Effect of Surface Evaporation and Slab Thickness on the Scaling Resistance of Concrete Containing Fly Ash

Laboratory tests (conducted in accordance with ASTM C 672) frequently indicate that concrete containing fly ash may be susceptible to scaling. Based on this concern, a few northern USA states restrict the use of fly ash during the late-fall pavement construction season. However, a survey of twelve northern states revealed that scaling is rarely observed in actual pavements. In an attempt to develop a more clear understanding of the potential reasons for the apparent discrepancy between the scaling resistance of laboratory concretes and field concretes, scaling studies were conducted on concrete containing 20% of a Class C fly ash. The main variables in the study were the surface water evaporation rate (water loss was ranged from 0 to 4.5 kg/sq m 2) and the thickness of the slab specimen (55 mm to 245 mm). The selected ranges of test variables were intended to represent moisture and temperature conditions that are likely to be encountered in the field. In addition to monitoring the mass of material lost from the surface due to scaling, the temperature gradient that developed inside the specimen during freezing was measured. In addition, scanning electron microscopy was performed to assess differences in porosity of the near surface concrete. The results indicate that ASTM C 672 is too severe when compared to the environmental conditions that are likely to be encountered in the field.