Research On Bond And Anchorage Properties Of Concret With Strands Of Large Diameter

The assembled concrete structure formed by precast concrete members is one of the main supporting structural forms in the process of building industrialization in China.However,in the past,most of the prefabricated elements used in China are small and medium prestressed members with steel wire as prestressing tendons.Although the imported technology from abroad uses the nominal diameter of 12.7mm small diameter steel strand as the leading prestressed tendon of the SP plate and the double T-plate and other large span components,but targeted to the nominal diameter of 15.2mm,17.8mm,21.6mm as the leading prestressed tendons of large prefabricated units of the mechanical properties,In particular,the bond between the larger diameter steel strand and the concrete and the required anchorage length and the prestress transfer length are lagging behind.Moreover,the calculation method of bonding anchorage between steel wire and small diameter strand and concrete is applied directly to the suitability of larger diameter steel strand.Therefore,it is necessary to carry out the research on the bond performance of large diameter steel strand,and then put forward the calculation method of reasonable anchorage length,which will provide some theoretical basis for the popularization and application of larger precast steel wire strand with large diameter.In this paper,the bond performance of the larger diameter steel strand of 15.2mm,17.8mm and 21.6mm three kinds of nominal diameters is studied by drawing test,and the following work is done:?1?The bond mechanism between larger diameter steel strand and concrete is analyzed through the static load test of 56 larger diameter steel strand drawing parts and the comparison test of 16 deformed steel bars with similar nominal diameters.The measured results of the bond strength of larger diameter steel strand are obtained,and the basic anchorage length of the larger diameter strand is mainly based on the nominal diameter of 15.2mm,17.8mm and 21.6mm,and the embedded length of the steel strand and the thickness of the concrete cover are emphatically investigated.The influence of eccentric load and stirrup ratio on the bond properties of steel strand,the slip relationship curve between drawing test and strand is obtained,and the stress characteristics of five stages of bond slip of larger diameter steel strand are analyzed.It is found that the critical bond strength of the larger diameter steel strand determined by the Pull-out test method should be marked by the slip curve reaching the turning point of the second stage,and the ultimate bond strength should be marked by the slip curve reaching the end point peak load of the third stage.The ratio of the thickness of the protective layer to the nominal diameter c/d₀,the ratio of embedded length to nominal diameter and the ratio of stirrup spacing to nominal diameter s_sv/d₀ the l_an/d₀ of the ultimate bond strength of large diameter steel strand with basic parameters,and the expression of the limit strength of these three main parameters is given.?2?Through the static load bonding test of 36 strand concrete buried in the horizontal beam,the bonding and anchoring properties of the prestressing tendons and concrete at the end of the large prefabricated units are simulated to perfect the uniformity of the fullness degree of the bond stress along the anchorage length which is difficult to be considered by the tensile specimen.The Drawing test-slip relation curve is obtained,the rationality of the bonding mechanism based on the Pull-out test is verified,and the bonding failure process is found to be in good agreement with the corresponding stage of the drawing part test.The law that the bond anchorage length corresponding to the nominal yield strength of the strand decreases with the increase of nominal diameter is revealed,and the calculation method of the bond strength of the steel strand with the embedded length as the basic parameter and considering the degree of the bond stress distribution is presented.?3?The approximate method is used to analyze the reliability,the design value of the critical anchorage length of large prefabricated units with larger diameter steel strand as the leading prestressed tendon is determined,and the limit anchorage length design value which can be used for the safety evaluation of bonding and anchorage failure of existing precast components is determined.