Compaction Effects on Uniformity, Moisture Diffusion, and Mechanical Properties of Asphalt Pavements

Field compaction of asphalt mixtures is an important process that influences performance of asphalt pavements; however there is very little effort devoted to evaluate the influence of compaction on the uniformity and properties of asphalt mixtures. The first part of this study evaluated relationships between different field compaction patterns and the uniformity of air void distribution in asphalt pavements. A number of projects with different asphalt mixture types were compacted, and cores were taken at different locations from these projects. The X-ray Computed Tomography (X-ray CT) system was used to capture the air void distributions in these cores. The analysis results have revealed that the uniformity of air void distribution is highly related to the compaction pattern and the sequence of different compaction equipment. More importantly, the efficiency of compaction (reducing air voids) at a point was found to be a function of the location of this point with respect to the compaction roller width. The results in this study supported the development of the "Compaction Index (CI)," which quantifies the degree of field compaction. The CI is a function of the number of passes at a point and the position of the point with respect to the compaction roller width. This index was found to correlate reasonably well with percent air voids in the pavement. The CI calculated from field compaction was also related to the slope of the compaction curve obtained from the Superpave gyratory compactor. This relationship offers the opportunity to predict field compactability based on laboratory measurements. The compaction of longitudinal joints was investigated, and recommendations were put forward to improve joint compaction. The air void distributions in gyratory specimens were related to the mixture mechanical properties measured using the Overlay and Hamburg tests. The second part of this study focused on studying the relationship between air void distribution and moisture diffusion. A laboratory test protocol was developed to measure the diffusion coefficient of asphalt mixtures. This important property has not measured before. The results revealed that the air void phase within the asphalt mixtures controls the rate of moisture diffusion. The measured diffusion coefficients correlated well with the percent and size of connected air voids. The measured diffusion coefficient is a necessary parameter in modeling moisture transport and predicting moisture damage in asphalt mixtures. The last part of this study investigated the resistance of asphalt mixtures with different percent air voids to moisture damage by using experimental methods and a fracture mechanics approach that accounts for fundamental material properties.