Detection and Profiling of Accelerated Chloride Penetration in Concrete Using Near-field Microwave Techniques

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Concrete normally provides reinforcing steel with adequate corrosion protection. When steel is encased in concrete, a protective iron oxide film forms at the steel-concrete interface due to the high pH of the concrete. This film protects the steel from corrosion. However, the intrusion of chloride ions in reinforced concrete can cause this protective film to be destroyed. Additionally when moisture and oxygen are present in the concrete, the steel will corrode through an electrochemical process. Once the steel begins to corrode, the concrete will deteriorate. This occurs because the by products of corrosion occupy a greater volume than the steel itself, which exerts a substantial stress on the surrounding concrete. Chloride can be introduced into concrete in many ways. It may be introduced by the individual concrete ingredients; aggregates, cement, admixtures and mixing water; or through exposure to deicing salts, seawater, or salt air environments. Since chloride can be introduced through many mechanisms, placing limits on any one ingredient (e.g. mixing water) will not always limit the total amount of chloride in the concrete. Therefore, it is important to be able to measure the chloride content of concrete to yield an indication of the potential of corrosion of any embedded steel. Currently, there are two standard tests used to measure the chloride penetration into concrete; namely, the "rapid chloride permeability test" and the "90-day ponding test". The former test does not necessarily replicate the transport of chloride ions in real-life situations, nor does it have a sound scientific basis. The results of this test have also been shown to be influenced by chemical admixtures and additives since they may contain some chloride. This test is not conducted in-situ and is destructive. Using the latter test takes a long time to obtain results. The test subjects a concrete, specimen to a chloride sloution over a given amount of time. Results from this test take at least 90 days to produce, but with the advent of newer high strength mixes, results may take longer due to the slow ingress of chloride from this test. A new test method that can predict the chloride ion penetration in concrete for use with in-place concrete, would be invaluable to the concrete and construction industry. This is especially true for the Federal Highway Administration (FHWA) and the Department of Transportation where there have been major problems with chloride ingress and steel corrosion in pavements treated with deicing salts. A near-field microwave nondestructive testing technique, employing open-ended rectangular waveguide probes, is proposed to predict chloride content and the extent of chloride penetration in concrete based materials. The investigating team proposesto use accelerated techniques to introduce chloride in cement paste, mortar and concrete. The objectives of this investigation are to detect the presence and the content of chloride in cement based materials in addition to closely estimating the extent or depth to which chloride has penetrated in these materials. The investigating team has been successfully using such a microwave nondestructive testing technique to study and correlate the near-field microwave reflection properties of cement based materials (hardened paste, mortar and concrete) to their w/c ratio, s/c ratio, coarse aggregate volume and size distributions, ans their compressive strenths. On a preliminary basis they have been able to detect chloride content, present in the mixing water, in cement paste and mortar as well as the influence of chloride on the curin of these specimens. It is intended to subject cement paste, mortar and concrete to accelerated chloride penetration using an enclosed tank filled with chloride solution under pressure and controlled temperature. Upon successive exposure to chloride solution, the reflection properties of these specimens will be measured at several microwave frequencies. The results will then be correlated to the chloride content as well as the depth to which it has penetrated. Using a discrete model describing the manner by which chloride has penetrated in these specimens, and existing electromagnetic model will be used to model the results and more importantly predict the amount of chloride and its penetrated depth into these materials.