Internal structure characterization of asphalt concrete using image analysis

The performance of asphalt concrete (AC) mixtures is influenced by the arrangement of aggregates and their associated air voids. Parameters to measure aggregate orientation, aggregate gradation, and air void distribution in AC mixes are proposed. Computer automated image analysis procedures were used to measure these parameters. The air void distribution was characterized using X-ray tomography images. The new parameters were used to study the evolution of the internal structure of AC mixes during laboratory compaction by the Superpave Gyratory Compactor and in the field. The preferred orientation of the aggregate structure in the laboratory was found to increase with compaction up to a certain compaction effort. Thereafter, the aggregate structure tended to have more random orientation. Percent voids measured on X-ray tomography images compared well with percent voids measured in the laboratory. The void distribution in the specimens was found to be nonuniform. More internal voids were concentrated at the top and the bottom portions of the gyratory compacted specimen. The gyratory compacted specimens reached the initial aggregate orientation of the field cores at a higher number of gyrations whereas they reached the percent air voids in cores at a lower number of gyrations. Coarse aggregate gradation of gyratory compacted specimens was well captured using the image analysis techniques. There was no change in gradation with compaction.