Predicting Damage in Concrete Due to Expansive Aggregates: Modeling to Enable Sustainable Material Design
Syeda Rahman and Zachary Grasley, Texas A&M University, April 2012, 139 pp. (476660-00027-1)
A poroelastic model is developed that can predict stress and strain distributions and, thus, ostensibly damage likelihood in concrete under freezing conditions caused by aggregates with undesirable combinations of geometry and constitutive properties. Sensitivity of the stress distributions to the aggregate and matrix constitutive parameters are assessed to allow improved concrete design. The proposed model does not account for the viscoelastic stress relaxation and may over-predict the stress results. The model is evaluated experimentally through acoustic emission analysis under freeze-thaw cyclic loading, which reveals that air-entrained concrete may undergo durability cracking (D-cracking) if deleterious materials are present. It is determined that high-porosity, low-permeability aggregates with fine pore structure are the most vulnerable to D-cracking in non-air-entrained concrete, and the destructive tensile stress is generated at the aggregate boundary by the Mandel-Cryer effect. On the other hand, low-porosity, high-permeability aggregates relax the pore liquid pressure rapidly and prove to be beneficial for the non-air-entrained concrete. Reduction in aggregate size is found to be effective in quickly relaxing the tensile tangential stress, which eventually helps mitigate D-cracking of concrete. The difference between the coefficients of thermal expansion of the coarse aggregate and the matrix in which they are embedded should not be too high since it may cause tensile stress at the aggregate boundary or interfacial transition zone. Low water-to-cement mass ratio and addition of pozzolans help increase the bulk modulus, reduce the porosity of the porous body, and improve durability. It is also observed that increase in cooling rate decreases concrete durability under freezing temperatures through the reduction in time available to relax pore pressure buildup and the related tangential stresses in the aggregate and matrix.
Keywords: D-Cracking, Poroelastic, Aggregate Size, Porosity, Permeability, Bulk Modulus, Coefficient of Thermal Expansion, Mandel-Cryer Effect, Acoustic Emission
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