Effects of aggregate coatings and films on concrete performance

Abstract

Coarse aggregates containing surface coatings were collected throughout Wisconsin and characterized to determine composition. Based on the results of these tests, a subset of the sample aggregates was selected to further study the effects of the coatings on concrete performance. It was hypothesized that coatings consisting of clay material are more deleterious than coatings consisting of either dust or carbonate material. The 10-batch concrete mixing plan tested coarse aggregates from three different sources and in the following three conditions: (1) coated aggregates that were sampled in the field; (2) aggregates washed in the laboratory; and (3) aggregates with added coatings. The effect of each aggregate coating on concrete performance was assessed from changes in compressive strength, tensile strength, drying shrinkage, freeze-thaw durability, and chloride ion penetrability as influenced by washed versus coated aggregates. In general terms, this research suggested that the effects of aggregate coatings as sampled in Wisconsin are minor, but could not rule out the possible existence of problem aggregates not included in the small sample. The test results confirmed that coatings with a high clay content are more deleterious to concrete strength and durability than coatings that consist largely of dust or carbonate material. While the carbonate coatings appeared to slightly improve performance, the clay coatings appeared to significantly decrease strength and durability. Although current field washing procedures do not need to be changed, it was recommended that the Wisconsin Department of Transportation consider adoption of the California Test 227.