Study On Axial Compression Performance Of Recycled Concrete Restrained By Stirrups After Freeze-Thaw

With the accelerating urbanization process,recycled concrete occupies an increasingly important position in the process of urbanization development.For areas such as Northeast China and North China where freeze-thaw damage is more serious,recycled concrete is resistant when it is put into formal use.The experimental and theoretical research on the freezing performance needs to be continuously improved.This paper will be based on experiments,the change parameters are the replacement rate of recycled aggregates(0%,30%,70%,100%),the number of freeze-thaw cycles(0,50,100,150,200)and Bracing method(unconstrained,rectangular hoop restraint,spiral stirrup constraint).The main research contents are:Firstly,the axial compression and splitting tensile tests were carried out on 120 cubic specimens with the replacement ratio of regenerated aggregate and the number of freeze-thaw cycles.The compressive strength of the aggregate and the number of freeze-thaw cycles were analyzed.And the effect of splitting tensile strength.The results show that the mass of cube specimens increases first and then decreases with the increase of freeze-thaw cycles under different substitution rates,but the variation of water absorption rate with freeze-thaw cycles is not obvious.The tensile strength of recycled concrete varies with the number of freeze-thaw cycles.The increase is in a decreasing trend,and as the replacement rate of recycled aggregate increases,the extent of the decrease is smaller;the tensile-compression ratio remains basically unchanged at N<100 times,and after N>100 times,As the number of freeze-thaw cycles decreases,the tensile compression ratio of the RC-70 specimens is the highest’ after the same number of freeze-thaw cycles,indicating that the crack resistance is best after freeze-thaw;the cubic compressive strength is obtained by fitting.The relationship between the tensile strength and the coefficient of freezing and thawing and the substitution rate is better.Secondly,the axial compression test of 48 rectangular hoop-constrained recycled concrete specimens is carried out.The test results show that as the number of freeze-thaw cycles increases,the peak strength and initial stiffness of the confined recycled concrete decrease continuously,while the peak strain and ductility coefficient decrease.With the increase of the number of freeze-thaw cycles,the peak stress of the recycled concrete is increasing with the increase of the replacement rate of the recycled aggregates,indicating that the increase of the replacement rate of the recycled aggregate can make the freeze-thaw Constrained recycled concrete specimens have better frost resistance;due to the existence of stirrup restraint,the effect of freeze-thaw on the strength of restrained recycled concrete is less than that of plain concrete.With reference to the Sargin model and the uniaxial compression stress-strain constitutive model of Zhenhai-Zhang Xiuqin,the axial stress-strain constitutive relation of the reinforced concrete with hoop-reinforced reinforced concrete after freezing and thawing is established.The damage mechanics theory and the Weibull strength probability model were used to analyze the damage of the recycled concrete after freeze-thaw.The experimental results show that the initial damage variable of the RC specimens after freezing and thawing is less than that of the NC specimens.The initial damage of the specimens is more Large,the slower the rate of development of the damage.Finally,36 kinds of reclaimed concrete specimens with different hooping methods(unconstrained,rectangular stirrups and spiral stirrups)were subjected to axial compression test,and the different hoops were obtained after 100 times of freezing and thawing.Reconstruction concrete stress-strain full curve between the modes,and using the over-tension model to fit it;based on the existing concrete octahedron strength theory,establish the theoretical relationship of the strength of the confined recycled concrete after 100 cycles of freezing and thawing;Based on the calculation formula of axial compressive bearing capacity of existing concrete structure design specifications,the relationship between bearing capacity of confined recycled concrete after freeze-thaw is derived.The theoretical value is consistent with the experimental value and is safe.It can be used as freeze-thaw.Calculation of bearing capacity of post-constrained recycled concrete.