Study On Bond Performance Between NPR Steel Bar And Marine Concrete

With the continuous development of reinforced concrete structures to high-rise,long-span and heavy load.The engineering community puts forward new requirements for the material properties of steel: higher strength,better ductility,stronger corrosion resistance,lower cost,resource conservation,energy conservation and environmental protection.Compared with ordinary steel bars,high-strength steel bars not only have higher yield strength but also the use of high-strength steel bars is of great significance for saving resources and reducing construction costs.However,the currently used high-strength steel bars generally have the problems of increased strength and decreased ductility,and the corrosion resistance has not been improved compared with ordinary steel bars.NPR steel bar is a new type of high-strength,high-ductility and corrosion-resistant steel.It not only overcomes the contradiction between high yield strength and high uniform elongation of ordinary high-strength steel bars but also improves the corrosion resistance.In this paper,the basic mechanical properties,corrosion resistance,mechanical properties after corrosion and bonding properties with marine concrete of NPR steel have been carried out experimentally studied and theoretically analyzed.Meanwhile,the bond-slip constitutive relationship between NPR steel bars and marine concrete and the calculation formula of basic anchorage length were deduced based on the experimental data.The main research contents and conclusions are as follows:(1)The mechanical properties of NPR steel bars were studied by static load tensile test at room temperature.During the test,the necking phenomenon of NPR steel and the macro fracture morphology after breaking was observed and recorded,and compared with HRB400 steel bars.The main conclusions of the static load tensile test are: NPR steel has no obvious necking phenomenon in the tensile process,and the macro fracture morphology shows ductile fracture characteristics.In addition,the stress-strain curve of NPR steel shows that its yield platform disappears and there is no obvious yield strength.The nominal yield strength,tensile strength,elongation after fracture,and total elongation at maximum force of NPR steel are significantly improved compared with HRB400 steel bars.Finally,based on the test results,the measured stress-strain curve of NPR steel was fitted by the Ramberg-Osgood model,and the fitted curve was in good agreement with the measured curve.(2)The corrosion resistance of NPR steel was studied by a natural environment accelerated corrosion test,and compared with HRB400 steel bar.The test results show that under the same corrosion time,the corrosion rate of NPR steel is lower than that of ordinary steel bars.The nominal yield strength and ultimate strength of corroded NPR steel are still higher than those of corroded ordinary steel bars.Moreover,with the increase of corrosion time,the ductility of NPR steel with a diameter of 18 mm gradually decreased to be close to or even lower than that of ordinary steel bars.(3)The bond properties of NPR steel bar and marine concrete were studied by a central pull-out test.The failure modes of the specimens were observed;The effect of the types of steel bars(NPR steel bars,ordinary steel bars),the strength grades of marine concrete(C30,C40,C50),steel bar diameter(8mm,18mm)and bond length(5d,7d,d is steel bar diameter)on bond strength and bond stress-slip curve were analyzed and discussed.The test results show that the type of steel bar has a significant effect on the failure form,bond strength,and bond stress-slip curve.When other conditions are the same,the bond strength of NPR steel bars is lower than that of ordinary steel bars.In addition,the bond strength of NPR steel bars increases with the increase of the strength grade of marine concrete,while the influence of the diameter of NPR steel on the bond strength varies with the bond length and the strength grade of marine concrete.(4)The measured bond-slip curve of NPR steel and marine concrete was fitted by a three-stage model;The fitting results showed that the measured curve was in good agreement with the fitted curve.At the same time,the critical anchorage length of NPR steel and marine concrete was calculated according to the requirements of the "Code for Design of Concrete Structures" GB 50010-2010(2015 Edition);The calculation results show that: the "Code for Design of Concrete Structures" GB 50010-2010(2015 Edition)Recommendations for NPR steel and marine concrete anchorage lengths are safe and conservative.