Research On Static Performance Of Precast U-shaped RPC Permanent Beam Formwork And Its Composite Beam

The prefabricated components are mainly prefabricated components of prefabricated concrete structures,which are assembled and connected by prefabricated components,which meet the requirements of green building.Permanent formwork can improve the quality of construction,reduce the use of temporary formwork,speed up construction efficiency,reduce dependence on manpower and environmental pollution,and its application in the field of civil engineering has broad prospects.There are many types of permanent beam formwork,such as steel mesh cement "U"-shaped permanent beam formwork,polyvinyl chloride(PVC)plastic,profiled thin-wall steel plate,fiber reinforced composite material(FRP),textile reinforced concrete(TRC)and other forms.Reactive powder concrete(RPC)has the advantages of high strength,high durability,and reduced weight of concrete structures.RPC with high strength and good durability can reduce the weight of the concrete structure.In this study,RPC was applied to the production of frame structure beam formwork.On the basis of improving the overall performance of the structure,the support was reduced,and the construction efficiency of the prefabricated structure was accelerated.In this paper,through a series of experiments,the performance of the prefabricated Ushaped RPC permanent beam formworks and the composite beams formed by the in-place ordinary concrete(NC)has been studied and theoretically analyzed.The main content involves the following work:(1)Combined with the current research status at home and abroad,the structural form of the prefabricated U-shaped permanent beam formwork was determined.Under the condition of meeting the prefabricated construction conditions of the factory,the volumetric method was used to try out the mix ratio of RPC,which provided a basis for the production of prefabricated U-shaped RPC permanent formwork.The basic mechanical properties of RPC,NC,and steel bars were obtained through experimental tests on accompanying test blocks.The full curve of the RPC axial tensile test was obtained,and the tensile stress-strain curve equation was obtained by fitting,which provided a basis for the subsequent theoretical analysis.(2)Completed the bending performance test of six prefabricated U-shaped RPC permanent beam formwork,taking the thickness of bottom plate and side wall and longitudinal reinforcement as the parameters to influence the stress process,crack development and distribution state,strain change of steel bar and concrete of the permanent beam formwork,Failure modes and load-deflection changes were analyzed.Experiments showed that increasing the thickness of the bottom plate could make full use of the tensile properties of the RPC material,increased the cracking load of the formwork,and reduced the deflection under normal use.The thickness of the base plate of the prefabricated U-shaped permanent beam formwork was increased by 10 mm,and the cracking moment could be increased by30.93%?41.11%.Based on the bending test data of the prefabricated U-shaped RPC permanent beam forwork and considering the tensile performance of RPC,a theoretical model for calculating the cracking moment of the prefabricated U-shaped RPC permanent beam formwork was proposed.Compared different codes and methods of calculating cracking moment by scholars,introduced the influence coefficient of section plastic resistance moment,a simplified calculation formula for cracking moment of precast U-shaped RPC permanent beam formwork was proposed,and it was recommended that the section plastic resistance moment influence coefficient was 0.8.In the actual hoisting,transportation,and construction process,the U-shaped permanent formwork is required to have no cracks during the construction process,and the cracking strength can be checked by the formula suggested in this article.(3)Completed the shear performance test of four prefabricated U-shaped RPC permanent formworks,using the sidewall thickness,stirrup ratio and shear span ratio as parameters to determine the stress process,stirrup and longitudinal strain changes of the permanent beam formworks.Development of cracks was analyzed in terms of changing trends and failure modes.Experiments showed that increasing the thickness of the sidewall of the prefabricated U-shaped RPC permanent beam formwork could improve the shear strength and the diagonal crack cracking shear force of the formwork.The influence of steel fiber on its shear capacity and the diagonal crack cracking load needed to be considered.The shear cracking moment calculation method proposed by ACI 544.4R could conservatively calculate the shear cracking moment of the precast U-shaped RPC permanent beam formwork.Based on the statistical analysis method and the simplified calculation method of the U-shaped section,a calculation model for the shear bearing capacity of the prefabricated U-shaped RPC permanent beam formwork was proposed,which was in good agreement with the experimental values and provides a theoretical design basis for engineering applications.(4)The NC was poured into the prefabricated U-shaped RPC permanent beam formwork to form a composite beam.The shear-span ratio,core concrete strength grade,and hoop ratio are used as parameters to test the shear performance of eight composite beams and one reinforced concrete beam.Research showed that the failure mode of composite beams was similar to that of reinforced concrete beams,presenting three stages: the elastic stage before cracking,the crack development stage after cracking to the yield of the steel bar,and the failure stage after yielding.The prefabricated U-shaped RPC permanent formwork composite beam has higher shear strength and crack resistance.Compared with the NC beam,the bending cracking load,stirrup yield load and peak load of the composite beam were increased by51.54%,88.02% and 78.38%,respectively,under the same section and the same reinforcement.(5)Combining the shear test data of the composite beam,the calculation methods of the shear capacity,cracking shear force,and maximum diagonal crack width under normal use of composite beam were studied.Considering the contribution of the concrete in the compression zone on the upper part of the beam,based on the truss-arch model,a softening factor suitable for the calculation of the shear capacity of the U-shaped RPC-NC composite beam was proposed.Based on the simplified calculation of the U-shaped section,a formula for calculating the shear capacity of the U-shaped RPC-NC composite beam was established.Based on the statistical analysis method,combined with the calculation method of the shear bearing capacity of the precast U-shaped permanent beam formwork,considering the contribution of the inner core NC,a calculation model for the shear bearing capacity of the RPC-NC composite beam was established.The calculation model was not only in good agreement with the experimental values studied in this article,but also applicable to the research data of other scholars,and the coefficient of variation was small.Based on the theoretical model of the oblique section shear resistance of reinforced steel fiber concrete beams,a calculation model for the maximum oblique crack width of the precast U-shaped composite beam under normal use was established.Using the method based on experimental statistics and parameter regression fitting,the calculation method of the laminated beam's anticracking shear force and the simplified calculation model of the maximum diagonal crack width were proposed,which provided convenience for practical engineering applications.