Hazardous waste materials are being produced and generated in large quantities causing an increasing threat to the environment. Hazardous materials are classified as chemical, toxic or non-decaying material accumulating with time. Hazardous materials are found in the form of liquid, solid, contained gases, or sludge. Tires are non-decaying material which is in solid form. So it is considered as hazardous waste material. Most of the hazardous materials are disposed in landfills or other containment areas. If these hazardous waste sites are not properly designed or managed, their contents will released into the surrounding environment, posing a threat to public health. Hazardous wastes are harmful to human health and to the environment. Every year, major health problems result from hazardous waste. Increasing amounts of hazardous waste have caused increasing health problems.
Industrial processes commonly uses over 80,000 types of chemicals. These chemicals are very harmful to human's health. Sometimes these dangerous chemicals are found near to living places. Insufficient research has been done to provide data on the effects of every chemical. The Expert Groups stressed that more research is needed about chemicals and toxics. This is to prove the potential hazards posed to man by chemicals . It will also be necessary to learn how combinations of these chemicals affect human health because waste chemicals often mix together. For example a school in New Jersey in 1989 had to be closed. The students there had been exposed to chromium excessively. It was found that large amounts of chromium had been dumped nearby, and had blown over to the school area. Hazardous waste materials are so dangerous which can cause humans life terribly .
The disposal of waste tires is becoming a major problem to the environment. Waste tires are considered as non-decaying materials that disturb the surrounding environment. One of the most popular methods is to pile used tires in landfills, as due to low density and poor degradation they cannot be buried in landfills. These tires can also be placed in a dump, or basically piled in a large hole in the ground . However these dumps serve as a great breeding ground for mosquitoes and due to the fact that mosquitoes are responsible for the spread of many diseases, this becomes a dangerous health hazard.
Worldwide, the use of rubber products increases every year. In Malaysia alone in year 2002, the numbers of vehicles that discarded waste tires have been estimated around 8.2 million or 57,391 tonnes. At least 60% of the waste tires are disposed via unknown routes .
As for in 2010, we our own self can witness the amount of vehicles that had increased. This is due to more human growth in Malaysia and further development in technologies. More over the Malaysia economy have been very good so far. So this set to increase, in line with the growth in road traffic and car ownership because everyone manages to own a vehicle for them. It is estimated that millions of car and truck tires are being discarded annually.
Most of the tires dealers are facing problems and most of the time they go through considerable pressure when the waste tires accumulates in their premises. They often get penalties from local authority or personnel organization for improper storage of waste tires. They usually make use of private rubbish collectors to dispose their waste tires. They do not get the correct direction to dispose the waste tires. Although the people in the private rubbish collectors had proper settlement to dispose the waste, but it is not confirmed where this waste tires are disposed and in what way.
Burning of waste tires is also a dangerous solution. An open tire fires which release the burning air is proven to be more toxic. Experiment have been conducted and the results shows that open tire includes pollutants such as carbon monoxide (CO), oxides of nitrogen (NOx), sulfur oxides (SO2), and volatile organic compounds (VOCs). They also contain hazardous air pollutants such as polynuclear aromatic hydrocarbons (PAHs), dioxins, furans, hydrogen chloride, benzene, polychlorinated biphenyls (PCBs).
Photo 3: Scrapped Tyres Burned
Source : Tire On Fire
So based on the level and degree of exposure, it may cause health to human such as irritation of the skin and eyes, and mucous membranes, respiratory effects, central nervous system depression and cancer. The melting tires also produce large quantities of oil, which cause contamination of soil and ground water. So burning of tires is considered a bad decision to be made. It has to avoid before major problems occur.
Tire Derived fuel
Unless this, researchers have found that tires can be used as tire-derived fuel. Tire-derived fuel is composed of shredded tires. Tires are a hydrocarbon-based (polymerized rubber) derived from oil and gas. Their heat is 20-40% higher than coal: 7800 to 8600 kcal/kg [14,037 - 15,476 BTU/lb] for tires compared to 5550 to 7200 kcal/kg [9988 - 12,957 BTU/lb] for coal . The moisture of the tires is very low and a lower ash content than coal. This means higher energy use efficiency. The ratios of volatile in tires are higher to fixed carbon, which improves their ability to burn rapidly. Tire produces the same amount energy as petroleum and approximately 25% more energy than coal .
