| 500MPa grade steel is a new type of hot-rolled ribbed bar with a high strength and good ductility, which is researched and exploited by the metallurgical industry in our country. In place of the 335MPa and 400MPa grade steel, the 500MPa grade steel can significantly reduce the amount of reinforced bar, improve the quality of building and save resources and energy, so it has great social and economic benefits. Hot-rolled ribbed steel bar can be divided into two kinds——low alloy HRB500 steel and fine-grained HRBF500 steel, HRB500 steel needs to add vanadium, titanium and some other alloy elements during its production, while HRBF500 steel is produced by control rolling and control cooling and it doesn't need this alloy elements or simply need a little. It mainly has the metallographic structure of ferrite and pearlite, and its grain size does not exceed degree 9. The appearance and mechanical performances of HRB500 and HRBF500 reinforced bar are almost the same, but HRBF500 steel can save the precious alloy resources and reduce steel price, therefore its social and economic benefits will be more remarkable.Concrete reinforced members with 500MPa grade steels have higher working stress under the service states, but the calculation formulae of crack width are established mainly based on the test results of members with lower strength reinforced bars(335MPa grade and 400MPa grade). When using 500MPa grade steels in the concrete members, the crack width calculated by "Code for design of concrete structures"(GB50010-2002) is too large to meet the requirements of crack control, and it will stop the popularization and application of the high strength steel. In order to establish a suitable formula on crack width of reinforced concrete members with 500MPa grade steels, the experiment on the concrete reinforced axial tension members with HRBF500 steels has been carried out, and the following researches have been completed in this paper:(1) Based on the test of 15 concrete reinforced axial tension members with HRBF500 steels, the reinforced stress at cracking moments and the development process of cracks are analyzed, and the comparison on cracking characteristics are given between two kinds of axial tension cracks, one of them is the transfixion crack that can reach the whole section or one side face of the member, another is the crack that can not reach the edge of the section.(2) According to the test results, the changing regularity of cracking space and cracking width are analyzed systematically, which will be affected by the strength of concrete, the distance from measure point to reinforced bar, diameter of reinforced bar, reinforcement ratio and som other factors; The modificatory formulae on average cracking space, average cracking width and maximum cracking width of concrete reinforced axial tension members with 500MPa grade steels are put forward by statistical analysis, and the results calculated by modificatory formulae are compared with that calculated by formulae in current code; The rationality of modificatory formulae is proved through the comparison of experimental data.(3) Based on the crack modificatory formulae, the maximum cracking width simplified formulae are deduced and the axial tensile force per unit area with no need for cracking width check of concrete reinforced axial tension members with 500MPa grade steels are listed to give some references on fast calculation of cracking width control in engineering design.The results of the research in this paper will provide experimental and analytic basis for the modificatory calculation of cracking width in "Code for design of concrete structures" (GB50010-2002).The topic for the 863 plan to subsidize the subject (2007AA03Z550)... |
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