Mechanical Properties Of Compound Concrete Containing Demolished Concrete Lumps At Room Temperature And After Freeze-thaw Cycles

It is indicated that the compound concrete made of fresh concrete(FC)and coarsely-crushed demolished concrete lumps(DCLs)is an effective approach in recycling demolished concrete(DC).Some preliminary studies about the compressive and creep behaviors of the compound concrete containing normal-strength or high-strength FC have been conducted by our research group.The mechanical behaviors(compressive,flexural and shear behaviors),seismic properties,and creep behaviors of the structural members cast using the compound concrete have been also experimentally investigated.This kind of structural members(columns,beams,and slags)have been successfully employed in several multi-storey buildings in recent years.To further improve the theoretical system of compound concrete made of DCLs and FC,the tensile-splitting and shear behaviors of the compound concrete at room temperature,and the compressive and tensile-splitting behaviors of the compound concrete after freeze-thaw cycles were experimentally researched in this paper.The main achievements include that:1.The tensile-splitting tests for one hundred and twelve conventional concrete and compound concrete cubes,and the double-direct shear tests for one hundred and sixteen conventional concrete amd compound concrete prims were carried out.The influences of the replacement ratio of DCLs,the characteristic size of DCLs,the size of specimens and the strength of FC on the tensile-splitting and shear strength of specimens have been investigated.It is revealed that the liner strength assumption is applicable to predict both tensile-splitting strength and shear strength.It is found that:(1)the adverse effect of DCLs on the tensile-splitting strength of the compound concrete does not increase significantly with the increasing of the difference between the compressive strength of FC and that of DC,and the difference between the shear strength of FC and that of the compound concrete is obviously smaller than that between the compressive strength of FC and that of DC;(2)for compound concrete containing normal-strength FC,the ratio of the tensile-splitting strength to the compressive strength is generally close to that for FC,and the ratio of the shear strength to the compressive strength is about 0.91 times of that for FC,but for compound concrete containing high-strength FC,the ratio of the tensile-splitting strength to the compressive strength is about 1.1 times of that for FC,and the ratio of the shear strength to the compressive strength is about 1.08 times of that for FC;(3)the influence of the characteristic size of DCLs on the tensile-splitting and shear strength of the compound concrete can be ignored approximately;(4)and the size effect on the tensile-splitting or shear strength of compound concrete is similar to that on corresponding strength of conventional concrete.2.The freeze-thaw tests for forty-eight 300 mm cubes of compound concrete and seventy-two 100 mm cubes of FC and DC were performed,and the compressive and splitting-loading failure strength of the frozen and thawed specimens were measured.The microstructure and pore structure of FC and DC were also studied by using both scanning electron microscopy(SEM)and mercury intrusion porosimetry(MIP).Any surface scaling,mass loss of the frozen and thawed specimens were monitored.In addition,the influences of the number of freeze-thaw cycles and the replacement ratio of DCLs on the compressive and tensile-splitting strength of frozen and thawed cubes were studied.The liner strength assumption was also confirmed to be applicable to predict the compressive strength of the frozen and thawed cubes,but it cannot be used to predict the tensile-splitting strength,so a modified formula was established.It is found that:(1)the compressive and tensile-splitting strength of the compound concrete decrease gradually not only with an increasing number of freeze-thaw cycles,but also with increasing the replacement ratio of DCLs,and the negative influences of including DCLs and freeze-thaw action on tensile-splitting strength of the frozen and thawed specimens are more remarkable than those on compressive strength;(2)with an increasing number of freeze-thaw cycles,the tensile-splitting strength of compound concrete decreases more rapidly than that of fresh concrete,but this trend is not very clear for the compressive strength;(3)the ratio of tensile-splitting strength to compressive strength for the frozen and thawed compound concrete was only about 78% of the ratio for FC;(4)and the negative effect of freeze-thaw action on the compressive and tensile-splitting strength of the FC generally bears only limited relation to the size of the cubes tested within a range of 100 to 300 mm.3.The freeze-thaw tests for fifty-four fresh concrete cubes,demolished concrete cubes or bonding cubic specimens made of FC and DC,and fifty-four fresh concrete prisms,demolished concrete prisms or bonding prismatic specimens made of FC and DC were carried out.The tensile-splitting and shear failure strength of the frozen and thawed specimens were measured.The influence of the number of freeze-thaw cycles on the mechanical behaviors of the interface between FC and DC without any binder has been preliminarily investigated.It is found that:(1)both the tensile-splitting and shear strength of interface between FC and DC decrease with an increasing number of freeze-thaw cycles;(2)the variations of tensile-splitting and shear strength with number of freeze-thaw cycles for the interface between FC and DC are generally close to those for demolished concrete;(3)and,the ratio of tensile-splitting strength to compressive strength and the ratio of shear strength to compressive strength for the interface between FC and DC range from 0.81 to 0.94 and 0.23 to 0.36,respectively.