Properties of concrete containing scrap-tire rubber - an overview
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Waste Management 24 (2004) 563–569
www.elsevier.com/locate/wasman
Properties of concrete containing scrap-tire rubber – an overview
Rafat Siddique, Tarun R. Naik *
Department of Civil Engineering and Mechanics, College of Engineering and Applied Science, UWM Center for By-Products Utilization,
University of Wisconsin–Milwaukee, P.O. Box 784, Milwaukee, WI 53201, USA
Accepted 2 January 2004
Abstract
Solid waste management is one of the major environmental concerns in the United States. Over 5 billion tons of non-hazardous
solid waste materials are generated in USA each year. Of these, more than 270 million scrap-tires (approximately 3.6 million tons)
are generated each year. In addition to this, about 300 million scrap-tires have been stockpiled. Several studies have been carried out
to reuse scrap-tires in a variety of rubber and plastic products, incineration for production of electricity, or as fuel for cement kilns,
as well as in asphalt concrete.
Studies show that workable rubberized concrete mixtures can be made with scrap-tire rubber. This paper presents an overview of
some of the research published regarding the use of scrap-tires in portland cement concrete. The benefits of using magnesium
oxychloride cement as a binder for rubberized concrete mixtures are also presented. The paper details the likely uses of rubberized
concrete.
Ó 2004 Published by Elsevier Ltd.
1. Introduction meet the challenge of tire disposal problem have long
been in development. The promising options are:
More than 270 million scrap-tires are produced in (1) use of tire rubber in asphaltic concrete mixtures;
United States each year (Rubber ManufacturerÕs As- (2) incineration of tires for the production of steam and
sociation, 2000). In addition to this, more than 300 (3) reuse of ground tire rubber in a number of plastic
million tires are currently stockpiled throughout the and rubber products. In addition, scrap-tires have been
United States (Rubber ManufacturerÕs Association, used as a fuel for cement kiln, as feedstock for making
2000). These stockpiles are dangerous not only due to carbon black, and as artificial reefs in marine envi-
potential environmental threat, but also from fire haz- ronment (Paul, 1985). Because of high capital invest-
ards and provide breeding grounds for rats, mice, ver- ment involved in it, using tires as a fuel is technically
mines and mosquitoes (Naik and Singh, 1991; Singh, feasible but economically not very attractive. The use of
1993). Over the years, disposal of tires has become one rubber-tires in making carbon black eliminates shred-
of the serious problems in environments. Landfilling is ding and grinding costs, but carbon black from tire
becoming unacceptable because of the rapid depletion pyrolysis is more expensive and has lower quality than
of available sites for waste disposal. For example that from petroleum oils (Paul, 1985). Table 1 shows
France, which produces over 10 million scrap-tires per some of the facts and figures published in 2000, as re-
year, will have a dwindling supply of landfills starting ported by the Rubber ManufacturerÕs Association
from July 2002, due to a new law that forbids any new (2000).
landfill in the country. Used tires are required to be A tire is a composite of complex elastomer formula-
shredded before landfilling. Innovative solutions to tions, fibers and steel/fiber cord. Tires are made of plies
of reinforcing cords extending transversely from bead to
bead, on top of which is a belt located below the thread.
*
Corresponding author. Fax: +1-414-229-6958. Table 2 lists typical types of materials used to manu-
E-mail address: tarun@uwm.edu (T.R. Naik). facture tires.
0956-053X/$ - see front matter Ó 2004 Published by Elsevier Ltd.
doi:10.1016/j.wasman.2004.01.006564 R. Siddique, T.R. Naik / Waste Management 24 (2004) 563–569
Table 1 Table 3
Some facts and figure concerning tires in USA (Rubber Manufac- Typical composition of manufactured tires by weight (Rubber Man-
turerÕs Association, 2000) ufacturerÕs Association, 2000)
Facts Figures Composition (wt%) Automobile Truck
tire tire
Number of scrap-tires generated annually 270 million
Approximate weight of scrap-tires 3.6 million tons Natural rubber 14 27
Number of scrap-tires in stock piles 300 million Synthetic rubber 27 14
Number of tires processing facilities 498 Carbon black 28 28
Scrap-tires used in civil engineering applications 30 million Steel 14–15 14–15
Scrap-tires processed into ground rubber 18 million Fabric, filler, accelerators and 16–17 16–17
Scrap-tires used for fuel 125 million antiozonants
Number of states with scrap-tires legislation/ 48
regulations
Number of states that ban whole tires from 33
landfills
2.3. Shredded/chipped tires
Number of states that ban all scrap-tires from 12
landfills Tire shreds or chips involve primary, secondary or
Number of states with no landfill restrictions 5 both shredding operations. The size of the tire shreds
produced in the primary shredding process can vary
from as large as 300 to 460 mm (12–18 in.) long by 100–
Table 2 230 mm (4–9 in.) wide, down to as small as 100–150 mm
Typical materials used in manufacturing tire (Rubber ManufacturerÕs (4–6 in.) in length, depending on the manufacturerÕs
Association, 2000) shredder model and the condition of the cutting edges.
