Cementitious Composites Containing Recycled Tire Rubber: An Overview of Engineering Properties and Potential Applications

Content:

Moncef Nehdi1 and Ashfaq Khan2

Cementitious Composites Containing Recycled
Tire Rubber: An Overview of Engineering
Properties and Potential Applications

pose a potential environmental threat, but also are fire hazards and
provide breeding grounds for mosquitoes (Tantala et al. 1996).
The practice of disposing of scrap tires in landfills is becoming
unacceptable because of the rapid depletion of available sites for
waste disposal. Moreover, tires can even “rise from the grave”—
floating upward through a sea of trash to break through landfill
covers—sometimes with explosive force (Tantala et al. 1996). Innovative solutions to cope with the tire disposal problem have long
been in development. Among the most promising alternatives are:
reuse of ground tire rubber in a variety of rubber and plastic products, thermal incineration of worn-out tires for the production of
steam or electricity, and use of tire rubber in asphalt mixes. In addition, waste tires can be used as fuel for cement kilns, as feedstock
for producing carbon black, and as reefs in marine environments
(Paul 1985; Takallou and Takallou 1991; O’Keefe 1984). Because
of the high capital investment involved, using tires as fuel is technically feasible but economically unattractive (O’Keefe 1984; Lee
1995). The use of rubber tires in the production of carbon black
eliminates shredding and grinding costs, but the carbon black from
tire pyrolysis is more expensive and has lower quality than that
from petroleum oils (Paul 1985). Unfortunately, the generation of
waste tires far exceeds its current uses. In addition, environmental
concerns and public resistance have greatly impeded the option of
incinerating waste tires. Although an economically attractive solution, the negative impact on the environment caused by tire incineration makes this alternative a compromise at best.
Early studies on the use of worn-out tires in asphalt mixes were
very promising. They showed that rubberized asphalt had better
skid resistance, reduced fatigue cracking, and achieved longer
pavement life than conventional asphalt (Adams et al. 1985; Esch
1984; Estakhri 1990; Khosla and Trogdon 1990). However, the initial cost of rubberized asphalt is 40 to 100% higher than that of conventional asphalt, and its long-term benefits are uncertain (Fedroff
et al. 1996). Likewise, the asphalt industry can currently absorb
only 30 to 40% of the scrap tires generated (Anonymous 1993).
Moreover, when pavements incorporating these materials are
themselves recycled, disposal of the embedded rubber could itself
become a serious environmental hazard (Fedroff et al. 1996).
Although the use of recycled tire rubber in asphalt pavements
was emphasized in several publications, not much attention has
been given to the use of rubber from scrap tires in portland cement
concrete (PCC) mixtures, particularly for highway applications.
However, large benefits can result from the use of worn-out tire
rubber in PCC mixtures, especially in circumstances where properties like lower density, increased toughness and ductility, higher
impact resistance, and more efficient heat and sound insulation are

REFERENCE: Nehdi, M. and Khan, A., “Cementitious Composites Containing Recycled Tire Rubber: An Overview of Engineering Properties and Potential Applications,” Cement, Concrete, and Aggregates, CCAGDP, Vol. 23, No. 1, June 2001, pp.
3–10.
ABSTRACT: One of the major environmental challenges facing
municipalities around the world is the disposal of worn out automobile tires. To address this global problem, several studies have
been conducted to examine various applications of recycled tire
rubber (fine crumb rubber and coarse tire chips). Examples include
the reuse of ground tire rubber in a variety of rubber and plastic
products, thermal incineration of waste tires for the production of
electricity or as fuel for cement kilns, and use of recycled rubber
chips in asphalt concrete. Unfortunately, generation of waste tires
far exceeds these uses. This paper emphasizes another technically
and economically attractive option, which is the use of recycled tire
rubber in portland cement concrete. Preliminary studies show that
workable rubberized portland cement concrete (rubcrete) mixtures
can be made provided that appropriate percentages of tire rubber are
used in such mixtures. Achievements in this area are examined in
this paper, with special focus on engineering properties of rubcrete
mixtures. These include: workability, compressive strength, splittensile strength, flexural strength, elastic modulus, Poisson’s ratio,
toughness, impact resistance, sound and heat insulation, and freezing and thawing resistance. The benefits of using magnesium oxychloride cement as a binder for rubberized concrete mixtures are
discussed. Various applications in which rubcrete could be advantageous over conventional concrete are described.
KEYWORDS: compressive strength, freezing and thawing, recycling, rubcrete, solid waste, tire rubber, toughness, workability

