Investigation of Conditions for Moisture Damage in Asphalt Concrete and Appropriate Laboratory Test Methods

Moisture damage is the progressive deterioration of asphalt mixes by loss of adhesion between asphalt binder and aggregate surface and/or loss of cohesion within the binder due to water. It is a complex phenomenon affected by a variety of factors, and has not been fully understood in the pavement community with major knowledge gaps in three areas: major contributing factors to moisture damage in the field, appropriate laboratory test procedures, and the effectiveness of treatments. Both field investigation and laboratory investigation were performed in this study to fill up some of the major gaps. Statewide condition survey and field sampling were conducted to identify factors contributing to moisture damage. Statistical analysis revealed that air-void content, mix type, pavement structure, cumulative rainfall, and pavement age significantly affect the extent of moisture damage. Laboratory experiments revealed that high air-void contents not only allow more moisture to enter mixes, but also significantly reduce the fatigue resistance of mixes wet conditions. Reduction in the binder content from the optimum binder content may significantly reduce the moisture resistance of asphalt mixes under repeated loading. The effectiveness of Hamburg wheel tracking device (HWTD) test to determine moisture sensitivity of asphalt mixes was evaluated by both laboratory prepared specimens and field cores. Results revealed that the current test procedure does not clearly distinguish mixes with different moisture sensitivities. The test tends to overestimate the performance of mixes containing conventional binders and underestimate the performance of mixes containing polymer modified binders. Several ways to improve the prediction accuracy of the HWTD test were suggested. As a new approach of testing, a fatigue based test procedure for evaluating moisture sensitivity was explored in this study. A typical test procedure was determined for comparative evaluation of different mixes, which is a controlled-strain flexural beam fatigue test performed at 20°C, 10 Hz and 200με on specimens pre-saturated under 635 mm-Hg vacuum for 30 minutes and preconditioned at 60°C for one day. An extension of the test procedure for use in the pavement design was also discussed. The long-term effectiveness of both hydrated lime and liquid antistripping agents in improving the moisture resistance of asphalt mixes was evaluated by both the tensile strength ratio (TSR) test and the fatigue beam test. Results showed that both treatments are effective even after one year’s moisture conditioning.