Experimental Study On Bond Behavior Between Green High Performance Fiber Reinforced Cementitious Composites And Steer Bars After High Temperature

Fire hazard is one of the hazards that have the highest probability of occurrence and the largest time and spatial spans worldwide.As one of the most commonly used structures in China,the performance of reinforced concrete structures degrades in fire or high temperature,due to the degradation of mechanical properties(i.e.compressive strength,tensile strength,and stiffness)of concrete and steel,reduction in bond strength between concrete and steel,and changes in internal stress and deformation associated with the thermal gradient.Therefore,the studies on the mechanical properties and bond strength of reinforced concrete after exposure to high temperature are of great importance.Green high performance fiber reinforced cementitious composites(GHPFRCCs)are developed based on Engineering Cementitious Composites(ECCs).The tensile strain limit is up to 5%,and crack width is around 100?m.In order to extend the engineering practice of GHPFRCC,the bond between GHPFRCC and steel needs to be investigated.In this study,pull-out test is carried out after exposure to high temperature,and the effects of anchorage length,bar diameter,bar type,protective layer thickness,cooling method on the bond performance and the failure mechanism of bond.The study includes:(1)Bond behaviors of HRB400 rebar from 16 GHPFRCC mixtures are characterized by pull-out tests.The failure mode is shear failure,and the slip corresponding to the maximum load is up to 2.5 mm.The effects of five mix proportioning variables(fly ash content,PVA fiber content,sand-binder ratio,superplasticizer content,and water-to-binder ratio)of GHPFRCC on the bond performance are evaluated using Minitab software.The influence degrees from high to low in turn correspond to fly ash content,PVA fiber content,sand-binder ratio,superplasticizer content,and water-to-binder ratio.(2)The influence factors on the bond performance of GHPFRCC at room temperature are investigated by pull-out test.With increases in anchorage length,the ultimate load increases but the bond strength decreases.Deformed rebar has higher bond strength than plain rebar.Bond strength increases with the protective layer thickness with a limit.As the rebar diameter increases,the ultimate load increases but the bond strength decreases.Splitting fracture failure can occur due to the insufficient protective layer thickness,rebar diameter,or anchorage length.(3)The effects of high temperatures(200 ?,400 ?,600 ?,800 ?)on the bond performance are evaluated.The bond strength decreases with temperature.At 200 ?,the bond strength is sustained;after the temperature goes above 400 ?,the influence of mix proportions on the bond strength becomes insignificant,the bond strength approximately linearly decreases with temperature,and spring cold water furthers the reduction in bond strength.At 200?,the bond strength between GHPFRCC and plain rebar can be reduced by 50%,while deformed rebar demonstrates excellent bond performance.After the temperature reaches 400 ?,splitting fracture failure occurs except when plain rebar is used.(4)The load-slip curves are characterized and the mechanisms are analyzed at normal and after high temperatures.Before exposure to elevated temperatures,the load-slip curve can be divided into three stages:stable rise phase,decline phase,and residual phase.At 200 ?,the load-slip curve is similar as that at room temperature.From 400 ?,it becomes hard to capture the decline phase in the load-slip curve.Only when the anchorage length is extremely short,after the ultimate load is reached,slip can continue without splitting the specimen suddenly,while the other specimens are fractured abruptly.