness of rubberized

concrete. (σ-ϵ) curves were established for repeated loading which was

applied by loading, unloading, and reloading phases. When the load

reached approximately 85% of the maximum concrete carrying capacity,



the loading was stopped. Brittleness index (BI) can be calculated

according to energy measured

from the areas under the (σ-ϵ) curves. As seen in Fig. 1, the A1 area

shows the irreversible plastic energy consumed during the failure and

never recovered again (plastic energy capacity); and the A2 area shows

the recovered deformation energy to be obtained

just before fracture (elastic energy capacity). BI of concrete specimens in

compression is defined as the ratio of the elastic deformation energy to

irreversible deformation energy, shown as A2/A1. When the ratio A2/A1

approaches zero, all energies become irreversible, while when the ratio

A2/A1 goes to infinity all energies become reversible. For brittle materials

such as concrete, which has a larger elastic energy capacity than the

plastic energy

capacity, the BI value, in general, is higher compared to other ductile

materials. The smaller the brittleness index value, the more ductile

deformation the material has. Addition of rubber in concrete can reduce

the brittleness index values and improve ductility of concrete, changing

the concrete from brittle material to considerably ductile material.

4.EXPERIMENT STUDY

4.1MATERIALS

A target strength of 30 MPa was selected for rubberized concrete in the study because the

strength value is commonly used in infrastructural applications. To meet the target

compressive strength, a control mixture with a compressive strength of 40 MPa was selected.

In developing rubberized concrete mixtures, all mixture proportions were kept constant

except for the coarse aggregate constituents, meaning that the fine aggregate (sand), cement



content, water-cement ratio, and admixture were kept constant. Type I portland cement was

used in all mixtures. Sand was river sand, which had 5 mm maximum size and 2.5%

moisture content. Crushed stone of 31.5 mm maximum size was used as coarse aggregate.

NF-5 was applied as a water reduce admixture. Two groups of rubber were used in the study

as shown in Fig. 2. One group (GR-8) is rubber powder of ground tires, as shown in Fig. 2(a).

(GR-8) represents ground rubber powder of 8-mesh, which means 80% of powder is smaller

than the size of 2.6 mm. The other group is crushed rubber or used tyre chips

(CR-40), shown in Fig. 2(b). The surface of each chip is rough and jagged due to the cutting

process used. The tyre-chip group of rubber has particles that ranges in size from about 15 to

4 mm, with the steel belt wires included and extended. Rubber replacements of 15, 30, and

45% by the volume of the coarse aggregate were used and were named Set-15, Set-30, and

Set-45, respectively. The mixture proportions of concretes and specific gravities

for each raw material are listed in Table 1.



4.2SPECIMENS

Cylinder specimens that were of 150 mm in diameter and 300 mm in height were used to get

the cylinder compressive strength, the static modulus of elasticity, and the hysteresis loops.



Cube specimens with dimensions of 150 mm were used to acquire the dynamic modulus of

elasticity of rubberized concrete with the pulse velocities tested. After that, the cube

compressive strength was obtained. For each rubberized concrete set, four specimens were

tested for the dynamic modulus of elasticity and compressive strength, and two specimens

were tested for the static modulus of elasticity and hysteresis loops. The compressive

strengths of specimens were obtained at 28 days of concrete age. The average compressive

strengths were determined from at least three replicate specimens.

4.3EXPERIMENTAL SETUP

The U-Sonic ultrasonic detection system was used to measure the two kinds of wave

velocities (Vp and Vs) in rubberized concrete specimens. The test setup of the longitudinal

wave (P wave) and the transverse wave (S wave) velocities are shown in Figs. 3(a

and b), respectively. Velocity measurements were made by clamping two disk shaped

ultrasonic transducers (one source, one receiver) onto opposite sides of the specimen. The

longitudinal wave (P wave) transducer was coupled with Vaseline and the transducers were

coupled with tin foil leaf. Each measurement was made three times at different positions on a

specimen to improve accuracy. A pulse generator and oscilloscope monitor completed the

experimental equipment. The test process was based on ASTM E 494 (ASTM 2005). An

INSTRON universal testing machine was used to get the compressive strength, modulus of

elasticity, and hysteresis loops of rubberized concrete. The compressive strength and modulus

of elasticity were tested according to ASTM C 39/C 39M (ASTM 2002a) and ASTM C469

