Experimental Study On Flexural Mechanism Of UHPC Formwork-GFRP Bar Reinforced NC Composite Beams

Under the strategic guidance of the Belt and Road Initiative and the Maritime Silk Road,it is urgent to construct infrastructures such as ports,hospitals,shopping malls and roads in the marine environment to meet the continuous advancement of the strategy.However,due to the low tensile strength of concrete,it is easy to crack under adverse external loads,which seriously affects the durability of concrete structures.In addition,when the concrete structure is in the marine environment of high temperature,high humidity and high salt,it will aggravate the corrosion of concrete structure.The corrosion of concrete not only seriously affects the service life of the structure,but also threatens people’s life and property safety.Fiber Reinforced Polymer(FRP)and Ultrahigh-performance concrete(UHPC),which have developed rapidly in recent years,provide the possibility to solve the corrosion problem of reinforced concrete structures.GFRP bars are used to replace the tensile reinforcement in concrete beams,which not only solves the problem of steel corrosion,but also exerts the characteristics of high tensile strength of GFRP bars.Prefabricated permanent formwork made of UHPC can reduce the crack width so as to improve the durability of concrete structure on the one hand,and improve the bearing capacity of concrete beams on the other hand,which also greatly improves the problems of excessive deflection and excessive crack width of GFRP reinforced concrete beams.The effective bonding between UHPC template and post-cast concrete is the basis to ensure the joint force of the two.At the same time,due to the good tensile strength of UHPC and GFRP bars,it is difficult to play the excellent performance of the material when the two work together.The existing research results show that UHPC formworkFRP reinforced concrete beams have low material utilization rate and poor bonding performance.Therefore,a new composite beam of prefabricated U-shaped UHPC template-GFRP bar-NC was proposed in this paper.Different interface treatments were set on the inner surface of the prefabricated UHPC permanent template to increase the interface roughness so as to ensure the excellent bonding performance of the two.In addition,in order to prevent the premature crushing of concrete at the top of the composite beam from leading to the low utilization rate of GFRP bars,this paper sets UHPC composite layer at the top of the composite beam,which can give full play to the excellent tensile properties of UHPC and GFRP bars.The research results provide the possibility to solve the problem of premature degradation of reinforced concrete structures in the harsh marine environment.The research work and conclusions of this paper are as follows:(1)Tensile tests were carried out on GFRP bars with different diameters,UHPC and ordinary concrete were prepared,and their ultimate tensile strength,elastic modulus,ultimate strain and stress-strain curves were tested,which provided the basis for numerical simulation and theoretical calculation.(2)Four-point bending tests were carried out on eight UHPC formwork-GFRP bars-NC composite beams.The test variables include the presence or absence of Ushaped UHPC formwork,the reinforcement ratio of GFRP bars,the interface treatment method,the reinforcement of the top laminated layer and the thickness of U-shaped formwork.The test results show that compared with ordinary concrete beams,UHPC formwork reinforced concrete beams have higher cracking load and ultimate load,and a large number of fine cracks appear on the surface of composite beams,which effectively inhibits the formation of excessive crack width.With the increase of reinforcement ratio,the cracking load and ultimate load of composite beams increased by 30.8%,35.4%and 19.6%,28.1%,respectively.The increase of reinforcement ratio can effectively improve the stiffness,cracking load and ultimate load of composite beams after cracking.For composite beams with different interface treatments,no stripping of formwork and core concrete was observed,and UHPC and concrete could work together.From the comprehensive consideration of construction difficulty and engineering practical application,foam interface treatment is the most ideal interface treatment method.After setting UHPC composite layer at the top,the ultimate load of the composite beam increases by 24.9%,and the ultimate tensile strain of GFRP bars increases by 28%,which effectively improves the ultimate load of the composite beam and the utilization rate of GFRP bars.When the bottom UHPC template thickness was 45 mm,the cracking load and ultimate load increased by 13.0%and 18.3%,respectively,indicating that the increase of bottom UHPC template thickness effectively improved the bearing capacity of composite beams.(3)The finite element analysis of U-shaped UHPC formwork-GFRP bar-NC composite beam is carried out.The simulation results show that the model established in this paper can better simulate the bending performance of the composite beam.The bearing capacity and stiffness of the composite beam are in good agreement with the experimental results.(4)A simplified calculation formula for cracking load and ultimate load of Ushaped UHPC template-GFRP bar-NC composite beam is proposed.Considering the strength degradation of UHPC template under large load,the reduction coefficient of UHPC template under large load is given.The calculation results are in good agreement with the experimental results.