Effect of Curing Methods and Conditions on the Performance of Fly Ash Concrete in De-king Salt Scaling

Abstract

Seven air-entrained concrete mixtures including reference concrete without fly ash, concrete incorporating 25 and 35% fly ash, and a high-volume fly ash concrete mixture with 58% fly ash were made in this study. The water-to-cementitious materials ratio of the mixtures ranged from 0.32 to 0.45, and the fly ash used was an ASTM Class F, low calcium fly ash. Concrete slabs were cast and used for the determination of the resistance of concrete to the de-icing salts scaling. Most slabs were cast in horizontal moulds and finished using a wood trowel but a number of slabs were cast in vertical moulds. The slabs were either moist cured or cured using a curing compound before being subjected to drying prior to the scaling test. Different moist-curing and air-drying periods were used. Also, the water absorption of the surface of the slabs was determined immediately before the scaling test. For the same water-to-cementitious materials ratio, the fly ash concrete showed more scaling than the reference concrete. However, concretes incorporating up to 35% fly ash by mass and having a W/(C+FA) of 0.40 or less performed well in the scaling test when tested using the standard 14-day moist-curing and 14-day air-drying periods. Extended moist-curing periods beyond 14 days do not insure increased resistance to de-icing salt scaling for concrete, and this is particularly so for fly ash concrete. The drying affects significantly the surface of the slabs and makes them more vulnerable to scaling, possibly through the development of microcracking; this effect seems to be more severe for the fly ash concrete. The performance of the slabs cast vertically was not significantly different from that of the slabs cast horizontally. The use of the curing compounds greatly improved the scaling resistance of all concretes but was more beneficial for the fly ash concretes.