(ASTM 2002b), respectively. To evaluate the modulus of elasticity, both longitudinal and

transverse strain gauges were attached to the prepared cylinder specimens. Prior to evaluating

the elastic modulus, four specimens of each set were loaded to obtain the average ultimate

compressive strength so that 40% ultimate strength could be determined. When conducting

hysteresis loops of rubberized concrete, two longitude strain gauges were attached

symmetrically to the specimens. Before the hysteresis loop was drawn, minor loads were

applied and the specimen was adjusted until the readings of the two strain gauges were very

close to avoid the eccentricity of the loading. Upon loading, unloading, and reloading on the

prepared specimens, the hysteretic loops were obtained (Sinha et al. 1964). Generally, the

loading was stopped when loading reached approximate 85% of the maximum load carrying

capacity. However, some specimens failed before the load reached the target loading and the



last loop could not be closed. In that situation, the final loading stress-strain curve had to be

eliminated and the hysteresis loops were counted only to the last closed loop.

5.TEST RESULTS AND ANALYSIS

5.1CHANGES ON UNIT WEIGHTS

The densities of each set of specimens were measured before testing for mechanical

properties. Because of low specific gravity of rubber particles, unit weight of rubberized

concrete decreased with the increase in the percentage of rubber content. The effect of rubber

content on the unit weight of concrete is shown in Fig. 4. For GR-8 rubberized concrete, the

average unit weight decreased from 2,399 kg/m3 of the control set to 2,245, 2,130, and 2,006

kg/m3 with the rubber powder content of 15, 30, and 45%, respectively. For CR-40

rubberized concrete, the average unit weight decreased to the lowest value of 2046 kg/m3 at

45% rubber content. At 15 and 30% rubber content, the average unit weight of CR-40

rubberized concrete was very close to that of GR-8 rubberized concrete. Variation of the

rubber type had less influence on unit weight of mixtures than on rubber content.

5.2COMPRESSIVE STRENGTH

The values of cylinder compressive strength for rubberized concrete are

given in Fig. 5. As shown in the figure, the strength values of the

rubberized concrete decreased with the increasing amount of rubber. The

average cylinder compressive strength of the normal concrete at 28 days

was determined to be 38.8 MPa. The strength of group CR-40 at the

content of 15, 30, and 45% decreased to 30.1, 21.0, and 18.1 MPa, with

the a decrease of

22.3, 45.8, and 53.3%. Although there is also a decrease for GR-8

rubberized concrete compressive strength, down to 33.5, 25.8, and 19.6

MPa at the rubber content of 15, 30, and 45%, respectively, the decrease

was not as much as that for CR-40. It can be concluded that crushed

rubber affects cylindrical compressive strength more than ground rubber.

The values of cube compressive strength for rubberized concrete are



given in Fig. 6. As seen in this figure, the strength values of the rubberized

concrete decreased considerably with the increasing amount of rubber.

For GR-8 rubberized concrete, the control compressive strength of 53.8

MPa reduced to 46.4, 35.8, and 27.3 MPa with the rubber content of 15,

30, and 45%, respectively. For CR-40 rubberized concrete, the control

compressive strengths were 48.5, 28, and 25.2 MPa with the rubber

content of 15, 30, and 45%, respectively. Again, crushed rubber caused

more reduction on cube compressive strength than ground rubber.