Photo 4: Processing Tire Derived Fuel
Source : American Recycler
Tire-Derived fuel is usually consumed in the form of shredded or chipped material with most of the wire from the tire's steel belts removed. The content of the fuel are very high, with an average heat value of 15,500 BTUs per pound of fuel. Normally this fuel burns at 550 to 650 degrees Fahrenheit. Complete combustion is achieved with flame temperatures of 1202 degrees Fahrenheit .
Although it is a good way to implement waste tires but there some controversial problems. It is found that dangerous toxins are produced such as dioxins and furans during the combustion process. Dioxins and furans are chemical compounds as polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans. Dioxins and furans are not biodegradable. It poses a major health risk. The common human effect causes by this toxins is chloracne which is a skin disorder.Extreme exposures also lead to other effects on the skin, liver, immune system, reproduction system, senses and behaviour. This toxin also effect animals with serious health problems such as weight loss, immune system damage, impaired liver function and some other damages . So we conclude that tire-derived fuel is a major risk to the environment. It is not a good way to implement the use of waste tires.
Photo 5: Tire Fuel Handling System
Source : TMI System
Recent research has highlighted the need to identify developing recycling routes for used tires. As shown in Figure 1.1, tire recycling is predicted to remain flat until 2012 with the only growth being in energy recovery. A major use in energy recovery is as a fuel source, where tires produce 20% more energy than coal. A considerable shortfall in the capacity to reprocess its used tires is also predicted due to the lack of economically viable alternatives to landfill. Currently only about 4.5% of tires are recycled in engineering applications. These tend to be small-scale applications in single projects. However, the potential market in engineering applications is enormous. For example, about 1.8 billion is spent annually on concrete products .
Figure 1 Predicted Tire Reprocessing Capacities
Recent research found that steel fibers from waste tires are a good way to be used in concrete. The application of recycled steel fibers has shown interesting potentiality because of their ability to improve the mechanical performances of the concrete. It is called rubberized concrete. It is a way to use scraped tires from waste tires to replaced natural aggregates in concrete 
As for now this rubberized concrete have been suggested to be used as foundation pad for rotating machinery and railway station as vibrations damper or where resistance to impact or blast is required .
Some researchers found that rubberized concrete may be used for sound barriers in highway constructions and as earth quake shockwave absorber. They also have proposed the employment of rubber crumb in flow able fill for construction applications such as trench hills and foundation fills. A researcher has carried out an ultrasonic analysis by the ultra echo technique in order to investigate sound absorption and the ultra-sonic modulus of the rubberized concrete. It is concluded that rubberized concrete is an effective absorber of sound and shaking energy .
Even though these methods are implemented so that the amount of waste tires will decrease but there are some thoughts are going on. These methods cannot be used for a long term period. So at last it will end up as waste tire again. Although these methods are not that effective but the idea of using waste tires in concrete for building constructions should be considered. Research should be done for this method.
General Characteristics and Constituents of Concrete
Review about the concrete and the mixture of the concrete. Discussions about the properties of the concrete also have been reviewed.
Concrete is a vital material for the construction industry. Its use has increased considerably over the past few decades. Concrete is a mixture of cement, water, sand and aggregates like stones and crushed rocks. This lead too much quarrying of natural materials used for the production of concrete. Consequently, there has been increasing public concern about the quarrying of natural resources in recent years .
Photo 6: Concrete Mixture
Source: Levitt M. The philosophy of testing concrete
The aggregates usually constitute between 50% and 80% of the volume of conventional concrete and may therefore greatly influence its properties. Artificial and replacement aggregates have long been used in concrete, such as waste materials from industrial processes e.g. pulverized fuel ash. More recently, the requirements of sustainability have led to the development of replacement aggregates such as recycled concrete and glass cullet .
Concrete strength is greatly affected by the properties of its constituents and the mix design parameters. Because aggregates represent the major constituent of the bulk of a concrete mixture, its properties affect the properties of the final product. Aggregate has been customarily treated as inert filler in concrete. However, due to increasing awareness of the role played by aggregates in determining many important properties of concrete, the traditional view of the aggregate as inert filler is being seriously questioned. Certain aggregate characteristics are required for proportioning concrete mixtures. These include density, grading and moisture state .
Porosity or density, grading, shape and surface texture determine the properties of a fresh concrete mixture. The mineralogical composition of aggregate affects its crushing strength, hardness, elastic modulus and soundness which in turn influence the strength and durability properties of hardened concrete.
