Chloride penetration in concrete subject to wet/dry cycling: influence of moisture content

Abstract

Concrete structures suffer from corrosion if chloride ions in the environment penetrate to the depth of reinforcing steel; surfaces periodically wetted with chloride solution are at most risk, when chloride ions can enter concrete by combined diffusion and absorption. BS8500-1 recommended minimum cover requirements primarily assume that chloride ingress is diffusion controlled, and absorption is only partly accounted for, so cover values may be inadequate. This paper investigates factors that influence absorption of chloride ions into concrete and how this affects distribution of chloride at different depths from the surface. Concrete cubes (CEM I cement; water/cement ratio 0 . 45) are subjected to repeated wetting (with 50% saturated salt solution) and drying cycles, during which their mass is monitored. The effective porosity, weight and distance sorptivity and chloride penetration depths are calculated for a number of curing, conditioning and drying conditions. Samples are extracted from the cubes at a range of depths and analysed for chloride content. Results show that the quantity of chloride entering the concrete, and in particular surface chloride content, is very sensitive to effective porosity/drying conditions immediately before wetting; also as much as 31% of the protection provided by concrete cover can be lost after exposure to just one wet/dry cycle, thereby significantly reducing time to corrosion of concrete structures.

Chloride penetration in concrete subject to wet–dry cycling: influence of pore structure

“…14% Cl value estimated by Hobbs and Matthews (1998). This discrepancy in C s levels may be attributable to various reasons including (a) leaching of chloride due to rainfall (b) disparity in concentration of the salt solution (c) difference in effective porosity (d ) C s is very sensitive to sampling technique, as discussed elsewhere (Arya et al, 2014).…”

“…Full details of the methods used to assess absolute and effective porosity can be found in a previous paper (Arya et al, 2014). The paper also gives the background to the test method used in this work to determine the absorption properties, which involved subjecting the test specimens to repeated cycles of wetting (2 days) and drying (12 days), and monitoring weight changes.…”

“…05 Cl (per cent by weight of concrete) (Life-365, 2008) -was estimated using chloride/depth profiles. Full details of all the test methods and procedures used in this work can be found in a previous paper (Arya et al, 2014).…”

“…The moisture content of concrete is influenced by, among other factors, conditioning time and temperature prior to the first wetting cycle, curing time and the degree of drying between wetting events. The effect of these factors on sorptivity and chloride distribution is reported elsewhere (Arya et al, 2014). This paper primarily focuses on the role of pore structure on this transport process.…”

“…Results obtained by Abdul Razak et al (2004), Chan and Ji (1999) and McCarter and Watson (1997) using a variety of test methods including electrical conduction measurements, ISAT and the water absorption test detailed in BS 1881-122 (BSI, 2011 all found that the use of silica fume (SF) additions reduces the sorptivity of concrete when compared with plain OPC concrete. However, the results of tests on mixes containing pulverised fly ash (pfa) and ground granulated blastfurnace slag (GGBS) are sometimes contradictory when compared with each other or plain OPC concrete, perhaps because of differences in Unfortunately, the results of these experiments cannot be used to model chloride ingress in any meaningful way and hence estimate the depth of concrete cover needed to prevent corrosion in structures exposed to cyclic wet-dry chloride environments because (a) the absorbing solution used is normally water, which behaves somewhat differently to salt solution because of factors such as pore blocking due to chloride binding and/or crystallisation of chloride salts (Chrisp et al, 2002), and differences in the densities of the two liquids (Emerson and Butler, 1997) (b) in many cases the moisture content of test specimens is unlikely to be representative of field conditions and this can significantly affect sorptivity (Arya et al, 2014).…”