Possible reasons attributed to the strength reduction might include that:

(1) replacing coarse aggregate with softer rubber particles results in an

obvious quantity reduction of strong load carrying material because

coarse aggregate is the most important concrete component for load-carrying

capacity; (2) the bonding between rubber and mortar is not as good as the bonding between

the aggregate and mortar; and (3) stress concentrations in the paste at the boundaries of the

rubber aggregate cause the strength reduction. The ratio of cylinder compressive strength and

cube compressive strength for normal concrete is 0.72, which is close to the commonly

accepted cylinder/cube strength ratio for normal concrete, 0.75 (Neville 1993). For GR-8

rubberized concrete at rubber content of 15, 30, and 45%, the ratio became 0.68, 0.71, and

0.75, respectively, while for CR-40, the ratios were 0.64, 0.75,

and 0.64, respectively. The result indicates that rubberized concrete has a slightly lower

cylinder /cube strength ratio than that of normal concrete.



5.3STATIC MODULUS OF ELASTICITY

The change in the static modulus of elasticity (Es) for two types of rubber, GR-8 and CR-40,

is shown in Fig. 7. The static modulus of elasticity of rubberized concrete was lower than that

of normal concrete. The average value of modulus of elasticity of normal concrete was 31.8

GPa, while for rubberized concrete with ground rubber of 15%, the static modulus of

elasticity is 27.1 GPa, which is 81% of normal concrete. The modulus of elasticity decreased

with the increase of rubber content for both ground and crushed rubberized concrete

compared to concrete without any rubber. While rubber content varied from 15 to 45%,

modulus of elasticity for ground rubberized concrete reduced from 14.8 to 29.9%, while for

crushed rubberized concrete the value reduced from 27.4 to 49.4% compared to the control

concrete. As clearly shown in Fig. 7, crushed rubber caused more reduction on static modulus

of elasticity of rubberized concrete at a higher percentage replacement of aggregates by

rubber. In the ACI 318-2005 building code, the relationship between modulus of elasticity

and compressive strength of concrete is



where E=modulus of elasticity; wc=unit weight; and fc =cylinder compressive strength.

Using the ACI equation, the static modulus of elasticity of normal concrete (without rubber)

and rubberized concrete were calculated. For normal concrete, the unit weight was 2,399

kg/m3 and the compressive strength was 38.8 MPa. The calculated ACI value of modulus of

elasticity was 31.5 GPa, which was very close to the obtained experimental value of 31.8

GPa. For rubberized concrete, the unit weight ranged from 2,006 to 2,245 kg/m3 and average

compressive strength varied from 16.1 to 31.4 MPa. The calculated modulus of elasticity of

rubberized concrete based on the ACI equation is in the range of 15.8–31.5 GPa. Then, the

calculated results were compared with the actual test results of modulus of elasticity for

rubberized concrete. As shown in Fig. 8, for both GR-8 and CR-40 rubberized concrete,

similar trends on the modulus of elasticity were observed. Modulus of elasticity increased

with the increase in compressive strength. Further study showed that the type of rubber

particles influenced the change of modulus of elasticity. With the increase of compressive

strength, the modulus of elasticity of ground rubber increased slightly slower than that of

crushed rubberized concrete. Based on this study, the ACI equation could reasonably predict

the relationship between modulus of elasticity and compressive strength for rubberized

concrete; however, it underpredicted the modulus of elasticity of rubberized concrete.



5.4DYNAMIC MODULUS OF ELASTICITY

The values of dynamic modulus of elasticity (Ed) for rubberized concrete tested by pulse

velocities are shown in Fig. 9. As shown in the figure, the dynamic modulus of elasticity of

rubberized concrete decreased with the increase of rubber content. For ground rubber



concrete, the decrease in dynamic modulus was from 5.7 to 28.6% as the rubber amount

increased from 15 to 45%, while for crushed rubberized concrete, the decrease was

from 16.5 to 25.0% with the same amount of increase in rubber content. The crushed rubber

concrete introduced more reduction on dynamic modulus of elasticity of rubberized concrete

than ground rubber concrete.



