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Asphalt modifications either involved commercial or recycled waste materials modifiers help optimise and improve the deficient properties of asphalt mixtures relative to current standards. This manuscript reviews the recycling of waste materials generated from various sources such as household, agricultural and industrial to minimise natural resources exploitation, lower energy demand, and enhance overall pavement performance. The actual development and implementation of recycled asphalt pavement in numerous countries in Asia, North America, South America, Europe and Africa are emphasised, and their influence on the performance of asphalt pavements is reviewed. Modified pavement showed satisfying performance for years as maintenance and restoration are not required. Generally, solid wastes are required to undergo pre-requisite treatments via mechanical, biological stabilization, thermal or chemical methods before the implementation as asphalt modifiers to ease the handling process. Moisture damage issues have been spotted in the implementation of several types of solid wastes and consequently reduced the strength and durability of pavement. Therefore, potential hydrophobic treatment can be adopted to improve the quality of asphalt mixtures by modifying the hydrophilic characteristics that are contributed by the hydroxyl groups on the cellulose nanocrystals (CNCs) or providing a superhydrophobic coating onto the particles. The treatments greatly weakened the moisture sensitivity of modified materials by reducing their water absorption and increasing their water contact angle. The modified mixtures are necessary to have identical or enhanced pavement performance to the conventional mixtures. Besides, future researchers are encouraged to broaden the research field in pavement engineering by considering chemical treatments in improving material properties to achieve cost-saving, efficient waste management, and higher commercial feasibility. Before implementing any virgin materials, waste materials and by-products as alternative substances or modifiers, it is strongly advised to consider the environmental and economic impacts of the material properties and their optimal incorporation.
Wong et al. (Sat,) studied this question.