The effects of aggregate coatings on the performance of Portland cement concrete

Abstract

Specifications which limit the amount of fine material that adheres to coarse aggregates tend to be vague. For example, the Wisconsin state specifications require coarse aggregates used in portland cement concrete to be free of deleterious substances and adherent coatings; however, neither the washing procedure nor the desired end result of aggregate washing is specified. The purpose of this research was to identify which aggregate coatings in Wisconsin are deleterious and to determine what degree they affect concrete durability and strength. To this end, coarse aggregates containing surface coatings were collected throughout the state and characterized with the California cleanness test, methylene blue adsorption test, and x-ray diffraction. Based on the results of these tests, a subset of the sampled aggregates was selected to further study the effects of the coatings on concrete performance. It was hypothesized that coatings containing clay material are more deleterious than coatings containing either dust or carbonate material.

The 10-batch mixing plan tested coarse aggregates fiom three different sources and in the following three conditions: (1) coated aggregates that were sampled in the field, (2) washed aggregates that were washed in the laboratory to remove the existing coating, and (3) coated aggregates that were manufactured with dust and clay fines to increase the extent of the coating. The effect of each aggregate coating on concrete performance was assessed by comparing the relative changes in compressive strength, tensile strength, drying shrinkage, freeze-thaw durability, and chloride ion penetrability between batches containing the washed and coated aggregates. The test results confirmed that coatings with high clay contents 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 washing procedures do not need to be changed, it was recommended that the WisDOT require the California cleanness test whenever aggregate coatings are suspected of influencing strength or durability during concrete construction.