Solid waste management has been a major environmental concern in cities around the globe. Recent studies indicate that roughly
4.6 billion tons of nonhazardous solid waste materials are produced
annually in the United States (Epps 1994; Amirkhanian and Manugian 1994; Collins and Ciesielski 1994), among which waste tires
constitute a significant portion. Indeed, there are more than 240
million scrap tires (200 million passenger tires and 40 million truck
tires; 2.1 million tons and 1.9 million tons, respectively) generated
each year in the United States alone (Epps 1994). In addition, about
3 billion used tires are currently stockpiled throughout the country
(SHR 1992). These stockpiles are dangerous not only because they
1
Assistant professor, Department of Civil & Environmental Engineering,
The University of Western Ontario, 1151 Richmond Street, London, Ontario,
N6A 5B9, Canada.
2
Graduate research assistant, Department of Civil & Environmental Engineering, The University of Western Ontario, 1151 Richmond Street, London,
Ontario, N6A 5B9, Canada.

© 2001 by the American Society for Testing and Materials
3

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CEMENT, CONCRETE, AND AGGREGATES

desired. The use of recycled tire rubber in PCC mixtures would not
only make good use of an otherwise waste material and help alleviate disposal problems, but can also improve certain properties of
concrete for particular design applications. It would also address
the growing public concern about the need to preserve natural resources (such as aggregates) used in the production of concrete that
are depleting rapidly due to excessive quarrying. The scope of this
paper is to present a critical overview of results obtained in this area
with special focus on engineering properties of rubberized portland
cement concrete mixtures and their potential applications.
Properties of Fresh Concrete
Slump
Khatib and Bayomy (1999) investigated the workability of
rubcrete mixtures. They observed a decrease in slump with increased rubber content by total aggregate volume (Fig. 1a). Their
results show that at rubber contents of 40% by total aggregate vol-

ume, the slump was near zero and the concrete was not workable
by hand. Such mixtures had to be compacted using a mechanical vibrator. Mixtures containing fine crumb rubber were, however,
more workable than mixtures containing either coarse tire chips or
a combination of crumb rubber and tire chips. In another study conducted by Raghavan et al. (1998), it was found that mortars containing rubber shreds achieved workability comparable to or better
than a control mortar without rubber particles. It is not clear, however, whether the effect of rubber particles on the workability of
concrete is attributed to a reduction in the density of concrete or to
actual changes in the yield value and plastic viscosity of the mixture. Rheological measurements using fundamental techniques
(e.g., rheometers) rather than the highly empirical slump test are
therefore needed to clarify the effect of the rubber-aggregate content and particle size distribution on the rheology of fresh concrete.
Unit Weight
Due to the low specific gravity of rubber, the unit weight of
rubcrete mixtures decreases as the percentage of rubber increases
(Fig. 1b). In addition, the increase in rubber content increases the
air content (see section below), which in turn further reduces the
unit weight (Fedroff 1995). However, the decrease is almost negligible for rubber contents lower than 10 to 20% of the total aggregate volume. Figure 1b shows that data of unit weight versus rubber addition for rubberized concrete fits a straight-line curve when
fine crumb rubber, coarse tire chips, or a combination of these is
used as fine and/or coarse aggregate replacement in concrete.
Air Content
According to Fedroff et al. (1996), and Khatib and Bayomy
(1999), the air content increased in rubcrete mixtures with increased amounts of ground tire rubber (Fig. 1c). Although no airentraining agent (AEA) was used in rubcrete mixtures, higher air
contents were measured as compared to control mixtures made
with an AEA (Fedroff et al. 1996). The higher air content of
rubcrete mixtures may be due to the nonpolar nature of rubber particles and their ability to entrap air in their jagged surface texture.
When the nonpolar rubber is added to the concrete mixture, it may
attract air as it repels water. The air may adhere to the rubber particles or perhaps gets trapped in their jagged texture. Therefore, increasing the rubber content results in higher air contents of rubcrete
mixtures (Fedroff 1995). When a mixture of rubber, sand, and water was placed in a roll-a-meter, a large portion of the rubber floated
to the top of the meter (Fedroff et al. 1996). Since rubber has a specific gravity of 1.14, it is expected to sink rather than float. However, if air gets trapped in the jagged surface of the rubber particles,
it could cause them to float, which supports the theory discussed
above.
Plastic Shrinkage

