STRESS-STRAIN RELATIONSHIP ANALYSIS OF PVC-FRP CONFINED STEEL REINFORCED CONCRETE COLUMNS UNDER AXIAL COMPRESSIVE

To address construction quality challenges caused by dense reinforcements in steel-reinforced concrete structures, this study introduces a novel PVC-FRP pipe-confined steel-concrete composite structure, replacing the conventional steel skeleton with PVC-FRP pipes. Axial compression tests were conducted on nine composite columns to investigate failure mechanisms, mechanical behavior, strain development and, material interactions under axial loads. The effects of concrete strength, of specimen size, of FRP spacing and, of steel ratio on the axial compression performance of the specimens are also analyzed. Results indicate that larger specimen dimensions, higher concrete strength, greater steel ratios and, reduced FRP layer spacing significantly enhance the bearing capacity, with the maximum improvement reaching 38.8%. The maximum value of the transverse deformation coefficients ranged from 0.60 to 0.85, reflecting the confinement level provided by the PVC-FRP pipe. For specimens with higher concrete strength, the PVC-FRP pipe's confining effect is initially low but intensifies as the load increases, whereas increase FRP spacing, larger specimen dimensions and enhance steel ratios inversely affect the variation of the confining effect. The stress-strain curves exhibit a bilinear trend, with an inflection point at approximately 80% of the peak load. The initial slope of the stress-strain curve increases with higher concrete strength and smaller specimen dimensions and, the ultimate axial strain is most significantly affected by the specimen size, while ultimate axial strain improves by 17.4% (from 0.109 to 0.128) as the specimen diameter decreases from 250 mm to 200 mm. Using experimental results and existing stress-strain models for confined concrete, the characteristic parameters of the stress-strain curve were revised to account for the influence of H-shaped steel within the specimens. The revised model demonstrates high accuracy in predicting the mechanical behaviors of this kind of composite structures under axial compression.