Use of Stiffness of Hot-Mix Asphalt as a Simple Performance Test

The objective was to determine whether the stiffness of a mix could be used as a simple performance test (SPT) parameter to complement the Superpave® volumetric mix design. This was investigated by a statistical analysis of the strength of the correlation between different mixture stiffness parameters and field performance (rutting, thermal, and fatigue cracking). A total of 30 mixtures were tested with laboratory-fabricated specimens. The studied stiffness parameters were compressive dynamic (complex) modulus |E*|, simple shear tester (SST) shear (complex) modulus |G*|, and dynamic elastic modulus Ed, obtained from ultrasonic wave propagation. Also, computed stiffness factors |E*|/sin ϕ and |G*|/sin ϕ for rutting and |E*|sin ϕ for cracking were studied as analogous to the Superpave binder specification. Research indicated that the correlation to rutting varied based on test temperature and frequency; it peaked at 54.4°C and 5 Hz. At peak conditions, |E*|/sin ϕ had better statistical correlation to rutting than |E*|, but correlations reversed at lower frequencies. Although |E*| and |G*| had similar correlation to rutting, analysis of test data indicated that the SST shear testing gave lower stiffness values and higher phase-angle values than the compressive dynamic modulus testing, even when Poisson’s ratio effects were considered. This was especially true at high temperatures. Because of these and other reasons, the dynamic modulus |E*| was recommended as the SPT parameter for rutting as well as for fatigue cracking. None of the studied parameters turned out to be adequate performance indicators for thermal cracking.