FIG. 1—Effect of rubber type and rubber content on properties of fresh
concrete (after Khatib and Bayomy 1999).

Preliminary results of a study conducted by Raghavan et al.
(1998) suggest that the addition of rubber shreds to mortar reduced
plastic shrinkage cracking compared to a control mortar. The use of
rubber shreds in mortar allowed multiple cracking to occur over the
width of mortar specimens compared to a single crack in a mortar
specimen without rubber shreds. In spite of the occurrence of multiple cracking, the total crack area in the case of the rubber-filled
mortar decreased with an increase in the rubber mass fraction. Despite their apparently weak bonding to the cement paste, rubber
shreds provided sufficient restraint to prevent microcracks from

NEHDI AND KHAN ON RECYCLED TIRE RUBBER

5

propagating. It was observed (Raghaven et al. 1998) that the control mortar specimen developed a crack having an average width of
about 0.9 mm, while the average crack width for specimens with a
mass fraction of 5% rubber shreds was about 0.4 to 0.6 mm. It was
also found that the onset time of cracking was delayed by the addition of rubber shreds; the mortar without rubber shreds cracked
within 30 min, while the mortar with a mass fraction of 15% rubber shreds cracked after 1 h. The higher the content of rubber
shreds, the smaller the crack length and crack width, and the more
the onset time of cracking was delayed. Although additional studies are necessary to confirm these observations, it appears that the
addition of rubber shreds could be beneficial for reducing plastic
shrinkage cracks of mortar and probably of concrete.
Mechanical Strength
The compressive strength of rubberized concretes was studied
using different sizes and shapes of specimens. Cylindrical specimens of 75, 100, or 150 mm in diameter were used by Rostami et
al. (1993), Ali et al. (1993), and Eldin and Senouci (1993), respectively. Topcu (1995) used both 150 mm diameter cylinders and 150
mm cubes. The compressive strength of ordinary concrete obtained
from cube tests is higher than that obtained from cylinder tests
(Neville 1997). Indeed, standards such as the European ENV-206
1992 include tables of equivalence of strengths for the two types of
specimens. However, Topcu’s (1995) results for rubberized concretes unexpectedly indicated the reverse. This discrepancy remains to be explained.
Results of various studies indicate that the mechanical strength
of rubcrete mixtures is greatly affected by the size, proportion, and
surface texture of rubber particles, and the type of cement used in
such mixtures. The effect of these parameters is discussed below.

FIG. 2—Effect of replacement of coarse aggregate by tire chips on (a)
compressive strength, and (b) split tensile strength at 7 and 28 days (after
Eldiu and Senouci 1993).

Effect of Rubber Content and Particle Size
Various published results show that coarse grading of rubber
granules lowered the compressive strength of rubcrete mixtures
more than fine grading. For instance, results obtained by Eldin and
Senouci (1993) indicate that there was about 85% reduction in
compressive strength and 50% reduction in tensile strength when
the coarse aggregate was fully replaced by coarse rubber chips
(Fig. 2a,b). However, specimens lost up to 65% of their compressive strength and up to 50% of their tensile strength when the fine
aggregate was fully replaced by fine crumb rubber (Fig. 3a,b).
Topcu (1995), and Khatib and Bayomy (1999) also showed that the
addition of coarse rubber chips in concrete lowered the compressive strength more than the addition of fine crumb rubber. However, results of tests carried out by Ali et al. (1993), and Fatuhi and
Clark (1996) indicate the opposite trend. All results (Khatib and
Bayomy 1999; Ali et al. 1993; Eldin and Senouci 1993; Fatuhi and
Clark 1996) show that the greater the rubber content used in
rubcrete mixtures, the lower the compressive and tensile strengths
achieved (Figs. 2 and 3).
Effect of Surface Texture of Rubber Particles
Various studies show that the rougher the rubber particles used
in concrete mixtures the better the bonding they develop with the
surrounding matrix and, therefore, the higher the compressive
strength achieved. For instance, Tantala et al. (1996) argued that if
the bond between rubber particles and the surrounding cement
paste is improved, then significantly higher compressive strength
rubcrete mixtures could be obtained (Fig. 4). To achieve enhanced

