Comparison of Shape Parameters for Selected Coarse Aggregates in Oklahoma

Abstract The performance of pavement can be significantly influenced by the gradation and shape parameters of the aggregates (i.e., angularity, texture, form, and sphericity). An accurate measurement of these properties is important for the development of specifications for the quality control and quality assurance of aggregates. The present study was undertaken in order to compare the shape parameters of different types and sizes of selected coarse aggregates that are commonly used in Oklahoma. Three different types of aggregates—granite, rhyolite, and limestone—were collected from stockpiles. Each type of aggregate was divided into three different sizes of coarse aggregates. These sizes include the following: passing through a 19 mm sieve and retained on a 12.5 mm sieve (CA1), passing through a 12.5 mm sieve and retained on a 9.5 mm sieve (CA2), and passing through a 9.5 mm sieve and retained on a 4.75 mm sieve (CA3). CA1 type aggregates had the largest size, followed by CA2 and CA3. An automated aggregate imaging measurement system was used to measure the shape parameters of the aggregates. The results show that the aggregates become smooth (low texture) and elongated (high form and low sphericity) as their size decreases. No relation was observed between the particle size and the angularity of the aggregates. A comparison of different types of aggregates indicated that, in general, granite aggregates were highly rough (high texture) and cubical (low form and high sphericity) compared to rhyolite and limestone aggregates. Similarly, the angularity of granite aggregates was found to be significantly different than that of rhyolite and limestone aggregates. The ranking of the aggregates was done based on the composite shape index factor (CI) for angularity, form, texture, and sphericity. The CI considers the combined effect of the gradation and shape parameters of aggregates. In addition, the ranking of aggregates was further verified by estimating the dynamic modulus (|E*|) of the mixes composed of granite, rhyolite, and limestone aggregates. The aggregate shape-based model developed by the authors was used to estimate |E*| of a mix using the aggregate shape parameter, frequency, viscosity of the asphalt binder, and volumetric properties of a mix. |E*| of a mix is considered as an important parameter in estimating the performance of flexible pavements in terms of fatigue, rutting, and low temperature cracking. Therefore, the ranking of aggregates based on |E*| of the mixes provides a better understanding of the performance of the aggregates. The results show that both methods for ranking the aggregates (i.e., CI factor and |E*|) showed similar results. Overall, the mix with rhyolite aggregate was ranked first, followed by the mixes with limestone and granite aggregates.