Fecha de cambio

The environmental problems and health hazards of discarded automobile tyres are well recognised around the world. A huge amount of scrap tyres are generated every year across the world, many of them are illegally disposed or stockpiled. Many countries have prompted strategic regulation for management of scrap tyres to minimise the illegal disposal and reduce the environmental hazards of these tyres. One option of tyre recycling is “thermal recycling” by which the discarded tyres are incinerated as a fuel supplement. However, the use of scrap tyre rubber in applications that are beneficial to society is a sensible choice from an environmental and economic point of view. Thus, using recycled tyre rubber as a modifying agent for bitumen would significantly alleviate the environmental problems and open a large market for scrap tyres because a large quantity of recycled tyres can be used in the pavement industry. Additionally, this approach has been broadly recognised to improve the performance characteristics of asphalt pavements and has increased service life in comparison to conventional asphalt. The recycled rubber has been incorporated into asphalt mixtures by two distinct processes known as “dry process” and “wet process”:

Dry Process

In this technology, the recycled crumb rubber is added to the aggregate before blending with the bitumen. Typically, coarser crumb rubber particles up to 9.5mm are used to replace a small portion of fine aggregate, Figure 2.10. The percentage of crumb rubber usually used is from 1 to 3% of total weight of the mixture, or, roughly as 15 to 35% by the weight of the binder. The grading of aggregate is modified by replacement of one or two aggregate gradations with crumb rubber particles. The degree of interaction between the binder and crumb rubber is influenced by many factors such as the grading, size of crumb rubber, mixing temperature, mixing time and the specific area of rubber particles. Generally, using coarser crumb rubber with less specific surface area results in very slight interaction, and has no significant effect on the properties of the bitumen in comparison to the wet process (Rahman 2004).

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Figure 2.10: Field schematic of dry process technology (Memon 2011)

There are two common types of dry process technology used in the industry; PlusRide and the Generic dry process. PlusRide was developed in the United States in the late 1960’s by modifying a gap-graded mixture; the modification is achieved by replacement of up to 3% of aggregate by a coarse granulated crumb rubber. The interaction between crumb rubber and bitumen is kept at a minimum level so the rubber particles maintain their physical and elastic properties within the asphalt mixture which are important to modify the stability of a gap-graded mixture (Rahman 2004).

The generic dry process, also known as the “TAK” system, on the other hand, was developed in the late 1980’s by modifying dense-graded and gap-graded mixtures (Takallou and Hicks 1988). Both coarse and fine rubber are added to the dense graded mixture by a percentage up to 3% of total mixture mass. The philosophy behind the Generic dry process is to allow a greater binder modification by the fine rubber, while the coarse particles act as a flexible replacement to the aggregate that improves the elastic properties of the mixture.

Although larger quantities of scrap tyres are consumed in mixtures produced using the dry process and minimal or no modification is required in an asphalt plant, the dry process is not widely used and increasingly is being abandoned. Inconsistency in field performance of pavements made with the dry process is the main reason for this (Emery 1995, Rahman 2004).

26 Wet Process

In this process and according to McDonald’s technique, the crumb rubber is blended with the bitumen for a period of time (45 to 60 min) at elevated temperature ~160 °C, and the rubber particles are swollen by absorbing the light frictions of the bitumen. Figure 2.11 shows a schematic of production of rubberised bitumen in the field by means of the wet process. The interaction between crumb rubber and bitumen leads to an increase in the viscosity of the matrix and imparts enhancement to the engineering characteristics of the product.

Figure 2.11: Field schematic of the wet process technology (Memon 2011) The definition of recycled tyre rubber modified bitumen RTR-MB according to ASTM D8 is “a blend of bitumen and crumb rubber and certain additives, where the crumb rubber is at least 15% by the mass of total blend. The mixture has been sufficiently reacted to cause swelling of the crumb rubber particles”. Moreover, a number of RTR-MB specifications have been introduced to specify each of the crumb rubbers used as a modifier (content and properties), the base binder and the final product. The most commonly used specifications are ASTM D6114, CalTrans Bitumen Rubber User Guide, SABITA Manuel 19, VicRoads and APRG Report No. 19 and Austroads User Guide; a comparison between them is presented in Table 2.5 (Memon 2011).

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However, the properties of RTR-MBs vary significantly with the interaction process, and the extent of this interaction also depends on a number of variables such as blending temperature, blending time, type and amount of mechanical mixing, crumb rubber type, size, content and specific surface area of the crumb rubber, extender oil and the type of bitumen. Therefore, it is not always possible to define a generic type of RTR-MB for a very specific product.

The main shortcomings of RTR-MBs are the phase separation and the construction challenges due to the high viscosity of the binder. Because of these limitations, many researchers have investigated producing a product with extended storage or shelf life characteristics by adopting much severer curing conditions and fine rubber particles (Zanzotto and Kennepohl 1996, Billiter, Chun et al. 1997, Billiter, Davison et al. 1997, Glover, Davison et al. 2000, Attia and Abdelrahman 2009). But, the key issue is still how to produce material with no minimal phase segregation problems and at the same time maintaining an acceptable level of modification.

RTR-MBs have been successfully used in hot bituminous mixture applications (gap and open graded hot mixes), chip seals or spray applications, and asphalt rubber stress absorbing membrane interlayers (SAMI-R). RTR-MBs applied to wearing course and base course mixtures have shown improved rutting resistance when tested by the full-scale field accelerated loading (ALF) (Mohammad, Huang et al. 2000). In addition, the improvement when using the wet process has been evident in both laboratory and field performance. Many studies have shown that both low-temperature properties and high-temperature properties of asphalt mixtures are improved by the incorporation of RTR-MBs (Glover, Davison et al. 2000, Wong and Wong 2007, Lee, Akisetty et al. 2008). Finally, asphalt-rubber pavement could be considered a more cost-effective option than a conventional pavement when the long-term performance is taken into account (Jung, Kaloush et al. 2002).

28 Table 2.5: Available specifications for RTR-MBs (Memon 2011)

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