Effect of aggregate porosity on chloride ingress into concrete

Chloride ingress into structural concrete is generally considered to be controlled primarily by the properties of the paste fraction, with aggregate particles considered to be inert inclusions. Indeed, the key codes of practice for structural concrete make no specific reference to the characteristics of aggregates that may affect chloride ingress. However, a theoretical analysis by Hobbs has recently challenged this accepted view and has demonstrated that if the aggregate had an interconnected porosity then it could be up to 1000 times more permeable than the surrounding paste. Furthermore, the interfacial zone between the aggregate and the paste is also an important influence over the movement of chloride ions through concrete. This paper reports a laboratory-based experimental programme that studied the performance of concretes containing different sources of aggregate with a wide range of water absorption. It is demonstrated that there is indeed a significant effect of aggregate permeability over chloride ingress into concrete. These results show that when specifying concrete for exposures to chlorides, more consideration should be given to specifying additional controls on the properties of the aggregate, either directly or indirectly. The current specification approach of minimising chloride ingress into structural concrete by limitation on the water/cement ratio, is in isolation, therefore inadequate to ensure the long term durability of structures exposed to chloride environments.