Bioubakhsh, S.; (2011) The penetration of chloride in concrete subject to wetting and drying: measurement and modelling. Doctoral thesis , UCL (University College London).

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Abstract

Corrosion of reinforcing steel arising from contamination by chlorides from de-icing salt is the major cause of deterioration of concrete bridges in the UK and many parts of the world. Those elements of structures exposed to cyclic wetting and drying (BS 8500-1, XD3) have proven to be the most vulnerable to corrosion damage. Penetration of chloride in concrete exposed to wet/dry environments occurs by diffusion and absorption. Diffusion is a relatively slow and quite well understood process. However, absorption is a relatively rapid transport mechanism and there is a lack of understanding of the role of this mechanism on chloride ingress as studies on chloride penetration in concrete exposed to wet/dry cycles ignore the effect of this mechanism on chloride ingress. In addition, chloride penetration prediction models are mostly based on Fick’s laws of diffusion, ignoring the effect of absorption on chloride ingress. The aims of this work are: to develop a more detailed understanding of chloride penetration in concrete subjected to wet/dry cycles and identify the effect of absorption on chloride ingress; to produce reliable numerical model for chloride penetration due to this transport process; to recommend values of the minimum thickness of concrete cover to steel reinforcement relevant to this service environment or identify exposure conditions which require alternative methods of protection. The absorption test method used in this work is a cyclic regime as developed by TRL [Emerson and Butler, 1997] to represent site conditions. Concrete cubes, 100 mm³, were subjected to wet/dry cycles with the suction surface in contact with NaCl solution. Drying temperature was found to be the most critical factor influencing sorptivity and depth of chloride penetration. The salt solution concentration also had a significant effect on chloride penetration via the apparent surface chloride content. Moreover, the depth of chloride penetration was found to be proportional to the square root of exposure time. Two approaches to predict chloride penetration in concrete exposed to wet/dry cycling are proposed. The first is based on the relationship obtained in the present study between equilibrium sorptivity of concrete, Se, depth of chloride penetration, d, and time, t, which leads to the following general expression, where A, B and C are constants for a given cement type and salt solution concentration d = A×√t + B×Se + C The second method is simply based on the well known solution of Fick’s second law but utilises values of apparent diffusion coefficient appropriate to this transport process. The first model suggests that, initially, absorption has a significant effect on chloride ingress but that diffusion dominates long-term behaviour. The outputs from these models suggest that the thicknesses of concrete cover in structures exposed to class XD3 should be higher than those currently recommended in BS 8500-1 (2006). However, there are practical limits as to what can be specified for thickness of concrete cover and therefore alternative methods of protection such as coatings or cathodic protection should be adopted when the concrete cover does not provide sufficient protection.

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