Investigation of Deflection Basins to Identify Structural Distresses within Flexible Pavements

Abstract

Cracking in pavements is often considered a random phenomenon that is difficult to predict and diagnose. It would be beneficial to pavement managers to identify pavements that are likely to have cracking in the near future for planning purposes. The mode of cracking is also an important factor for road managers because the type of distress should dictate maintenance or rehabilitation decisions. The objectives of this work were to utilize falling weight deflectometer (FWD) testing to identify structural changes in flexible pavements prior to cracking and to determine the mode of cracking once cracking has occurred. These objectives were met by comparing FWD deflection basins to theoretical deflection basins of simulated distress modes created using linear-elastic analysis. FWD deflection basins on pavement structures with simulated delamination between asphalt concrete (AC) layers, top-down cracking (TDC), and bottom-up fatigue cracking (BUFC) were simulated using BISAR 3.0. The TDC and BUFC simulations were generated by lowering the AC moduli values for the cracked layers. Deflection basin parameters (DBP) were used to capture changes in the pavement structure without backcalculation. DBPs generated from the simulated cracking modes were compared to DBPs from FWD testing data from the NCAT Test Track. Each section had fixed FWD testing stations and FWD data were scrutinized for 165 unique testing locations within 19 sections. Prior to making the comparison between field and theoretical DBPs, it was essential to determine when a given FWD testing location was cracked. An investigation was conducted to determine the distance at which a discontinuity (crack) begins to influence the deflection basin measured by the FWD. Based on the results, an FWD station was classified as cracked if cracking was observed within 1 foot on a given date. The comparison of the change in field DBPs to the theoretical change in DBPs was successful for the delamination and BUFC sections. The TDC comparison was not successfully conducted but the methodology showed promise based on a limited amount of TDC in the other sections. Seven DBPs that focused on the AC were utilized and their effectiveness was assessed. D0 was found to be the most sensitive DBP and AREA and F1 were the least sensitive. In summary, the assessment of DBPs from deflection basins measured at the same location over time provided the ability to predict when cracking was going to occur and if the cracking was due to an issue in the entire AC structure.