| The anchorage length in concrete can be effectively reduced by welding the headed bars at the end of the high strength reinforcement.When the high strength reinforcement with headed bars is applied,there are the bond stress between the reinforcement and concrete and the pressure force from the headed bars.The main problems affect the application of reinforcement with headed bars in concrete structure were carried out:(1)When the loading end of the reinforcement is subjected to tensile yield,the distributive relationship of anchorage force relative to reinforcement and headed bars is the first problems concerned in this paper.(2)The concrete under the headed bars not only affected by the pressure from the headed bars,but also affected by the bond stress between the reinforcement and concrete,which makes the local bearing capacity of concrete under the the concrete under the headed bars take on new stress characteristics,which is the second problem concerned in this paper.(3)When the reinforcement with headed bars arrangement is relatively dense,there is a phenomenon that the concrete areas for distributing bearing under the headed bars will overlap with each other,and coupled with the bond stress between the reinforcement and concrete.It makes the calculation of the local bearing capacity of the concrete under the headed bars with relatively dense more complicated,which is the third problem concerned in this paper.Therefore,it is important theoretical significance and practical value to carry out the distributive relationship of anchorage force relative to reinforcement and headed bars and the calculation of the local bearing capacity of the concrete under the headed bars.The following studies were carried out:(1)In order to study the distributive relationship of anchorage force between the bond stress between the reinforcement and concrete and the pressure force from the headed bars,the pull-out test of 120 concrete prismatic specimens with cross section measurements of 150 mm×150 mm were completed.The reinforcement with headed bars is arranged on the centroid axis of the section.In the tests,the steel grades for the reinforcements comprised HRB500 and HRB600,and the diameters of the reinforcements were 20 mm,22 mm,and 25 mm.The concrete strength grades can be divided into five strength class of C30,C40,C50,C60,C70,and the anchorage length of reinforcement can be divided into five categories that approximately 0.3,0.4,0.5,0.6,0.7 times the critical anchorage length.The critical anchorage length of reinforcement refers to the anchorage length of reinforcement in concrete corresponding to the state of reinforcement to be pulled out but not pulled out under the steel tensile yield in the control section.The experimental results show that when the steel tensile yield in control section,the ratio of the pressure force from headed bars to the anchorage force at the loading end(Fp/Fy)decreases linearly with the increase of the ratio of the anchorage length of reinforcement to the diameter(lah/d),and increases linearly with the increase of the ratio of the yield strength of reinforcement to the tensile strength of the concrete(fy/ft).The formulas for calculating the ratio of the pressure force from headed bars to the anchorage force at the loading end(Fp/Fy)are established,which take the ratio of the anchorage length of reinforcement to the diameter(lah/d)and the ratio of yield strength of reinforcement to the tensile strength of the concrete(fy/ft)as independent variables.(2)It is proposed that the anchorage length when the free end of the reinforcement is about to slip is the stable anchorage length of the reinforcement lasunder the steel tensile yield in control section.Based on the relationship between compressive stress and strain of concrete,tensile stress and strain of reinforcement and the bond-slip constitutive relation between reinforcement and concrete,the stress balance equations and deformation coordination equations of reinforcement and concrete are established.The critical anchorage length and stable anchorage length between reinforcement and concrete are calculated by program iteration.(3)Based on the stress balance equations and deformation coordination equations of reinforcement and concrete under the boundary condition that the slip of reinforcement at the end of headed bars is zero.The numerical analysis of(2)is used to calculate and analyze the stress behavior of reinforcement with headed bars in concrete.Based on the pull-out test results of 120 concrete prismatic specimens,the accuracy of the program is verified.Based on the theoretical calculation results,an equation was established to calculate the stable anchorage length of reinforcement lasby using the ratio of yield strength of reinforcement to the tensile strength of the concrete(fy/ft)and the relative protective thickness(c/d)as independent variables.Analytical results showed that the changing trend in the ratio of the ratio of the pressure force from headed bars to the anchorage force at the loading end(Fp/Fy)with the ratio of the anchorage length of reinforcement to the stable anchorage length of the reinforcement(lah/las)was only related to the relative protective thickness(c/d)at the yield moment for the reinforcement.(4)In order to study the influence of the bond stress between reinforcement and concrete on the local bearing capacity of concrete,the pull-out tests of 30 concrete specimens with cross section measurements of 150 mm×150 mm and 200 mm×200mm were completed.The standard value of yield strength of the reinforcement was980MPa and 1080MPa,and the diameter of reinforcement was 32mm and 25mm respectively.The concrete strength grades can be divided into three strength class of C30,C40,C50,and the anchorage length of reinforcement can be divided into five categories that approximately 0.3,0.4,0.5,0.6,0.7 times the critical anchorage length.The experimental results show that the influence range of the bond stress between reinforcement and concrete is a ring area,which is concentric with the reinforcement and the diameter of the ring area is about 1.25-1.35 times of the diameter of reinforcement.The formulas for calculating the local bearing capacity of concrete is established considering the influence of the bond stress.(5)The concrete areas for distributing bearing under the headed bars will overlaps with each other.In order to investigate the influence of the overlap area from the headed bars on the concrete local bearing capacity,6 specimens with the cross section measurements of 200 mm×300 mm×660 mm,and the overlap area of concrete areas for distributing bearing under the headed bars were completed in the pull-out test.The standard value of yield strength of the reinforcement was 1080MPa and the diameter of reinforcement was 25mm.In the test,the net distance between the headed bars was 10mm,25 mm,40 mm,55 mm,70 mm and 85 mm.The concrete strength grade was C40in the test.Based on the concrete local bearing capacity values with a net distance of 0mm and 2 times the side length of the headed bars,the calculation formulas of the local bearing capacity of concrete are established.The calculation formulas take the ratio of the net distance c to the side length of the headed bars b as the independent variable. |
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