1. Synthetic rubber Production of tire chips, normally sized from 76 (3 in.)
2. Natural rubber to 13 mm (0.5 in.), requires both primary and secondary
3. Sulfur and sulfur compounds shredding to achieve adequate volume (quantity) re-
4. Phenolic resin
5. Oil
duction (Read et al., 1991).
(i) Aromatic
(ii) Naphthenic 2.4. Ground rubber
(iii) Paraffinic
6. Fabric Ground rubber for commercial applications may be
(i) Polyester
(ii) Nylon
nominally sized as large as 19 mm (or 3/4 in.) to as small
7. Petroleum waxes as 0.15 mm (No. 100 sieve). It depends upon the type of
8. Pigments size reduction equipment and intended applications. The
(i) Zinc oxide processed used tires in ground rubber applications are
(ii) Titanium dioxide typically subjected to two stages of magnetic separation
9. Carbon black
10. Fatty acids
and to screening. Various size fractions of rubber are
11. Inert materials recovered (Heitzman, 1992). Some processes/markets
12. Steel wires term 30 mesh rubber as crumb rubber.
2.5. Crumb rubber
2. Classification of scrap-tires
Crumb rubber consists of particles ranging in size
2.1. Scrap-tires from 4.75 (No. 4 Sieve) to less than 0.075 mm (No. 200
Sieve). Generally, the following methods are used to
They can be managed as a whole tire, as slit tire, as convert scrap-tires into crumb rubber. These methods
shredded or chopped tire, as ground rubber or as a are: (i) cracker mill process, (ii) granular process and
crumb rubber product. A typical automobile tire weighs (iii) micro-mill process. The cracker mill process tears
20 lb, whereas a truck tire weighs about 100 lb. Table 3 apart or reduces the size of tire rubber by passing the
gives the major materials used to manufacture tires by material between rotating corrugated steel drums. This
percentage of total weight of the finished tire that each process produces an irregularly shaped torn particles
material represents. having large surface area. The size of these particles
varies from 5 to 0.5 mm (No. 4–No. 40 Sieve) and is
2.2. Slit-tires commonly known as crumb rubber.
Granular process shears apart the rubber with re-
These are produced in tire cutting machines. These volving steel plates, producing granulated crumb rubber
machines can slit the tire into two halves or can separate particles, ranging in size from 9.5 (3/8 in.) to 0.5 mm
the sidewalls from the thread of the tires. (No. 40 Sieve) (Heitzman, 1992).R. Siddique, T.R. Naik / Waste Management 24 (2004) 563–569 565
3. Management option in slump with increase in rubber content as a percentage
of total aggregate volume. They further noted that at
3.1. Disposal rubber contents of 40%, slump was almost zero and
concrete was not workable manually. It was also ob-
Disposing of scrap-tires in landfills is becoming un- served that mixtures made with fine crumb rubber were
acceptable because of rapid depletion of available sites more workable than those with coarse tire chips or a
for waste disposal. Approximately 45% of 270 million combination of tire chips and crumb rubber.
tires are disposed off in landfills, stockpiles or illegal
dumps. As of the year 2000, 48 states of the 50 United 4.1.2. Air content
States have legislation/regulations related to landfilling. Fedroff et al. (1996) have reported higher air content
Thirty three states ban whole tires from disposal in in rubcrete mixtures than control mixtures even without
landfills. Twelve states ban all forms of scrap-tires from the use of air-entraining admixture (AEA). Similar ob-
being disposed off in landfills. Five states have no servations were also made by Khatib and Bayomy
landfill restrictions on disposal of tires. Thirty states (1999). This may be due to the non-polar nature of
charge for landfilling tires, whereas seven states allow rubber particles and their tendency to entrap air in their
monofills (Rubber ManufacturerÕs Association, 2000). rough surfaces. Also when rubber is added to a concrete
mixture, it may attract air as it has the tendency to repel
3.2. Recycling water, and then air may adhere to the rubber particles.
Therefore, increasing the rubber content results in
According to the data available for the year 2000, higher air contents in rubcrete mixtures, thereby de-
about 15 million of the 270 million scrap-tires generated creasing the unit weight of the mixtures.
