Experimental Study On The Properties Of Concrete With Different Number Of Recycling Circles Under Coupling Effect Of Loading And Freeze-thaw Cycles

Researchers not only need to study and solve the practical problems of making recycled concrete from recycled aggregate of construction waste and realizing the resource utilization of construction waste.It is also necessary to predict and study new problems that may be brought by the widespread use of recycled coarse aggregate in the near future.That is to solve the problem of how to deal with the recycled concrete structure after its service life is removed,so as to realize the recycling of construction waste.In this paper,the C40 strength grade natural aggregate concrete(NAC)is mixed with natural coarse aggregate(NA),and then the primary recycled aggregate(RA1)is mixed with primary recycled aggregate concrete(RAC1).The secondary recycled aggregate concrete(RAC2)is mixed with secondary recycled coarse aggregate(RA2)from primary recycled concrete Until three times of regeneration(RA3,RAC3).The coarse aggregate with different regeneration times and concrete with different regeneration times are systematically studied.The main contents are as follows:(1)The apparent density,bulk density,water absorption and crushing indexes of natural aggregate and aggregate with different regeneration times were determined.The surface morphology of coarse aggregate was analyzed by SEM.The results showed that with the increase of regeneration times,the apparent density and bulk density of coarse aggregate decreased linearly,and the water absorption increased successively.But there was little difference between RA2 and RA3,which were both about 10%.The crushing value of reclaimed coarse aggregate was lower than that of natural coarse aggregate,and the crushing value of RA2 was the lowest among reclaimed coarse aggregate.Scanning by SEM.It is found that the surface of reclaimed aggregate is coarser than that of natural aggregate.With the increase of regeneration times,cracks of mortar in reclaimed aggregate increase significantly.(2)The compressive strength tests of ordinary concrete and concrete with different regeneration times were carried out.The results show that the strength of recycled concrete is NAC>RAC2>RAC3>RAC1,and the strength difference among different recycled concrete is small.The size effect of RAC2 and RAC3 is weaker than that of NAC.The strength development of recycled concrete at early age is slower than that of ordinary concrete,and is positively correlated with the regeneration times.(3)The stress-strain curves of concrete with different stress levels and different regeneration times under load freezing-thawing coupling were studied.It is found that the peak stress and elastic modulus of concrete decrease with the increase of stress level,But the compressive stress level of 0.4f_ccan improve the frost resistance of concrete to some extent.The peak strain and limit strain of NAC,RAC1,RAC2and RAC3 are not affected by freeze-thaw loading at different stress levels,and the correlation with stress level is not obvious.The regeneration times of concrete have obvious correlation with elastic modulus,peak strain and ultimate strain of concrete.The increase of regeneration times will reduce elastic modulus of concrete,and increase peak strain and ultimate strain of concrete.(4)By fitting the stress-strain curve,the parameters A and K in the ascending section of the curve are obtained to characterize the plasticity and ductility of concrete.With the increase of regeneration times,the plasticity first increases and then decreases.RAC2 has the best plasticity.At the same time ductility is better than ordinary concrete,RAC2 has the best ductility under load-freezing-thawing coupling.