It is recommended to use rubber in concretes. The use of rubber as a partial replacement for the natural aggregates in concrete will therefore make a necessary investigation of the changes in the properties of the concrete. The mixture of rubber in concrete is known as rubberized concrete.
The reasons to use rubber as a natural aggregate have been discussed further. The required tests which are made that can show the results and effectiveness of rubberized concrete are also discussed further.
Characteristics and Constituents of the rubberized concrete
A review about the rubberized concrete and describes the physical characteristics of the constituents of the rubberized concrete in previous investigation for example rubber aggregate, mineral aggregate and cement.
Rubberized concrete is the concrete mixed with waste rubber added in different volume proportions and is an infant technology. Partially replacing the coarse or fine aggregate of concrete with some waste tires which are tuned into rubber can improve qualities such as low unit weight, high resistance to abrasion, absorbing the shocks and vibrations, high ductility and brittleness and so on to the concrete. Moreover the inclusion of rubber into concrete results in higher resilience, durability and elasticity. In constructions that are subject to impact effects the use of rubberized concrete will be beneficial due to the distorted state of its properties .
Rubber aggregates are produced by reduction from scrap tires to aggregate sizes using two general processing technologies which are done by mechanical grinding at ambient conditions or cryogenic grinding .
Mechanical grinding uses an abrasive wheel to final machine precision components to fine tolerances. Mechanical grinding machine is the most common process under gone by waste tires. This method consists of using a variety of grinding techniques to mechanically break down the rubber shred into small particle sizes ranging from several centimeters to fractions of a centimeter. The steel bead and wire mesh in the tires is magnetically separated from the crumb during the various stages of granulation and sieve separated the fiber in the tire .
Cryogenic processing is performed at a temperature below the glass transition temperature. This is usually accomplished by freezing of scrap tire rubber using liquid nitrogen. The cooled rubber is extremely brittle and is fed directly into a cooled closed loop hammer-mill or multi-state screener to be crushed into small particles with the fiber and steel removed in the same way as in mechanical grinding .
The whole process takes place in the absence of oxygen, so surface oxidation is not a consideration. Because of the low temperature used in the process, the crumb rubber derived from the process is not altered in any way from the original material. Unlike mechanically processed rubber, the cryogenic process is an efficient means of obtaining rubber aggregate which is steel and fabric-free, uniformly geometric in shape and finely ground .
Researchers used different types of rubber aggregate for the experiments. They described various methods to process scrap tires into rubber and presented typical comparisons between the chemical compositions of truck and car tires. Some have used buff rubber obtained by mechanical grinding of the tire head. They also used smaller size rubber crumb obtained from cryogenic processes, which has a gradation close to that of typical sand and two types of coarse rubber aggregate where one type was long angular chips obtained by mechanical grinding and the other was round particles produced by cryogenic grinding .
According to this investigator, the rubber source and grinding process can influence the amount of steel and textile fiber in the rubber as well as the shape and texture of the rubber, and ultimately the properties of rubberized concrete .
Some researchers used different size and grading of rubber aggregates. They used three grading of rubber with a maximum size of less than 4.76mm and one type contained textile fiber . Coarse rubber aggregates were also used and the rubber was graded into three groups of 38, 25 and 19 mm maximum sizes. They also used one grading passing a size 2 mm sieve. Rubber have graded based on the ASTM C 136 method. They indicated that it was not possible to determine the gradation curve for their tire chips, as for normal aggregates, because they are elongated particles that range in size from about 10 to 50mm [17, 19].
The density of the rubber aggregates reported in the previous studies varied. It is reported that the unit weight of the rubber used varied between 800 and 960 kg/m3. Also, the specific gravity of rubber used in the different investigations varied widely for example 0.65, 0.80, 1.06 to 1.09 and 1.12. Reports also suggested that the variations in specific gravity could be due to varying rubber quality and experimental error .
Cement is still an essential material in making concrete, but, in some modern concretes it is no longer the most important material because these concretes are composite materials. In a composite material, it is impossible to decide which is the most important material because, by its nature, a composite material has properties that are always much better than the simple arithmetical addition of the individual properties of each component. For the investigation to the rubberized concrete, Ordinary Portland Cement (OPC) was used in concrete mixes in all of the investigations. The reason for using Ordinary Portland Cement in their investigations is that this is by far the most common cement in use and is highly suitable for use in general concrete construction when there is no exposure to sulfates in the soil or groundwater [11, 14, 18, 19].
