Research On Mechanical Properties Of Recycled Concrete After Freeze-Thaw Under Cyclic Loading And Unloading

In the alpine regions of my country freeze-thaw damage to concrete buildings is not uncommon.At the same time,these buildings are also subjected to cyclic loads such as earthquakes,wave impacts,and vehicle rolling.At present,the research results on the cyclic loading and unloading of recycled concrete after freezing and thawing are very lacking.Therefore,it is of great significance for the popularization and application of recycled concrete to study the damage and failure laws of recycled concrete under the coupled action of freeze-thaw cycles and loading and unloading cycles.In this paper,the freeze-thaw cycle test,uniaxial compression test after freeze-thaw,and cyclic loading and unloading test after freeze-thaw are carried out by combining experiments and theoretical analysis.The frost resistance and uniaxial compressive properties of recycled concrete,the basic mechanical properties and dynamic mechanical properties of recycled concrete under cyclic loading and unloading were studied.The performance differences between recycled concrete and ordinary concrete were compared and analyzed,and the applicability of recycled concrete with different replacement ratios of recycled coarse aggregate was verified.The main research contents and conclusions are as follows:（1）By comparing the performance include mass loss rate,relative dynamic elastic modulus,compressive strength loss rate and uniaxial compressive stress-strain curves changes of the three specimens of NC（ordinary concrete）,RC50（recycled coarse aggregate replacement rate of 50%）and RC100（recycled coarse aggregate replacement rate of 100%）can be found that when the freeze-thaw cycles are within 100 times,the frost resistance and compressive performance parameters of RC50 are similar to NC,while RC100 is different from NC larger.When the freezethaw cycles do not exceed 100 times,from the perspective of frost resistance and uniaxial compressive performance,recycled concrete with a replacement rate of 50%of recycled coarse aggregate can be a good substitute for ordinary concrete.（2）By strain rate(10^-5s^-1,10^-4s^-1,10^-3s^-1),number of freeze-thaw cycles（0,25,50）,replacement rate of recycled coarse aggregate（0,50%,100%）as control variables,the basic mechanical properties,stress-strain curve and outer envelope of recycled concrete under cyclic loading and unloading are studied.It was found that the peak stress decreased with the increase of the number of freeze-thaw cycles and the replacement rate of recycled coarse aggregate,and the change rule was in a linear relationship.When the degree of freeze-thaw deterioration is small,the ability of ordinary concrete and recycled concrete to resist cyclic loading and unloading is similar.When the degree of freeze-thaw deterioration is large,the ability of recycled concrete to resist cyclic loading and unloading is more weakened than that of ordinary concrete.（3）Cyclic loading and unloading tests at different strain rates were carried out on ordinary concrete and recycled concrete,and it was found that the peak stress changes showed a certain strain rate sensitivity.With the increase of freeze-thaw cycles,the rate sensitivity of NC specimens gradually weakened,and the rate sensitivity of NC specimens without freeze-thaw cycles was significantly different from that of 25 and 50 freeze-thaw cycles.With the increase of the number of freeze-thaw cycles,the RC50 specimen and the RC100 specimen showed a certain discreteness in the change of the rate sensitivity.When not subjected to freeze-thaw cycles,the rate sensitivity decreases with an increase in the replacement rate of regenerated coarse aggregate.（4）The residual deformation,hysteresis effect and elastic-plastic strain energy of recycled concrete under dynamic cyclic loading and unloading are studied,and the energy evolution law during cyclic loading and unloading is analyzed.Introducing cyclic loading and unloading damage variables based on energy method.From the change of damage variables,it can be obtained that when the freeze-thaw cycles are less than 50 times,the RC50 specimen has a better ability to resist the damage of cyclic loading and unloading,while the RC100 specimen is relatively poor.From the perspective of resistance to cyclic loading and unloading,recycled concrete with a replacement rate of 50%of recycled coarse aggregate has a good substitute for ordinary concrete.