Study On Mechanical Behavior Of Square Steel Tube Concrete Beam With Fiber Reinforced BMSCC Permanent Formwork

In this paper,a new type of fiber reinforced BMSCC（basic magnesium sulfate cement base composite）permanent formwork—concrete-filled square steel tube composite structure has been proposed.The structure uses a BMSCC as a U-shaped prefabricated shell with a glass fiber woven mesh inside to improve its ability to limit cracks.The raised lateral ribs of the side walls were combined with perforated square steel pipes to improve bonding between the precast permanent formwork and the interior postcast concrete.The U-shaped prefabricated shell was combined with the built-in perforated square steel pipe to form a permanent formwork that requires no disassembly.The formwork does not need to be removed after the concrete is poured,and was integrated into the concrete after pouring.Not only the construction was simple,shorten the construction period,but also improve the mechanical properties of the concrete beam.Model test and finite element simulation analysis were used to study the bending and shear resistance of fiber reinforced BMSCC permanent formwork concrete—filled square steel tubular beam.The main research contents and conclusions are as follows:（1）Study on the optimum proportion and performance of the base material of the prefabricated shell.In this paper,by comparing several groups of material performance test data,the best ratio is obtained:the molar ratio of each material is7:1:22（Mg O:Mg SO₄·7H₂O:H₂O）,the ratio of cement sand is 1:2.5（Mg O:sand）;Properties of materials in the best proportion:compressive strength（28d）is 45.1MPa,flexural strength（28d）is 22.5MPa,tensile strength（28d）is 6.05MPa,fluidity 90mm,initial setting time is 7.3h,final setting time is 11h.（2）Experimental study on flexural behavior of normal section of concrete-filled square steel tube beams with fiber reinforced BMSCC permanent formwork.In this paper,static flexural tests are carried out on one reinforced concrete beam Z-W,one composite beam B-W₁and a composite beam B-W₂with increased steel content in the longitudinal tension zone The test results show that the flexural capacity of B-W1composite beam is increased by 64.7%compared with that of the integral beam Z-W,and the flexural capacity of B-W2 composite beam is increased by 9.6%compared with B-W1 after increasing the amount of steel used in the longitudinal tensile zone.In the test loading process of composite beam,the precast shell and core concrete have good bonding effect,and no slippage phenomenon occurs,showing a typical failure mode suitable for reinforcement.Combining with the test data,the formula for calculating the flexural capacity of the normal section of the composite beam was established by introducing the constraint stress coefficientα₀and the stiffness contribution coefficient?.The experimental results showed that the theoretical values had a good agreement with the experimental values,and the relative error was less than 6%.（3）Finite element simulation analysis.In this study,ABAQUS is used to simulate the bending and shear resistance of composite beams,and the simulation results are in good agreement with the test values.