Study On Mechanical Properties Of Steel Fiber Reinforced Concrete Beams Based On Fiber Pullout Mechanism

The addition of steel fibers can effectively limit the development of cracks and significantly improve the mechanical properties of concrete,especially the residual tensile strength and tensile toughness after cracking.There is a disconnect between the current research on steel fiber reinforced concrete(SFRC)structures and materials,and the design method of SFRC structural members may cause waste of SFRC materials.Therefore,it is necessary to link the mechanical performance design method of concrete beams with the material properties of SFRC.The key connection between them is to establish a reasonable SFRC constitutive model,and the fiber pull-out mechanical performance is the core of establishing the SFRC constitutive model.In this study,the pullout behavior of single steel fiber in concrete matrix is firstly studied,and the mesoscopic constitutive model is established based on the pullout behavior of single steel fiber.The flexural strength and the shear strength of SFRC beams in ultimate limit state,the crack width and deformation of SFRC beams in serviceability limit state are carried out systematically,and the established calculation methods are verified widely.The main research contents and achievements can be summarized as follows:(1)The pullout process of the steel fiber in the concrete matrix is divided into three stages for force analysis,including complete bonding,debonding,and pull-out slippage.According to the force characteristics of the steel fiber at different stages,the bonding stress-end displacement relationship between the fiber and the concrete matrix is established.The calculation models for pullout properties of inclined straight and hooked-end steel fiber are developed by comprehensively considering the spalling of the matrix and snubbing effect caused by fiber pullout orientation angle,the plastic deformation of the steel fiber,and additional residual friction,respectively.(2)Based on the established calculation model for pullout behaviour of inclined steel fiber,the calculation method of the fiber bridging stress at the crack interface is proposed considering the fiber pullout orientation angle,the number of fibers and the fiber embedding length.Also,a bilinear model for tensile stress-crack width relationship of the matrix is proposed considering the influence of the steel fibers.Finally,a post-cracking meso-mechanical constitutive model of SFRC is developed,which can reasonably optimize the parameters of fiber shape and matrix strength.Considering the influence of fiber aspect ratio,fiber volume content,and steel fiber pullout behavior,a compressive constitutive model of SFRC is established.(3)Based on the developed constitutive models and some reasonable assumptions,the calculation method for flexural strength of SFRC beams is established,and the specific calculation program is compiled.The calculation methods for the equivalent stress coefficientα_b and the equivalent rectangular height coefficientβ_b of the equivalent rectangular stress pattern in the compression zone are derived,and the values of the above coefficients in the range of the commonly used concrete compressive strength and fiber factor are given.According to the established calculation method,the average stress reduction factor in the tension zone is determined,which is mainly related to the longitudinal reinforcement ratio and the fiber factor.Finally,a design formula for flexural strength of SFRC beams is also developed.The calculation method and design formula are extensively verified by the collected test data.The mean ratio of the predicted value to the experimental data of the calculation method and design formula is 0.98 and 1.03,and the coefficient of variation is 0.08 and 0.10,respectively(4)Three shear resistance mechanisms of SFRC beams are systematically investigated,including the residual stress of SFRC,the dowel action of longitudinal reinforcement,and the concrete in the shear compression zone.The Mesoscopic multi-mechanisms shear model(MMSM)for the shear design of SFRC beams without stirrups is established,which can not only accurately predict the shear strength of SFRC beams,but also the location of the critical shear crack.On the basis of the shear model of SFRC beams without stirrups,the MMSM for the shear strength of SFRC beams with stirrups is established considering the effect of the stirrups.It is found that the position of the critical shear crack is located in the range of 0.35a-0.5a from the support under different design parameters.The shear databases containing 473 SFRC beams are established.And the established calculation models and the existing shear models are evaluated and verified by these shear databases.The proposed shear models in this study can accurately predict the shear strength of SFRC beams.The effects of compressive strength,fiber coefficient,reinforcement ratio,shear-span ratio and stirrup coefficient on different shear resistance mechanisms and shear strength are investigated.(5)Based on the developed constitutive models and the concept of fracture mechanics,a calculation method for the bending moment-crack width of SFRC beams is established.The established method and five existing methods suggested by different codes are used to predict the crack widths of SFRC beams.Compared with the experimental results,it can be found that the established method can not only accurately predict the crack width of SFRC beams in serviceability limit state,but also can effectively estimate the crack width of SFRC beams during the whole bending process.Considering the residual tensile stress of the cracking interface,the method for calculating the crack width recommended by CECS 38-2004 is revised.The predicted results of the revised formula are in good agreement with the test data and the coefficient of variation is small.(6)Based on the stiffness analysis method and the effective moment of inertia method,the short-term stiffness and mid-span deflection of SFRC beams in serviceability limit state are comprehensively analyzed,and the corresponding calculation methods are also established.Comparing with a large number of test results,it can be found that the effective moment of inertia method has higher calculation accuracy and is safer than the stiffness analysis method,while the stiffness analysis method calculation results will overestimate the stiffness of the beam.Considering the effect of tension stiffening,the stiffness analysis method is modified.The accuracy of the modified method is extensively verified.The mean ratio of the predicted value to the experimental value of the modified method is 1.04,and the coefficient of variation is 0.18,indicating that it can accurately predict the mid-span deflection of SFRC beams.