Study On Bond Anchorage Performance Of Bundled Bars

With the improvement of seismic grade,seismic design requirements increase the amount of steel used in building,which often leads to the increase of reinforcement ratio of ordinary steel bars,the density of steel bars in joint area,the difficulty of construction and quality control,and the reduction of economy,especially with the promotion and application of high strength concrete,this problem becomes particularly prominent.In view of the problem that the construction is difficult to guarantee the construction quality when the steel bars are used intensiently in engineering practice,the form of reinforcing bars binding into bundles is put forward,which is called bundled reinforcing bars,but after the bundled reinforcing bars,the effective contact area between the steel bars and concrete decreases,and the bonding performance is relatively poor.This paper analyzes the research status of bundled bars at home and abroad,focusing on the bond anchorage performance of bundled bars.In order to study the bond anchorage properties of bundled bars,the bond anchorage tests of 57 pull out specimens and 6 beam specimens were tested.Were considered in the test and the number of reinforcement(single steel bar,and two-bundle steel bar,and three-bundle steel bar),test method(eccentric pull out,beam test),diameter of rebar(16mm,20 mm,25mm),the strength of concrete(C30,C50,C70),anchor length(5d,10d),reinforced position(central,angle)of six factors on properties of reinforced bond-slip effect.Based on the bond anchorage performance of single steel bar,the bond anchorage performance and force characteristics of bundled bars are analyzed through comparative tests.The results show that the number of reinforcements,the test method,the diameter of reinforcement and other factors do not affect the bond anchorage force of reinforcement.The nominal bond strength decreases with the increase of the number of bundle bars.The influence of test method,anchoring length,diameter of steel bar,strength of concrete and position of steel bar on bond strength of bundled bars is similar to that of single bars,so the bond anchoring performance of bundled bars can be analyzed by using the test results of single bars.Based on national standard bond strength experience formula and experimental data analysis,it is suggested that the equivalent diameters of 2 and 3 bundled bars should be 1.41 and 1.73,respectively,so as to be used for the values of anchorage length and thickness of cover.The experimental data in this paper are evaluated by comparing with the previous results on bond slip of parallel bars.When the thickness of cover is reduced,the anchorage length of the bundled bars can be appropriately extended.On the basis of the test in this paper,a numerical statistical analysis was made on the previous test data of bond anchorage for single reinforcement and bundled reinforcement,and the calculated values of bond strength of several empirical formulas(Xu Youling,ACI 408,FIB model code)were compared.On this basis,a rationality analysis was proposed,and the safety and applicability of the three empirical formulas were evaluated.Empirical formula which is based on the bond anchorage database with Xu Youling’s empirical formula,increase the steel bar diameter and larger protective layer thickness of two influence factors,puts forward Suggestions on the steel bond strength calculation formula,and respectively with single steel bar and bundled reinforcement comparing the test results the results show that the empirical formula has better accuracy,discreteness of greatly reduced.Finally,summarize the research results,and prospect the future research.