Bond Mechanism Of Steel Bar Embedded In High Ductile Concrete And The Calculation Model Of Crack Width Of Reinforced HDC Beam

High Ductile Concrete(HDC)is a new kind of building material with high strength,high toughness and high damage tolerance ability,which is characterized by an excellent strain hardening behavior with multiple fine cracks when subjected to tensile loading.Due to the outstanding mechanical properties,HDC has been widely utilized in strengthening and repairing the new and existing structures at home and abroad.Bond behavior is the foundation of combined action between HDC and reinforcing bar in various operating environment and stress conditions.Research on earthquake disasters showed that the bond degradation under cyclic loading was a main factor resulting in the loss of strength and stiffness of joint zone.Due to the fiber bridging effect,HDC material after cracking can still bear a greater tensile stress,effectively controlling the opening of cracks.Therefore,the bond behavior and degradation mechanism between steel bar and HDC under monotonic and cyclic loading as well as the calculation method of crack width of reinforced HDC beam were investigated in this paper.The research contents and main conclusions were drawn as follows.(1)The bond behavior of plain round bar and HDC were investigated.The influence of HDC compressive strength,HDC flexural toughness,cover thickness and embedded length on the ultimate bond strength and residual bond strength were analyzed.Results indicated that the HDC compressive strength was the main factor affecting bond performance.Finally,the calculation formula of the ultimate bond strength was built,and then a bond slip constitutive relation model for plain round rebar in HDC was proposed.(2)The bond performance of the deformed bar and HDC were investigated.The differences between HDC specimens and concrete specimens were discussed in terms of failure mode,failure mechanism and bond slip curve.Experimental results suggested that the failure of concrete specimens was caused by the splitting of concrete cover.Due to the fiber bridging effect preventing the opening and propagation of cracks,the pullout or splitting-pullout failure occurred for HDC specimens,which exhibited an obvious ductility.Through analyzing the effect of various parameters,a calculation formula of ultimate bond strength was built,and a bond slip constitutive model was proposed for the deformed bar in HDC.(3)Based on the thick cylinder theory of elastic mechanics and fictitious crack model of fracture mechanics,the stress in HDC cover was analyzed.According to the tensile strain hardening behavior and multiple cracking development criteria of HDC materials,the cohesive force-crack width relationship of HDC after cracking were built.Then a calculation method of bond strength for the deformed rebar was established which considered the fiber bridging effect.(4)The degradation law of bond performance of steel bar in HDC under cyclic loading was evaluated.Effects of HDC compressive strength,flexural toughness,cover thickness and embedded length were analyzed in terms of ultimate bond strength,residual bond strength,ultimate bond strength degradation,residual bond strength degradation and energy dissipation capacity under cyclic loading.Results indicated that under the cyclic loading,the degradation degree of chemical adhesion was more serious than that of friction for plain round bar,and the degradation degree of mechanical interlock force was smaller than that of friction for deformed bar.(5)The degradation mechanism of bond behavior between the ribbed rebar and HDC under the cyclic loading was analyzed.Damage factors were introduced to reflect the influence of HDC damage development on the degradation of positive peak bond stress.Considering the influence of slip on the degradation laws of unloading stiffness,horizontal friction resistance and reverse peak bond stress,the calculation model of bond slip constitutive relationship between the ribbed rebar and HDC under cyclic loading was proposed.And the model curves were in good agreement with the test curves.(6)Reliability analysis of anchorage of steel bars was carried out to obtain the basic anchorage lengths of the plain round bar and the ribbed steel bar in HDC members.Compared with the anchorage length calculated in accordance with the code for design of concrete structures(GB50010-2010),the design suggestion of the anchorage length of steel bars in HDC members was put forward.According to the degradation law of bond properties under cyclic loading,the design suggestion of the anchorage length considering the influence of earthquake action was put forward.(7)To investigate the crack development process,four reinforced HDC beam specimens and one ordinary reinforced concrete beam specimen as comparison were designed.The effects of HDC ultimate tensile strain and reinforcement ratio on average crack spacing and maximum crack width were discussed.Results indicated that the average crack spacing and maximum crack width of reinforced HDC beam were less than that of ordinary reinforced concrete beam due to the fiber bridging effect.Considering the contribution of fiber bridging effect to tensile stress,the calculation methods of average crack spacing and maximum crack width of reinforced HDC beam were proposed.