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

Moisture damage in asphalt pavements is a complex phenomenon affected by a variety of factors, and has not been fully understood, with major knowledge gaps in three areas: major factors contributing to moisture damage in the field, appropriate laboratory test procedures, and the effectiveness of treatments. Both field and laboratory investigations were performed in this study to provide additional information in these three areas. Statewide condition survey and field sampling were conducted to identify factors contributing to moisture damage, other than aggregate source. Statistical analysis revealed that air-void content, pavement structure, cumulative rainfall, mix type (DGAC vs RAC-G), use of anti-strip additive (lime or liquid), 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 in wet conditions. Less than optimum binder contents also reduce the moisture resistance of asphalt mixes under repeated loading. The effectiveness of the Hamburg Wheel Tracking Device (HWTD) test to determine moisture sensitivity of asphalt mixes was evaluated by testing both laboratory-fabricated specimens and field cores. It was found that the test can correctly identify the effect of anti-strip additives; its results generally correlate with field performance except that the test may sometimes fail mixes that perform well in the field and, in a very few cases, provide false positive results. A fatigue-based test procedure for evaluating moisture sensitivity was explored in this study. A test procedure was developed for comparative evaluation of different mixes. Application of the test procedure for use in pavement analysis/design is suggested for expensive projects. The long-term effectiveness of both hydrated lime and liquid anti-strip agents was evaluated by both the tensile strength ratio (TSR) test and the fatigue beam test. Results showed that both types of treatment are effective in preventing moisture damage for up to one year’s continuous moisture conditioning in the laboratory. A database with all field and laboratory results has been prepared for Caltrans.