Photo 7: Portland cement
Source: Ordinary Portland cement 42
Many different sizes of course aggregate were used in the investigation. Most of the researchers replaced either the fine or coarse aggregates in the concrete mixes partially or wholly by volume of rubber aggregate.
Properties of the Fresh Rubberized concrete
Properties of the fresh rubberized concrete have been discussed on this topic. Rubberized concrete contains good aesthetics, acceptable workability and a smaller unit weight than ordinary concrete.
Rubberized concrete showed good aesthetic qualities. The appearance of the finished surfaces was similar to that of ordinary concrete and surface finishing was not problematic. However, the authors reported that mixes containing a high percentage of larger sized rubber aggregate required more work to smooth the finished surface. They also found that the colour of rubberized concrete did not change noticeably from that of ordinary concrete.
Investigations have been done on the workability of rubberized concrete. They observed a decrease in slump with increased rubber aggregate content by total aggregate volume. Their results show that for rubber aggregate contents of 40% by total aggregate volume, the slump was close to 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 rubber aggregate or a combination of crumb rubber and tire chips .
In general the rubberized concrete batches have been reported that it showed acceptable performance in terms of ease of handling, placement and finishing. However, they found that increasing the size or percentage of rubber aggregate decreased the workability of the mix and subsequently caused a reduction in the slump values obtained. They also observed that the size of the rubber aggregate and its shape (mechanical grinding produces long angular particles) affected the measured slump. The slump values of mixes containing long, angular rubber aggregate were lower than those for mixes containing round rubber aggregate (cryogenic grindings). Round rubber aggregate has a lower surface/volume ratio. Therefore less mortar will be needed to coat the aggregates, leaving more to provide workability. They suggested that the angular rubber aggregates form an interlocking structure resisting the normal flow of concrete under its own weight; hence these mixes show less fluidity. It is also possible that the presence of the steel wires protruding from the tire chips also contributed to the reduction in the workability of the mix .
The replacement of natural aggregates with rubber aggregates tends to reduce the density of the concrete. This reduction is attributable to the lower unit weight of rubber aggregate compared to ordinary aggregate. Previous studies have found that the unit weight of rubberized concrete mixtures decreases as the percentage of rubber aggregate increases. Results indicated that concrete densities were reduced to 87% and 77% of their original values, respectively, when the maximum amounts of rubber aggregate were used in the investigations. It is also reported that a reduction in density of up to 25% was observed when ordinary aggregate was replaced by coarse rubber aggregate. A researcher found that the density of rubberized concrete was reduced by around 10% when sand was replaced by crumb rubber to the amount of 33% by volume [18-20].
When rubber aggregate was added to the concrete, the air content increased considerably up to 14%. Investigators observed that the air content increased in rubberized concrete mixtures with increasing amounts of rubber aggregate. Although no air-entraining agent (AEA) was used in the rubberized concrete mixtures, higher air contents were measured as compared to control mixtures made with an AEA. The higher air content of rubberized concrete mixtures may be due to the non-polar nature of rubber aggregates and their ability to entrap air in their jagged surface texture. When non-polar rubber aggregate is added to the concrete mixture, it may attract air as it repels water. This increase in air voids content would certainly produce a reduction in concrete strength, as does the presence of air voids in plain concrete. Since rubber has a specific gravity of 1.14, it can be expected to sink rather than float in the fresh concrete mix. However, if air gets trapped in the jagged surface of the rubber aggregates, it could cause them to float. This segregation of rubber aggregate particles has been observed in practice [21, 23-25].
Properties of the Hardened Rubberized Concrete
Properties of the hardened rubberized concrete have been discussed in this topic mainly about the strength of the rubberized concrete. Some testing has been done on the rubberized concrete.
Compressive Strength Testing
Compressive strength measures the largest compression force the material can withstand before it loses its shape or fails. The compressive strength of concrete is the most common performance measure used by the engineer in designing buildings and other structures. It is calculated from the failure load divided by the cross sectional area resisting the load. The test results are primarily used to determine that the rubberized concrete mixture as delivered meets the requirements of the specified strength. Compressive strength tests are widely accepted as the most convenient means of quality control of the concrete produced. Generally, two types of compressive test specimens are used: cubes and cylinders. Standard cubes of 100 mm are used in UK, Germany and many other countries in Europe. Cylindrical specimens of 300 mm high and 150 mm in diameter are the standard specimens used in the United States, France and Canada [25, 26].
