Measurement of HMA shear resistance potential in the lab: The Simple Punching Shear Test

Rutting or permanent deformation (PD) is one of the major distresses occurring in hot-mix asphalt (HMA) pavements. The primary mechanism of HMA rutting is shear deformation often caused by large stresses in the HMA layers under traffic loading, particularly at elevated temperatures. The current HMA rutting and PD tests, though have a fairly proven history of successfully identifying and screening HMA mixes that are prone to rutting, are not inherently designed to capture the HMA shear properties such as shear strength, shear strain, and shear modulus. As supplement to the traditional PD and rutting tests, a new HMA test method, namely the Simple Punching Shear Test (SPST), was explored as a potential routine laboratory test to evaluate and characterize the HMA shear properties. The SPST protocol and the input parameters were established through a series of comprehensive trial testing of HMA mixes commonly used in Texas. SPST data analysis models were derived to evaluate various HMA shear parameters including the shear strength, shear modulus, and shear strain energy (SSE). The corresponding results indicated that the SPST has promising potential to routinely differentiate and screen HMA mixes in terms of their shear resistance potentials. The test is fairly simple to run and very repeatable (coefficient of variation of the measured HMA shear strength are less than 10%) with measured shear strength values within range of typical HMA shear strengths as reported in the literature. Additionally the test demonstrated high sensitivity to HMA volumetric design parameters such as the asphalt binder content with HMA shear resistance decreasing with increasing asphalt binder contents. A comparison with the Hamburg Wheel Tracking Test (HWTT), which is a traditional HMA rutting test, showed that while, in general, the HMA rutting/shear properties obtained from the two tests are qualitatively comparable, some differences do exist in terms of the shear performance prediction from the two tests. This evidently indicates that the SPST is able to capture and expose certain shear characteristics of the HMA that the HWTT is not able to capture and vice versa. Overall, this study has shown that the SPST can be a useful tool for characterizing the HMA shear properties and has promising potential to be used as a surrogate rutting/shear test.