Shift Factors for Asphalt Fatigue from Full-Scale Testing

Asphalt fatigue lives determined in the laboratory must be shifted to predict the performance of in-service pavements. Determination of the shift factor to be applied constitutes one of the main challenges of flexible pavements design. Experimental data from four sections tested for 28 months at the CEDEX test track provided detailed information regarding accumulation of asphalt fatigue damage as a function of loads and temperature. The data were used to draw conclusions about the damage process and to determine shift factors for two cracking levels in the field. An effort was made to quantify the beneficial effect of rest periods in the field through use of a variable shift factor that incorporates the methodology for studying this effect and the concept of reduced rest period that was developed in NCHRP 9-38 and 9-44. Results showed that rest periods were not only the main factor responsible for the need for shift factors but also the primary explanation for why most damage took place at medium to low temperatures and not during the warmest season. The asphalt fatigue model in the CalME design procedure has been used successfully to reproduce actual performance observed in the tested sections in terms of modulus reduction, which was previously determined from falling weight deflectometer backcalculation.