Study On Mechanical Behaviors Of Axially-and Eccentrically-Loaded Partially Encased Composite Columns Cast With Demolished Concrete Blocks And Fresh Concrete

To simplify the recycling process of demolished concrete, the structural members casted with coarsely-crushed demolished concrete blocks(DCBs) and fresh concrete(FC) have been proposed and systematically studied by the authors and co-researchers in past several years. To extend the species of such structural members and provide better technical support for engineering applications, three parts of research work are involved in this dissertation:1. Tests of nine full-scale specimens under axial loading were conducted, and the influences of some parameters(i.e., replacement ratio of DCBs, thickness of flange of the builtup H-shaped steel section, and spacing of transverse links) on the specimens’ axial behaviors have been discussed. According to the existing formulas for partially encased composite column, the axial strengths of the specimens were calculated and compared with the test results. It is found that:(1) the influences of the replacement ratio of DCBs within a range of 0~33% on the specimen’s axial strength, initial stiffness and ductile ratio are all limited;(2) the specimen’s ductility and axial strength increase with the decreasing of the spacing of transverse links;(3) the formulas proposed by ACI318-14 and Tremblay et al. are still available, and the specimens’ axial strength can be better predicted by using latter formulation consideration of the confinement effect of transverse links and flange to concrete.2. Test of seven full-scale specimens under eccentrically compressive loading and two fullscale specimens under concentrically compressive loading were conducted, and influences of the load eccentricity, thickness of flange of the built-up H-shaped steel section, spacing of transverse links and replacement ratio of DCBs were investigated. According to foreign design codes, the compressive strengths of the specimens were calculated and compared with the test results. Test and calculation results show that:(1) in case that the strength of DCBs is significantly lower than FC, the compressive strengths of specimens with DCBs were less than those without DCBs, but the former specimens reveal similar initial stiffness and more gently descending branch;(2) with an increasing of the load eccentricity, the initial stiffness and the compressive strength decreased significantly;(3) the reduction of the spacing of transverse links can improve the compressive strength and ductile ratio, while the influence to initial stiffness can be ignored;(4) the formulas proposed by ACI318-14、AISC and EC4 are suitable to predict the compressive strengths of the eccentrically loaded specimens, and predicted value calculated using AISC provided the best accuracy.3. Base on ABAQUS, numerical simulation on the axial behaviors was carried out, and compared to test results in chapter 2. Under the condition that the consumption of steel remains constant, the influences of the different distribution of steel on the axial behaviors and the difference of axial behaviors between(PEC) partially encased composite columns and CFST(concrete filled square tubular) column were investigated. It is found that:(1)under the condition of constant steel consumption, the influences of the different distribution of steel on the PEC column’s axial strength and initial stiffness is limited;(2) the stiffness of descending branch is distinctly influenced by the spacing of transverse links, and the optimal values of the spacing is 150 mm;(3) compared to CFST columns with the same consumption of steel, the initial stiffness of PEC columns is greater on the whole, the stiffness of descending branch and axial strength can also be comparative with the appropriate distribution of steel.