yearly are exported, 10 million are recycled into new
products, 20 million are processed into ground rubber, 4.1.3. Unit weight
125 million are used as tired-derived fuel and 30 million Because of low specific gravity of rubber particles,
in civil engineering applications (Rubber Manufac- unit weight of mixtures containing rubber decreases with
turerÕs Association, 2000). Ground rubber is used in the increase in the percentage of rubber content.
making rubber products such as floor mats, carpet Moreover, increase in rubber content increases the air
padding, vehicles mudguards, plastic products and as a content, which in turn reduces the unit weight of the
fine aggregate addition (dry process) in asphalt courses. mixtures. The decrease in unit weight of rubcrete is
Crumb rubber is also used as an asphalt binder modifier negligible when rubber content is lower than 10–20% of
(wet process) in hot mix asphalt pavements (Naik et al., the total aggregate volume (Khatib and Bayomy, 1999).
1995; Naik and Singh, 1995). Over 30 million tires go to
the retreaders, who retread about one-third of the tires 4.2. Hardened properties
received (Scrap Tire Management Council, 1998).
4.2.1. Compressive and tensile strength properties
Several authors (Ali et al., 1993; Rostami et al., 1993;
4. Research findings Eldin and Senouci, 1993; Topcu, 1995) reported the
compressive strength results of rubberized concrete.
Early investigations on the use of discarded tires in Results of various studies indicate that the size, pro-
asphalt mixtures have been very encouraging. Results portions and surface texture of rubber particles notice-
showed that rubberized asphalt had better skid resis- ably affect compressive strength of rubcrete mixtures.
tance, reduced fatigue cracking and longer pavement life Eldin and Senouci (1993) reported that concrete mix-
than conventional asphalt (Khosla and Trogdon, 1990; tures with tire chips and crumb rubber aggregates ex-
Raghvan et al., 1998; Khatib and Bayomy, 1999; hibited lower compressive and splitting tensile strengths
Fedroff et al., 1996). than regular portland cement concrete. There was ap-
proximately 85% reduction in compressive strength and
4.1. Fresh concrete properties 50% reduction in splitting tensile strength when coarse
aggregate was fully replaced by coarse crumb rubber
4.1.1. Slump chips. However, a reduction of about 65% in compres-
Raghvan et al. (1998) have reported that mortars sive strength and up to 50% in splitting tensile strength
incorporating rubber shreds achieved workability (de- was observed when fine aggregate was fully replaced by
fined as the ease with which mortar/concrete can be fine crumb rubber. Both of these mixtures demonstrated
mixed, transported and placed) comparable to or better a ductile failure and had the ability to absorb a large
than a control mortar without rubber particles. amount of energy under compressive and tensile loads.
Khatib and Bayomy (1999) investigated the work- Khatib and Bayomy (1999) and Topcu (1995) also
ability of rubcrete and reported that there is a decrease showed that the addition of coarse rubber-chips in566 R. Siddique, T.R. Naik / Waste Management 24 (2004) 563–569 concrete lowered the compressive strength more than TALC (tire-added latex concrete) as a substitute for fine the addition of fine crumb rubber. However, results re- aggregates or styrene–butadiene rubber (SBR) latex ported by Ali et al. (1993) and Fattuhi and Clark (1996) while maintaining the same water–cementitious materi- indicate the opposite trend. als ratio. TALC showed higher flexural and impact Studies have indicated that if the rubber particles strengths than those of portland cement, latex modified have rougher surface or given a pretreatment, then concrete and rubber-added concrete. Pictures taken us- better and improved bonding may develop with the ing the SEM seem to support that there was better surrounding matrix, and, therefore, that may result in bonding between crumb rubber and portland cement higher compressive strength. Pretreatments may vary paste due to latex. TALC showed potential as a viable from washing rubber particles with water to acid etch- construction material that is less brittle than other types ing, plasma pretreatment and various coupling agents of concrete. (Naik and Singh, 1991). In acid pretreatment, rubber Biel and Lee (1996) reported that the type of cement particles are soaked in an alkaline solution (NaOH) for noticeably affects the compressive strength of rubcrete. 