Study On Fatigue Behaviors Of Recycled Lump Concrete

It is effective to recycle and reuse demolished concrete with the form of recycled lump concrete(RLC),which has many advantages including relatively small crushing energy consumption,significantly reduced cement consumption,and remarkably alleviated cracking problems caused by cement hydration.Up to now,the RLC has been applied in a variety of members(e.g.,reinforced concrete(RC)beams,RC slabs,RC columns,CFSTs,U-shaped steel beams filled with RLC,composite slab with profiled steel sheet cast using demolished concrete,and pile platform)in over ten practical projects.The knowledge of the fatigue behavior of RLC,however,is still lacking.In order to promote the application of RLC in the structures suffering from fatigue loads(such as concrete pavement,concrete bridge,and industrial building),this study has carried out preliminary tests and analysis on the fatigue behavior of RLC.The main achievements include that:1.The constant amplitude compressive fatigue tests for sixty cylindrical RLC specimens have been carried out.The effects of the replacement ratio of demolished concrete lumps(DCLs)and the maximum stress level on the compressive fatigue behavior of RLC have been investigated.It is found that:(1)at a significance level of 0.05,the influence of DCL replacement ratio on the fatigue life of RLC is not significant;(2)as DCL replacement ratio increases,the lateral expansion of RLC increases under a given stress level,but the dissipated energy in one cycle is almost unchanged;(3)DCL replacement ratio can hardly influence the secondary strain rate of RLC,and the relationships between the secondary strain rate and the fatigue life of RLC are well described by two linear formulas based on S_max and S_min.2.The initiation and propagation of surface cracks for ten planar specimens under compressive static load and fifteen planar specimens and three cylindrical specimens under compressive fatigue load has been observed using digital image correlation(DIC)technique.The effects of DCL replacement ratio and strength difference between new and old concrete on the crack growth of RLC subjected to compressive fatigue load have been investigated.It is found that:(1)the number of cracks initiating on the RLC under fatigue loading is relatively more than that on the conventional concrete(CC),but the cracks on the former are thinner and shorter;(2)when the strength difference of new-old concrete is less than±10.0MPa,the interface between DCLs and fresh concrete(FC)is not evident weak zone in RLC under compressive fatigue loading,which clarifies the mechanical reason for no significant decrease observed for RLC's compressive fatigue life;(3)when the DCL replacement ratio is not more than 33%,as the strength difference of new-old concrete reduces from 44.4 MPa to8.9 MPa,then-9.2 MPa,finally-21.1 MPa,the main damage zones of RLC specimens under static loading or fatigue loading gradually evolved from only demolished concrete(DC)to both DC and FC,and finally focused on the interface between DCLs and FC.3.The constant amplitude flexural fatigue tests for seventy-seven beam RLC specimens have been performed.The effects of DCL replacement ratio and maximum stress level on the flexural fatigue behavior of RLC have been investigated.It is found that:(1)the mean of the logarithm of RLC beams'fatigue lives declines gradually with increases in the DCL replacement ratio,though the rate of decrease slows;(2)when the DCL replacement ratio is less than 33%,the flexural fatigue behavior of RLC is almost identical to that of CC under the stress level in practical engineering;(3)a formula relating the beam's fatigue life with the secondary strain rate on the beam's top surface has been established with good results.4.The initiation and propagation of surface cracks on the beam RLC specimens under flexural static or fatigue load has been observed using DIC technique.The influence of DCL replacement ratio on the crack growth of RLC subjected to flexural fatigue load has been investigated.It is found that:(1)there is no clear relationship between the mean cracking stress level of RLC specimens under flexural static load and the DCL replacement ratio,and the mean maximum crack width related to the flexural strength of RLC and CC are similar,as well as the mean crack length;(2)the expression relating flexural fatigue life of RLC to crack length has been established,as well as the expression between crack width and RLC's fatigue life,and the two expressions have no clear relationship with DCL replacement ratio;(3)the relationships relating maximum crack width to corresponding crack length under both static loadings and fatigue loadings have been obtained,and those relationships are suitable for both RLC and CC;(4)the mean ratio of the critical value of cycle number to the RLC's fatigue life is about 0.49,which is basically similar to that mean ratio for CC,and from the average angle,the critical value of crack length for the specimen under fatigue loading is approximate to the crack length at peak load for the specimen with the same type of concrete under static loading.5.The bending fatigue tests for five reinforced CC beams and eight reinforced RLC beams have been carried out.The effects of DCL replacement ratio on the cracking moment,fatigue life,crack width,and mid-span deflection of reinforced RLC beams have been investigated.It is found that:(1)when the strength of FC is similar to that of DC,the cracking moment of RLC beams does not have a monotonic trend with DCL replacement ratio,and the cracking moment of RLC beam is basically similar to that of CC;(2)at the same load level,the fatigue life of RLC beam is not lower than that of CC beam on the whole;(3)under the same number of cycles,the effects of DCL replacement ratio on the maximum residual crack width and the mean residual crack width are not significant;(4)when the FC's strength is similar to the DC's,the influence of DCL replacement ratio on the mid-span deflection of RLC beam under both static loading and fatigue loading is not clear,as well as the increasing rate of the mid-span deflection.