Different shapes and sizes of specimens were used for the determination of the compressive strength of the control and rubberized concretes. Cylindrical specimens 75, 100, or 150 mm in diameter were used. However, an investigator used 150 mm diameter cylinders and 150 mm cubes. It is generally accepted that the compressive strength of ordinary concrete obtained from cube tests is higher than that obtained from cylinder tests. The size of rubber granules appeared to have an influence on the compressive strength of concrete. All investigators replaced coarse or fine aggregates in concrete by varying amounts of rubber. The researcher concluded that larger reductions in the compressive strengths were observed when the coarse aggregate rather than fine aggregate was replaced by rubber .
Two types of rubber, obtained from the shredding processes of truck tires, were also used to investigate compressive strength. They used 1 mm to 16 mm rubber crumb which is normally processed further to produce high quality rubber crumb and low grade rubber which contains textile fiber and dust. Both rubber materials were used as supplied. However, their quality for use in concrete may be improved if they were to be washed. Various surface treatments of the rubber were conducted. They soaked and thoroughly washed the rubber with water in an attempt to remove any contaminates, while some attempted to clean the rubber by using water. The results of the latter showed that concrete containing washed rubber was about 16% higher in compressive strength than concrete containing rubber as received [11,22,29,32].
The results shows that a reduction in compressive strength when rubber was added to the concrete. The decrease in strength is possibly due to the rubber acting as voids which carry negligible load relative to the surrounding matrix which is due to its low elastic modulus relative to that of the matrix and at the same time possibly increasing lateral strains and consequently causing early disruption of the matrix. The results shows that the appearances of selected cubes after crushing. The reductions in the compressive strength of rubberized concrete mixes were much larger than those for normal concrete. However, factors such as the formation of micro cracks around coarse aggregates during loading, and the fact that concrete contained less cement may have contributed to this behavior. Concretes made with low-grade rubber or rubber crumb, and having a similar density, appeared to show very similar compressive strengths. The concretes containing either type of rubber had similar compressive strength when the density of the concrete was below 2150 kg/m3. Above this density, the compressive strengths of concretes containing rubber crumb were slightly higher, with a maximum increase of about 10%. Generally concrete containing rubber crumb had a higher compressive strength than concrete containing low-grade rubber for a similar rubber/cement ratio with a maximum difference in strength of about 15%. However, the difference in compressive strength appeared to be negligible for concretes with rubber/cement ratios of 0.45 and higher [11,22,29,32].
So based on the study the reductions in compressive strength depended on the amount of rubber added, and the trend of compression test results were consistent with those obtained from other researchers. Further work is needed to characterize the rubber in terms of origin, size, shape, grading, density and the ratio of concrete needed to produce better rubberized concrete.
Photo 8: Compressive Test 1
Photo 9: Compressive Test 2
Flexural strength, also known as bend strength, a mechanical parameter for brittle material, is defined as a material's ability to resist deformation under load. The flexural strength represents the highest stress experienced within the material at its moment of rupture. Some investigations have been carried to determine the flexural strength of the rubberized concrete.
The aggregate consisted of crushed stone coarse aggregate of a maximum size of 19 mm (3/4 in) with a specific gravity of 2.65, and concrete sand with a maximum size of 4.76 mm (3/16 in). The specific gravity of the regular concrete sand was 2.68 and the fineness modulus was 256. The water to cement ratio was 0.5. Shredded rubber tires with a maximum size of 12.7 mm (l/2 in) and a specific gravity of about O-61, were used. The rubber tire chips were free of steel wires. The particle-size distributions of all aggregates were graded and the particle-size grading of each was within the limits of ASTM C 33. Four different contents of rubber aggregate were used to replace the mineral aggregate, 25, 50, 75 and 100% by volume. No mineral or chemical admixtures were added. The flexural strength specimens, measuring 100 x 100 x 350 mm (4 x 4 x 14 in) were prepared. The four-point bend tests were conducted using an Instron testing machine. The specimens were loaded at a crosshead speed of 0.5 mm/min. Load and displacements were digitally recorded at a rate of 10 data points per second .
