Study On Mechanical Properties Concrete And Microstructure Of Recycled Under Salt-freeze Cycle

As a green and sustainable resource,the promotion and application of recycled concrete can not only bring economic benefits,but also solve the problem of construction waste pollution.The experimental study of recycled concrete under the complex environment of salt erosion and freeze-thaw will provide theoretical support for its application and extension in the northern region.Therefore,this paper studies the mechanical properties and microstructure of recycled concrete in the environment of cold and salt erosion in Inner Mongolia.(1)Under the condition of 3.7%Na Cl solution,the salt-frost resistance of recycled concrete was worse than that of ordinary concrete.The damage pattern of recycled concrete after salt freezing is similar to that of ordinary concrete.The higher the strength grade,the better the salt-frost resistance.After 90 cycles of salt-freezing,the mass,dynamic elastic modulus and cubic compressive strength of RC30 group decreased by 4.41%,62.33% and 38.8% respectively,while RC50 group only decreased by 0.83%,20.93% and 4.6%.The sulfate group with 3.7% concentration had better frost resistance in the early stage of freezing-thawing cycle,but the damage accumulated in the later stage and the frost resistance decreased rapidly.Damage models of chlorine and sulfate under freezing-thawing cycle were established respectively.(2)The failure modes of concrete under uniaxial compression are shear failure and split failure.The stress-strain curves of each group are similar in shape.The peak strain and limit strain of RC40 group were greater than NC40 group,and the peak stress was less than NC40 group.The compressive strength ratio of recycled concrete prisms to cubes ranges from 0.74 to 1.NC40 group had better elastic deformation than RC40 group after salt-freeze cycle.The higher the strength grade of recycled concrete,the better its resistance to plastic deformation.After freezing-thawing cycle,the peak stress and limit strain of SC40 group first increased and then decreased.During 0-60 cycles,SC40 group showed significant difference in peak stress,peak strain and elastic modulus from RC40 group,and the elastic deformation performance was better than RC40 group.After 90 cycles,the characteristic values of stress-strain curves of SC40 group were almost close to each other.Poisson’s ratio decreases with the increase of freezing-thawing cycle,and ordinary concrete is more likely to be damaged than recycled concrete.The damage resistance of sulfate group is better than that of chlorine group.The constitutive equation of recycled concrete after salt-freeze cycle can be adopted the Gouzhenhai model.As the number of freeze-thaw cycles increases,the fitting parameter a decreases and the fitting parameter B increases.(3)By means of SEM and XRD,the internal structure morphology and phase composition of recycled concrete under the coupling of salt erosion and freeze-thaw cycle were analyzed.Before and after freezing-thawing cycle,the internal structure density and bonding performance of the transition zone of ordinary concrete are better than that of recycled concrete.The lower the strength grade of recycled concrete,the looser the internal structure and the wider the crack width after freeze-thaw.The chlorine enters the concrete to produce Na Cl crystals.When the sulfate enters the concrete,the calcium-vanadite generated by the freeze-thaw cycle in the early stage fills the cracks and pores,and the amount of calcium-vanadite generated in the later stage increases,causing serious damage to the recycled concrete.The failure mechanism of recycled concrete under salt freezing-thawing cycle is described.