Experimental Study On Flexural Behavior Of Recycled Concrete Beams

Recycled concrete is a new structural material made of recycled aggregates.Because of the inherent defects of recycled aggregates(high porosity and high water absorption),the study on the mechanical properties of recycled concrete is the basis of its application.In this paper,54 concrete specimens and 3 test beams with the same strength class but different replacement ratios(0%,50%and 100%)were prepared by using the recycled aggregate made of waste concrete beams and the reasonable mix proportion.The mechanical properties of recycled concrete and conventional concrete were comparatively tested.The mechanical properties of compression strength,tensile strength,flexural strength and elastic modulus of concrete specimens were tested.The experimental results show that the strength of recycled concrete is lower than that of conventional concrete,and the elastic modulus is reduced by 19%-25%.The failure mode of recycled concrete specimens are similar to that of conventional concrete,but the failure interface is not as flat as conventional concrete.The failure does not only occur at the interface between the recycled aggregate and the cement mortar,but also occurs inside the recycled aggregate.The static loading test of 3 concrete beams shows that the bending process of recycled concrete beams is basically the same as that of the conventional concrete beam,and the average strain measured on the cross-section obliged to the plane section assumption.The cracking moment of the recycled concrete beam is about 66.0%of the conventional concrete beam.The cracking development of recycled concrete beams is more turbulent and dense.The average bending moment is 11.7%lower than that of the conventional concrete beam.Numerical model for beams was established by using finite element software ANSYS,and the loading process of the beams were simulated.The results show that the trend of the load-deflection curves are basically the same as that of the test results.The cracking distribution are similar to the cracking area of the loading process.The influences of reinforcement ratio and section size on the bending resistance of recycled concrete beams were analyzed with reference to the fully replacement rate model.Based on the experimental results,the modifications of the compression zone height and cracking moment of recycled concrete are proposed.Comparing the calculated value and test value of the mid-span deflection of recycled concrete beams,the modification of the bending stiffness of recycled concrete beam is proposed.In order to obtain the recycled concrete beams required to reach the same performance as the conventional concrete beams,combined with the requirements of JTG D62-2012 and the various compliance checked with the bending of ultimate limit state,the deformation and cracking of serviceability limit state and durability,conversion criteria for concrete beams with recycled aggregates(concerning its structural performance)are established.The conversion formulas are tested in case studies by using the test beams.