Lattice Discrete Particle Modeling of acoustic nonlinearity change in accelerated alkali silica reaction (ASR) tests

Alkali silica reaction (ASR) in concrete is a complex, multi-scale chemo-mechanical problem characterized by expansion and cracking of concrete meso-structure. Driven by various environmental factors such as humidity and temperature levels, it causes microscopic to macroscopic cracking of concrete resulting in degradation of concrete mechanical properties. Many standardized test methods have been proposed to investigate concrete vulnerability to ASR and they are characterized by time durations varying from only 2 weeks (accelerated mortar bar test) to up to two years (concrete prism test). While these tests can give some insight on the susceptibility of new structures to ASR damage, the assessment of the residual loading carrying capacity of existing ASR affected structures mostly rely on destructive evaluations. This approach is not only cost ineffective, but also difficult or even dangerous to implement for sensitive structures such as, nuclear power plants. Promising alternatives are ultrasonic nondestructive evaluation techniques that have been adopted successfully to detect metallic materials fatigue damage and debonding in composites. These techniques can detect early damage stages that cannot be captured accurately by using simple linear measurements but do not provide a direct measurement of the damage characteristics (e.g. statistics of crack opening) and, much less, of the deterioration of mechanical properties such as stiffness and strength. This can be achieved, however, by integrating non destructive measurements with accurate computational modeling of the damage mechanisms associated with ASR. This paper pursues such integration within the framework of the Lattice Discrete Particle Model (LDPM), a mesoscale model for concrete with superior modeling capability of fracturing behavior that was recently extended to account for ASR damage. The numerical simulations carried out in this study demonstrate (1) the ability of LDPM to replicate ultrasonic nonlinear phenomena and (2) that a strong correlation exists between these phenomena, local characteristics of cracking evolution, as well as stiffness and strength reduction.