Mechanistic-empirical models for top-down cracking initiation of asphalt pavements

Mechanistic-empirical models are developed in this study to characterise top-down cracking (TDC) initiation of asphalt pavements. TDC initiation phase is defined as a stage for micro-cracks to initiate and coalesce into a macro-crack which appears at pavement surface. Micro-fracture mechanics and Miner’s hypothesis are applied as the mechanistic approach to calculate numbers of axle load cycles at different load spectrum levels and corresponding cumulative micro-crack damage in the TDC initiation phase. Long-term pavement performance (LTPP) data including traffic load, pavement distress, material properties, pavement structure and temperature are collected and analysed. Traffic load is characterised using a load spectrum model and TDC initiation time is predicted from historical distress observations. A mathematical model is proposed to characterise the initial air void distribution within surface layer. The LTPP data are utilised to develop mechanics-based prediction models to compute a TDC initiation energy parameter and initiation time. Unaged surface layer modulus, m-value of relaxation modulus of surface layer and pavement structure are identified as key factors for the initiation energy parameter. Traffic load, initiation energy parameter and temperature are critical to the initiation time. As illustrated in the development and validation process, the prediction models show reasonable agreement with TDC field performance.