Numerical Analysis of the Freezing Behavior of Saturated Cementitious Materials with Different Amounts of Chloride.

The freezing behavior of cement paste saturated with different chloride concentrations is investigated numerically with a coupled 3D hygro-thermo-mechanical FE analysis. The mathematical formulation of the freezing processes in the context of poromechanics takes into account the water (hydraulic) and ice pore pressures, as well as the distribution of heat (temperature) and strains. These quantities are calculated numerically based on three coupled differential equations, namely the static equilibrium equation and the equations for the transport of water and heat. The coupling between the mechanical (loading) and the non-mechanical processes (freezing) is performed using a staggered solution scheme. The proposed numerical approach is first validated using numerical and experimental studies from the literature dealing with two different cement pastes saturated with different amounts of chloride. The validated model is then used to investigate the effects of liquid water permeability, total porosity and pore size distribution on the freezing behavior of hardened cement paste. The results show that liquid water permeability has a strong effect on the pore pressure and deformation of the hardened cement paste. It is also shown that by decreasing the total porosity, the material becomes denser and contracts more as the temperature decreases, leading to a decrease in freezing strain. The results of this paper will provide important findings for the development of a simplified engineering model to investigate the mechanism that leads to freeze-thaw salt-induced damage to concrete structures in the framework of the DFG-funded research project.

Impact Analysis of Thermally Pre-Damaged Reinforced Concrete Frames.

In the present study, the influence of thermally induced damage of reinforced concrete (RC) frames on their static and dynamic response is experimentally and numerically investigated. In the experimental test, the RC frame is first pre-damaged through fire exposure and then loaded from the side with the impact of a steel pendulum. To verify the recently developed coupled thermo-mechanical model for concrete, transient 3D FE simulation is carried out. The rate and temperature-dependent microplane model is used as a constitutive law for concrete. It is first shown that the simulation is able to realistically replicate the experimental test. Subsequently, the numerical parametric study is performed where the dynamic and static response of RC frame is simulated for both hot and cold states. It is shown that the pre-damage of RC frame through fire exposure significantly reduces the resistance and changes the response. Finally, it is demonstrated that for the impact load the rate sensitive constitutive law of concrete significantly contributes to the response of RC frame.