Analysis For Chloride Diffusion In Recycled Aggregate Concrete By Considering The Effect Of Double Interface Transition Zone

Recycled aggregate concrete as a sustainable development of green building materials,achieves the recycling of resources and accords with the demand of sustainable development.Due to the presence of old mortar on the surface of recycled coarse aggregate,there are double interfacial transition zone(ITZ),which affects the durability of recycled concrete.In this paper,recycled concrete is seen as a five-phase material,which includes new mortar,old mortar,original coarse aggregate,new ITZ in new mortar-old mortar and old ITZ in old mortar-original aggregate.The law of chloride diffusion in recycled concrete was analyzed from the finite element numerical model and the equivalent diffusion coefficient calculation model.The numerical model and the calculation model were validated by natural diffusion test.The main research and conclusions of this paper are given as follows:(1)A five-phase model for chloride diffusion in recycled concrete with random aggregate was established.Based on the bidirectional walk method of random recycled aggregate,the five-phase model and the composite of three-phase and five-phase model of recycled concrete were studied,and the parameters of the model were discussed.The results show that the five-phase model can be considered to evaluate the chloride diffusion behavior in recycled concrete by considering the influence of old mortar and double ITZ.(2)Based on the theory of aggregate gradation distribution,the area fraction calculation model of double ITZ in recycled concrete is proposed.Combined with the chloride diffusion coefficient of double ITZ,the upper and lower limits of diffusion coefficient of recycled concrete were analyzed based on Viogt-Reuss model.On this basis,a calculation model for calculating the equivalent diffusion coefficient of recycled concrete was established.The results show that the calculated accuracy of the area fraction of the double ITZ in the recycled concrete is high,and the results of the equivalent diffusion coefficient calculation model are in good agreement with the numerical simulation results when aggregates are distributed evenly in model.(3)Based on the factors of coarse aggregate content,water-binder ratio,recycled coarse aggregate substitution ratio,fly ash content,slag content and coarse aggregate gradation,eight groups of recycled concrete were designed and prepared.And then the natural diffusion test of recycled concrete has been launched.The results show that the reduction rate of recycled coarse aggregate can improve the anti-chloride corrosion resistance of recycled concrete.The corrosion resistance of recycled aggregate concrete with 45%aggregate content is better.The control of coarse aggregate particle size is 10?15mm and the decrease of water-binder ratio can improve the resistance to salt corrosion of recycled concrete.The durability of recycled concrete with single fly ash is lower than that of recycled concrete with mixed fly ash and slag and recycled concrete with single mixed mineral powder.(4)Based on the relationship between the chloride concentration and the diffusion depth in the recycled concrete,the experimental data were contrasted to five-phase model and equivalent diffusion coefficient model.The result indicated the accuracy and efficiency of these two proposed model.