Research On The Performance Of HRB500 Bars Reinforced Prestressed Ultra-high Performance Concrete Beams

As a new kind of cementitious composites, ultra-high performance concrete(UHPC) is becoming a very popular material in civil engineering due to its high mechanical properties and excellent durability, however, there is still a lack of sufficient study on its structure behavior. HRB500 bar is a high-strength hot-rolled ribbed bar, its use has made it possible to reduce the amount of steel reinforcement and easy the reinforcement layout. The combination of UHPC and HRB500 makes it possible to optimize their properties, and obtain advantages such as reducing dead load, simplifying bars arrangement, increasing structure spanning capability and durability.An experimental study and the corresponding theory analysis are carried out to study the behavior of UHPC beams reinforced with HRB500 bars. The purpose of this study is to explore the suitability of the combination of HRB500 and UHPC, and investigate the UHPC beams mechanical behavior and design philosophy. The main content involves the following work:(1) The UHPC uniaxial compression test, uniaxial tension test, flexural prism test etc were conducted, factors affecting the UHPC mechanical properties including curing regime and vibrating methods were analyzed. The rising stage of uniaxial compression stress-strain curve of UHPC was obtained and corresponding equation was fitted. The tests show that the compression peak stress corresponding strain is much higher than that of ordinary concrete, close 3000×10-6, the steel fibers crossing the cracks bear tensile stress post-crack, and uniaxial tensile stress decreases with the increasing crack width and the steel fibers pull-out.(2) Six post-tensioned UHPC beams subjected to bending were tested. The detailed analysis on the flexural behaviors, the characteristics of the load-deflection curves, the developing of cracks and the failure modes are analyzed. The test results show that HRB500 can work well with UHPC, their high strength can be fully used; the bending cracks are fine and small spacing, and the maximum of crack width is less than 0.15 mm in serviceability limit states; the displacement ductile factor of UHPC beams is greater than 3. The UHPC uniaxial compression and tension stress-strain curves and corresponding equations were proposed based on experimental results; Considering the increase of the plasticity degree in tension zone caused by steel fiber, the anti-cracking effect coefficient is introduced to modify the cracking moment formula in “JTG D62-2004 Code for design of highway reinforced concrete and prestressed concrete bridges and culverts”, and the calculated values by the modified formula agree well with the test results. The crack-correction coefficient is introduced to modify the maximum crack width formula in “GB50010-2010 Code of concrete structure design”, and the calculated values agree well with the test values. The ultimate bearing capacity formula is established, and the results of the formula are compared with test data and show a good correlation, which can be used for flexural theoretical analysis and design of UHPC beams.(3) Nine UHPC beams were tested in shear. From the results of diagonal cracking characteristics, it is observed that with increase in shear span-to-depth ratio, the failure mode transforms from diagonal compression failure to shear-compression, then to diagonal tension failure; the shear diagonal cracks are fine and small spacing, and the maximum diagonal crack width is less than 0.2mm in serviceability limit states; the strength of HRB500 stirrups can be fully used. The main factors affecting the shear capacity include shear span-to-depth ratio, prestress and stirrup ratio, etc. Based on shear test results, calculation formulas for shear cracking force and the maximum diagonal crack width in serviceability states are suggested. It will lead to far too conservative results using existing code to calculate the shear strength of UHPC beams; Based on the modified compression field theory, considering the shear contribution of concrete in compression, shear stress transmitted across crack interface, steel fibers and stirrups, a method and its simplified method of the UHPC beams shear capacity are developed. It is found that consisitency between the calculated data and the test data are well. Finally considering of shear span-to-depth ratio, stirrup ratio and prestress, a statistical formula for shear capacity of UHPC beams is suggested, the calculated values agree well with the test values, and the coefficient of variation is small.(4) To study the internal force redistribution of UHPC continuous beams, a calculation program for moment redistribution coefficients is coded using nonlinear analysis methods. Through the numerical analysis, the moment redistribution coefficients of 22 simulated HRB500 bars reinforced UHPC continuous beams by applying concentrated load at midspan are obtained, and a calculation formula for the moment redistribution coefficient is proposed in which the independent variable is the relative height of compressive region. Finally, considering the limit of the maximum crack width is less than 0.2mm in serviceability limit states, the calculation method of moment redistribution coefficient of HRB500 reinforced UHPC beams is proposed.