Study On Splitting-pulling Rate Effect Of Freeze-thaw Deteriorated Concrete Under Freezing Condition

China’s high latitude and high altitude cold areas account for most of the land area,and a large part of them are still in high intensity earthquake areas,freeze-thaw deterioration is the main disease of concrete hydraulic structures in these areas.Because of the influence of water,temperature and other environmental factors,many concrete buildings suffer frost damage.Under this kind of environmental factors,concrete buildings not only bear static loads,but also inevitably suffer from dynamic loads such as wind and earthquake.It is also possible that hydraulic concrete buildings will be frozen when subjected to dynamic loads such as impact and earthquake.However,the importance of large hydraulic concrete structures is self-evident,once the accident consequences are unimaginable.Therefore,it is of great academic value and broad engineering application prospect to strengthen the research on dynamic characteristics and rate effects of freeze-thaw deteriorated concrete under freezing condition.Based on the above understanding,the splitting tensile tests of concrete specimens under freezing state after different freeze-thaw cycles at different loading rates were carried out by using multifunctional static and dynamic triaxial apparatus and TR-TSDRSL freeze-thaw apparatus.The dynamic mechanical properties of freeze-thaw degraded concrete under freezing state are explored,the dynamic damage mechanism of freeze-thaw degraded concrete under freezing state is revealed,and the splitting-tension dynamic constitutive model of freeze-thaw degraded concrete under freezing state is constructed.The concrete research conclusions are as follows.1)With the increase of strain rate,the peak splitting tensile stress of frozen concrete increases linearly,while the peak strain and modulus of elasticity do not change significantly.With the deepening of deterioration,the peak splitting tensile stress and modulus of elasticity of concrete in frozen state decrease linearly,and the peak strain has no obvious change.The analysis of performance difference between frozen state and frozen-thawed state shows that,compared with frozen-thawed state,the splitting tensile peak strain of frozen state concrete is generally lower and the elastic modulus is slightly higher,which indicates that negative low temperature can improve the brittleness of concrete.The peak splitting tensile stress of frozen concrete is higher than that of freeze-thaw concrete as a whole,and the increasing proportion of peak splitting tensile stress increases with the deepening ofdeterioration degree,which indicates that the deeper the deterioration degree is,the more significant the effect of low temperature is.The change speed of relative splitting tensile peak stress value of frozen concrete is less than that of frozen-thawed concrete,which indicates that the rate sensitivity of frozen concrete decreases in negative low temperature environment.2)With the increase of the stress-level ratio,the tangent modulus shows the overall change characteristics of first decreasing,then unchanged and then decreasing.When the stress level ratio is low,the worse the degradation degree is,the steeper the curve is.When the stress level ratio is high,the higher the strain rate is,the steeper the curve is.Damage variable D is defined by the degradation of tangent modulus.Damage can be divided into four stages: structural compaction stage,elastic stage,damage instability stage and rapid damage development stage.Based on the calculation results of stress-level ratio and tangent modulus and damage evolution law,the damage threshold is studied.With the increase of strain rate,the damage threshold increases,and with the deepening of deterioration,the damage threshold decreases.3)Assuming that the rate effect of concrete with different deterioration degree is the same,an expression of peak stress and deterioration degree and strain rate is proposed.After determining the reliability of the expression parameters,the Mazars model is improved and a dynamic constitutive model of deterioration degree,strain rate and splitting tension is proposed.The improved constitutive model is used to fit and analyze the experimental data in this paper,and the fitting effect is good.The test curves and fitting curves are analyzed from the deterioration degree and strain rate.The curves can better express the mechanical properties of concrete under freeze-thaw environment.