Behavior of partially and fully FRP-confined circularized square columns under axial compression

Fiber-reinforced polymer (FRP) jacketing has become an attractive strengthening technique for concrete columns. Existing research has demonstrated that FRP confinement is effective for circular columns, whereas it is less effective for square columns. The lower FRP confinement effectiveness in a square column is predominantly attributed to the non-uniform FRP confinement in the column, while the concrete in an FRP-confined circular column is uniformly confined. Although rounding the corners of a square column can enhance the effectiveness of FRP confinement, its benefit is still not satisfactory as the radius of rounded corners is usually limited owing to the presence of internal steel reinforcement. An appropriate approach to enhancing the confinement effectiveness of FRP strengthening technique for square columns is to circularize a square column before FRP jacketing. This paper aims to study the compressive behavior of circularized square columns (CSCs). A total of 33 column specimens were prepared and tested under axial compression in this paper. The column parameters include FRP wrapping schemes (including fully wrapped and partially wrapped), FRP volumetric ratio, sectional shapes and unconfined concrete strength. The test results have indicated that the section circularization of square columns can significantly improve the effectiveness of FRP confinement, and strengthening square columns using section circularization in combination with partial FRP confinement is a promising and economical alternative to the fully FRP strengthening technique. Comparisons between the theoretical predictions and the test results were conducted, and the accuracy of existing partially FRP-confined concrete models were also examined.