Effect of Curing on the Strength and Permeability of PFA Concrete

Abstract

This paper describes how tests were carried out on a series of concrete mixes that were designed to equal workability and compressive strength with a range of pulverized fuel ash (PFA) levels in order to study the effect of curing on the strength and permeability of PFA concrete. Concrete specimens were subjected to a range of moist-curing periods prior to air storage. Compressive strength was determined at various ages and permeability to oxygen and water was determined at 28 days. Results confirm the importance of curing, with reductions in the curing period resulting in lower strength, more permeable concrete. The strength of the PFA concretes appears to be more sensitive to poor curing than ordinary portland cement (OPC) concrete, the sensitivity increasing with increasing PFA content. However, despite exhibiting lower strengths, PFA concretes moist-cured for only one day were, generally, no more permeable to water and substantially less permeable to oxygen than similarly cured OPC concretes. As the period of curing increased, the PFA concretes became considerably more impermeable to water and oxygen than the OPC concretes. These results are discussed in the context of the minimum periods of curing and protection recommended in BS 8110. It is argued that although the increased curing periods suggested for PFA concrete are justified on the basis of concrete strength, PFA concrete may require no more curing than OPC concrete to achieve equal durability, as measured by oxygen and water permeability.