Study On The Flexural Properties Of Ultra-high Performance Concrete-normal Concrete Composite Reinforced Beams

The RC(Reinforced Concrete)structure subjected to overload,fatigue load,and environmental corrosion would lead to the degradation of performance and the mechanism of decrease in bearing capacity.The repair and reinforcement materials are selected wisely for strengthening reinforced concrete structures in service,or the ordinary concrete structure is converted into new composite material structures.It has theoretical and practical engineering significance in improving the durability and performance of concrete structures.UHPC(UltraHigh Performance Concrete)is a kind of ultra-high strength cement-based material with ultrahigh durability and physical and mechanical properties.The new composite material structure of high structural efficiency and long service life would be formed when UHPC combines steel bars and ordinary concrete,which would meet the durability requirements of concrete structures in harsh environments.Reasonable allocation of the three materials,improvement of the degree of coordination among them,and improvement of structural efficiency are the main concerns of engineering designers.However,the research results of UHPC-NC(Ultra-High Performance Concrete Reinforced Normal Concrete)and RUHPC-RC(Reinforced Ultra-High Performance Concrete Strengthened Reinforced Concrete)composite reinforced beams are still rare.The macro-scale UHPC-NC composite reinforced beams are taken as the research object in this paper.The mechanical property of static bending and flexural fatigue for UHPC-NC composite reinforced beams have been studied through experiments and theoretical methods.The results are as follows :(1)The experimental and theoretical study on the flexural bearing capacity of UHPC-NC reinforced composite beams with rectangular sections is carried out.Through the four-point bending test of 5 rectangular section test beams,the bending behavior of UHPC-NC composite reinforced beams with different layer thicknesses is studied from the aspects of crack distribution and propagation,failure pattern,load-deflection curve,section strain distribution,the contribution rate of UHPC to bearing capacity and bending stiffness.The results show that the cracking load and yield load of UHPC-NC composite reinforced beams are increased by at least 30% compared with ordinary reinforced concrete test beams.In addition,the multi-linear UHPC meso-constitutive model is used to establish the calculation method of cracking moment and the ultimate moment of rectangular section UHPC-NC composite beam and UHPC beam,considering the effect of steel fiber and residual tensile strength.Compared with the experimental results,the calculated results agree with the experimental results.(2)Based on meso-mechanics,the area of steel fiber between cracks is equivalent to the area of the steel bar.Adopting the conversion section method establishes the calculation methods of whole section bending stiffness and the cracked section of short-term bending stiffness.The bending stiffness and displacement of tested beams were calculated using different modulus theories,conversion section method,frequency method," Specifications for Design of Highway Reinforced Concrete and Prestressed Concrete Bridges and Culverts " and " Standard for the design of steel fiber reinforced concrete structures " respectively.Compared with the test results,it is suggested that the converted section method should be used to calculate the flexural stiffness of the whole section stiffness and the cracked section of short-term stiffness.Based on the flexural stiffness of the whole section,the overall flexural stiffness of UHPC-NC composite reinforced beams is studied,and it is proposed that the optimal UHPC layer thickness is about h/3.(3)The flexural bearing capacity of RC-RUHPC composite beams with rectangular sections strengthened after the damage was studied.The bending performance of the composite beam is studied through the four-point bending test of three RC-RUHPC composite beams with a rectangular section.The formulas for calculating the cracking moment,ultimate moment and stiffness of rectangular section RC-RUHPC composite beams considering the effect of damage are proposed,compared with the experimental results,calculated results agree with the experimental results.It is suggested that the conversion section method should be used to calculate the cracked section of short-term stiffness,the stiffness of the whole section should be reduced according to the damage,and the reduction coefficient should be 0.5 for safety considerations.(4)According to the highway bridge and culvert design drawings-16 m reinforced concrete T-beam,six test beams were made using the scale of 1/5,with the same reinforcement and stirrup ratios.The bending performance of UHPC wet pouring and UHPC reinforced cracked T beams was studied through the static load test of three T-section test beams.The calculation methods of the cracking moment and ultimate moment of T-section UHPC-NC composite reinforced beam and RC-RUHPC considering the damaging effect were proposed.As verified with the experimental results,the calculated results agree with the experimental results.(5)The fatigue performance of UHPC-NC reinforced composite beams were studied.The bending fatigue characteristics of the NC-RUHPC composite beam,RC-RUHPC strengthened beam and UHPC beam with T-section were studied through the constant amplitude low cycle fatigue tests of three T-section test beams.The relationship between fatigue deflection,bending stiffness,fatigue cracks width,fatigue cracks width growth rate and fatigue life,and the bending fatigue damage mechanism were obtained.The experimental phenomena and results show that in the later stage of fatigue loading,only one main crack is formed near the middle span.Compared with the micro-cracks under initial load,the other bending cracks extend slowly and shortly,mostly micro-cracks,and the fatigue fracture of steel bars all occurs more than 3.7million times.The fatigue bending stiffness of RC-RUHPC strengthened beams decreased slightly in 3 million cycles and RUHPC beams in 2 million cycles.However,after 3.7 million cycles,the fatigue bending stiffness decreased significantly,47% lower than the initial bending stiffness.During the fatigue fracture of steel bars,there are two kinds of fracture surface:intergranular fracture and transgranular fracture.After fatigue,the mechanical properties of the steel bar,the strength index,and the plasticity index of 600 high-strength steel bar decreased slightly.In contrast,the strength index of the HRB400 steel bar increased somewhat,but the plasticity index fell slightly.