Prepared by: Asst. Lect. Ahmed Saeed Hakim Department of Civil Engineering – College of Engineering – Al-Mustaqbal University The Problem of Waste Tires End-of-life tires are among the most difficult solid wastes to dispose of. They are bulky, slow to decompose, and unsuitable for ordinary landfilling because they tend to float and rise within landfills. The number of tires reaching the end of their service life worldwide each year is estimated at more than one billion. These tires accumulate in open sites, where they become a breeding ground for insects and rodents and pose a severe fire hazard that is difficult to extinguish and produces toxic smoke and oily residues that contaminate soil and groundwater. For this reason, most countries have enacted legislation banning the landfilling of whole tires and encouraging their recycling and use in beneficial engineering applications, including civil construction, which can absorb large quantities of this waste. Tire Composition and Processing Methods A tire consists of natural and synthetic rubber (about half of its weight), carbon black added to increase durability and abrasion resistance, steel wires and textile fibers, as well as fillers and processing agents such as sulfur and zinc oxide. Because of the vulcanization process, which cross-links the rubber chains, rubber cannot be remelted and reshaped as plastic can, so it is usually reused in the form of particles. The recycling process begins with primary shredding into large pieces, followed by tearing and grinding, with steel wires separated by magnets and textile fibers separated by air or screening. Processing yields graded sizes, classified as follows: First: Chips and shreds (Tire Derived Aggregate), ranging from a few centimeters to about 30 cm, used in lightweight fills and drainage layers. Second: Crumb rubber, ranging approximately between 0.075 and 4.75 mm, used in concrete and asphalt. Third: Fine rubber powder, with smaller particle sizes, used in the chemical modification of bitumen and in some rubber products. Grinding is carried out by two main methods: ambient grinding, which is the most common and produces rough-surfaced particles that bond well with cementitious and bituminous media, and cryogenic grinding with liquid nitrogen, which produces finer, smoother particles but at a higher cost. Use of Rubber in Concrete Principle of use: Crumb rubber is used in concrete mostly as a partial replacement for fine aggregate (sand), and sometimes for part of the coarse aggregate, in proportions calculated by volume. The resulting material is known as rubberized concrete. Since the specific gravity of rubber is only 1.1 to 1.2, compared with 2.6 to 2.7 for natural aggregate, adding it reduces the density and unit weight of the concrete in proportion to the replacement ratio. Effect on fresh properties: Workability generally decreases as the rubber content increases because of the rough surface of the particles and increased friction, and the light particles tend to float to the surface when mixing is poor or vibration is excessive. Entrapped air content also increases. Workability can be improved by adding plasticizers or superplasticizers. Effect on hardened properties: Compressive strength, tensile strength, and modulus of elasticity decrease as the rubber content increases. Figures vary between studies and designs, but in general the reduction in compressive strength ranges from about 10% to 30% when 10% to 20% of the aggregate is replaced by volume, and it may exceed 80% when the coarse aggregate is replaced entirely, according to pioneering studies. This loss is attributed to weak bonding between the rubber surface and the cement paste, to the low stiffness of the rubber particles, which act as flexible voids within the concrete mass, and to increased entrapped air. On the other hand, other properties improve markedly, including ductility and energy absorption capacity after reaching peak load. Instead of sudden brittle failure, rubberized concrete shows larger deformations before complete failure. Impact resistance, resistance to shrinkage cracking, damping capacity, and vibration absorption also improve, thermal conductivity decreases so insulation increases, and sound insulation improves. Improving bond: To reduce the loss of strength, rubber particles are treated before mixing. The best-known treatments include washing them with sodium hydroxide solution to remove oils and surface impurities and roughen the surface, treating them with silane coupling agents, or coating them with cement paste or fine materials such as silica fume. Results vary according to the type and conditions of treatment, but in general they improve bond and recover part of the strength loss. Suitable applications: Because of its reduced strength, rubberized concrete is mostly used in non-structural or lightly loaded elements, including sidewalks and pedestrian paving, concrete barriers and sound barriers along roads, playground surfaces, floor slabs requiring shock absorption, foundations for vibrating equipment, and some railway elements. Many studies address its possible use in structural elements at small replacement ratios, but this requires special design and special testing. Use of Rubber in Asphalt Mixtures The road sector is one of the largest consumers of recycled tire rubber, and there are two methods: Wet process: Crumb rubber is blended with hot bitumen in advance and left to react with it at high temperature (about 170 to 200 °C) for roughly 45 to 60 minutes. The rubber swells and absorbs the light components of the bitumen, forming a modified, high-viscosity binder known as asphalt rubber. The American specification ASTM D6114 sets the requirements for this type of binder, including a minimum rubber content of about 15% by weight of the binder. Dry process: Crumb rubber is added to the aggregate in place of a small portion of the fine aggregate, usually in a small proportion (about 