Alkali silica reaction (ASR) is a complex, multi-scale chemomechanical process which can lead to the expansion and damage of concrete structures by forming microcracks, gradually reducing the structure's mechanical properties such as strength and stiffness. This work investigates the effect of the ASR induced gel present within a microcrack on the material's nonlinear elastic constants. Therefore, significant changes such as the existence of lubrication need to be considered in addition to the contact force when deriving the stress state. The lubrication force depends on the viscosity and transport properties of the ASR gel and each of these needs to be modeled respectively. The characteristics of this force lead to the fact that nonlinearity due to microinhomogeneities is altered over time compared to the pure elastic contact force which is repulsive and tries to open the microcrack. Firstly, the contact force is approximated by a Hertzian contact between asperities, characterized by spherical tops located at different heights whereas the introduced lubrication force is derived in terms of the distance between irregularities and the gel volume present within a microcrack. As a second step, appropriate asperity distribution functions are used to derive the internal stress state within the crack which is then superimposed with the far field stress and the additional acoustic stress in order to express the nonlinear stressstrain relationship including the overall elastic constants of the solid. The higher order elastic constants are much more sensitive to microcracks than the Young's Modulus and Poisson's ratio which are altered only slightly due to the presence of microcracks. Nonlinear ultrasonic measurement techniques are capable of measuring second order waves which typically occur in nonlinear materials and relating these to the acoustic nonlinearity parameter . The direct dependence of on the quadratic and cubic nonlinearity parameters allows the derivation of a nonlinearity parameter relationship to the ASR gel volume present in a microcrack.
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