Study on the flexural behaviour of the concrete filled square steel tube beam with the basic magnesium sulfate cement-based composite shell system

A new type of stay-in-place formwork (SIPF) concrete beam was proposed; this beam consists of a U-shaped shell made of basic magnesium sulfate cement-based composite (BMSCC), a perforated square steel pipe and core concrete. The prefabricated shell and square steel pipe form a composite SIPF, the steel pipe replaces the longitudinal reinforcement and hoop reinforcement, and the combination beam can be formed after pouring the core concrete inside. Four-point bending tests of two SIPF beams and one reinforced concrete (RC) comparison beam were conducted using longitudinal steel consumption as a parameter. The results showed that the SIPF beams exhibited a typical ductile damage pattern and that the precast shell was reliably bonded to the core concrete without spalling. The cracking load was increased by 198.25 %, the yield load by 66.3 %, and the ultimate load by 64.7 % for the same amount of steel used in the steel pipe instead of the reinforcing cage. After the equivalent longitudinal reinforcement rate of 1 % at the bottom of the SIPF beam was increased to 1.25 %, the flexural capacity increased by 9.65 %. By introducing the restraining stress coefficient and stiffness contribution coefficient, a formula for calculating the flexural capacity of the positive section of the SIPF beam was established.