Cracking characteristic of asphalt rubber mixtures

The Arizona Department of Transportation (ADOT) has used Asphalt Rubber (AR) modified binders since the early 1970’s. The primary purpose for using AR is to reduce reflective cracking in Hot Mix Asphalt (HMA) rehabilitation overlays. The AR mixtures have also performed well in cold climate conditions. This research study had the primary objective of conducting a laboratory experimental program to obtain typical cracking properties for asphalt rubber mixtures used in Arizona and comparing the performance of these AR mixtures to other conventional asphalt mixtures. Gap and open graded mixtures were subjected to fatigue and indirect tensile cracking tests. All test specimens in this study were prepared using hot mix AR mixtures that were collected during construction. Fatigue testing of AR specimens was conducted at different test temperatures using the beam fatigue apparatus proposed by the Strategic Highway Research Program (SHRP). The indirect tensile strength and creep tests were carried out at three temperatures according to the procedures described in the draft indirect tensile test protocol developed for the new 2002 Design Guide. The results from the fatigue tests indicated that the AR mixtures would have longer fatigue life compared with the ADOT conventional dense graded mixtures. For the indirect tensile strength tests, the analysis for strains measured at failure showed that the AR mixtures have higher values than the conventional mixes. AR mixtures exhibiting higher strains at failure would have higher resistance to thermal cracking. The fracture energy results indicated that the AR mixtures are not as greatly affected by the decrease in temperature as compared to the conventional mixes. This relative insensitivity for changes in temperature makes the AR mixtures better resisting to thermal cracking in the field.