A significantly smaller reduction in flexural strength was observed as compared to compressive strength with increases in the tire chip contents. The flexural strength specimens lost up to 35% of their flexural strength. Load-deflections of the flexural beam specimens were continuously recorded digitally using a personal computer and a load frame manufacturer's software. Load-deflection curves for specimens containing 0, 50 and 100% rubber aggregate are received. As may be, the failure of specimens containing rubber tire chips exhibited a ductile mode of failure as compared to the control specimens. The specimens exhibited a higher capacity to absorb energy. The specimens were capable of withstanding measurable post failure loads and undergoing significant displacement [19, 29].
This was due to the ability of the rubber aggregate to undergo large elastic deformation before the failure of the specimen took place. The failure was initiated in the extreme fiber of the tension region of the beam specimens in which cracks propagated in the mortar until they reached the rubber aggregate. When cracks reached the rubber particles and, because of their elastic properties and low modulus of elasticity, the rubber particles prolonged and sustained a portion of the applied load, which leads to an increase in the area of the failure surface. The reduction in compressive and flexural strengths is due to the incorporation of rubber tire aggregate in concrete. The reduction in compressive strength was significantly higher than that in flexural strength and the relationship between the rubber aggregate content in concrete and percent of reduction in strength were not linear in either measure of strengths [19, 29].
Tensile strength of concrete was reduced with the addition of rubber in both groups however the mechanism of tensile failure for rubber concrete is similar to normal concrete. The reduction of tensile strength in the first group was two times that of the second group. The reduction in tensile strength with 7.5 percent replacement was 44 and 24 percent respectively in the first and second group as compared to the control sample .
The Griffiths theory describes the relationship between applied nominal tensile stress and crack length at fracture, for an example when it becomes energetically favorable for a crack to grow. Griffith was concerned with the energies of fracture, and considered the energy changes associated with incremental crack extension. When a brittle material exposed to a tensile stress, it undergoes incremental crack extension .
During this process the only contributors to energy changes are the energy of the new fracture surface which is two surfaces per crack tip and the change in potential energy in the body. The surface energy term represents energy absorbed in crack growth, while the some stored strain energy is released as the crack extends due to unloading of regions adjacent to the new surface energy. If the released energy is sufficient to cause the development of cracks, the conditions are predictors for immediate failure. If barriers were located in the crack development path, crack expansion is halted so that exerted pressure is increased. This process appears in concrete also as micro cracks grow and expand if stress is applied .
Crack expansion in the cement paste will stop advancing when it is confronted with barriers such as a large cavity, an un-hydrated cement particle or a soft material that requires greater energy to disintegrate. Tire rubber as a soft material can act as a barrier against crack growth in concrete. Therefore, tensile strength in concrete containing rubber should be higher than the control sample. However, the results are in contradiction with this hypothesis .
From previous studies was found that addition of rubber aggregate into the Ordinary Portland Cement concrete mixture produces a reduction in the mechanical strength of the rubberized concrete. It was found that with increasing the rubber aggregate volume content, the reduction in concrete strength also increased.
An experiment has been conducted to examine the strength of rubberized concrete mixtures. Three sets of experiments were performed, the first set using coarse rubber aggregate which are chipped tires of 19-38 mm size and the second and third sets using smaller diameter chips of 6 mm and 2 mm respectively. They found that when mixed with cement, the rubber aggregate tends to act as a large void and did not have a significant role in the resistance to applied external loading. Concrete containing rubber aggregate did not exhibit brittle failure and was able to absorb a significant amount of plastic energy. The authors suggested that there is good potential for using recycled rubber in OPC concrete mixtures because the rubber increases the fracture toughness [17, 31].
An author reported that a general reduction in the physical and mechanical properties of rubberized concrete made by using scrap tires was observed. The effect of the replacement of coarse aggregate by rubber aggregate has been investigated by conducting an experiment. Four different contents of rubber aggregate with a maximum size of 12.7 mm were used to replace the coarse aggregate at 25, 50, 75 and 100% by volume. Studies show that reduction in strengths increased with increasing the rubber aggregate content. He observed that the specimens containing rubber aggregate exhibited a ductile mode of failure as compared to the control specimens [18, 19].
