Influences Of Size And Shape Of Demolished Concrete Lumps On Mechanical Behavior Of Recycled Lump Concrete Made With High-strength Fresh Concrete

Recycled lump concrete（RLC）is the mixture of fresh concrete（FC）and demolished concrete lumps（DCLs）with size of 60 mm～300 mm.Compared to recycled aggregate concrete（RAC）,RLC has the advantage of significantly reducing the amounts of cement and other cementitious materials,and thus CO₂emissions.For more than a decade,most of the experimental studies and practical applications of RLC have focused on the roughly spherical DCLs.And the fluctuation of DCL size is limited in the same batch of specimens or in the same component.In practice,however,crushing waste concrete produces lumps with wildly various sizes and shapes.Assuming that the DCLs with different shapes and sizes can be mixed directly as seems convenient,the production of DCLs will be greatly simplified,promoting the application of RLC.Combining the idea of using high-strength FC with the concept of RLC,this paper conducted experimental studies on the effects of DCL shape and size on the compressive,flexural and direct-shear properties of RLC made with high-strength FC.Then,the mechanical properties of RLC containing mixed DCLs with different shapes or/and sizes were investigated.Finally,the compressive behavior of RLC was further studied by means of discrete element method.The main work and conclusions are as follows:1.The three-dimensional size parameters and shape indexes of DCLs were defined,and3 representative DCL shapes were determined.Then,39φ300 mm×600 mm RLC cylinders were tested to investigate the effects of DCL shape and characteristic size on the compressive behavior of RLC made with high-strength FC.Scanning electron microscope tests were conducted to reveal the effect of DCL shape on the microstructure of RLC.The results show that:（1）The influence of DCL characteristic size on the compressive strength and elastic modulus of RLC made with high-strength FC can be approximately neglected;（2）For a given lump size,employing oblate lumps produces a compressive strength about 10%lower than if spherical or slender DCLs are used,but the strength of RLC with slender DCLs is similar to that with spherical DCLs;（3）There is no significant coupling between the characteristic size and shape of DCLs in determining the compressive strength of RLC;（4）There are micro-cracks at the interfacial transition zones（ITZs）between oblate DCLs and FC,but no similar defects were found at the ITZs between spherical or slender DCLs and FC.2.The deformation during compression for 12 RLC specimens（295 mm×295 mm×60mm）was measured by using DIC technique,to investigate the effects of the shape,size,placement orientation and number of DCLs on the crack development of RLC made with high-strength FC.The results show that:（1）Although lateral strain concentrates at part of ITZs between a single spherical DCL and FC during the early and mid-loading stages,this phenomenon is no longer obvious as load increases;（2）When a single oblate or slender DCL is placed vertically,crack develops along the vertical ITZs between the DCL and FC,but this is not obvious when the DCL is placed horizontally or obliquely.In practical engineering,most oblate and slender DCLs are placed horizontally or obliquely,so the phenomenon mentioned-above can be effectively avoided;（3）Regardless of the shape of DCLs,cracks tend to go through the relatively thin FC layer between adjacent upper and lower DCLs;（4）The delamination of compressive strain is diminished in practical RLC members made of high-strength FC and plenty of DCLs.3.Flexural tests were conducted on 39 RLC prisms（250 mm×250 mm×900 mm）and compressive tests were conducted on 9Φ250 mm×500 mm RLC cylinders to study the effects of DCL shape and characteristic size on the flexural behavior of RLC made with high-strength FC.In addition,nanoindentation was performed on the ITZs between DCLs and FC.The results show that:（1）When the characteristic size of DCLs increases from 50 mm to133 mm,the flexural strength of RLC tends to decrease with a maximum decrease of about15%,and the decreases are least for oblate lumps,greatest for slender lumps,and intermediate for spherical lumps respectively;（2）When lump size is invariant,oblate DCLs give the highest flexural strength and slender DCLs give the lowest strength and the differences between the intermediate value and the others increase with the increase in lump size,with a maximum difference of about 10%;（3）The calculated values from the prediction formulas of RLC flexural strength show reasonable agreement with measured results.4.Compressive tests onφ300 mm×600 mm cylinders,flexural tests on 250 mm×250mm×900 mm prisms,and direct-shear tests on 300 mm×300 mm×600 mm prisms were conducted to investigate the effects of DCL shape and the mixing method of DCLs on the mechanical properties of RLC made with high-strength FC.The results show that:（1）Both mixing spherical DCLs with different sizes and mixing 100 mm DCLs with different shapes have limited effects on the compressive strength and modulus of elasticity of RLC,but lead to a reduction in RLC flexural strength of 6%to 12%;（2）Mixing spherical DCLs with different sizes has a negligible effect on the direct-shear strength of RLC;When the characteristic size of DCLs is 100 mm,the direct-shear strengths of the specimens containing only oblate DCLs or slender DCLs and the specimens containing mixed DCLs with different shapes are about10%higher than that of specimens containing only spherical DCLs;（3）When the DCLs with different shapes and sizes are mixed at same time,the previously established formulas for predicting the compressive,flexural,direct-shear strengths and modulus of elasticity of RLC can be applied generally and the basic mechanical properties of RLC in this case are approximate to that of RLC made with 100 mm spherical DCLs.5.Three-dimensional simulations of RLC cylinders were performed using discrete element method to investigate the effect of DCL shape on the compressive behavior of RLC under different situations and to reveal the mechanical behaviors of new and old concrete during compression.The results show that:（1）When the strength difference between new and old concrete ranges from-15 MPa to 30 MPa,the compressive strength of RLC specimens with slender DCLs is similar to that with spherical DCLs,which is slightly higher than that with oblate DCLs.This trend weakens as the DCL replacement ratio decreases;（2）The effect of DCL shape on the modulus of elasticity of RLC can be approximately neglected;（3）For the new or old concrete in RLC specimens,the ratio of its peak stress during compression to its compressive strength is little influenced by the strength of old concrete,but is significantly affected by the strength difference between new and old concrete;（4）Before the peak load,the proportion of the load borne by old concrete remains basically unchanged or only slightly decreases,but eventually increases to some extent;（5）When there is a large difference in strength between new and old concrete,the existing formula for predicting the compressive strength of RLC cylinders is not consistent with the actual mechanical behaviors of new and old concrete at peak load.Due to acceptable accuracy and high convenience,this formula can still be used for preliminary estimation.