Multi-scale computational modeling of coupled chemical, physical and mechanical phenomena in cementitious materials

Abstract

With the increasingly popular use of cement-based materials like concrete in buildings and infrastructure, it has become crucially important to reliably predict the aging and deterioration of cement-based materials. One of the major phenomena that cause this deterioration is the ingress of corrosive agents into concrete materials, e.g., chloride penetration and sulfate attack. This is a delicate yet challenging task to tackle because of the following nature of the problem to be considered (1) the mass transport in concrete is a phenomenon related to various length scales. First, the microstructure of the cement-based material significantly affects the penetration process. Second, the corrosion induced degradation initiates at the mesoscale level. Finally, the overall transport problem needs to be solved at a macro-scale level to consider the internal and external conditions like temperature, relative humidity, and fracture. (2) The cementitious matrix is a chemical compound with ongoing hydration and membrane-like properties. These features make the mass transport involved in multiple chemical and physical reactions. (3) Evolution of mechanical properties including strength gain due to hydration and strength loss due to degradation, plays an important role in the formation of fracture in cement-based materials and thus in turn, affecting the mass transport in concrete. In an attempt to address the problem described above, a comprehensive multi-scale computational framework is proposed in this research and is applied to the mass transport problem in cementitious materials. The research covers four main projects. In the first project, a coupled multi-physics model is proposed to simulate chloride penetration in saturated and unsaturated concrete. This transport model considers different chemical and physical phenomena encountered in the diffusion process of a concentrated solution, and meanwhile characterizing the effect of microstructure as well as the mesoscale tortuosity. In the second project, moisture diffusion is taken as an example to study how the existence of cracks affects the mass transport. The water transport in cracked concrete is successfully simulated with the modified diffusivity and the consideration of disequilibrium condition between adsorption and desorption. The first two projects are both developed at mesoscale. To accurately characterize the effect of underlying mechanism at the microscale, a chemo-mechanical model, μLDPM, is developed in the third project by making use of the advantages of two state-of-the-art models: (1) microscale chemical reactions modeled by using μic, a vector-based model that simulates the formation of reaction products around idealized spherical particles, and (2) microstructural elastic and damage behavior modeled by a variant of the Lattice Discrete Particle Model (LDPM) known for its wide successes in modeling cementitious materials failure under various stress states. In the last project, a multi-scale coupling method is proposed to connect the microscale model and the transport models proposed in the first two projects. With this scheme, the values of parameters that depend on underlying microscale phenomena can be directly identified once the mixture is provided, thus avoiding empirical estimation due to lack of sufficient experimental data.