Development of a Reaction Signature for Combined Concrete Materials

Although concrete is widely considered a very durable material, if conditions are such, it can be vulnerable to deterioration and early distress development. Alkali-Silica Reaction (ASR) is a major durability problem in concrete structures. It is a chemical reaction between the reactive silica existent in some types of rocks and alkali hydroxides in the concrete pore water. The product of this reaction is a gel that is hygroscopic in nature. When the gel absorbs moisture, it swells leading to tensile stresses in concrete. When those stresses exceed the tensile strength of concrete, cracks occur. The main objective of this study was to address a method of testing concrete materials as a combination to assist engineers to effectively mitigate ASR in concrete. The research approach involved capturing the combined effects of concrete materials (water cement ratio, porosity, supplementary cementitious materials, etc.) through a method of testing to allow the formulation of mixture combinations resistant to ASR leading to an increase in the life span of concrete structures. To achieve this objective, a comprehensive study on different types of aggregates of different reactivity was conducted to formulate a robust approach that takes into account the factors affecting ASR; such as, temperature, moisture, calcium concentration and alkalinity. A kinetic model was proposed to determine aggregate ASR characteristics which were calculated using the System Identification Method. Analysis of the results validates that ASR is a thermally activated process and therefore, the reactivity of an aggregate can be characterized in terms of its activation energy (Ea) using the Arrhenius equation. Statistical analysis was conducted to determine that the test protocol is highly repeatable and reliable. To relate the effect of material combinations to field performance, concrete samples with different w/cm?s and fly ash contents using selective aggregates were tested at different alkalinities. To combine aggregate and concrete characteristics, two models were proposed and combined. The first model predicts the Ea of the aggregate at levels of alkalinity similar to field conditions. The second model, generated using the Juarez- Badillo transform, connects the ultimate expansion of the concrete and aggregate, the water cement ratio, and the fly ash content to the Ea of the rock. The proposed models were validated through laboratory tests. To develop concrete mixtures highly resistant to ASR, a sequence of steps to determine threshold total alkali in concrete were presented with examples. It is expected that the knowledge gained through this work will assist government agencies, contractors, and material engineers, to select the optimum mixture combinations that fits best their needs or type of applications, and predict their effects on the concrete performance in the field.