Study On Mesoscopic Moisture Transport Performance Of Unsaturated Concrete

Most of the durability problems of concrete structures are related to moisture,which acts as a carrier of harmful ions in the concrete invading process.Generally,the speed of the concrete structure degradation process depends on it.Therefore,it is significant to study moisture transport performance to improve its durability of concrete.In this paper,different temperature and humidity alternating moisture transport tests and saturation balance tests are designed.The response of concrete with different water-binder ratios and internal depths to the external environment is studied,and the moisture transport performance of water-binder ratio 0.45 cement-based materials under different temperature and humidity environments is studied.Based on the test results,the isothermal adsorption-desorption curves and water characteristic curves at different temperatures are obtained.And combined with the random aggregate model,the meso-scale moisture transport numerical model of concrete is established,further,which provided a basis for exploring the influence of boundary temperature,boundary saturation,and meso-scale factors on the moisture transport performance of concrete.The specific work and main conclusions of this paper are as follows:(1)The variation of water-binder ratio and internal depth relative humidity of concrete with different water-binder ratios are analyzed by conducting the moisture transport tests under different temperature and humidity alternation,and the moisture diffusion coefficient of concrete with different water-binder ratios is obtained based on Fick ’ s second law.The results show that the larger the water-binder ratio,the earlier the relative humidity response inside the concrete,the deeper the distance from the transport surface,the slower the relative humidity change rate,and the later the response to the external environment.When the relative humidity is less than 80 %,the moisture diffusion coefficient of different water-binder ratios is basically the same.When the relative humidity is greater than 80 %,the moisture diffusion coefficient increases exponentially,and the difference in moisture diffusion coefficient between different water-binder ratios is more and more obvious.(2)By saturation equilibrium relationship test,the moisture transport performance of water-binder ratio 0.45 cement-based materials under different temperature and humidity environments is studied,and the isothermal absorption and desorption curves of mortar and concrete at different temperatures are obtained.Besides,considering the effect of temperature,the relative humidity-saturation relationship model based on Simpson model is established,subsequently,the expression of the relationship between capillary pressure and saturation is derived from the Kelvin-Laplace equation,and the water characteristic curve of mortar is obtained.The results show that the isothermal absorption and desorption curves of mortar and concrete are the primarily same,but there is an obvious hysteresis in the process of moisture removal;in the process of moisture absorption,when the relative humidity is greater than55 %,the rate of moisture absorption is significantly accelerated,the internal moisture content is significantly increased,and the saturation degree rises exponentially.(3)Based on the Kozeny-Carman model,the inherent permeability coefficient is calculated by mercury intrusion test results of mortar.And the m value of the Mualem equation is fitted based on the water characteristic curve combined with the Van Genuchten model,at the same time,the expression of the relative permeability coefficient is determined.On the basis of the above,the theoretical model of moisture transport in the mortar with liquid water and water vapor is established.Then,the mathematical relationship between the moisture transport coefficient of mortar and interface transition zone is obtained based on local pore theory using the results of the scanning electron microscopy test.According to the concrete mesoscopic heat transport theory model,the mesoscopic moisture transport theory model of concrete is established,which considers the influence of temperature.(4)Based on the Monte Carlo method and Walraven equation,the random aggregate models of circular,elliptical,and polygon are established.And then,combined with the mesoscopic moisture transport theory model of concrete,the numerical model of concrete mesoscopic moisture transport is established by COMSOL software.On this basis,the influence of boundary temperature,saturation,and mesoscopic factors on the moisture transport performance of concrete are further explored.The results show that the higher the boundary temperature,the lower the equilibrium saturation reached in the process of absorption or desorption process.The higher the boundary saturation,the faster the absorption rate and the slower the desorption rate in the early stage;With the increase of aggregate volume fraction,the moisture transport rate gradually increases.When the volume fraction is 0.15 to0.45,the difference in moisture transport performance is the most obvious;With the increase of the interfacial transition zone’s thickness and transport coefficient,the moisture transport rate gradually accelerates.After 50 h,the saturation changes of the different interfacial transition zone’s thickness and transport coefficient are more and more obvious;The aggregate shape,particle size range,and distribution position have no significant influence on the moisture transport performance of concrete.