Evaluation of Coarse Aggregate in Cold Recycling Mixes Using X-Ray CT Scanner and Image Analysis

The performance of cold recycling (CR) mixtures has been widely evaluated, but these studies rarely focused on the internal structure of CR mixtures. The objective of this research is to investigate the distribution, orientation, and shape properties of coarse aggregates in CR mixtures with emulsion by processing a series of sectional images from an X-ray CT scanner. The uniformity index (UI) was derived to describe the distribution of coarse aggregates. The aggregate orientation was obtained by calculating the average angle of inclination theta and the coherence of orientation Delta. The morphological indices, such as the flat and elongated ratio (FER), angularity index (AI), and surface texture index (STI) were used to address the shape properties of CR mixtures. According to the results, the distribution uniformity of CR aggregates becomes worse with coarser gradation. Compaction methods significantly influence the aggregate orientation in CR mixtures. The superpave gyratory compaction (SGC) method makes the best coherence of aggregate orientation, but the static load compacted specimen shows the opposite result. The CR mix tends to have worse uniformity of aggregate distribution, larger angle of aggregate inclination, and more random aggregate orientation compared with the HMA mix. It is estimated that more crushed faces of aggregates are produced in the process of pavement milling and the surface texture of RAP materials decreases with the long-term effects of traffic loading and environment. Adding new coarse aggregates to CR mixtures disturbs the aggregate distribution, but it can improve the average surface texture of aggregates in CR mixtures.