Experimental Study And Theoretical Analysis On Chloride Diffusion Properties Of Cement-based Materials

As reinforced concrete structures are widely used in severe natural environments,more and more attention has been paid to the durability problem in engineering.Among the factors that cause the lack of durability of concrete,reinforcement corrosion is most important.The initiation of reinforcement corrosion is dependent on the chloride diffusivity of cement-based materials.Since there are still some defects in the existing experimental and theoretical studies on the chloride diffusivity of cement-based materials,this paper aims at analyzing the chloride diffusion characteristics.The main works are as follows.The variations of degree of cement hydration of net cement paste,fly ash cement paste,and silica fume cement paste with curing ages of up to 360 days are studied by experiment.The test results indicate that the degree of cement hydration increases with the increase of the curing age,the water/cement ratio,the fly ash content,and the silica fume content.The variations of chloride diffusivity of cement-based materials with curing ages of up to 1800 days are studied and the relationships between the chloride difusivity and the curing age,the water/cement ratio,the aggregate volume faction,the fly ash content,and the silica fume content are analyzed.The test results show that,the chloride diffusivity of cement-based materials decreases with the increase of the curing age,the aggregate volume fraction,the fly ash content,and the silica fume content but decreases with decreasing the water/cement ratio.The chloride diffusivity of net cement paste is calculated with a numerical method and an analytical method.In the numerical method,the two-dimensional microstructure of cement paste is reconstructed through computer simulation.By introducing an equivalent hydration cement particle,a random walk method is proposed for evaluating the chloride diffusivity.In the analytical method,the chloride diffusivities of the inner and outer hydration products composed of CSH and CH are first computed.The chloride diffusivity of the equivalent hydration cement particle composed of the unhydrated cement core and the inner and outer hydration products are then calculated.Finally,the effective medium method is adopted to obtain the chloride diffusivity of cement paste.Through comparison with experimental results,the validity of the numerical and analytical methods are verified.An equivalent ellipsoidal aggregate model is proposed and the equivalent interfacial transition zone(ITZ)thickness is derived analytically.A numerical method for the chloride diffusivity of concrete composed of equivalent ellipsoidal aggregates and cement paste is developed with the random walk algorithm.The validity of the numerical method is verified through comparison with experimental results and the effects of the aggregate aspect ratio,the ITZ thickness,and the aggregate volume fraction on the chloride diffusivity are quantified.Through uniform and unidirectional scaling for every aggregate,the mesostructure of concrete is generated for a given aggregate aspect ratio.The random walk algorithm and the fast Fourier transform method are used to compute the chloride diffusivity of concrete.In the random walk algorithm,it is directly applied to concrete with polygonal aggregates.The validity of the simulation method is verified through comparison with experimental results.The effects of the chloride diffusion coefficient and thickness of ITZ and the aggregate area fraction on the chloride diffusivity of concrete are quantified.In the fast Fourier transform method,the simulation element is discretized into a regular grid and each pixel is endowed with the corresponding physical property according to its location.By introducing depolarization fields and combining the Fourier transform method with the iteration method,a numerical method is developed for estimating the chloride diffusivity of concrete.Through comparison with experimental results,the validity of the simulation method is verified.The effects of the aggregate aspect ratio,the ITZ thickness,and the aggregate area fraction on the chloride diffusivity of concrete are quantified.