Experimental Study On The Mechanical Properties Of Multi-recycled Coarse Aggregate Concrete Under Triaxial Compression And Combined Compression And Shear

In this paper,natural coarse aggregate concrete(NAC)was first prepared using natural coarse aggregate(NA),and then by cmshing natural coarse aggregate concrete to recover coarse aggregate,preparing concrete,cmshing again to recover coarse aggregate,and preparing concrete again,the 1st generation recycled coarse aggregate(RA1),the 2nd generation recycled coarse aggregate(RA2),and the 3rd generation recycled coarse aggregate(RA3)were obtained in turn.Then,the 1st generation recycled coarse aggregate concrete(RACⅠ),the 2nd generation recycled coarse aggregate concrete(RACⅡ),and the 3rd generation recycled coarse aggregate concrete(RACⅢ)were prepared from the three types of recycled coarse aggregates obtained.Relevant studies were systematically carried out on coarse aggregates with different number of regeneration and concrete under triaxial compression and combined compression and shear loading with different number of regeneration,and the main results were as follows:(1)The apparent density,bulk density,water absorption rate and crushing index of natural coarse aggregate and three kinds of recycled coarse aggregate were tested,and the test showed that:with the increase of the number of coarse aggregate regeneration,the apparent density and bulk density of coarse aggregate showed a decreasing trend,and the water absorption rate and crushing index increased significantly.Among them,the crushing index and apparent density of RA3 increased by 55.4%and-4.17%,respectively,compared with RA1,and the void ratio of RA3 increased by 16.6%compared with RA1,while the increase of water absorption was very obvious,and the water absorption of RA3 was about 9 times that of NA.(2)The mechanical properties of natural coarse aggregate concrete(NAC)and Multi-recycled coarse aggregate concrete(Multi-RAC)under compression-shear combined stress were studied.Under different regeneration times N of coarse aggregates and compressive stress ratio K,the peak shear strength τp,peak shear displacement sp and failure mode were analyzed.The results show that the peak shear strength τp and peak shear displacement sp increase substantially with the increase of compressive stress ratio,for example,the peak shear strength at K=0 is around 5MPa and the peak shear displacement is around 1mm;while the peak shear strength at K=0.8 is between 25MPa and 30MPa and the peak shear displacement is around 3.5mm.When the number of coarse aggregate regeneration increased,the peak shear strength τp decreased slightly,and the peak shear strength of RACⅢdecreased within 5 MPa compared with that of NAC;while the peak shear displacement sp changed slightly,within 0.5 mm.Under the action of normal pressure,the peak shear strength τp is mainly composed of adhesive strength τc,contact friction strength τf and shear expansion strength τd,among which contact friction strength τf is more than half of peak shear strength τp.Based on different stress expressions,three strength failure criterions considering the number of coarse aggregate regeneration under planar stress were established for four types of concrete.Among them,the stress invariant failure criterion and the octahedral stress failure criterion in the form of quadratic parabolic function have high prediction accuracy.(3)The mechanical properties and energy evolution of natural coarse aggregate concrete(NAC)and Multi-recycled coarse aggregate concrete(Multi-RAC)under triaxial compression were studied.The effects of the number of coarse aggregate regeneration(N)and confining pressure(σ3)on the failure behavior,peak compressive strength,peak deformation,crack initiation threshold(σci),crack damage threshold(σcd),energy evolution law and damage evolution curve of concrete were analyzed.The results show that under uniaxial and triaxial compression,the number of coarse aggregate regeneration has little effect on the concrete properties,such as the change in peak compressive strength is about 15 MPa,peak deformation is within 10‰,and the change in crack initiation threshold(σci)and crack damage threshold(σcd)are within 20 MPa.While the confining pressure significantly affects the failure mode and mechanical properties of concrete.With the increase of confining pressure,the failure mode of concrete transitions from vertical splitting failure to oblique shear failure,the peak compressive strength and peak deformation increase,the crack initiation threshold(σci)and crack damage threshold(σcd)also increase,the peak elastic strain energy increases significantly,and the increase of confining pressure can effectively inhibit the development of damage evolution curve.In addition,three strength failure criterion models were selected to analyze the test data and achieved good results.