5 min and then rinsed with water. This treatment en- They used two types of cement, magnesium oxychloride hances the strength of concrete containing rubber par- cement and portland cement, in making rubcrete. The ticles through a microscopic (a very small) increase in percentage of fine aggregate substitution varied from 0% the surface texture of the rubber particles. Eldin and to 90% by weight. It was observed that 90% loss in Senouci (1993) soaked and thoroughly washed rubber compressive strength occurred for both portland cement aggregates with water to remove contaminants, while rubber concrete (PCRC) and magnesium oxychloride Rostami et al. (1993) used water, water and carbon cement rubber concrete (MOCRC) when aggregates tetrachloride solvent, and water and a latex admixture (90% of fine aggregate and 25% of total aggregate) were cleaner. Results showed that concrete containing water- replaced by untreated rubber. Magnesium oxychloride washed rubber particles achieved about 16% higher cement concrete exhibited approximately 2.5 times the compressive strength than concrete containing untreated compressive strength of portland cement concrete for rubber aggregates, whereas this improvement in com- both inclusion of rubber and without inclusion of rub- pressive strength was 57% when rubber aggregates ber in the concrete. In terms of splitting tensile strength, treated with carbon tetrachloride were used. portland cement concrete specimens made with 25% of Segre and Joekes (2000) have worked on the use of rubber by total aggregate volume retained 20% of their tire rubber particles as addition to cement paste. In their splitting tensile strength after initial failure, whereas the work, the surface of powdered tire rubber (particles of magnesium oxychloride cement concrete specimens with maximum size 35 mesh, 500 lm) was modified to in- the same rubber content retained 34% of their splitting crease its adhesion to cement paste. Low-cost proce- tensile strength. They further noted that use of magne- dures and reagents were used in the surface treatment to sium oxychloride cement may provide high strength and minimize the final cost of the modified material. better bonding characteristics to rubber concrete, and Among the surface treatments tested to enhance the rubber concrete made with magnesium oxychloride ce- hydrophilicity of the rubber surface, a sodium hydroxide ment could possibly be used in structural applications if (NaOH) solution gave the best result. The particles were rubber content is limited to 17% of the total volume of surface-treated with NaOH saturated aqueous solutions the aggregate. for 20 min before using them in concrete. Then, scan- ning electron microscopy (SEM) and measurements of 4.2.2. Shrinkage water absorption, density, flexural strength, compressive A limited amount of literature is available concerning strength, abrasion resistance, modulus of elasticity and the plastic shrinkage of concrete containing rubber fracture energy were performed using test specimens (W/ particles. Preliminary results reported by Raghvan et al. Cm, water-to-cementitious materials ratio as 0.36) con- (1998) suggest that incorporation of rubber shreds (two taining 10% of powdered rubber or rubber treated with different shapes of rubber particles as constituents of 10% NaOH. The test results showed that the NaOH mortar: (i) granules about 2 mm in diameter and (ii) treatment enhances the adhesion of tire rubber particles shreds having two sizes which were, nominally, 5.5 to cement paste, and mechanical properties such as mm 1.2 mm and 10.8 mm 1.8 mm (length diame- flexural strength and fracture energy were improved ter)) to mortar help in reducing plastic shrinkage with the use of tire rubber particles as addition instead cracking in comparison to control mortar. They further of substitution for aggregate. The reduction in the reported that control specimens developed cracks hav- compressive strength (33%) was observed, which is ing an average width of about 0.9 mm, while the average lower than that reported in the literature. crack width for specimens with a mass fraction of 5% Lee et al. (1998) developed ‘‘tire-added latex con- rubber shreds was about 0.4–0.6 mm. It was also re- crete’’ to incorporate recycled tire rubber as a part of ported that onset time of cracking was delayed by the concrete. Crumb rubber from used tires was used in addition of 5% rubber shreds. Mortar without rubber
R. Siddique, T.R. Naik / Waste Management 24 (2004) 563–569 567
shreds cracked within 30 min, while mortar with 15% in its body) seemed to decrease with an increase in
fraction by mass cracked after 1 h. It was further indi- rubber content. However, Topcu and Avcular (1997a,b)
cated that the higher the content of rubber shreds, the have recommended the use of rubberized concrete in
smaller the crack length and width, and the onset time of circumstances where vibration damping is required.
cracking was more delayed. Similar observations were also made by Fattuhi and
Clark (1996).
