A Pressure Forming Technique for Concrete-Filled Steel Tubes and Mechanical Properties of the Compressed Concrete

A device for pressure forming concrete-filled steel tubes (CFST) was designed, which can apply a certain amount of mechanical pressure on fresh concrete for a certain period of time. Using this device, 7 sets of compressed concrete specimens with different compressing times and pressure levels were fabricated and subjected to axial compression failure tests. The influence of compressing conditions on concrete strength, brittleness, and stress-strain relation was studied, and a strength fitting formula for compressed concrete was proposed. The collaborative mechanism between the steel tube and core concrete in pressure formed components was analyzed through numerical simulations. The results showed that after compressing, the strength of concrete could be increased by 12%-42%, and the strength of concrete increased with the increase of compressing time and pressure magnitude; the effect of compressing time on the strength of compressed concrete was more significant; the proposed formula for compressed concrete strength could fit the test results well, with most specimens having an error of less than 3% and some specimens having an error of less than 15%. The pressure forming technique mainly improved the axial compression performance of pressure formed components by improving the strength of the core concrete. The growth rate of the component's axial compression strength caused by the initial circumferential stress was less than 5%.