Prefabrication Technics And Mechanical Properties Of UHPC Stay-in-place Formwork-steel Reinforced Concrete Hybrid Structural Members

UHPC stay-in-place formwork-reinforced concrete composite member is a new type of assembly-stress integrated composite member.UHPC mold shell is prefabricated in factory to improve product quality and reduce errors.Because UHPC is only used in permanent mold shell,it has better economy and popularization.In the construction stage,UHPC formwork shell is used as the construction formwork of reinforced concrete members to realize the rapid construction of assembled integral structure.In the service stage,UHPC formwork shell becomes permanent formwork,participates in the structural stress,and improves the mechanical performance and durability of the structure.On this basis,the construction,tensile strength and ductility of UHPC thin-walled structure can be further improved by using continuous grids to strengthen UHPC,and the mechanical performance of composite members can be improved.Therefore,a continuous grid reinforced UHPC stay-in-place formworkreinforced concrete composite column is proposed in this paper.Aiming at its Prefabrication Technology and axial compression performance,the research is carried out according to the three-level route of "formwork material-reinforced formwork-composite member".The main contents and conclusions are as follows:(1)The optimum mix ratio and preparation technology to meet the construction and performance requirements of UHPC are determined.Through the design of eight groups of UHPC mix ratios with water-binder ratio and steel fiber content as parameters,the preparation process and performance test of UHPC materials were carried out,the mixing and maintenance process of UHPC was determined,and the physical indexes were measured.The results show that the water-binder ratio mainly affects the mechanical strength of UHPC;steel fibers can enhance the integrity of UHPC,avoid booming when destroyed,and improve the tensile strength of UHPC,but the excessive amount of steel fiber content will reduce the fluidity of slurry,and lead to excessive air content affecting the performance.UHPC with water-binder ratio of 0.18 and steel fiber content of 1% possesses certain self-compactibility and excellent mechanical properties.It is an optimum proportion for the performance and construction requirements of UHPC stay-in-place formwork.(2)The tensile mechanical properties of the gird-reinforced UHPC sheet were tested.Four kinds of commercially available finished grids were selected,and a suspension forming process was proposed.A gird-reinforced UHPC sheet was prepared by combining UHPC with 1% steel fiber.The effects of different gird types,gird processing methods and gird layers on the mechanical properties and crack morphology of gird-reinforced UHPC thin specimens were investigated by uniaxial tensile tests.The results show that the gird strength and the interface performance between the gird and the UHPC matrix are the main factors affecting the UHPC enhancement effect of the gird.Therefore,the carbon fiber gird has a significant effect on the UHPC reinforcement,but the gird needs to be impregnated with epoxy resin and adhered to the sand;the stainless steel gird has a better reinforcing effect,and the increase in the number of gird layers can greatly increase the ductility of the test piece;carbon-glass fiber The mixed gird and basalt gird are not ideal due to premature debonding of the adhesive layer and low selfsupporting capacity.The carbon fiber grid and the stainless steel grid were thus screened as composite column reinforcements.(3)UHPC stay-in-place formwork-steel reinforced concrete composite columns were tested under axial compression.Taking the thickness of stay-in-place formwork and the number of continuous gird layers and grid type as design parameters,the prefabricated composite columns with UHPC stay-in-place formwork as outer layer and ordinary steel reinforced concrete as core were designed and manufactured.The hanging forming method was further applied in the column gird forming process to form the preparation process of UHPC stay-inplace formwork-steel reinforced concrete composite columns,and the axial compression test of composite columns was carried out.The results show that the composite column can greatly increase the axial compressive capacity of the column and increase the overall elastic modulus,and the UHPC layer can remain the whole after failure.However,the compressive brittleness of composite columns increases significantly,and the ductility and toughness of columns can be improved by grid.The reinforcement effect increases with the increase of the number of grid layers,and the peak load falls sharply due to the yield or fracture of grid beams.The failure mechanism of stepped load-strain curves of grid-reinforced composite columns is the sequence of grid failure at two main cracks,the time-sharing fracture of different fiber bundles and the asynchronous failure of double-layer grids.(4)Fine finite element model and parametric analysis of grid-reinforced UHPC stay-inplace formwork-steel reinforced concrete composite members are carried out.The fine finite element model of UHPC stay-in-place formwork-steel reinforced concrete composite column,is established and solved explicitly by LS-DYNA.The simulation results and experimental results are compared.The influence of steel fiber content on the axial compression performance of columns under different grid reinforcement is further parameterized.The results show that the model can simulate the axial compression load-strain response of composite columns well.The toughening effect of steel fiber and grid reinforcement and failure effect can be realized.In addition,the premature failure of UHPC will occur when the steel fiber content is lower than the limit content(about 1%).At the same time,it is proved that the scheme of choosing 1% steel fiber content and using grid to reinforce is the best choice for balancing construction,performance and cost.