5.5BRITTLENESS INDEX OF RUBBERISED CONCRETE

Fig. 10 shows seven typical figures of hysteresis loops of specimens. Among them Fig. 10(a)

is hysteresis loop of normal concrete; Figs. 10(b–d) are the loops of ground rubberized

concrete with the rubber content of 15, 30, and 45%, respectively; and Figs. 10(e, g, and f)

are the loops of crushed rubber with content of 15, 30, and 45%. The brittleness index

calculated from hysteresis loops for the normal concrete is and rubberized concrete is shown

in Fig. 11. Test results of compressive strength and calculated BI of rubberized concrete are

shown in Table 2. As indicated in the previous section, a lower brittleness index signified

higher ductility of a material. It can be observed from the results obtained that the BI

values of rubberized concrete were lower than that of normal concrete. The observation can

be explained by considering rubber as a good energy absorbing material and the rubberized

concrete absorbing more energy that lead to plastic deformation at the time of fracture. For

rubberized concrete, the highest BI value was 0.93, which happened on CR-40 concrete with

a rubber content of 15%, compared to the average BI value of 1.27 for normal concrete.

The results clearly showed that rubberized concrete had higher ductility performance than

that of normal concrete. Brittleness index of both CR-40 and GR-8 rubberized concrete

decreased linearly with varying rubber content. The more rubber particles, the higher ductility

rubberized concrete had. It is very obvious that BI values of CR-40 crushed rubberized

concrete decreased faster than that of GR-8 ground rubberized concrete. When the rubber

content changed from 15 to 45% for GR-8 rubberized concrete, the brittleness index

decreased from 0.70 to 0.59, with only a 15.7% reduction, while for CR-40 rubberized

concrete, with the rubber content varying from 15 to 45%, the BI value decreased from 0.93

to 0.46, which was a 50.5% reduction. Although, CR-40 rubberized concrete with 45%

rubber content yielded a lower BI value than GR-8 with the same rubber content,

brittleness index of GR-8 at 15 and 30% rubber contents were much lower than that of CR-40

at 15 and 30% rubber contents. In addition to the BI value, consideration has to be given to

the other mechanical properties of rubberized concrete. In general, use of ground rubber with

a rubber content of 15% is more efficient in getting a lower brittleness index.

6.CONCLUSIONS

Based on this study, the following conclusions can be drawn:



1. The compressive strength, the static modulus of elasticity, and the dynamic modulus of

elasticity of the rubberized concrete decreased considerably with the increasing amount of

rubber content. Crushed rubber caused greater reduction on these three material properties

than ground rubber. Rubberized concrete had a slightly lower cylinder /cube strength

ratio compared with normal concrete.

2. For both GR-8 and CR-40 rubberized concrete, similar trends were observed on the

relationship of modulus of elasticity versus compressive strength. With the increase of

compressive strength, the modulus of elasticity of ground rubber increased slightly slower

than that of crushed rubberized concrete. The ACI equation could reasonably predict the

modulus of elasticity for rubberized concrete and was the lower bound for the modulus of

elasticity of rubberized concrete.

3. BI values of rubberized concrete were lower than that of normal concrete, which signified

that rubberized concrete had a higher ductility performance than normal concrete.Brittleness

index of both CR-40 and GR-8 rubberized concrete decreased linearly with the increase of

rubber content. BI values of CR-40 rubberized concrete decreased faster than that of GR-8

rubberized concrete. Use of 15% rubber content in GR-8 rubberized concrete yielded a

desirable brittleness index. For CR-40 rubberized concrete, since the results clearly showed

that high rubber content would cause a dramatic reduction in the strength and modulus of

elasticity, the optimal content for crushed rubber should be less than 30% for satisfactory

strength and deformation properties. In general, use of ground rubber is more efficient in

decreasing brittleness index.

4. The relationship between BI value and compressive strength revealed that the brittleness

index of GR-8 increased at a slower rate than that of CR-40 with the increase of compressive

strength. At the same stress level, the brittleness index of GR-8 was lower than that of CR-40.

7.REFERENCES

1. L Zheng, X. Sharon Huo and Y.Yuan(2008): “Strength, Modulus of Elasticity and

Brittleness Index of Rubberized Concrete” ,ASCE.

2. A.Turatsinze, S. Bonnet, L. Granju(2005): “ Potential of rubber aggregates to modify

properties of cement based-mortars: Improvement in cracking shrinkage resistance.


Documents

Seminar Rubber is Ed Concrete