Toughness and Failure Mode
Previous investigators have suggested that rubberized concrete exhibits enhanced toughness and a less brittle failure mode although the reduction in strength of rubberized concrete may limit their use in some structural applications. An investigator showed that when loaded in compression, specimens containing rubber did not exhibit brittle failure. A more gradual failure was observed, either of a splitting which is for coarse rubber aggregate or a shear mode which is for fine crumb rubber. Since the cement paste is much weaker in tension than in compression, the rubberized concrete specimen containing coarse rubber aggregate would start failing in tension before it reaches its compression limit [21, 22]
The generated tensile stress concentrations at the top and bottom of the rubber aggregate result in many tensile micro cracks that form along the tested specimen. These micro cracks will rapidly propagate in the cement paste until they encounter a rubber aggregate particle. Because of their ability to withstand large tensile deformations, the rubber aggregate will act as springs 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 [21, 22].
An experimental conducted by an investigator, it was found that the dynamic modulus 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 on 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 .
It is recommended to use rubberized concrete in circumstances where vibration damping is required, such as in buildings as an earthquake shock-wave 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% rubberized concrete cylinders. It was also found that the higher the rubber aggregate 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 non-destructive measurements of the dynamic modulus of elasticity may be used for estimating the compressive strength of rubberized concrete. A good correlation between compressive strength and the damping coefficient calculated from transverse frequency was also found, indicating that the damping coefficient of rubberized concrete may likewise be used for predicting the compressive strength [18, 28].
Impact Resistance, Heat and Sound Insulation
In an investigation, two slabs were made and tested simultaneously. One slab was made with ordinary concrete without rubber, while the other contained about 11% of low-grade rubber relative to the total solids content by weight (a rubber to cement ratio of about 0.44). The initial height of drop for the hammer was set at 1.0 m. However, after one drop, the slabs suffered hairline cracking only. The height was then increased to 2.0 m, and the hammer was dropped twice. Examination of the slabs showed that both suffered cracking in all directions. However, the slab containing rubber had a larger spread of cracks over the tension face. After the second drop, the maximum crack width in the ordinary concrete without rubber slab was 0.16 mm, while that for the slab containing rubber was 0.50 mm. After the third drop, the maximum crack widths at the same locations, it increased to 0.3 and 2.0 mm for the plain concrete and rubberized concrete slabs, respectively. This shows that both slabs sustained the impact of the drop hammer, despite the lower compressive strength of the rubberized concrete slab (about 30% of the strength of ordinary concrete). The wider rack widths that resulted in the rubberized concrete slab would require some attention in terms of reinforcement durability .
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. The increase became more prominent in concrete samples containing larger-size rubber aggregates. It was expected that acoustical tests would substantiate the applicability of rubberized concrete mixtures for roadway sound barriers to reduce the effects of acoustic emissions.
Wisconsin and Pennsylvania Departments of Transportation (DOTs) have studied the noise-absorption properties of whole rubber tires as sound barriers with moderate success. More research is required to study the sound insulation effects of the rubberized concrete 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. However, no pilot projects to take advantage of this property in practice are available in the open literature. Also, fire test indicated that the flammability of rubber in rubberized concrete mixtures was much reduced by the presence of cement and aggregates. Although more testing is needed, it is believed that the fire resistance of rubberized concrete mixtures is satisfactory .
Based on the previous reports and investigation adding rubber in the concrete has produced various results. The test that have been conducted are compressive strength testing, flexural strength, tensile strength, mechanical strength, toughness and failure mode and impact resistance, heat and sound insulation testing.
The studies show that adding rubber in the concrete reduces the strength of the rubberized concrete. Based on the studies the strength reduction is because the rubber aggregate is much softer. Then the bonding between the rubber and the cement is likely to be weak. The strength of the rubberized concrete is also greatly affected by the quality of mixture and size and proportion of the rubber. It is also known that the strength of the rubberized concrete depends greatly on the density, size and hardness of the rubber.
Consequently it was decided that in the present study, an experiment will be conducted to investigate the cooling effect of the rubberized concrete. This is experiment is more to heat transferred in the rubberized concrete. Some testing will be conducted on some specimens to obtain excellent results on the rubberized concrete. Thermal conductivity testing will be more focused in this study.
In the present study, it was therefore decided to concentrate on developing the most straightforward mix design and preparation techniques to produce rubberized concrete with acceptable properties in the fresh and hardened states. A mixture of cement, sand, crumb rubber and water will be used for the investigation. The effect of coating the crumb rubbers with cement paste was investigated as a potentially simple method of improving the performance of the material, thereby avoiding the use of additional or costly additives which may adversely affect the production costs.
In addition, the previous studies have shown that the workability of concrete containing rubber aggregate is reduced. This could affect the method of preparing the rubberized concrete samples and products and requires further study during the present investigation.