Description
One of the main requirements of designing perpetual pavements is to determine the endurance limit of Hot Mix Asphalt (HMA). The endurance limit is the strain or stress level below which no fatigue damage occurs or can be healed during unloading. If the pavement thickness is increased so that the strain at the bottom the asphalt layer is kept below the endurance limit, the pavement would endure indefinite load repetitions and would not experience bottom-up fatigue cracking. Field observation shows that endurance limit for HMA does exist. The endurance limit values were determined in the laboratory in the NCHRP Project 9-44A for conventional mixtures at different conditions. A concentrated research effort, however, is needed to validate the endurance limit concept for Asphalt Rubber (AR) mixtures. The purpose of this study was to determine the endurance limit values for AR-HMA using laboratory beam fatigue tests. The paper discusses the results of a pilot study that produced a preliminary estimation of the endurance limit for AR mixture placed in Sweden. This study included 24 beam fatigue tests conducted according to the AASHTO T321-03 test procedure. Two factors that affect the fatigue response of HMA were evaluated: the applied strain and the rest period between loading cycles. A model was developed to determine the stiffness ratio as a function of strain and rest period. The endurance limit was determined when no accumulated damage occurred indicating complete healing. Endurance limit values for this AR mixture ranged from 150 to 175 microstrains, which are significantly higher than those of conventional mixtures. Results from this study can be used in future design considerations as well as by agencies which seek to implement the asphalt-rubber technologies.
