Mesoscopic Simulation On Freeze-Thaw Behavior Of Recycled Aggregate Concrete Based On Multi-Field Coupled Model

At present,the research on the freeze-thaw performance of recycled concrete under coupling state mostly belongs to the physical test at the macro level.Due to the limitation of experimental conditions,the research results can only characterize the freeze-thaw performance of recycled concrete from the macro mechanical performance indexes such as crushing index,dynamic elastic modulus,strength loss rate and so on.Based on the multi field coupling model,the finite element model of recycled concrete is established for calculation and analysis.Through the changes of pore water pressure,first principal stress distribution,temperature difference,strain and other parameters of recycled concrete,the evolution of freeze-thaw performance of recycled concrete before and after multiple freeze-thaw cycles is studied from the micro level.The specific research work and main conclusions are as follows(1)The microcosmic composition of recycled concrete,the freezing law of pore water derived from Water ice phase transition theory and the calculation process of pore water pressure are introduced.The determination of pore water pressure is the focus of the follow-up study,which lays the foundation for the follow-up simulation.(2)The traditional random aggregate model is improved by adding the old mortar and two ITZs(interface transition zone)of recycled coarse aggregate on the basis of the aggregate,and the replacement rate of recycled aggregate replacing natural aggregate and the thickness of old mortar and ITZ can be controlled.Based on the aggregate gradation theory and random recycled aggregate model,the finite element model of recycled concrete is established,which includes the old mortar wrapped by recycled aggregate and the ITZ between the new and old interface.(3)The solid mechanics module,Darcy’s law module and porous media heat transfer module of COMSOL multiphysics are used to build the thermal hydro mechanical multi field coupling model of recycled concrete.Considering the volume fraction of aggregate,permeability of interface and old mortar,temperature change rate,cooling mode and external load,the model is used to simulate the influence of multi physical field coupling on the freeze-thaw performance of recycled concrete during freeze-thaw cycle.The results show that the volume fraction of aggregate has an effect on the first principal stress of the specimen,and recycled aggregate can improve the frost resistance of recycled concrete;the increase of interfacial permeability will greatly reduce the frost resistance of recycled concrete and aggravate the stress concentration at the old mortar;the larger temperature change rate will increase the ice content in the internal pores of recycled concrete,which has a negative impact on its performance Compared with the broken line cooling method,the sinusoidal cooling method is more prone to freeze-thaw damage,and the freeze-thaw damage often starts from the old ITZ;External load will accelerate the separation of old mortar and new mortar matrix at the transition zone of the new ITZ.