Study On Uniaxial Compressive Strain Rate Effect Of Freezing-thawing Deteriorated Concrete Under Freezing Condition

The northeast,northwest,some parts of north and the alpine regions in southwest of China,which account for nearly 30% of land area in China.The concrete buildings in the above areas will be crashed damaged to different degrees under repeated freeze-thaw cycles under the combined influence of temperature change and water environment.At the same time,earthquake action as another factor affecting the safety and reliability of buildings,which is directly relates to China’s modernization construction and the safety of people’s life and property.The moisture inside concrete is frozen under low temperature in winter,the physical propeties of each components of concrete is changed by the effect of low temperature,and the material stress state is changed by the existence of pore ice.So the mechanical properties and working properties of concrete material is changed inevitable.This paper with the support of the national natural science foundation of“The alpine dynamic characteristics of hydraulic concrete freeze-thaw environment and constitutive theory”（foundation project number:51579139）,the concrete uniaxial compression test with different freeze-thaw deterioration degree and different strain rate under freezing condition.And the main research contents of this paper are as follows:1)The pretreatment to concrete specimens were carried out with different freeze-thaw cycles（0 time,10 times,20 times,30 times and 40 times）,and make sure the concrete specimens were frozen.The concrete specimens which are under freezing condition and dryness condition the ice content in concrete were weighted to determine the correlations between the mass and freeze-thaw cycle.The correlations between the degree of freeze-thaw deterioration and the freeze-thaw cycles of concrete under freezing condition were established.The results show that with the increase of the number of freezing-thawing cycles,the mass of ice in the pores of frozen concrete specimens will gradually increase,and the deterioration degree of freezing-thawing of concrete materials will continue to deepen.2)The frozen concrete which experienced different freeze-thaw cycles（0 times,10 times,20 times,30 times,40 times）were carried out under the uniaxial compression test at different loading rate(10^-5/s,10^-4/s,5×10^-4/s,10^-3/s,5×10^-3/s),analysising the changing rule of mechanics characteristics,failure pattern and the brittle characteristics of the frozen concrete with the change of strain rate and freeze-thaw deterioration.Constructing the calculation model of mechanical parameter based on strain rate and freeze-thaw deterioration,research the sensitivity of mechanical properties to strain rate and freeze-thaw deterioration degree.The results show that the uniaxial compressive strength of concrete still has a rate effect under frozen condition,and the cohesive effect of pore ice on each component of frozen concrete will gradually change to separation and isolation effect with the increase of its content.The sensitivity of uniaxial compressive peak stress,elastic modulus and energy absorption capacity of frozen concrete to freeze-thaw deterioration degree is higher than that to strain rate.3)The mechanical properties of freeze-thaw deteriorating concrete under freezing and melting conditions are compared and analyzed,the influence and the mechanism of strain rate,freeze-thaw deterioration degree and ice heave force on the uniaxial compressive strength increment of concrete under frozen conditions is studied.The stiffness degradation and strain energy growth of concrete under the two conditions are analyzed.The results show that when the ice expansion force reaches a certain threshold,the increment of uniaxial compressive strength of frozen concrete would decreases with the increase of strain rate.The degree of freezing-thawing deterioration and ice expansion force in the internal pores of frozen concrete are the main factors affecting concrete strength increment under frozen condition.The stiffness degradation and strain energy growth of frozen concrete under the uniaxial compression loading process show different degrees of retardation compared with that under the molten condition.4)The Weibull-Lognormal constitutive model was selected to fit the uniaxial compressive stress-strain curve of concrete under freezing condition,the shape control parameters of the model was determined.The influence of strain rate and freeze-thaw deterioration degree on the characteristics of stiffness degradation threshold and post-peak stress drop of frozen concrete was analyzed.The uniaxial compressive constitutive model of concrete under freezing condition based on strain rate and degree of freeze-thaw deterioration was constructed.The results show that low temperature freezing is helpful to delay the stiffness degradation of concrete under uniaxial compressive loading.Under the frozen condition,the stress drop rate of freeze-thaw deteriorated concrete after uniaxial compressive loading to the peak point is faster than that of molten condition.The uniaxial compressive stress-strain dynamic constitutive model of freezing-thawing deteriorated concrete under frozen condition could describe the uniaxial compressive stress-strain relationship of frozen concrete well,and it also has good applicability under other working conditions.5)The dynamic uniaxial compression damage development curve of frozen concrete under different working conditions was calculated based on the improved Najar energy method.The influence law of strain rate and degree of freeze-thaw deterioration on uniaxial compressive damage evolution was studied,and the difference between the uniaxial compression damage evolution characteristics of concrete under freezing and meilting condition was compared.The influence of strain rate and freeze-thaw deterioration degree on the location of concrete damage boundary point and the length of damage range was analyzed.The results show that the development of dynamic uniaxial compressive damage of freeze-thaw deteriorated concrete under frozen condition can be divided into initial damage accumulation stage,sudden damage accumulation stage and complete damage development stage.The uniaxial compressive damage evolution of freezing-thawing deteriorated concrete under frozen condition is more inclined to centralized and rapid development than that under molten condition.