Flat and Elongated: Advances Using Digital Image Analysis

Coarse aggregate shape (flat and elongated) is an important issue in the highway industry. Elongated aggregates tend to break down during the asphalt emplacement process. Flat aggregates tend to lay flat, imparting anisotropic properties to the finished product. The new Superpave guidelines call for no more than 10% of coarse aggregates to have aspect ratios greater than 5:1, and there is a possibility of a new standard at 3:1. Digital image processing is a new approach to measuring these characteristics of aggregates. Currently manual measurements using calipers are employed for flat and elongated. These measurements are not only slow and laborious, but are also highly subjective. Because they are so inefficient and expensive, there tend not to be enough measurements to produce a statistically valid sample. Two-dimensional shape analysis is possible using such systems as WipFrag or the VDG-40, systems that were originally designed for granulometry rather than for shape measurement. The problem with this approach is that particles tend to lay or fall flat, consequently it is elongation that is measured, rather than the more critical flatness or flat and elongation. This two-dimensional approach has been rejected by the industry as being inadequate. A new measuring system called WipShape has recently been developed. It images each particle separately, and from two views. A mini-conveyor system has been developed to parade individual fragments past two orthogonally oriented synchronized cameras fed by aggregates from a hopper. This system designed for continuous automated operation, measures flat and elongated aggregates at ratios of 5:1, 4:1, 3:1, and 2:1. Simultaneously it will produce size distribution curves, and shape measurement summaries by size classes.