TOWARD AUTOMATING SIZE-GRADATION ANALYSIS OF MINERAL AGGREGATE

One of the most important properties of an aggregate blend is its size gradation, which defines the percentages (by weight or volume) of different particle sizes that are present in the blend. Since aggregate represents more than 90% of a hot asphalt mix, aggregate gradation profoundly influences the properties of the hot mix (such as air voids, workability, and asphalt binder required) and the properties of the pavement (such as stiffness, stability, and durability). Aggregate gradation is determined by the well-known and widely used sieve analysis method. One major drawback of this method is the consumption of time and effort. The time factor is a major barrier to implementing sieve analysis as an aggregate gradation control measure in asphalt plants where production rates are high and interruptions to the production process are very undesirable. There has been some progress in the automation of gradation analysis of the coarse portion of an aggregate blend. The gradation of the fine portion, however, has major effects on the properties of an aggregate blend. A simple and low-cost approach to automate the gradation analysis of fine aggregate is presented in this paper. The approach is based on the concept of differential settling of aggregate particles in a fluid medium because of differences in particle sizes. It is essentially a fractionating-column methodology. A prototype system was fabricated to test particles passing the No. 8 sieve (or < 2.38 mm in size). The system is described in the paper, along with some example results.