FIG. 3—Effect of replacement of fine aggregate by crumb rubber on (a)
compressive strength, and (b) split tensile strength at 7 and 28 days (after
Eldin and Senouci 1993).

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CEMENT, CONCRETE, AND AGGREGATES

gregate. The effects of using blended cements, fiber reinforcement,
chemical admixtures, polymer resins, and other additives in
rubcrete remain to be investigated.
Mechanisms of Strength Reduction

FIG. 4—Effect of washing rubber particles with water on compressive
strength of rubcrete mixtures (after Tantala et al. 1996).

adhesion, it is necessary to pretreat the rubber. Pretreatments vary
from merely washing rubber particles with water to acid etching,
plasma pretreatment, and various coupling agents (Tantala et al.
1996). The acid pretreatment involves soaking the rubber particles
in an acid solution for 5 min and then rinsing it with water. This increases the strength of rubcrete mixtures through a microscopic increase in surface roughness of the rubber, which improves its attachment to the cement paste. Eldin and Senouci (1993) soaked and
thoroughly washed rubber aggregates with water in order to remove any contaminants, while Rostami et al. (1993) attempted to
clean the rubber using water, water and carbon tetrachloride (CCl4)
solvent, and water and a latex admixture cleaner. Results show that
concrete containing washed rubber particles achieved about 16%
higher compressive strength than concrete containing untreated
rubber aggregates. A much larger improvement in compressive
strength (about 57%) was obtained when rubber aggregates treated
with CCl4 were used.

Khatib and Bayomy (1999) found that the 28-day compressive
strength of rubcrete mixtures was reduced by about 93% when
100% of the coarse aggregate volume was replaced by rubber, and
by 90% when 100% of the fine aggregate volume was replaced by
rubber. They hypothesized that there are three major causes for this
strength reduction. First, because rubber is much softer than the
surrounding cement paste, upon loading, cracks are initiated
quickly around the rubber particles due to this elastic mismatch,
which propagate to bring about failure of the rubber-cement matrix. Second, due to weak bonding between the rubber particles and
the cement paste, soft rubber particles may be viewed as voids in
the concrete mix. The assumed increase in the void content would
certainly cause a reduction in strength. The third possible reason
for the reduction in strength is that the strength of concrete depends
greatly on the density, size, and hardness of the coarse aggregate
(Mehta and Monteiro 1993). Because aggregates are partially replaced with relatively weaker rubber, a reduction in strength is anticipated. It was also found (Khatib and Bayomy 1999) that the
flexural strength of rubcrete mixtures decreased with an increase in
the rubber content in a fashion similar to that observed for compressive strength, perhaps due to similar mechanisms.

Effect of Using Special Cements
A study conducted by Biel and Lee (1996) suggests that the type
of cement used in rubcrete mixtures greatly affects the mechanical
strength. Recycled tire rubber particles were used in concrete mixtures made with both magnesium oxychloride cement and portland
cement. The percentage of fine aggregate substitution ranged from
0 to 90%, increasing by 15% for each set. It was observed that 90%
loss of the compressive strength occurred for both the portland cement rubber concrete (PCRC) and magnesium oxychloride cement
rubber concrete (MOCRC) when 90% of the fine aggregate (25%
of the total aggregate) was replaced by rubber (Fig. 5a). Whether
with or without rubber inclusion, the magnesium oxychloride cement concrete exhibited approximately 2.5 times the compressive
strength of the portland cement concrete (Fig. 5a). The portland cement concrete samples containing 25% of rubber by total aggregate
volume retained 20% of their splitting tensile strength after initial
failure, whereas the magnesium oxychloride cement concrete
samples with similar rubber content retained 34% of their splitting
tensile strength after initial failure. The ratio of the MOCRC tensile
strength to PCRC tensile strength rose from 1.6 to 2.8 with increased amounts of rubber. It was argued (Beil and Lee 1996) that
the high-strength and bonding characteristics provided by magnesium oxychloride cement greatly improved the performance of
rubcrete mixtures and that structural applications could be possible
if the rubber content is limited to 17% by total volume of the ag-

FIG. 5—Effect of cement type on (a) compressive strength, and (b) split
tensile strength (after Beil and Lee 1996).

