Effect of crumb rubber on the mechanical properties of asphalt mixtures

In recent years, the growing number of vehicles in both developed and developing nations has led to the annual generation of millions of used tires. With around 1.4 billion tires sold globally each year and their limited lifespan, tire waste has become one of the most significant and challenging environmental issues. Research and practical applications have shown that incorporating crumb rubber into asphalt is an effective way to enhance asphalt performance while providing an eco-friendly solution for recycling used tires in flexible pavement construction. This study investigated the properties of asphalt mixtures and modified asphalt containing varying proportions of fine crumb rubber (0.177 mm), following two different standards: the American Society for Testing and Materials (ASTM) and the Ministry of Road Transport and Highways (MoRTH). Marshall tests were conducted to determine the optimal bitumen content (OBC) and evaluate the modified mix properties. A total of 30 samples were prepared, with 12 used to establish the OBC and the remaining samples testing different crumb rubber percentages in the asphalt mixture. The results revealed an OBC of 5.6% for ASTM specifications and 5.25% for MoRTH specifications by weight of the asphalt mixture. The study examined the effects of crumb rubber additions (3%, 5%, 8%, 10%, 12%, and 15% by weight of bitumen) on stability, density, stiffness, and void properties. The findings indicated that crumb rubber effectively improves asphalt mixture performance, with optimal percentages of 3.8% (ASTM) and 5% (MoRTH) by weight of bitumen.