Numerical Simulation Of Water Transport In Unsaturated Cement-based Material

Durability of reinforced concrete structures is a worldwild hotspot of civil engineering.It is not only closely related to resource conservation and environmental protection,but it has also become a crucial issue of the sustainable development of society and economy.Most of the degradation mechanisms of cement-based materials can be attibuted in one way or another to distribution and transmission of moisture.On the one hand,water is the important media which carries corrosive agents,such as chloride and sulfate,into the interior of cement-based materials.On the other hand,water itself can also be involved in chemical reactions and physical damage in cement-based materials.The mechanisms of moisture movement in cement-based material mainly consist of permeation,diffusion and capillary absorption.Under different environmental condition,in addition,the transport mechanisms differ a lot.With the intention of accurately evaluating a required service life of concrete construction,the core problem is to ascertain the the moisture movement mechanisms and distributions in cementious materials.In that case,based on the concrete mesoscopic theory,the finite element method(FEM)regarded to be suitable for moisture transport is used in this thesis.Numerical simulation of moisture transport in homogeneous mortar,cracked cement-based materials,three-phase concrete composite and five-phase recycled concrete composite was systematically carried out.In this thesis,the following researches are involved.(1)According to the test and numerical method for moisture transport in cement-based materials,the result obtained by domestic and foreign scholars are systematically outlined and discussed.The water transport equation of capillary absorption in unsaturated cement-based materials are reasonably established,and the discretization in finite element algorithm are deduced by means of the Galerkin weighted residual method.The meso-scale finite element method,in that case,can be used to solve the water transport problem by COMSOL.(2)The specimens whose mix proportion is the same as the specimens of referenced experiment are prepared.By using these specimens,the adsorption-desorption test and the constant-head permeability test are conducted to obtain more accurate parameters of the governing equation.Accordingly,the characteristic of cement-based materials model can be better described and the accuracy of the subsequent numerical simulation can also be well guaranteed.(3)The reinforced concrete structures always work under service loads with cracks or microcracks.These cracks,which provide easier access for moisture and aggressive medium into concrete,accelerate the corrosion of steel bars and the deterioration of cementious meterials.Hence,the Richards Equation,which can well describe the behavior of water transport in unsaturated materials,is used to establish three kinds of moisture transport models in cracks.And the accuracy of the numerical simulation results is verified by experimental results.The redistributaion of moisture with water source interruption is also simulated and verified.Then,with the consideration of other factors of craced motar,the parameter sensitivity analysis is carred out.The results show that the moisture penetration depth are positively correlated with the crack length,width,matrix saturation,and the number of cracks,and negatively correlated with the inclination angle of cracks.(4)On the basis of the non-homogeneous characteristic of concrete,the five-phase RAC model consisting of old aggregate,old mortar paste,old ITZ,new motar paste and new ITZ is established and calculated.The comparation of concrete and RAC shows that bonding motar and resulting double ITZ massively speed up moisture transport rate in recycled concrete.And the parameter sensitivity analysis of RAC suggested that replacement rate of aggregates,thinkness of old motar paste and volume fraction of aggregates are the primary parameters affecting moisture penetration into recycled aggregate concrete.