Utilising Class F Fly Ash to Offset Non-ideal Aggregate Characteristics for Concrete in Chloride Environments

Sources of primary aggregate resources are under increasing pressure as construction activity continues to consume very large quantities world-wide. Equally quarry and gravel extraction is not popular with the public at-large and planning authorities are increasingly reluctant to give permission for new quarries. As a consequence, concrete producers are increasingly looking to use aggregates that may be regarded as having non-ideal characteristics, such as a high porosity, whether from natural sources or recycled.

Of the exposure conditions to which reinforced concrete is subjected, arguably the chloride environment is the most severe. Generally the ingress of chlorides is dominated by the paste phase and thus can be controlled by selection of cement type, content and/or water/cement ratio. Historically, therefore, engineers have utilized dense, low porosity aggregates and as a result the properties of aggregate per se are not usually specified. Recent work, however, suggests that aggregate interconnected porosity can have a significant influence on the rate of chloride ingress and ultimately the intensity of reinforcement corrosion resulting.

The use of fly ash in concrete is universally accepted as enhancing resistance to chlorides. Based on this, the premise of the research described in this paper was to determine whether fly ash could be used to allow the effective and economic use of aggregates with non-ideal properties for reinforced concrete in chloride exposures.

Aggregates with a wide range of properties were considered and their effect on the concrete microstructure and its ability to resist chloride diffusion in the cover zone and reinforcement corrosion, tested. It was found that Class F fly ash can indeed be used in this manner and, furthermore, the rate of reinforcement corrosion is also reduced. The paper explores the practical implications of the results.