Experimental And Theoretical Study On Mechanical Performance Of Ultra High Performance Concrete(UHPC) Corbels

Reinforced concrete corbels,which are important structural components supporting crane beams,joists,wall beams,etc.,be widely used in fabricated concrete structures.However,due to the small shear span ratio of corbels,brittle shear failures are often occurred.Ultra high performance concrete(UHPC)has excellent mechanical properties and has the advantages of high strength,high toughness,and high durability.The study found that the shear capacity,durability and ductility of corbels made of UHPC will be significantly improved,and the failure mode of corbels could changed from brittle shear failure to ductile bending failure.Nevertheless,there are still few studies on UHPC corbels,and the design method of UHPC corbels are not specified in the relevant design codes.For this reason,the mechanical properties of UHPC corbels were studied by combination of experiments and theoretical analysis,which mainly include the following three aspects:(1)An experimental study on the mechanical properties of the UHPC corbels was carried out.Twelve UHPC corbels were designed and casted,and the test results of crack development,failure mode,load-displacement curve,maximum crack width,longitudinal reinforcement strain and ductility of UHPC corbels were obtained through vertical loading.The effects of main parameters including shear-span ratio,steel fiber volume ratio and stirrup reinforcement ratio on the mechanical properties of corbels were explored.The test results indicated that by the addition of 1% volume of steel fibers,the failure mode of the corbels were changed from being shear failure to bending failure,and the cracking load of corbels increased significantly.By a reduction of the shear-span ratio,the ultimate strength of corbels was greatly increased and the crack width of the corbels was decreased,while the ductility of corbels was reduced.By the addition of secondary horizontal reinforcement,the ultimate strength of corbels was improved while the crack width and ductility of corbels were slightly effected.(2)A theoretical research on the ultimate strength of the UHPC corbels was carried out.According to the test results,the applicability of the existing corbel ultimate strength calculation models were evaluated.The calculation models such as domestic and foreign design codes and calculation models in existing literature were involved in the evaluation.The study found that the domestic and foreign codes and literature calculation models generally underestimated the bearing capacity of UHPC corbels,and could not accurately reflect the influence of the volume ratio of steel fibers on the ultimate strength of the corbels.Finally,the contribution of steel fibers to the ultimate strength of the corbel is considered reasonably,and an improved strut-and-tie model is proposed to calculate the ultimate strength of the UHPC corbels.The calculation results show that the average error coefficient and variation coefficient of the improved strut-and-tie model model are 1.08 and 0.04,respectively.The accuracy of the calculation results could accurately reflect the influence of the research variables on the bearing capacity of the corbel,and could provide a reference for engineering design.(3)An evaluation of the ultimate strength calculation method based on the SFRC corbel database was carried out.162 groups of steel fiber reinforced concrete corbels shear test data were collected and screened,and the SFRC corbel database was established to evaluate the applicability of domestic and foreign codes and literature calculation models and the improved strut-and-tie model proposed in this paper.The calculation results show that the existing calculation models could not meet the requirements of accuracy,while the average error coefficient and variation coefficient of the improved strut-and-tie model are 1.13 and 0.17,respectively,and the calculation results are in good agreement with the experimental values,the discreteness is lower than that of other calculation models,which can accurately reflect the influence law of main design parameters on the ultimate strength of the corbels,and good applicability of the improved strut-and-tie model is verified again.