Strengrh And Constitutive Relationship Of Recycled Aggregate Concrete Under Multiaxial Stress States

Recycled concrete as a new green building material,its application has good environmental and economic effects.Concrete structures are mostly in multiaxial stress state in practical engineering.The multiaxial strength and constitutive relationship of concrete are the most important basic theoretical problems and structural analysis foundation.Because of significant differences in microstructure between recycled concrete and ordinary concrete,it is not suitable for structure analysis of recycled concrete with the existing theories of ordinary concrete under multiaxial stress state.Study on mechanical properties of recycled concrete under multiaxial stress states is a basic research subject.Combined with the National Natural Science Foundation“The strength and constitutive relation of recycled concrete under multiaxial stress states"(No.51478126),experimental research and theoretical analysis for recycled concrete specimens under uniaxial and multiaxial stress states were carried out.Specific work is as follows:1.The microstructure of recycled aggregate and the interfacial transition zone of recycled concrete were observed by scanning electron microscope(SEM)and electron microscope.Voids and microcracks were found between the old and new interfacial transition zone of recycled concrete.The development of cracks in recycled concrete slices under different loading stages was gathered by using high distinct camera,and the influence of recycled aggregate on crack development was analyzed.According to the microscopic characteristics of recycled aggregate,the microstructure model is proposed,and the formula for calculating the volume fraction of recycled aggregate effect layer is established.2.The cubic compressive strength,prism compressive strength and tensile strength of concrete specimens with different replacement ratio of recycled aggregate under uniaxial stress were tested,and the conversion relationship between the strength values was given.The results show that the uniaxial strength of concrete decreases with the increase of recycled aggregate replacement rate.Based on the relationship between the strength value and the density of concrete and the replacement rate of recycled aggregate,a mathematical model of two variables for the strength of recycled concrete was put forward.The stress-strain curve of recycled concrete under uniaxial compression was tested.The higher the replacement rate of recycled aggregate,the smaller the elastic modulus and the initial slope of the ascent curve,and the steeper the descent curve.According to the characteristics of compressive stress-strain curves,a piecewise rational constitutive model and a damage model for recycled concrete are proposed.Meanwhile,the tensile stress-strain curve under uniaxial tension state with the influence of recycled aggregate replacement rate is tested,and an appropriate tension model is put forward.3.The mechanical properties of recycled concrete under biaxial compressive stress were studied.Biaxial strength,deformation and initial modulus of elasticity influenced by stress ratio and recycled aggregate replacement ratio were analyzed.The test results show that the biaxial compressive strength of recycled aggregate concrete is higher than that of uniaxial strength,and the strength value varies with the stress ratio:α0.5>α0.75>α1>α0.25,with the recycled aggregate replacement ratio:RAC30>RACO>RAC50>RAC70>RAC100.The strain in the direction of the maximum principal stress increases with the increase of recycled aggregate replacement rate,and decreases with the increase of the stress ratio.The modulus of elasticity decreases with the increase of the replacement rate of recycled aggregate,and increases with the increase of stress ratio.According to the calculation of variance,the effects of stress ratio and recycled aggregate replacement ratio on biaxial compressive strength,deformation and initial modulus of elasticity are quantitatively analyzed.Based on the data of strength and deformation,the strength and strain failure criterion of recycled concrete under biaxial compression are proposed.The stress-strain curves of recycled concrete under biaxial compression are tested,and the influence of recycled aggregate replacement rate on the curve shape is analyzed.Based on the characteristics of the curve,a biaxial constitutive model based on damage theory is proposed.4.Strength and deformation properties of recycled concrete under triaxial compression were tested,the failure modes,strength and deformation under triaxial compression influenced by stress ratio and recycled aggregate replacement ratio were analyzed.The results show that there are two kinds of failure modes,split fracture and oblique shear failure,the recycled aggregate replacement ratio has no obvious influence on failure mode.The strength value of recycled concrete under triaxial compression is about 4.5～5.5 times of the uniaxial strength value,the strength reach the maximum value at the stress ratio of-0.1:-0.5:-1,and the variation of strength value with the replacement ratio of recycled aggregate is RAC30>RACO>RAC50>RAC70>RAC100.The peak strain at the maximum principal stress direction reach the maximum at α=-0.1:-0.25:-1,and the strain value increases with the increase of recycled aggregate replacement ratio.According to the effects of stress ratio and recycled aggregate replacement ratio on triaxial strength and strain,the calculation formula of triaxial strength and strain with the the two factors are proposed.The influence of stress ratio and replacement ratio of recycled aggregate on strength and deformation was quantitatively analyzed by means of variance calculation.Based on the strength data,the expression of the spatial octahedral stress is given,and the four parameter failure criterion and constitutive model are proposed.