Effect of Curing Regime and Controlled Permeability Formwork on Early Chloride Penetration into Fly Ash Concrete

Abstract

Fly ash is often used as supplementing cementitious material for marine concrete structures. In these cases, the development of a dense microstructure is typically slower than for other types, and the resistance against chloride ingress at early age is at a low level, but will develop to a high level during the first 6 to 12 months.

Early chloride ingress into concrete blocks made with a binder composition of 75% low-alkali sulphate-resistant cement (CEM I 42.5 N) and 25% fly ash were investigated. The effect of wet curing and dry curing was tested on concrete blocks cast against plywood on one side and controlled permeability formwork on the other side. The development of resistance against chloride ingress was documented by means of chloride migration coefficients measured according to NT BUILD 492 at the age of 1 month and 6 months. Furthermore, chloride profiles were measured in a laboratory experiment after immersion in a 2% chloride solution for 2 months.

The investigation demonstrated that a significantly improved resistance against chloride ingress can be achieved for concrete cast against a controlled permeability formwork liner compared to concrete cast against plywood formwork. This was found to be the case both for concrete cured under wet and dry conditions. Overall, the investigation shows that the use of a controlled permeability formwork liner represents an approach associated with a good potential for extending the service life of chloride exposed concrete structures.