Measurement and Prediction of Fundamental Tensile Failure Limits of Hot Mix Asphalt (HMA)

The purpose of this research program is to provide an insight into key mechanisms and asphalt mixture properties that control fracture in asphalt materials. A Digital Image Correlation (DIC) (non contact, full-field, surface displacement/strain measurement technique) was developed on purpose for accurately capturing localized or non-uniform stress distributions in asphalt mixtures and as a tool for detecting first fracture. The experimental analysis of asphalt mixture cracking behavior was based on the “HMA Fracture Mechanics” visco-elastic crack growth law (Zhang et al. 2001; Roque et al.; 2002). Investigation of the asphalt cracking mechanism and identification of fundamental tensile failure limits were achieved performing multiple laboratory test configurations, namely the Superpave Indirect Tensile Test (IDT), the Semi-Circular Bending Test (SCB) and the Three Point Bending Beam Test (3PB). Both unmodified and polymer modified mixtures were tested. The results presented herein illustrate that the DIC method could be used to reliably determine asphalt tensile failure limits at first fracture. It was found that these failure limits are independent of the specimen geometry and of the test configuration. Also, importantly the tensile failure limits were shown to be sensitive to both presence and level of polymer modification. It is also shown that first fracture and crack growth in asphalt mixtures can be predicted effectively using a Displacement Discontinuity (DD) boundary element method, for various different boundary condition problems, and not just for the calibrated laboratory test conditions. The numerical simulations and the DIC results presented show that significant damage, stress redistribution and other changes following initial fracture make the analysis at peak load difficult to interpret meaningfully. The effect of polymer modification on crack localization was also investigated through the use of horizontal full-field strain maps obtained from the DIC. It was found that polymer modified mixtures exhibit high strains only up to the location of impending fracture, while unmodified mixtures exhibit highly distributed damage in both the critical area and around the point of fracture.