Study On Mechanical Properties Of Recycled Coarse Aggregate Self-compacting Concrete After Salt Freezing (Sulfate)

With the rapid development of China’s construction engineering industry,domestic sand and gravel resources are gradually depleted,and finding alternatives for sand and gravel resources is on the agenda.As a waste product of urban development,improperly disposed waste concrete can pollute the ecological environment.Recycled coarse aggregate self-compacting concrete is a new type of green concrete that has many advantages of self-compacting concrete prepared using recycled coarse aggregate made from waste concrete instead of stone,in line with the country’s sustainable development policy.China has a vast territory,and concrete buildings in cold regions often suffer from salt frost(sulfate)erosion.Studying the performance changes of recycled coarse aggregate self-compacting concrete after salt freezing(sulfate)erosion has important theoretical guidance and exploration significance for the engineering practice of recycled coarse aggregate self-compacting concrete in cold regions.In this paper,the effects of recycled coarse aggregate replacement rate(0,50%,100%),freeze-thaw environment(clear water,5%mass fraction of Na2SO4 salt solution,5%mass fraction of MgSO4 salt solution,5%mass fraction of Na2SO4 and 5%mass fraction of MgSO4 mixed salt solution),freeze-thaw cycles(0,25,50,75,100,125)on the mechanical properties of recycled coarse aggregate self-compacting concrete were studied.The main conclusions are as follows:(1)The higher the replacement rate of recycled coarse aggregate and the more freeze-thaw cycles,the worse the performance of concrete.The performance of concrete in 5%mass fraction MgSO4 salt solution and clear water freeze-thaw environment is the worst and the best.(2)The accuracy of four GM(1,1)models of six groups of recycled coarse aggregate self-compacting concrete was evaluated as grade I.By introducing the accuracy coefficient S2,it is concluded that the accuracy of the discrete model in GM(1,1)is the highest,and the accuracy of the original difference model in GM(1,1)is the lowest When the actual project requires high accuracy of the model,it is recommended to use the discrete model in GM(1,1)to predict the frost resistance of recycled coarse aggregate selfcompacting concrete.(3)The higher the ratio of recycled coarse aggregate to stone,the greater the loss rate of compressive and splitting tensile strength of concrete.When the replacement rate of recycled coarse aggregate is the same,the loss rate of compressive strength,splitting tensile strength and uniaxial compressive strength of concrete decreases most under the freeze-thaw environment of 5%MgSO4 salt solution.A new relationship between splitting tensile strength,uniaxial compressive strength and compressive strength was obtained by fitting.(4)With the increase of freeze-thaw cycles,the relative peak stress of concrete decreases gradually,the relative peak strain increases gradually,and the ultimate strain increases first,then decreases briefly and then increases continuously.At the same number of freeze-thaw cycles,the relative peak stress of NC-H and RC50-M is the largest and smallest,and the relative peak strain is the smallest and largest.The elastic modulus and Poisson’s ratio of concrete decrease with the increase of freeze-thaw cycles.For the same number of freeze-thaw cycles,the larger the replacement rate of recycled coarse aggregate,the smaller the elastic modulus and Poisson’s ratio of concrete.For the same number of freeze-thaw cycles,the elastic modulus and Poisson’s ratio of concrete in 5%MgSO4 salt solution and clear water freeze-thaw environment are the smallest and largest.(5)The uniaxial compressive stress-strain constitutive model was used to fit the stress-strain curve of recycled coarse aggregate self-compacting concrete before and after salt freezing(sulfate).The results show that as the number of freeze-thaw cycles increases,the curve gradually becomes smoother,the area enclosed by the curve and the horizontal axis gradually decreases,the fit parameter a gradually decreases and the fit parameter b gradually increases.Under clear water freeze-thaw conditions and with the same number of freeze-thaw cycles,the higher the replacement rate of recycled coarse aggregates,the smaller the value of a,and the larger the value of b.In the case of 50%recycled coarse aggregate replacement rate and the same number of freeze-thaw cycles,the a values of RC50-H and RC50-M are the largest and smallest,and the b values are the smallest and largest.