Deterioration of Concrete Exposed to Evaporative Transport Conditions

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Most deterioration mechanisms in concrete require that an outside agent enter the concrete through the connected pore system. This deleterious material could be sulfates that attack the concrete directly, chlorides that cause corrosion of the reinforcing steel, or simply water that can initiate alkali-aggregate reactions or contribute to freeze-thaw damage. Deterioration rates are highly dependent upon the rate of penetration of the liquid in question. Being able to quantify this rate is critical toward predicting deterioration rates and thus service lives of structures. Most current models rely on diffusion as the primary mass transport property. However, evaporative transport can produce a far higher rate of penetration, thus making a diffusion-based model highly inaccurate. Evaporative transport occurs when part of a structure is exposed to moisture while a nearby portion is exposed to the atmosphere. This creates an evaporation surface that tends to draw water into the concrete and toward this surface at a rate that depends upon the evaporation rate of the water from the surface. Most structures exposed to water match this description; bridge piers protruding from the ocean, floor slabs on grade, and earth retaining walls are a few examples. Prior research by the applicant using magnetic resonance imaging (MRI) has shown that evaporative transport is a very complex mechanism that needs further study before it can be introduced into a service life model. In addition, evaluating the deterioration rate of concrete requires a new approach that relies on mechanical properties as opposed to indirect measurements like expansion. The applicant intends to use the tensile properties of concrete as a quantification medium for mechanical property deterioration. Since most deterioration mechanisms in concrete involve the development of internal expansive stresses that are tensile in nature, tensile strength is the most appropriate mechanical property for quantifying the magnitude of damage and the residual strength. The proposed research will tie these issues together by further evaluating the phenomenon of evaporative transport in concrete, developing an exposure regimen to create evaporative transport based deterioration, and devising an evaluation protocol based on the degradation in tensile properties. In general, the proposed research program will provide a better understanding of a very common transport mechanism found in concrete structures throughout the world. One could argue that evaporative transport is the MOST prevalent mechanism in deterioration. Being able to quantify its effects on deterioration, coupled with a standardized exposure and testing regime, would result in much more accurate service life predictions. This would allow for more sustainable design of structures, dramatically reducing material usage and lifecycle costs.