Measurement of Flat and Elongation of Coarse Aggregate using Digital Image Processing

Coarse aggregate shape (flat and elongated) and angularity are important issues in the highway industry. Elongated aggregates tend to break down during the asphalt emplacement process, or align themselves, resulting in anisotropic pavements. The use of angular aggregate results in a greater interlock and strength of the asphalt product.

Measurement of particle shape is a tedious subjective procedure, resulting in a great deal of uncertainty about the results, and indeed of the ability of the tests to reasonably characterize the population of aggregates being produced.

Digital image processing is a new approach to measuring these characteristics of aggregates. 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. It also has the potential to report various shape factors, including principal axis ratios, roundness, sphericity and angularity. Simultaneously it will produce size distribution curves, and shape measurement summaries by size classes. Research will be conducted to determine if the percentage of fractured faces can also be measured.