Assessment of the Ability of Laboratory Compaction Methods to Simulate Field Compaction of Asphalt Mixtures Based on CT-Scan Analysis of Internal Structure

The study aimed to assess the influence of compaction methods and efforts on the creep behavior and the magnitude and distribution of air voids (AV) and voids in mineral aggregate (VMA) in hot-mix asphalt (HMA). Four distinct compaction methods were examined: the Marshall compactor (with 35, 50, and 75 blows per side), the Hveem mechanical kneading compactor (at 2.7 and 3.4 MPa compaction pressure), the Superpave gyratory (at a 1.25° angle of gyration), and a modified gyratory (at 1.25° and 6.0° angles of gyration). The AV and VMA characteristics in HMA specimens prepared using laboratory compaction methods were assessed through computed tomography (CT)-scan images and compared with those obtained from field-compacted HMA specimens. The processed images were analyzed using ImageJ Software to determine the magnitude and distribution of AV and VMA. The dynamic creep test was used to assess the effect of different compaction methods on the creep behavior of HMA. Analysis of the results revealed that the method of compaction has significant effects on creep behavior and magnitude and distribution of both AV and VMA. The effectiveness of laboratory compaction methods in replicating field compaction varies depending on the specific parameter under consideration, such as AV magnitude, AV average size, VMA magnitude, VMA average size, creep deformation, and the distribution of air voids with depth from the surface. The optimal compaction method for simulating field-compacted HMA specimens with a maximum nominal aggregate size of 19 mm was identified as M 19/75, particularly when considering AV magnitude or the variation of AV magnitude with depth. In terms of AV average area, MG 19/1.25° was determined to be the most effective method, while for both VMA magnitude and VMA average area, MG 19/6° was the superior choice. When evaluating creep deformation, SGC 19/1.25° emerged as the best method for simulating field-compacted specimens. Also, the results indicated that, regardless of the compaction method used, the AV and VMA magnitudes exhibit higher values near the mid-depth of the HMA specimens compared to those near the specimen surface. Although this result was true for all tested HMA specimens, those specimens compacted solely on one side achieved maximum AV and VMA magnitudes at a shallower depth compared to specimens compacted on both sides or field-compacted specimens. The results also showed that the impact of the shear component resulting from compaction force was more pronounced at larger nominal maximum aggregate sizes. Finally, it was found that creep deformation in HMA is highly dependent on AV average area rather than AV magnitude.