Effects of Coarse Aggregate Coatings on Concrete Performance

摘要

There is evidence that the presence of certain microfine coatings (≤ 75 μm) on coarse aggregates encountered in actual field conditions and accepted for state projects has adverse consequences for the properties of hardened concrete. The purpose of this research was to investigate the influence of natural coarse aggregate coatings on physical properties of concrete and to establish how each of the three common monitoring tests (ASTM C117, Caltrans Test No. 227, and AASHTO TP57) predict the influence of these coatings on the quality of the resulting concrete. Seven mixtures of concrete were prepared with coarse aggregate containing two types of coatings: natural field-coated aggregates, and manufactured coated samples of dust and clay fines. The effects of these coatings on concrete performance were assessed by comparing the relative changes in the physical and mechanical properties of the concrete against concrete made with washed aggregate. The study confirmed that the extent and nature of the impact depend on the quantity and nature of the microfine, and therefore the current ASTM C117 is not an adequate monitoring tool.