Analysis of South Texas Aggregates for Use in Hot Mix Asphalt

The primary locally available aggregates in the South Texas districts for highway construction are siliceous river gravels, which typically exhibit a rounded shape. The warm South Texas climate combined with heavy truck traffic aggravates any rutting problems that may occur due to the use of low angularity aggregates in hot mix asphalt (HMA) paving mixtures. The objective of this study was to determine the effect of aggregate angularity and nominal maximum size of siliceous aggregates on performance of selected HMA surface pavement mixtures. The research approach was guided by the realization that engineers can use the results for mixture design, especially for South Texas districts. Aggregates and mixture designs were obtained from the Corpus Christi, Laredo, and Pharr Districts. The selected aggregates were characterized using image analysis, crushed face count, flat and elongated particles, and Micro-Deval tests. Researchers assessed performance of the HMA based on the simple performance tests and the Hamburg test. Comparison of aggregate properties with mixture performance yielded important information about optimal design of mixtures containing siliceous gravels. These findings do not support the use of finer South Texas gravel mixtures (Type D and 9.5 mm) with more crushed faces in the coarse aggregate to maximize rutting resistance. In fact, the finer mixes most often demonstrated the least rutting resistance in the simple performance tests. A decrease in nominal maximum aggregate size (NMAS) may adversely affect HMA rutting resistance unless it is offset by improved aggregate shape characteristics. Type C and 12.5-mm materials generally demonstrated the optimum rutting performance in the simple performance tests, however, the fine-graded 12.5-mm mixture performed poorly in the Hamburg test. Cracking resistance of the mixes was not significantly affected by the change in NMAS.

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