A Comprehensive Laboratory and Field Study of High-Cure Crumb-Rubber Modified Asphalt Materials (1460-1)

This project was a comprehensive study of methods for incorporating ground tire rubber in asphalt and of the resulting properties and potential pavement serviceability. Potential benefits include improved pavement durability through reduced rates of oxidative hardening, improved Superpave performance grade on both the high and low end grades, and utilization of a waste material. The degree of performance grade improvement depends upon the amount of rubber and the extent of the cure. In general, higher cure results in less increase in both high and low end improvement. However, higher cure also results in improved stability to storage settling and improved high-temperature (hot-mix temperature) viscosity and thus improved mix compaction in dense-graded mixes. A higher degree of curing can be achieved by higher shear and higher temperature, but also by air curing, which also can produce significant increases in the upper end performance grade. Over-curing can result in reducing significantly the benefit of rubber on performance grade, but the positive effect on reducing hardening rates remains. Field tests of a high-cure asphalt rubber material at two levels of rubber in the binder (13.5 and 17.6 wt%) were placed in Bryan, Texas, in summer 1998. Economic analysis indicates a favorable life-cycle benefit if only a 15 to 20 percent improvement in pavement life is achieved, a level which is likely, based on laboratory aging tests. Field tests of two additional rubber contents in the binder (8 and 12 wt%) were placed in League City, Texas, in June 2000.