A Study On Mechanical Properties Of Concrete Under The Coupled Thermo-hygro-mechanical Loads

Concrete is composed of mortar, aggregates and the interfacial transition zone, In which there is a strong coupled thermo-hygro-mechanical processes. In this article,some relevant mechanical problems are studied based on micromechanics, and the main works are as follows:A coupled thermo-hygro-mechanical mathematical model for concrete is proposed considering the mismatch of eigenstrain in mortar and aggregates. In this method,concrete is assumed as a two-phase composite material composed of mortar and aggregates and mortar is assumed as an elastic damage material. The coupled model is built by introducing the stress caused by the mismatch of eigenstrain in mortar damage.The damage influences transfer of heat and moisture by changing the coefficients of conductivity, while the influences of humidity and temperature on mechanical state are due to the thermal stress, shrinkage stress and those caused by the mismatch of eigenstrain. The strains and stresses of mortar and aggregates are calculated by Mori-Tanaka method. We analyze the coupled thermo-hygro-mechanical processes of a road slab and the results show that the the mismatch of eigenstrain plays an important role.The influences of transfer of heat and moisture on the strength of concrete are analyzed with the above coupled method. As the practical calculation examples, we evaluate the strength and distribution of stress for cylinders in the case of desiccation and chilled environment. The results show that transfer of heat and moisture lead nonuniform distribution of stress in cylinders, the lower compressive strength is caused by longer diffusing time and smaller size.A simple hydration model is built with taking the composition of the cement and the initial water: cement ratio (w/c) into account explicitly. The conceptual basis for the models is a combination of Avrami equation and Bentz's model which based on simple spatial considerations. In this model, Avrami equation is used to determine the initial reaction while Bentz's model describes the following hydration stage. This method can differentiate the hydration rates of different mineral phases and give the volume fractions of all the hydration productions. The model needs only one experimental parameter to determine the undetermined parameters. It is easy use and can predict the long term hydration development. A method is built to calculate the percolation threshold considering solid volume fractions and w/c. Based on the micromechanical homogenization method and the volume fraction of percolated solid, a simple and impactful approach is proposed to predict the elastic properties of cement paste at early age. The theoretical predictions are consistent with experimental results. Moreover, for ordinary Portland cement, a simpler model is built by employing Powers hydrate model.The interfacial transition zone is the most weak phase in concrete. Based on micromechanical theory,the mechanical properties of interfacial transition zone as well as the influences on the mechanical behaves of concrete are investigated. The interfacial transition zone is assumed as a thick hollow sphere made of two phases of different materials, and both of whose volume fractions vary radially. To evaluate the mechanical properties of interfacial transition zone, we develop several analytical and numerical methods based on different micromechanical homogenization methods. In analytical approaches, using the parallel and series rules respectively, we obtain the solutions in elastic and plastic condition, as well as those of steady state heat conduction.In numerical approaches, the interfacial transition zone is divided into several hollow spheres, in which the volume fractions of the two component is fixed, and the mechanical properties are determined by parallel rule, series rule and Mori-Tanaka method respectively. The solutions are obtained using the continuous condition of radial displacement and stress between the spheres. The numerical methods, with small amount of calculation and high precision, are consistent with the analytical approaches.The results show that the influences of interfacial transition zone on the mechanical behaves of concrete depend on the size and volume fraction of the aggregates. The existence of interfacial transition zone leads a lower strength of concrete and a negligible effect on the initial damage load.