1% to 3% of the mix weight), before the bitumen is added, and it reacts partially with the bitumen during mixing and processing. It is relatively easy to implement, but its results are less consistent than those of the wet process. The main benefits of rubberized asphalt are improved rutting resistance due to increased viscosity and stiffness at high temperatures, improved resistance to thermal cracking and fatigue cracking thanks to the elasticity of the rubber, the possibility of reducing surface layer thickness in some designs, reduction of traffic noise by several decibels depending on the design, and longer pavement life with less maintenance. A single kilometer of road may also consume hundreds to thousands of tires, depending on layer thickness and lane width. Among the challenges, the high viscosity of asphalt rubber requires higher mixing, laying, and compaction temperatures, and the initial cost is higher compared with conventional bitumen. The mix also requires precise control of reaction time and temperature, smoke and odors may be produced during manufacturing, and storing asphalt rubber requires continuous agitation to prevent rubber separation. Geotechnical and Fill Applications Shredded tire pieces (TDA) are used in geotechnical works as a lightweight fill material, as their unit weight is about one-third to one-half that of conventional soil. They are useful for building embankments over weak soil to limit settlement, for backfilling behind retaining walls to reduce lateral pressure, for drainage layers in landfills, playgrounds, and roads because of their high permeability, as vibration and shock isolation in railway lines, and in impact-attenuating barriers. ASTM D6270 provides guidelines for using scrap tires in civil engineering applications, including limits on fill layer thickness to reduce the risk of self-heating and spontaneous combustion, since large masses of rubber pieces may heat up under certain conditions. Environmentally sensitive sites also require a study of the possible leaching of some elements, such as zinc, iron, and manganese, into groundwater. General Limitations and Challenges First: reduced mechanical strength in concrete, which usually limits its use to non-structural or lightly loaded applications. Second: weak bond between rubber and cement paste, which requires surface treatment that adds cost and complexity. Third: the need for precise control of mixing proportions and processing, and the variability of crumb rubber properties from one source to another. Fourth: the high cost of producing asphalt rubber and of the required equipment, and the need for special training of workers. Fifth: fire risks in large tire fills, and the need for environmental assessment of pollutant leaching. Sixth: limited local specifications and codes in many countries that lack an advanced recycling industry. Feasibility of Application in Iraq Iraq has a large number of accumulated waste tires, with multiple environmental problems, including their indiscriminate burning and the resulting air pollution. On the other hand, roads in many regions suffer from high summer temperatures that cause rutting and deformation of asphalt layers, conditions that suit the benefits of rubberized asphalt. There is an economic and environmental opportunity in establishing tire shredding and grinding plants that serve the road and construction sectors. Potential obstacles include the scarcity of specialized industrial recycling plants, the lack of national codes for rubberized concrete and asphalt, the need to train engineering staff, and the need for local laboratory studies to determine optimum proportions suited to local materials and climate. Based on the above, the proposed order of implementation begins with rubberized asphalt on pilot roads and then main roads, followed by the use of rubberized concrete in non-structural elements such as sidewalks and barriers, and then lightweight fills at suitable sites after studying environmental safety. Conclusions Recycling tire rubber is a dual-benefit solution: it alleviates a growing environmental problem and provides materials that can be used in construction. Rubberized concrete is characterized by light weight, ductility, damping, and insulation, but it loses part of its strength, so it is suitable for non-structural or lightly loaded applications. Rubberized asphalt is one of the most successful and widespread applications, owing to its resistance to rutting and cracking, longer service life, and reduced noise. Rubber chip fills offer a solution for weak soils and drainage works, provided that safety requirements are observed. The success of application remains dependent on control of proportions, treatment, and specifications. Recommendations We recommend establishing local plants for shredding and grinding tires to approved gradation specifications; constructing pilot sections of rubberized asphalt and monitoring their performance in hot conditions; conducting laboratory studies to determine the optimum rubber contents in concrete and the treatment methods suitable for local materials; using rubberized concrete in barriers, sidewalks, and vibration-absorbing elements; issuing national specifications for the use of rubber waste; training engineers and technicians; and banning the indiscriminate burning of tires while regulating their collection and storage. References Eldin, N. N., and Senouci, A. B., “Rubber-Tire Particles as Concrete Aggregate,” Journal of Materials in Civil Engineering, ASCE, Vol. 5, No. 4, 1993, pp. 478–496. Siddique, R., and Naik, T. R., “Properties of Concrete Containing Scrap-Tire Rubber – An Overview,” Waste Management, Vol. 24, No. 6, 2004, pp. 563–569. Khaloo, A. 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ASTM D6270: Standard Practice for Use of Scrap Tires in Civil Engineering Applications, ASTM International. ASTM D6114/D6114M: Standard Specification for Asphalt-Rubber Binder, ASTM International.