Evaluation of the Linear Damage Assumption in JPCP Bottom-up Fatigue Cracking

Current pavement design methods for Jointed Plain Concrete Pavements assume that bottom-up fatigue cracking occurs at one critical location at the mid-slab edge and that damage accumulates linearly and can be described via S-N curves. More recently, experiments have shown that fatigue does not accumulate linearly. This assumption will lead to an accumulation of prediction error which depends on the sequence of applied loads. In this paper, a modified Paris fatigue equation is used to simulate mid-slab edge fatigue damage. The fatigue equation accounts for the transient and steady state stages of cracking in addition to the peak and stress range effects. An interpolation scheme was used to determine the pavement stresses at the bottom mid-slab edge. The interpolation scheme was constructed from a total of 35,000+ FEM runs in EVERFE. Intra-axle peak and valley stresses were accounted for so load history and R-ratio effects could be assessed simultaneously. Two different climatic regions were considered: Miami, FL and Lansing, MI. The results of this study suggest that the dominant stress and R ratio fall between 0 and 0.1 for both climates. In addition, it seems that although high stress ratios (0.7-0.9) occur infrequently (less than 1% for both cases), the life of the pavement is strongly correlated with high stress ratio events. Finally, it is shown that damage may be under-predicted using a linear damage rule because it does not account for the over-load effect. This under-prediction can be corrected by re-calibrating the percent slabs cracked equation.