Experimental Study On Behavior Of Concrete With Different Strength Grades Experiencing Ultralow Temperature Freeze-thaw Cycle Action

As a mechanical material for the LNG storage tank structure,the concrete is subjected to the ultralow temperature freeze-thaw cycle action due to changes in the temperature of the tank caused by changes in the LNG inventory.And considering functional requirements,stress characteristics,the economical nature and the safety of the actual operation of the tank structure,the concrete strength grades used in different types of tanks and different parts of the tank will also be different.Therefore,it is necessary to study the performance of the concrete with different strength grades experiencing the ultralow temperature freeze-thaw cycle action.This article will explore the effect of the strength grade on the compressive performance and deformation properties of the concrete subjected to the ultralow temperature freeze-thaw cycle action,and compare with the conclusions of the conventional freeze-thaw cycle action.According to the actual operation of the LNG storage tank and the research results of the research group,the concrete design strength grades selected this paper are C40,C50 and C60,respectively,the investigated temperature ranges are 15°C~-120°C and 15°C~-190°C,and the number of the ultralow temperature freeze-thaw cycle action are 0,16 and 30 times respectively.A total of 120 test pieces are produced in this test,and all tests are carried out by the self-developed ultralow temperature concrete performance test devices.Through the ultralow temperature test furnace,the concrete thermal deformation index(thermal residual deformation,thermal expansion deformation coefficient)is obtained,and then by the associated hydraulic testing machine to apply the axial compression load,the change rule of the concrete upper and lower limits compressive strength properties and compressive deformation properties(modulus of elasticity,peak strain,stress-strain curve)can be learned.From the test results,it is shown that the strength grade has a significant effect on the performance of the concrete after the ultralow temperature freeze-thaw cycle action,but there are also differences for different ultralow temperature intervals and the number of the action.For a given ultralow temperature freeze-thaw test condition,the strength grade has a significant influence on the damage status of the concrete at the upper limit temperature,but is not obvious at the lower limit temperature.The compressive strength and elastic modulus of the concrete have similar variation rules with the increase of its strength level.However,the change mechanisms of the peak strain,the stress-strain curve and the thermal deformation are more complicated,the change trends with different strength grades under different test conditions are different.At the same time,the performance variation of the concrete at the lower limit temperature is both affected by the freeze-thaw cycle damage and the low-temperature enhancement effect,so that the change rule of the concrete with the increase of the strength grade is closely related to the condition of the ultralow temperature freeze-thaw cycle action.Comparing with the existing experimental results of the the conventional freezethaw cycle action,it is shown that the effect of the ultralow temperature freeze-thaw cycle action on the performance index of the concrete with different strength grades is more serious and there are also obvious differences.The results provided of this paper can be used in the design and the safety assessment of concrete structures at the ultralow temperature such as the LNG tanks.