Adaptation and validation of stochastic limiting strain distribution and fatigue ratio concepts for perpetual pavement design

Traditional perpetual pavement thickness design is based, in part, on controlling strain levels at the bottom of the asphalt concrete layer below an endurance limit to prevent bottom-up fatigue cracking (FC). A field-based limiting strain threshold was developed from cumulative distributions of field-measured tensile strains in the 2003 and 2006 research cycles at the National Center for Asphalt Technology Pavement Test Track to understand the limiting strain necessary to control FC. Additionally, the fatigue ratio, the ratio of the nth percentile strain to the fatigue endurance limit, was developed. Both the tensile strain distributions and fatigue ratios showed a clear difference between sections that experienced bottom-up FC and those that did not. However, it is necessary to adapt these thresholds to strains predicted by perpetual pavement design tools. PerRoad, a stochastic perpetual pavement design programme, was used to predict strains for the same 2006 sections. Previously developed strain distributions and fatigue ratios were adjusted to reflect observed differences in predicted and measured strains. Cumulative distributions and fatigue ratios based on predicted strains for the 2009 research cycle validated the updated limiting strain distribution and maximum fatigue ratios for designing perpetual pavements to resist bottom-up FC.