Study On The Damage Degradation And Mass Transfer Of Recycled Concrete Subjected To Loading And Salt Freezing-thaw Cycles

In order to achieve the double carbon targets of the construction industry and meet the needs of major foundation projects,the preparation of waste concrete into recycled aggregates can not only reduce the consumption of non-renewable resources such as sand and gravel,but also decrease the carbon emissions of the construction industry,which is of great significance for the transformation and upgrading of the construction industry and the realization of green and sustainable development.However,due to the old mortar attached to the surface of the recycled coarse aggregate(RCA)and the mechanical damage during the manufacturing process,the mechanical and durability of the recycled aggregate concrete(RAC)is generally lower than that of natural aggregate concrete(NAC).In service,RAC components often suffer from load,carbonization,chloride erosion,freeze-thaw cycles or other factors,which accelerates the durability deterioration of structures.In particular,the other harsh environments such as external loads and freeze-thaw cycles tend to promote the development of pores and cracks in RAC,providing a more convenient channel for the intrusion of moisture and other corrosive ions,which in turn leads to more serious damage and deterioration of the material.Therefore,it is of great significance to study the influential mechanisms of RAC damage deterioration and mass transport under the coupling action of load,chloride erosion and freeze-thaw cycle for improving the durability design and life prediction theory of RAC structure in cold areas.Based on this,this paper investigates the damage deterioration and mass transport performance of RAC under the coupling action of load and salt frosting.The effects of sustained compressive stress level,RCA replacement rate and freeze-thaw cycle number on the microstructure and transport performance of RAC are analyzed,and the influential mechanism of the coupled damage of load and salt frost on the mass transport performance is elucidated.The specific work and main conclusions are shown as follows:(1)The experimental study on the damage deterioration of RAC and its microstructural characterization after salt frost was carried out.The effects of RCA replacement rate(0%,50%,100%)and the number of freeze-thaw cycles(0,25,50,75)on the damage deterioration of RAC under water and salt frost conditions were contrastively analyzed.The model of RAC damage deterioration under salt frost was further established.Combining with the meso/micro scale observation test including scanning electron microscopy(SEM),mercury intrusion porosimetry(MIP),X-CT and laser confocal scanning microscope(LSCM),the influential mechanism of freeze-thaw damage on the microstructure of RAC in chloride salt environment was explored.The results show that the frost resistance of concrete decreases with the increase of the number of freeze-thaw cycles and the replacement rate of RCA.Among them,the compressive strength of R0and R100 test blocks decreased by 16.25%and 41.99%,respectively,and the modulus of kinetic elasticity decreased by 75.92%and 44.19%,respectively.Moreover,the damage deterioration of RAC under salt frost condition is more serious compared to water frost condition,causing the more loose microstructure.Moreover,the most probable pore size can be significantly enlarged,and the interface between the aggregate and mortar in the R100 group of specimens have a serious separation phenomenon.(2)According to capillary water absorption and chloride content tests,the effects of different RCA replacement rates and freeze-thaw cycles on the performance of water absorption and chloride transport in RAC under water and salt frost conditions were contrastively analyzed.The constitutive model of chloride erosion RAC damaged by salt frost was developed.The numerical calculation of chloride content distribution in RAC subjected to different freeze-thaw cycles was carried out.The study found that under the same freeze-thaw cycle conditions,the initial sorptivity of concrete increases with the increase of RCA replacement rate and cyclic freeze-thaw number.Compared with the R0group,the initial sorptivity of R100 and R50 is increased by 70.4%and 110.2%,respectively.Through the numerical calculation of chloride content distribution,the rationality and accuracy of the established model of chloride erosion in RAC damaged by salt frost were further verified combining with experimental results.(3)Based on the designed loading device for real-time monitoring of stress loss,the coupled effects of the sustained compressive load and salt frost damage on the mass transport performance in RAC was experimentally investigated.The effects of different stress levels(λ_c=0.1,0.3 and 0.5),RCA replacement rate and freeze-thaw cycles on the damage degradation of RAC were studied from the viewpoint of multiscale analysis,and the model of damage deterioration was further established.Combined with the models of water absorption and chloride transport,the numerical calculation of mass transport in RAC subjected to loading and salt frost damage was carried out.It can be found that the capillary water absorption and chloride transport performance of RAC show a trend of first decreasing and then increasing with the increase of sustained compressive stress level.Compared to no stress state,when the stress level equals to 0.5,the initial sorptivity of R100 is increased by 15.65%after water frost of 55 cycles.At the same stress level,the amount of absorbed water,the initial sorptivity and chloride content at the same depth are significantly greater than that of water frost under the same stress level.Based on the model of damage deterioration under the coupling of load and freeze-thaw,the comparative analysis show that the calculation results of chloride content agrees well with the experimental data.