The hydration of cement in concrete is exothermic, which means it gives off heat. In large elements, the heat caused by hydration can dissipate at the surface, but is trapped in the interior, resulting in potentially large thermal gradients. The thermal expansion of concrete is greater at higher temperatures, so if the temperature differential between the surface and the interior becomes too great, the interior will expand more than the exterior. When the thermal stress from this mis-matched expansion exceeds the tensile strength of the material, the concrete will crack. This phenomenon is referred to as thermal cracking. Accurate characterization of the progress of hydration of a concrete mixture is necessary to predict temperature gradients, maximum concrete temperature, thermal stresses, and relevant mechanical properties of concrete that will influence the thermal cracking risk of concrete. Calorimetry is the most direct test method to quantify the heat evolution from a concrete mixture. There is currently no model, based solely on calorimetry, which completely describes the effects of mixture proportions, cement and SCM chemistry, and chemical admixture dosages on the temperature sensitivity and adiabatic temperature rise of concrete. The objective of this study is to develop a comprehensive model to describe these effects. First, the temperature sensitivity of the hydration reaction (described with activation energy, E[subscript a]) is needed to accurately predict the behavior of concrete under a variety of temperature conditions. A multivariate regression model is from isothermal calorimetry testing to describe the effects of water-cementitious materials ratio, cement chemistry, supplementary cementing materials, and chemical admixtures on the E[subscript a] of portland cement pastes. Next, a multivariate regression model is developed from semiadiabatic calorimetry testing that predicts the temperature development of concrete mixtures based on mixture proportions, cement and SCM chemistry, and chemical admixture dosages. The results of the models are validated using data from literature. The final model provides a useful tool to assess the temperature development of concrete mixtures, and thereby reduce the thermal cracking risk of the concrete structure.

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published_or_final_version / abstract / toc / Mechanical Engineering / Master / Master of Philosophy

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The thermal stability of two distinct blended polymer systems was examined. A model for polyethylene was used to investigate the vulnerability of polyethylene to premature crosslinking in industrial crosslinking conditions. Careful experiments were conducted to gather evidence of the interaction between a peroxide crosslinking agent and a specific antioxidant additive. Multiple lines of evidence were combined to propose a complete mechanism of interaction between the two species. The mechanism was further tested and a hypothesis was proposed for the reduction in premature crosslinking exhibited when the two species are present in polyethylene blends. A specific aspect of the proposed mechanism warranted further investigation on its own. The acid-catalyzed degradation of the peroxide initiator was thoroughly investigated. The thermal degradation of polyvinyl chloride was also studied. Model compounds were reacted with carboxylates to determine the relative rates of stabilization at various polymer defect sites. These model studies were combined with weight loss and color change investigations of bulk polymer systems. The knowledge gained from the model and polymer studies allowed for the proposal and examination of two novel stabilizing salt systems. The efficacy of the new stabilizers is presented.

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