NEHDI AND KHAN ON RECYCLED TIRE RUBBER

Toughness and Failure Mode
Although the reduction in strength of rubcrete mixtures may
limit their use in some structural applications, one can rather appreciate their future potential in their enhanced toughness and
failure mode. Eldin and Senouci (993) showed that when loaded
in compression, specimens containing rubber did not exhibit brittle failure. A more gradual failure was observed, either of a splitting (for coarse tire chips) or a shear mode (for fine crumb rubber). It was argued that since the cement paste is much weaker in
tension than in compression, the rubcrete specimen containing
coarse tire chips would start failing in tension before it reaches its
compression limit. The generated tensile stress concentrations at
the top and bottom of the rubber aggregates (Fig. 6a) result in
many tensile microcracks that form along the tested specimen
(Fig. 6b). These microcracks will rapidly propagate in the cement
paste until they encounter a rubber aggregate. Because of their
ability to withstand large tensile deformations, the rubber particles will act as springs (Fig. 6c), delaying the widening of cracks
and preventing full disintegration of the concrete mass. The continuous application of the compressive load will cause generation
of more cracks as well as widening of existing ones. During this
process, the failing specimen is capable of absorbing significant
plastic energy and withstanding large deformations without full
disintegration. This process will continue until the stresses overcome the bond between the cement paste and the rubber aggregates.
Biel and Lee (1996) reported that the failure of plain concrete
cylinders resulted in explosive conical separations of cylinders,
leaving the specimens in several pieces. As the amount of rubber in
concrete was increased, the severity and explosiveness of the failures decreased. Failure of concrete specimens with 30, 45, and 60%
replacement of fine aggregate with rubber particles occurred as a
gradual shear that resulted in a diagonal failure plane. The cylinders did not separate and continued to sustain load after the initial
failure. Upon release of the load, the cylinders rebounded back to
near their original shape. The samples containing 75 and 90% fine
aggregate substitution with rubber failed through a gradual compression that appeared like a true crushing, resulting in a post failure material that was sponge-like and elastic in nature.

7

In another experimental study conducted by Goulias and Ali
(1997), it was found that the dynamic moduli of elasticity and
rigidity decreased with an increase in the rubber content, indicating
that a less stiff and less brittle material was obtained. The damping
capacity of concrete (a measure of the ability of the material to
decrease the amplitude of free vibrations in its body) seemed to decrease with an increase in the rubber content. Conversely, Topcu
and Avcular (1997a), and Fatuhi and Clark (1996) recommended
using rubberized concretes in circumstances where vibration
damping is required, such as in buildings as an earthquake shockwave absorber, in foundation pads for machinery, and in railway
stations. Results of Poisson’s ratio measurements indicated that
cylinders with 20% rubber had a larger ratio of lateral strain to the
corresponding axial strain than that of 30% rubber concrete cylinders (Goulias and Ali 1997a). It was also found (Goulias and Ali
1997) that the higher the rubber content, the higher the ratio of the
dynamic modulus of elasticity to the static modulus of elasticity.
The dynamic modulus was then related to compressive strength,
providing a high degree of correlation between the two parameters.
This suggests that nondestructive measurements of the dynamic
modulus of elasticity may be used for estimating the compressive
strength of rubcrete. A good correlation between compressive
strength and the damping coefficient calculated from transverse
frequency was also found, indicating that the damping coefficient
of rubcrete may likewise be used for predicting the compressive
strength.
Khatib and Bayomy (1999) observed that as the rubber content
increased, rubcrete specimens tended to fail gradually in either a
conical or columnar shape failure mode. The samples sustained
much higher deformations than the control mix without rubber.
With a rubber content of 60% by total aggregate volume (fine
and/or coarse), the samples exhibited significant elastic deformation, which was retained upon unloading. Thus, flexibility and ability to deform at peak load were increased significantly by rubber
addition. Experimental results of Schimizze et al. (1994) showed
that the elastic modulus of a concrete mixture containing coarse
rubber granules replacing 100% of the coarse aggregate volume
was reduced to about 72% of that of the control mixture, whereas
for a concrete containing fine rubber granules replacing 100% of
the fine aggregate volume, the elastic modulus was reduced to

