The purpose of this study is to investigate the effect of size distribution and concentration of crumb rubber on the performance characteristics of porous asphalt mixture. The recycling of scrap tires in asphalt pavements appears as an important alternative providing a large-scale market. The characteristics of bitumen are very important with regard to service life of porous asphalt pavement. The experimental study consists of two main steps. Firstly, the mixture design was performed to determine the optimum bitumen content. In the latter step, the mixtures were modified by dry process using crumb rubber in three different grain size distributions of #4~#20, #20~#200, and #4~#200 and rubber content of 10%, 15%, and 20% as weight of optimum bitumen. The permeability, Cantabro abrasion loss, indirect tensile strength, moisture susceptibility, and resilient modulus tests were carried out on the specimens. Test results show that #20~#200 sized rubber particles reduced air voids and coefficient of permeability, while they increased the Cantabro abrasion loss. In general, increasing the crumb rubber size and content decreased the performance characteristics of the porous asphalt mixtures. 1. Introduction The worn-out tires from vehicles leave billions of waste tires every year becoming a significant source of waste materials. Scrap tires are still a serious environmental and financial issue for many countries in the world occupying landfill spaces and becoming a threat for health and safety hazards to the community. The scrap tires consist of rubber, carbon black, steel, and so forth potentially to be very useful in various applications which have been evaluated effectively as a valuable resource. There are some different recycling strategies developed for waste tires. The main markets in the assessment of waste tires include tire-derived fuel (TDF), civil engineering applications, and ground rubber. The recycling of waste tire has a good trend. However, it has still potential to consume more waste tires. In addition, TDF corresponds to 54% of the total scrap tires which may not represent an ideal application in the current recycling methods for waste tires from environmental conservation perspective [1]. More value-added alternatives are required to be discovered in order to motivate public and private agency to recycle them. One of the approaches of recycling scrap tires is to use crumb rubber from the tires as a component in asphalt pavement mixtures. The crumb rubber is combined in asphalt mixtures to improve the performance of asphalt concrete pavements.
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