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TopIntroduction
Hot Mix Asphalt (HMA) is the common material for road surface. It consists of crushed stone aggregates, asphalt binder and mineral filler. The asphalt binder acts as glue and binds the aggregate particles together to form a relatively stable cohesive mass. The aggregates provide the stone skeleton to resist the repetitive traffic load applications. Due to viscoelastic nature of asphalt binders, temperature susceptibility and aging greatly affect the asphalt binder performance (Shen & Kirkner, 2001).
Chen (2009) identified rutting and alligator cracking as the root causes of pavement failures. Rutting (permanent deformation) is characterized by the surface depression along the wheel path with or without the pavement uplift along the sides of the rut. Each time when a load is applied to the pavement, a small amount of permanent deformation occurs in it. The accumulation of these small deformations after many load applications causes pavement failure. Rutting depths are also dependent upon pavement temperatures. Severe ruts can cause accumulation of water in them which is one of the reasons for hydroplaning. Rutting can also be caused due to permanent deformation in the pavement subgrade or any other layer due to the consolidation or lateral movement of the materials. Severe rutting in the pavement is an indication of its structural failure (Brown & Cross, 1992).
Rutting has become one of the major flexible pavement distresses in Pakistan in the recent years. There are many factors that contribute to the premature failure of pavement due to rutting which include heavy illegal axle loading, high local temperatures, poor quality control during construction and poor applications of mix design procedures to meet the requirements of Pakistani climate (Khan & Kamal, 2008). In this study, we have used modifiers (crumbed rubber and Low Density Polyethylene (LDPE)) to improve resistance to rutting in a flexible pavement. Both these modifiers are waste products of many industries in Pakistan.
Polyethylene is a thermoplastic polymer and the most commonly used plastic in the world. It is obtained by the polymerization of ethane. It is a semi-crystalline material having a very simple structure. It has a wide range of properties including good fatigue and wear resistance (Malpass, 2010). Crumbed rubber is usually acquired from recycled rubber of automotive and truck scrap tires.
The objectives of this research are; (i) studying and comparing of the performance of polyethylene and crumbed rubber modified bitumen against HMA and (ii) proposing the rutting resistant asphalt mixture suitable for local climate and loading conditions. Moreover, this study also investigates the extent to which conventional bitumen can be applied in terms of temperature. The study will be beneficial for the engineers and administrative bodies to decrease the life-cycle cost of transportation infrastructure.
TopPrevious Work And Research Significance
Hesp and Coomarasamy (1997) used crumbed rubber as modifier to test the performance of pavement against rutting at 60°C using the wheel tracking machine test. They used thermal restrained specimen test for low temperature cracking. The 30 and 80 mesh crumbed rubber was used in this research and they concluded that the crumbed rubber modified bitumen performance is significantly better at high temperatures as compared to traditional bitumen. They also observed moderate improvements in low temperature thermal cracking resistance.
Huang et al. (2002) tested crumb rubber in laboratory as well as in the field and concluded that the conventional mixtures exhibited higher initial strength characteristics in the laboratory testing than the crumb rubber mixtures. However, the pavement sections constructed with crumb rubber asphalt mixtures showed overall better performance indices (rut depth, fatigue cracks, and IRI numbers) than the corresponding control sections after five to seven years of traffic.
Gawel et al., (2006) also mentioned reduction in reflective cracking of the pavement, noise reduction and reduction in propensity for failure at low pavement temperature as benefits of using crumbed rubber in bitumen. The above conclusions have also been found valid in other studies (Shankar & Prasad, 2009).