The paper describes a study undertaken to determine the effect of pulverized fuel ash (PFA) and its characteristics on air entrainment, the air void system and the freeze/thaw durability of concrete. The results demonstrate that the admixture demand of PFA concrete was higher than that of Portland cement (PC) concrete and greatly influenced by the type of air-entraining admixture, the level of air required and the characteristics of the PFA used. While PFA fineness had little influence on admixture demand, PFA with high loss on ignition required dosages in excess of two times that of PC. However, PFA was found to have little influence on the rate of air loss with handling and reduced the variability of air content at a given admixture dosage when combined with PC from different sources. Tests to assess the reliability of the ASTM method for examining the air void system confirmed this. The results indicate that improvements in air void parameters were obtained with increasing air content in the concrete. However, similar or slightly enhanced parameters were measured for PFA concrete compared to those of PC concrete. In this case, the characteristics of the PFA had no effect. Following on from this, tests for freeze/thaw durability (ASTM C666: Procedure A) indicated that the critical factors influencing deterioration were the air content and design strength of the concrete. In this respect, no difference was observed between PC and PC/PFA concrete, and all concretes, irrespective of design strength, exhibited very good freeze/thaw resistance above an air content of 3·5%. A nomogram was developed to demonstrate possible routes to material selection/admixture dosages for the practical achievement of durable concrete in freeze/thaw conditions. |
