Experimental Study On The Influence Of The Shape Of High Strength Steel Bar On The Bond Between Steel Bar And Concrete

With the deepening of the concept of green building,in the reinforced concrete structure,the high-strength reinforcement with its advantages of high strength,good ductility,energy conservation and environmental protection has been more and more promoted.However,due to the lack of theoretical and applied researches on the reinforcement with a strength above630 MPa in China,the promotion and usage of this kind of reinforcement are slowed down.The performance of high-strength reinforcement depends largely on the effectiveness of bond anchorage between reinforcement and concrete.Therefore,the research on bond anchorage between high-strength reinforcement and concrete is particularly important in the application and popularization of high-strength reinforcement.In China,crescent reinforcement is the most widely used deformed reinforcement in the field of construction engineering,but there is a few research on the shape change of crescent rib.In this paper,the bond-anchorage performance of 630 MPa reinforced concrete with widened transverse rib is studied systematically from the aspects of failure mode,influencing factors that affecting bond-anchorage performance,bond-slip constitutive relations,anchorage length calculation and reliability analysis.The specific contents are as follows:Through the design of the beam bond test of the 42 high strength reinforced concrete specimens of 630 MPa reinforcement with widened transverse rib and standard crescent rib respectively,the failure mode,the various influencing factors,the bond strength and the bond-slip curve of the two kinds of specimens were compared and analyzed.The results show that the failure modes of the two types of specimens can be divided into pull-out failure,splitting failure and compound failure,and the failure modes of the specimens with the same parameters are identical.The bond strength ratio of the two kinds of specimens with the same parameters fluctuates around 1.The influencing factors such as concrete strength,anchorage length and steel bar diameter on the bond strength of the two kinds of steel bars are basically the same.The bond-slip curves of the two kinds of bar can be divided into five parts:microslip section,slip section,splitting section,descending section and residual section.Except for some differences in the peak slip amount,the curve trend and characteristics of thespecimens with the same parameters are basically the same.According to the comprehensive comparison,there is no significant change in the bond performance of the 630 MPa high strength steel bar with the widened distance of two adjacent transverse rib and the standard shape,so it is not necessary to treat it differently in the practical application engineering.According to the steel strain data measured in the test,the distribution of bonding stress in different positions is analyzed,and the variation law of bonding stress of high-strength steel reinforcement in concrete within the anchorage length range is studied,which is reflected in the form of position function.Based on the research of the distribution of slip along anchorage length and the relative bond stress,the basic bond-slip function is obtained.The slip constitutive relation between 630 MPa steel bar and concrete is expressed by the product of basic bond-slip function and bond-slip position function.Compared with the experimental results,the practicability of the constitutive relation is verified.Based on the analysis of beam test results and the statistics of test data,the formula for calculating the bond strength of 630 MPa high strength steel bar is put forward.Based on the reliability analysis using the method of FOSM,the design value of anchor length for 630 MPa high strength steel bar and concrete is proposed.The results show that the anchorage length design formula in the current code of concrete structure design(GB50010-2010)is still applicable to the 630 MPa high strength steel bar,but it is not economic enough.On this basis,it is suggested that the shape factor α of anchorage steel bars(ribbed steel bars)in the basic anchorage formula in the code be taken as 0.12.