4.2.3. Toughness and impact resistance Topcu and Avcular (1997a,b) reported that the im-
Tantala et al. (1996) investigated the toughness pact resistance of concrete increased when rubber ag-
(toughness is also known as energy absorption capacity gregates were incorporated into the concrete mixtures.
and is generally defined as the area under load–deflec- The increase in resistance was derived from the en-
tion curve of a flexural specimen) of a control concrete hanced ability of the material to absorb energy. Eldin
mixture and rubcrete mixtures with 5% and 10% buff and Senouci (1993) and Topcu (1995) also reported
rubber by volume of coarse aggregate. They reported similar results.
that toughness of both rubcrete mixtures was higher Hernandez-Olivares et al. (2002) have reported that
than the control concrete mixture. However, the addition of crumb tire rubber volume fractions up to 5%
toughness of rubcrete mixture with 10% buff rubber (2–6 in a cement matrix does not yield a significant variation
mm) was lower than that of rubcrete with 5% buff of the concrete mechanical features, either maximum
rubber because of the decrease in compressive strength. stress or elastic modulus.
Based on their investigations on use of rubber shreds
(having two sizes which were, nominally, 5.5 mm 1.2 4.2.4. Freezing and thawing resistance
mm and 10.8 mm 1.8 mm) and granular (about 2 mm Savas et al. (1996) carried out investigations to study
in diameter) rubber in mortar, Raghvan et al. (1998) the rapid freezing and thawing (ASTM C 666, Proce-
reported that mortar specimens with rubber shreds were dure A) durability of rubber concrete. Various mixtures
able to withstand additional load after peak load. The were made by incorporating 10%, 15%, 20% and 30%
specimens were not separated into two pieces under the ground rubber by weight of cement used for the control
failure flexural load because of bridging of cracks by mixture. Based on their studies, they concluded that:
rubber shreds, but specimens made with granular rubber (i) rubcrete mixtures with 10% and 15% ground rubber
particles broke into two pieces at the failure load. This (2–6 mm in size) exhibited durability factors higher than
indicates that post-crack strength seemed to be en- 60% after 300 freezing and thawing cycles, but mixtures
hanced when rubber shreds are used instead of granular with 20% and 30% ground rubber by weight of cement
rubber. could not meet the ASTM standards (durability factor);
Khatib and Bayomy (1999) reported that as the (ii) air-entrainment did not provide improvements in
rubber content is increased, rubcrete specimens tend to freezing and thawing durability for concrete mixtures
fail gradually and failure mode shape of the test speci- with 10%, 20% and 30% ground tire rubber and (iii)
men is either a conical or columnar (conical failure is increase in scaling (scaling gives an evaluation of the
gradual, whereas columnar is more of shreds having two surface exposed to freezing and thawing cycles as mea-
sizes which were, nominally, 5.5 mm 1.2 mm and 10.8 sured by the loss of weight) increased with the increase
mm 1.8 mm (length diameter) sudden failure). At a in freezing and thawing cycles.
rubber content of 60%, by total aggregate volume, the Benazzouk and Queneudec (2002) studied the freeze–
specimens exhibited elastic deformations, which the thaw durability of cement–rubber composites through
specimens retained after unloading. the use of two types of rubber aggregates. The types of
Eldin and Senouci (1993) demonstrated that the the aggregates were: compact rubber aggregate (CRA)
failure mode of specimens containing rubber particles and expanded rubber aggregates (ERA). Volume-ratio
was gradual as opposed to brittle. Biel and Lee (1996) of the aggregates ranged from 9% to 40%. The results
reported that failure of concrete specimens with 30%, showed improvements in the durability of the composite
45% and 60% replacement of fine aggregate with rubber containing 30% and 40% rubber by volume. Improve-
particles occurred as a gradual shear that resulted in a ment in the durability of the composite containing ERA
diagonal failure, whereas failure of plain (control) con- type aggregates is better than composite made with
crete specimens was explosive, leaving specimens in CRA aggregates. The finding is more distinct for ERA
several pieces. type.
Goulias and Ali (1997) found that the dynamic Paine et al. (2002) investigated the use of crumb
modulus of elasticity and rigidity decreased with an in- rubber as an alternative to air-entrainment for providing
crease in the rubber content, indicating a less stiff and freeze–thaw resisting concrete. Three sizes of crumb
less brittle material. They further reported that damp- rubber, 0.5–1.5, 2–8 and 5–25 mm, were used. Test re-
ening capacity of concrete (a measure of the ability of sults showed that there is potential for using crumb
the material to decrease the amplitude of free vibrations rubber as a freeze–thaw resisting agent in concrete. The568 R. Siddique, T.R. Naik / Waste Management 24 (2004) 563–569
crumb rubber concrete performed significantly better Also, the tensile and flexural strengths of the TRA
under freeze–thaw conditions than plain concrete, and mortar specimens were higher than those of the control
the performance of crumb rubber concrete in terms of specimens.
scaling was similar to that of air-entrained concrete.
