Effective Strain in FRP Jackets on Circular RC Columns

ABSTRACT

Fiber Reinforced Polymer (FRP) composite materials have been widely used and have been extensively studied in the last decades in the form of jacketing to enhance axial strength as well as ductility, and their effectiveness has been extensively proven in many research programs investigating confined concrete column behavior. The existing models available for confined concrete assessment both in terms of ultimate capacity and of stress-strain relationships rely on an assumed value of the ultimate FRP strain. It is commonly assumed that FRP fails when hoop strain in the jacket reaches its ultimate tensile strain determined according to flat coupon tests. However, FRP confined concrete experimental results showed that in most cases, FRP experimental ultimate tensile strain is clearly not reached at the rupture of the FRP jacket. The discrepancies may include misalignment or damage to jacket fibers, residual strains or uneven tension during lay-up, cumulative probability of weaknesses in the FRP jackets since they are much larger than tensile coupons and, more likely, the radius of curvature in FRP jackets on cylinders as opposed to flat tensile coupons and the multiaxial stress state due to the transfer of loads through the bond with concrete. Since for confined sections the average absolute error of all models showed a remarkable decrease when the confining device effective strain is inserted in the equations, it is very important to assess the effective FRP jacket strain capacity. A criterion to directly evaluate the FRP strain efficiency factor as the strain ratio between effective FRP failure and straight coupon test outcomes has been formulated. Multiaxial failure criteria have been adopted (i.e. Tsai-Wu criterion for FRP) considering axial, circumferential and radial stresses. Results of theoretical analyses and experimental tests (experimental data available in literature) showed that a good agreement was achieved.