Microwave Evaluation and Modeling of Rapid Chloride Ingress in Mortar

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ABSTRACTSteel corrosion and the resulting deterioration of reinforced concrete structures is one of the primary causes of increasing damage to infrastructure. Corrosion of the reinforcing steel is implicated as a cause of damage to the majority of the 226,000 reinforced concrete bridges described by the Federal Highway Administration (FHWA) as deficient. In 1991, the United States Department of Transportation estimated rehabilitation costs for these damaged bridges at $90.9 billion. 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. However, the presence of a sufficient concentration of chloride ions can compromise the passive layer. In the presence of moisture, the steel will corrode. Development of a reliable test method for in-situ measurement of the chloride ion penetration depth and concentration in concrete structures would be invaluable to the concrete and construction industry. From an economic standpoint, accurate measurements of the concentration of chlorides and penetration depth in concrete structures, would allow agencies to more effectively allocate funding for repair of those structures with the most critical needs. Thus, the overall objective of the proposed investigation is to gain a more complete understanding of the fundamental implications of microwave measurements of cement-based materials exposed to chloride solutions. A methodical research plan is proposed in order to correlate microwave measurements to the salt content and chloride penetration depth in mortar. Mortar samples will be cyclically ponded with salt solution. After the samples are oven dried, a very accurate and sensitive microwave characterization technique will be used to evaluate their dielectric properties using four independent complex reflection and transmission coefficients. This information is then used in conjunction with a discrete layered electromagnetic model and a dielectric mixing formula to render information about the salt volume content as a function of depth into the samples. This process is repeated for a number of soaking cycles, giving information about salt penetration gradient and total content as a function of depth into the sample, sample make-up and soaking cycle. The anticipated outcome of this proposed investigation constitutes a significant and comprehensive leap forward in the path to evaluating chloride ingress in reinforced concrete structures.