FIG. 6—Modeling the behavior of rubcrete specimens under compression (after Eldin and Senouci 1993).

8

CEMENT, CONCRETE, AND AGGREGATES

about 47% of that of the control mixture. The reduction in the elastic modulus indicates higher flexibility, which may be viewed as a
positive gain in rubcrete mixtures that could be used in stabilized
base layers of flexible pavements. Tantala et al. (1996) conducted
a comparative study of the toughness of a control concrete mixture
and rubcrete mixtures with 5 and 10% buff rubber by volume of
coarse aggregate. It was found that the toughness of both rubcrete
mixtures was higher than that of the ordinary concrete mixture.
However, the toughness of the rubcrete mixture with 10% buff
rubber was lower than that of the rubcrete mixture with 5% buff
rubber because of the decreasing ultimate compressive strength. It
was also found (Tantala et al. 1996) that acid etching of rubber particles replacing the coarse aggregate lowered the toughness of
rubcrete mixtures. Results by Topcu and Ozcelikors (1991) show
that 10% rubber-chip addition increased the toughness of concrete
by 23%.
Raghavan et al. (1998) conducted an experimental study on the
use of rubber shreds and granular rubber in mortar. They found that
mortar specimens with rubber shreds were able to withstand additional load after peak load. The specimens did not physically separate into two pieces under flexural loading because of bridging of
cracks by rubber shreds. However, specimens with granular rubber
broke into two pieces when the peak load was attained. Therefore,
the post-crack strength seemed to improve when rubber shreds
were used instead of granular rubber.
Impact Resistance, Heat and Sound Insulation
According to Topcu and Avcular (1997b), the impact resistance
of concrete increased when rubber aggregates were added to the
mixture. It was argued that this increased resistance was derived
from an increased ability of the material to absorb energy and
insulate sound during impact (Eldin and Senouci 1993;
Topcu 1995; Rad 1976; Acar 1987). The increase became more
prominent in concrete samples containing larger-size rubber
aggregates.
It was expected that acoustical tests would substantiate the applicability of rubcrete mixtures for roadway sound barriers to reduce the effects of acoustic emissions (Tantala et al. 1996). Wisconsin and Pennsylvania Departments of Transportation (DOTs)
have studied the noise-absorption properties of whole rubber tires
as sound barriers with moderate success (Tantala et al. 1996). More
research is required to study the sound insulation effects of rubcrete
in buildings and other structures.
Rubber inclusion in concrete also makes the material a better
thermal insulator, which could be very useful especially in the
wake of energy conservation requirements (Tantala et al. 1996).
However, no pilot projects to take advantage of this property
in practice are available in the open literature. Also, fire tests
(Topcu and Avcular 1997a) indicated that the flammability of
rubber in rubcrete mixtures (if any) was much reduced by the
presence of cement and aggregates. Although more testing is
needed, it is believed that the fire resistance of rubcrete mixtures
is satisfactory.
Freezing and Thawing Resistance
Savas et al. (1996) investigated the freezing and thawing durability of rubcrete. Various mixtures were obtained by adding 10,
15, 20, and 30% ground rubber by weight of cement to the control
concrete mixture. Freezing and thawing tests in accordance with
ASTM C 666, Procedure A, Test Method for Resistance of Con-

