Fatigue and Fracture Properties of Asphalt Mixes Containing Low Content of Crumb-Rubber Modifier

The California Department of Transportation has proposed to add a low amount of crumb-rubber modifier into dense-graded asphalt mixes, in addition to current policies of using gap- and open-graded mixes with a high content of rubber (around 20% by weight of binder) as the surface layers. The objectives are to reduce landfill disposal of scrap tires and to obtain equal or better performance compared to current dense-graded mixes containing unmodified binders. This paper investigated a new type of rubberized dense-graded mix containing 5% and 10% of the crumb-rubber modifier. The rubberized binder and the unmodified binder were compared at the same performance grade to meet the specific climate requirement. Their corresponding mixes met the Superpave volumetric design requirements. The laboratory experiment evaluated the rubberized mix and the conventional mix in terms of fatigue and fracture properties using a flexural bending fatigue test and semicircular bending test, respectively. A long-term oven-aging protocol and the uniaxial thermal stress and strain testing were used to evaluate the mix resistance to low-temperature cracking. Those laboratory test results indicated that the rubberized dense-graded mix had an extended fatigue life, higher fracture resistance, and better thermal cracking resistance than the standard dense-graded mix. Mechanistic-empirical (ME) design using the California ME design software simulated the performance of these mixes in various pavement structures and climate/traffic conditions to assess the interactions of stiffness and strain-controlled fatigue property for those mixes. The simulation results revealed that the new rubberized dense-graded mix mostly provided the same or better field performance as the conventional mix. The ME design analysis should be performed to optimize the application of those mixes in a case by case scenario.