Monitoring the corrosion and remediation of reinforced concrete on-site: An alternative approach

Abstract

Difficulties associated with the interpretation of site data collected over long periods of time from commonly used electrochemical corrosion monitoring techniques often make it difficult to assess the corrosion of reinforcing steel. An alternative approach for the interpretation of data is proposed, based on a model of the quality of passive film upon the steel surface. This model leads to a representation of the corrosion state by means of the relationship, over a long period of time, of the corrosion potential and the logarithm of the Linear Polarization Resistance, since both are functions inter alia of corrosion rate. It is shown that for the reinforced concrete panels tested, data points representing this relationship closely fitted a family of results; allowing the development of a “monitoring control diagram”, MCD. The MCD reveals that for a fixed geometry and experimental conditions, a relationship between the corrosion potential and polarization resistance of steel exists, facilitating a useful monitoring tool for assessment of both the corrosion and remediation of reinforced concrete structures. Particular emphasis is placed on the latter in this work. Monitoring the corrosion of steel embedded within concrete presents certain challenges, especially in terms of physical inspections. On-site electrochemical techniques that have been developed to monitor corrosion processes that may be occurring at the reinforcement surface, but of which there is as yet no evidence on the concrete surface, include: half cell potential measurements and linear polarization resistance measurements [1]. Such techniques are in good relation to the technical and financial scope of those conducting routine nondestructive testing. Up to the present, measuring the corrosion potential, E corr , of reinforcing steel is the commonest of the corrosion monitoring methods available [2], and is widely used in practice as an indication of the possible corrosion risk. Corrosion potential monitoring is outlined in ASTM C876 [3], in which the value of E corr is only claimed to indicate the relative risk of corrosion, with is no provision of quantitative information regarding corrosion rates. The standard suggests that passivity of the reinforcement may be lost at potentials more negative than � 150 mV (Cu/CuSO4) [3]. In the LPR technique (originally developed for metals in solution in the late 1950’s [4]), the potential of the steel is scanned over a small range in the vicinity of the corrosion (open-circuit) potential. As the current densities and potential shifts associated with the technique are small, it was considered in the original derivation of the theory that mass transport effects could be neglected. The slope of the polarization curve (viz. the potential change divided by the applied current) at the point at which the applied current equals zero is defined as RP, the Polarization Resistance, and is considered a measure of the opposition to metallic dissolution. Each of these techniques provides some information as to what is going on within the concrete, but none give the complete story [5, 6]. In particular the changes in the environment that are an inevitable feature of site measurements introduce random variations into the data that can often mask underlying fundamental changes in the corrosion process [7 – 9]. As opposed to the presentation of alternative techniques to replace E corr and LPR monitoring, this paper attempts to introduce an approach to analyzing data from such techniques; which aims to lead to more meaningful conclusions. The paper will illustrate this technique by using it in the context of ranking different corrosion remediation techniques.