crete to Rapid Freezing and Thawing were conducted on the various mixtures. The following conclusions were drawn:
• As the percentage of mechanically ground waste tire rubber in
concrete was increased, the freezing and thawing durability
was decreased. Rubcrete mixtures with 10 and 15% ground
tire rubber by weight of cement exhibited durability factors
higher than 60% after 300 freezing and thawing cycles. However, mixtures with 20 and 30% ground tire rubber by weight
of cement did not meet this minimum acceptable limit set forth
by the ASTM standard.
• For rubcrete mixtures with 10, 20, and 30% ground tire rubber,
air entrainment did not provide significant improvements in
freezing and thawing durability.
• During freezing and thawing tests, scaling (as measured by the
reduction in weight) increased with the increase in the number
of freezing and thawing cycles and amount of ground rubber
in concrete.
A target air content of 5 to 7% is often selected to provide adequate freezing and thawing resistance for ordinary concrete mixtures. However, it was found that rubcrete mixtures with compressive strength lower than 28 MPa (4000 psi) are not considered
resistant to freezing and thawing whether they are air-entrained or
not (ACI 1991). It should be noted that although rubcrete mixtures
usually have high air contents, the large-size and nonuniform distribution of trapped air voids might be a possible reason for their
lack of freezing and thawing resistance, especially for mixtures
with high contents of rubber (Topcu and Avcular 1997b).
Potential Applications of Rubcrete
The unique qualities of rubcrete (e.g., light unit weight) suggest
that it may be suitable for architectural applications such as nailing
concrete, false facades, stone backing, and interior construction. It
is also anticipated that rubcrete will find new areas of usage in
highway construction as a shock absorber, in sound barriers as a
sound absorber, and also in buildings as an earthquake shock-wave
absorber (Topcu and Avcular 1997a). However, more research is
required before such recommendations can be made.
Fatuhi and Clark (1996) suggested various interesting applications where cement-based materials containing rubber could possibly be used. These include areas:
• Where vibration damping is required, such as in foundation
pads for machinery, and in railway stations.
• Where resistance to impact or explosion is required, such as in
jersey barriers, railway buffers, and bunkers.
• For trench filling and pipe bedding, in artificial reef construction, for pile heads and paving slabs.
The present authors are currently conducting research exploring
new areas of application for rubcrete mixtures, which potentially
can use large volumes of this material. Focus is particularly on
emerging geotechnical construction methods in which buffer layers of a low-strength high-ductility material are required to absorb
excessive delayed deformations.
Concluding Remarks
Although a rubcrete mixture generally has a reduced compressive strength that may limit its use in certain structural applications,

NEHDI AND KHAN ON RECYCLED TIRE RUBBER

it possesses a number of desirable properties, such as lower density, higher toughness, higher impact resistance, enhanced ductility, and more efficient sound and heat insulation compared to conventional concrete. Such engineering properties are advantageous
for various construction applications. If rubcrete is used to its potential in projects that require these unique attributes, this can contribute to alleviating the exacerbated solid waste disposal problems
associated with worn out rubber tires. However, this requires a
paradigm shift from the traditional misconception that the most important property of concrete is its compressive strength to a design
by function approach, which will allow rubcrete to be viewed as a
preferred solution over conventional concrete for particular
projects.
Structural applications involving rubcrete may still be possible if
appropriate percentages of rubber aggregates are used. It is also
possible to produce relatively high-strength rubcrete mixtures by
using magnesium oxychloride cement, which achieves better bonding characteristics to rubber and greatly improves the performance
of rubcrete mixtures. In addition, the adhesion between the rubber
particles and other constituents of the rubcrete matrix can be improved by pretreating the rubber aggregates. For instance, acid
etching of rubber granules improves compressive strength
markedly. However, further research is still needed to optimize the
effect of the percentage of rubber, particle size distribution, type of
cement, pretreatment method of rubber particles, and use of chemical and mineral admixtures on the properties of rubcrete.
Rubcrete mixtures usually absorb significant plastic energy and
undergo relatively large deformations without full disintegration.
This property can be utilized in various structural and geotechnical
projects in which the deformation at peak load is a primary design
concern. Using rubcrete as a flexible sub-base for pavements, as
pipe bedding, for tunnel linings, and other major construction work
has the potential to make good use of the billions of worn-out tires
stockpiled worldwide. However, further studies are needed before
one can draw final recommendations and set forth design guidelines for such applications.
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9

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