Application of waste materials in asphalt mixtures

Road networks are increasingly expanding all over the world. Over the past decades, the buoyant economy, growing population and increasing freight volumes have created high demand for new road pavements as well as the maintenance of current road networks. The construction and maintenance of the road pavements require large amounts of aggregates. Due to considerable usage of various natural aggregates for constructing roads, these materials have started to deplete gradually. At the same time, the generation rate of solid waste in the society is increasing with the increase of population, technological development, and changes in the life style of people. The management of solid wastes has become an acute problem due to enhanced economic activities and rapid urbanization. Moreover, according to World Commission on Environment and Development (1987), sustainable development is defined as the development that meets the needs of the present without compromising the ability of future generations to meet their own needs. Therefore, referring to the importance of the sustainability and all the above mentioned issues, the utilization of recycled materials and waste materials in the manufacture of new asphalt mixtures is addressed in this research, in order to follow the sustainability concept. Using waste materials in asphalt mixtures conserves natural aggregates, reduces the impact on landfills, decreases energy consumption, can provide cost savings, and generally results in significant economic and environmental benefits. In spite of the benefits mentioned above, designing asphalt mixtures containing waste materials can be challenging and complex. In this regard, it is necessary to review a wide variety of engineering and geotechnical considerations for asphalt mixture design, as the application of wastes should not influence the structural and functional aspects of the pavements. Among various factors, which would be considered in designing an asphalt mix, permanent deformation, and stress/strain characteristics are the most important factors from the perspective of structural and geotechnical stability. Evaluation of asphalt mixtures in terms of these engineering properties is necessary as they define the structural performance and strength of an asphalt pavement. In fact, these properties influence the ability of the mix in withstanding the destructive effects of traffic and environment. In light of this, many researchers have studied the behaviour of asphalt mixtures containing waste materials through different tests and approaches to predict the performance of the asphalt surface layer under traffic load and environmental conditions. Today, there are many published reports about the application of various waste materials in construction of different layers of pavements including asphalt surface layer. The research studies have been conducted from simple and primary tests to the advanced laboratory investigations, field tests, and constitutive modelling of asphalt surface layer. Due to the wide range of materials that fit into the definition of pavement, this proposed research does not intend to cover the investigation of the application of all types of materials available. Instead, it focuses on the assessment of the utilization of some of the waste materials including Recycled Construction Aggregate (RCA), Glass, plastic, and rubber in Dense Graded Asphalt (DGA) mixture, and it aims to propose a mixture with proper response to traffic loads and environmental conditions as well as appropriate resistance to rutting, cracking, and weathering. In this report, a critical and comprehensive literature review about the performance of the asphalt mixture incorporating selected waste materials is provided. Moreover, this report includes a brief description of the relevant tests commonly specified to provide confidence of material suitability for the intended application as well as the results of these tests. Based on these tests, it is concluded that these waste materials have serious shortcomings in accounting for the asphalt mixtures components. They are also unable to account for designing asphalt mixture individually, and therefore, it is required that combination of some waste materials in certain percentages be considered in designing the asphalt mixture. One of the goals of the proposed research is to determine the optimum percentage of RCA and glass in the mixture, and also to investigate the influence of the application of rubber and plastic in binder characteristics. This will be done through a series of preliminary and primary tests to evaluate the properties of individual components of asphalt mixtures and the volumetric properties of the mixture. The information and data collected from these tests will be used to select the adequate samples for further assessment through advanced tests such as resilient modulus test. Since the variability in the behaviour of RCA used in different construction projects indicates the variability in RCA composition. Therefore, this research investigates the composition and variability of RCA through classification of aggregate samples collected from a recycling centre in Sydney over one year. Furthermore, to demonstrate the effect of the utilization of waste materials on the internal structure of the asphalt mixtures, microstructure studies of RCA and the image analyses on different asphalt mixtures is carried out using a scanning electron microscope (SEM). This research will also include the simulation of the asphalt surface layer in order to understand the permanent deformation of asphalt surface layer. In addition, the life cycle assessment (LCA) and cost analysis of designed asphalt mixtures will be evaluated in order to compare them with conventional asphalt mixtures. The results suggest feasible use of RCA in a certain amount as partial aggregate substitution in hot mix asphalt in terms of engineering properties and also environment and economic aspects of asphalt production. Furthermore, the test results reveal that glass waste is a viable material for improving the shortcomings of asphalt mixtures containing RCA