Development of Thin HMA Overlay Crack Initiation and Progression Probabilistic Models

This report presents the results of a study to evaluate fatigue cracking performance models for hot-mix asphalt (HMA) overlays placed on existing HMA pavements. Previously, data from the Washington State Department of Transportation (WSDOT) pavement management system (PMS) were used to develop separate models for crack initiation and progression. Later, these two models were combined into a combined model for the entire cracking process. The study presented in this report evaluates the combined model using the limited California PMS data available. This was done to assess the model’s ability to predict the performance of HMA overlays of cracked asphalt pavement in California. An earlier study identified that data in the WSDOT PMS primarily reflects the performance of overlays of 0.15 ft (45 mm) on pavement with low levels of existing cracking. However, a sensitivity analysis performed in this study indicates that the performance trends for California overlays predicted by the model are reasonable. In a performance comparison of crack initiation between Washington State and California using PMS data from each state, the model underpredicted the performance of California overlays of similar thickness. Although there was insufficient data available to validate the model for crack progression, comparison results indicated that the model likely overpredicts the performance of California overlays for crack progression because of differences in pavement preservation practices conducted by Caltrans and the WSDOT in the late 1990s and early 2000s. As a result, this study recommends performance of a recalibration of the variable coefficients once better California PMS data is available. Revised coefficients would reflect differences in WSDOT and Caltrans practice and conditions, including the use of thicker overlays in California and the historic Caltrans practice of placing overlays at more advanced states of cracking in the existing pavement, California’s more benign climate conditions (less rain and fewer freeze-thaw cycles), and possible differences in scheduling of pavement preservation activities. To accomplish these changes, it is recommended that the Caltrans PMS database be populated with information collected over consistently segmented sections, and with accurate information regarding overlay material types and thicknesses of overlays and existing pavement structures.