7. Use of scrap-tire rubber in flowable fill
5. Uses of rubber concrete (rubcrete)
Pierce and Blackwell (2003) in their recently published
Fattuhi and Clark (1996) have suggested that rub-
paper have highlighted the use of crumb rubber in
crete could possibly be used in the following areas:
flowable fill. In their investigation, they replaced sand
(i) where vibration damping is needed, such as in foun-
with crumb rubber to produce flowable fill. Experimental
dation pad for rotating machinery and in railway
results indicate that crumb rubber can be successfully
stations;
used to produce a lightweight (1.2–1.6 g/cm3 ) flowable fill
(ii) for trench filling and pipe bedding, pile heads, and
with excavatable 28-day compressive strengths ranging
paving slabs and
from 0.02 to 0.09 MPa. Based on their investigation, they
(iii) where resistance to impact or blast is required such
have concluded that a crumb rubber-based flowable fill
as in railway buffers, jersey barriers (a protective
can be used in a substantial number of construction
concrete barrier used as a highway divider and a
applications, such as bridge abutment fills, trench fills
means of preventing access to a prohibited area)
and foundation fills.
and bunkers.
Rubcrete because of its light unit weight (density
ranges from 900 to 1600 kg/m3 ) may also be suitable for
architectural applications such as: (i) nailing concrete; 8. Conclusions and recommendations
(ii) false facades; (iii) stone backing and (iv) interior
construction. 1. The reduction in compressive strength of concrete
Topcu and Avcular (1997a,b) have suggested that manufactured with rubber aggregates may limit its
rubber-concrete may be used in highway construction as use in some structural applications, but rubberized
a (i) shock absorber, in sound barriers; (ii) sound concrete also has some desirable characteristics such
boaster (which controls the sound effectively) and (iii) in as lower density, higher impact and toughness resis-
buildings as an earthquake shock-wave absorber. tance, enhanced ductility and better sound insula-
However, research is needed before definite recommen- tion. The properties cited in the prior sentence can
dations can be made. be advantageous for various construction applica-
tions (driveways and roadway applications, and
flowable fill as subbase material). If tire-rubber could
6. Use of scrap-tire ash in mortar be used in applications that demand concrete charac-
teristics associated with rubberized concrete, then
Al-Akhras and Smadi (2002) studied the properties disposal of used tires could be reduced to a large
of tire-rubber ash (TRA) mortar. Tire rubber chips were extent.
obtained and burned at a controlled temperature of 850 2. It is also possible to make high-strength rubber con-
°C for 72 h. The residue of tire-rubber chips (ash) was crete using magnesium oxychloride cement, which
collected. TRA was utilized as partial replacement of gives better bonding characteristics to rubber and sig-
sand in five percentages ranging from 0% to 10% with nificantly improves the performance of rubcrete.
an increment of 2.5% by weight of sand. Based on the Moreover, adhesion between rubber particles and
test results, they concluded that: (1) as the TRA content other constituentÕs materials can be improved by
increased, the workability of the fresh mortar decreased. pre-treating the rubber aggregates with magnesium
This behavior is due to the increase in the cementitious oxychloride.
materials in the mortar mix, due particularly to the 3. More research is required to optimize the particle
large surface area of the added TRA; (2) both initial size, percentage of rubber, type of cement, use of
and final setting time increased with the increase in chemical and mineral admixtures, and methods of
TRA content. The initial setting time increased from pretreatment of rubber particles on the characteristics
145 min for the control paste mix to 220 min for 10% of concrete.
TRA paste mix. The final setting time increased from 4. Ash from rubber obtained from combustion tires
270 min for control paste mix to 390 min for 10% TRA could be used in mortar and concrete. However, sig-
paste mix; (3) The TRA specimens showed higher nificant research is needed in this direction.
compressive strengths at various curing periods up to 90 5. Scrap-tire rubber could be successfully used in flow-
days compared with those of the control specimens. able fill (Pierce and Blackwell, 2003).R. Siddique, T.R. Naik / Waste Management 24 (2004) 563–569 569
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