Revamping aggregate property requirements for portland cement concrete
Current Texas Department of Transportation (TxDOT) procedures for evaluating coarse aggregate for portland cement concrete (PCC) have been in place for over 39 years. Item 421 in the TxDOT "Standard Specifications for Construction and Maintenance of Highways, Streets, and Bridges" describes the tests and test limits that must be met by aggregates before they can be approved for use in portland cement concrete applications. The intention of Item 421 is to ensure that only strong, durable aggregates are used in concrete so that the life of concrete is not cut short by common distress mechanisms which ultimately lead to costly repairs and replacements. The two main tests currently used by TxDOT to evaluate aggregates are the magnesium sulfate soundness test and the Los Angeles abrasion and impact test. These tests are meant to characterize the overall soundness and resistance to abrasion and impact of an aggregate respectively. Unfortunately, past research has shown that the magnesium sulfate soundness and test and the Los Angeles abrasion and impact test are not able to successfully predict the field performance of an aggregate in concrete. The requirements of item 421 have thus far done a reasonably good job of ensuring long-lasting concrete; however the current tests and test limits may be unnecessarily precluding the use of some local materials. As high quality aggregate sources are depleted and transportation costs increase, it will become more necessary to distinguish good performers from marginal and poor performers in the future. If aggregate tests can be found that demonstrate better correlations with field performance, it may be possible to use more local aggregate sources and still provide the desired level of reliability for pavements, bridges, and other TxDOT concrete applications. Researchers are in the processing of collecting coarse and fine aggregates commonly used in Texas and testing these aggregates on a variety of alternative tests. Researchers will attempt to relate this test data to concrete behavior and ultimately recommend tests for improved TxDOT aggregate specifications.