Experimental Study On Damage Evolution Of Natural Aggregate In Recycled Coarse Aggregate Under Different Initial Damage

The natural aggregate in the recycled coarse aggregate will suffer certain freezing-thawing damage and mechanical damage in the service process of the raw concrete,and at the same time,mechanical damage will also occur to the aggregate in the process of crushing and processing into the recycled coarse aggregate.It is of great significance to study the initial damage and evolution process of natural aggregate in recycled concrete to reveal the damage evolution mechanism of recycled concrete.This article selects similar to natural aggregate performance characteristics of limestone as the research object,separately the freeze-thaw cycle test(considering freeze-thaw cycle 25 times,50 times,75 times,100 times,125 times and 150 times)and different stress level loops and unloading test(60%,70% and 80% stress levels,loops and uninstall 3 times,5 times and 20 times),in order to form different initial damage,the change of physical and mechanical properties of limestones with different initial damage is analyzed,and the damage mechanism is revealed through micro and fine tests.The main research contents and conclusions are as follows:(1)With the increase of the number of freezing-thawing cycles,the longitudinal wave velocity,uniaxial compressive strength,elastic modulus and splitting tensile strength of limestone show a linear decreasing trend,indicating that the damage accumulation in freezing-thawing process increases linearly with the increase of the number of freezing-thawing cycles.(2)Cyclic loading and unloading tests of limestone with different stress levels are carried out,considering three factors: stress level,number of stress cycles and load holding time,and the specimens with mechanical damage are experimentally studied.It is found that when the number of force cycles is small,the longitudinal wave velocity,uniaxial compressive strength,elastic modulus and splitting tensile strength of limestone all increase,indicating that the physical and mechanical properties of limestone are strengthened,and the strengthening effect of limestone is more and more significant after increasing the stress level.When the number of force cycles is large,the resistance to load and deformation of limestone appear fatigue,and the physical and mechanical properties of limestone decline,The performance of limestone decreases more obviously when the stress level is increased;At the same stress level,with the increase of the number of stress cycles,the physical and mechanical properties of limestone tend to strengthen first and then deteriorate;When the number of force cycles is large,the performance of limestone is further deteriorated after increasing the load holding time.(3)The damage amount is defined by the average loss rate of macro parameters(longitudinal wave velocity,uniaxial compressive strength,elastic modulus and tensile strength).The damage amount of freeze-thaw cycle and cyclic loading and unloading at different stress levels is compared and analyzed.It is found that the damage amount of limestone after 20 cycles of loading and unloading at 70% stress level is roughly close to that of about 25 cycles of freeze-thaw cycle;The damage amount of 20 loading and unloading cycles with 70% stress level for 30 s is close to that of 44 freeze-thaw cycles,and 20 loading and unloading cycles with 80% stress level is close to that of 42 freeze-thaw cycles;The damage of 80% stress level cycle for 20 times and holding load for 30 s is close to that of freeze-thaw cycle for 58 times.(4)Through mercury injection test and SEM test,it is found that the porosity of limestone increases after freeze-thaw cycle,which is mainly caused by pore expansion caused by water erosion and frost heaving.The pore volume with a pore diameter of more than 100 microns increases rapidly,and the pore fractal dimension also increases with the increase of freeze-thaw times.After 5 cycles of 80% stress level,the porosity of limestone decreases,increases after 20 cycles,and continues to increase after the increase of load holding time.Among them,the change of pore size above 100 microns is the most obvious,and the pore fractal dimension decreases after5 cycles of freeze-thaw and increases after 20 cycles.SEM observation shows that the micro damage of limestone after cyclic load is mainly caused by a large number of layered cracks formed inside.