Resistivity and water absorption of concrete

Corrosion is a significant cause of the deterioration of reinforced concrete structures. The main cause of corrosion is the ingress of aggressive chemicals such as chloride ions from salts. However, there is a lack of knowledge and understanding regarding the relationship between compressive strength, resistivity and water absorption of different concrete types - all of which are critical parameters influencing the chloride ingress rate and development of corrosion in reinforced concrete. Concrete cubes and cylinders with varying proportions of water-cement (w/c) ratios, Pulverised Fly Ash (PFA) (10-40% replacement), Ground Granulated Blastfurnace Slag (GGBS) (20-70% replacement) and Silica Fume (SF) (5-15%) contents were cast, and tested for compressive strength, hardened density, bulk and surface resistivity, and water absorption. The results showed that increasing the PFA, GGBS or SF replacement contents significantly increased both the surface and bulk resistivity of the concrete (e.g. with the 70% GGBS replacement, up to 9 times greater when compared to the control concrete). The addition of SF or GGBS had a considerable positive effect on the water absorption (even at low dosages), lowering it by up to a factor of 10. The PFA however, had little, or even an adverse, effect on the water absorption. Cement replacements such as PFA, GGBS and SF can therefore contribute significantly to improving the resistivity of concrete, and hence the resistance of concrete to chloride ingress, and should therefore be seriously considered for the majority of concretes designed for aggressive environments.