Cusatis Group Research

 

Discrete Modeling of Concrete Thermo-Hydro-Chemical-Mechanical Coupling Behavior at High Temperature

Thermal spalling can significantly increase the risk of concrete structure and so far its mechanism still remains in dispute. To gain a better understanding, a discrete hydro-thermo-chemical (HTC) model of concrete at high temperature is proposed and a two-way coupling scheme between the discrete HTC model and lattice discrete particle model (LDPM) is performed. This two-way coupling scheme is featured with the effect of pore vapor pressure and temperature on mechanical response and the impact of cracking behavior on mass and heat transport. The good agreements achieved in the calibrations of high-performance concrete and ordinary concrete indicate the precision of this model. Interestingly cracking behavior is found to have a significant effect on the local pore pressure built-up. Based on this coupling model, the spalling phenomenon is successfully reproduced and the mechanism study shows that thermal stress relatively plays a more important role in thermal spalling, although pore vapor pressure can form a macro-crack parallel to the heating surface.

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Concrete thermal conductivity evolution during tensile and compressive failure

Thermal Conductivity (ETC) due to the cracking behavior of concrete changes the temperature profile in concrete structures, indirectly inducing the redistribution of thermal stresses. To study this phenomenon, a mini-scale numerical method within the framework of finite element method is proposed for both tensile and compressive cracked concrete and this method is applied to obtain quantitative relationships between tensile or compressive strain and ETC. Results show that (a) for tensile dominated failure, concrete ETC decreases by 23% during the plastic stage whereas little decrease is found at complete failure; (b) for compressive dominated failure, ETC decreases by 30% during the plastic stage, and then becomes stable afterwards. In the softening stage ETC linearly decreases with the increase of compressive strain; (c) it is the interfacial thermal resistances induced by the micro-cracks between aggregates and mortar rather than the macro-cracks that play the dominant role in this phenomenon; (d) concrete ETC becomes anisotropic when cracks appear. The experiments show that compressive cracked concrete’s ETC vertical to cracks dramatically decreases by 20–25% at plastic drop stage and then becomes stable at the plastic steady stage. The numerical results are used to determine the interfacial thermal resistance factor in Wang Jiajun model. The proposed formulation provides results that are in excellent agreement with experiments.