Volume 28, Issue 2 (March 2000)
Comparison and Evaluation of Tests for Coarse Aggregate Particle Shape, Angularity, and Surface Texture
Particle shape, angularity, and surface texture are critical properties in assessing aggregate usage for asphalt concrete. Fractured and flat and/or elongated particles are used in most specifications to assure quality. The particle index and uncompacted void tests indirectly measure particle shape, angularity, and surface texture and offer alternative options. Several methods were used to test crushed gravels and crushed stones. Test results were compared and evaluated. Fractured and flat and/or elongated particle tests are somewhat subjective for evaluating aggregate structural quality. The basis for determining percentages (mass or particle count) for fractured faces did not make any difference. However, the basis (mass or particle count) for computing percentages of particles with particular dimensional ratios (flat and/or elongated) should be clearly stated in specifications since the results are distinctly different. Procedures in ASTM D 4791 contain two methods for assessing particle shape, i.e., flat or elongated particles and flat and elongated particles. Percentages of flat and elongated are larger than flat or elongated.
Particle index (ASTM D 3398) and uncompacted void (proposed) tests provide comparable measures of aggregate particle shape, angularity, and surface texture. They are somewhat more analytical and precise than the “fractured particle” and “flat and/or elongated” tests. Flat and/or elongated particles have a contrary influence on the assessment of aggregate quality with particle index and uncompacted voids. Flat and/or elongated particles are generally thought undesirable, but increase both particle index and uncompacted voids, which is generally thought desirable.