Repair and strengthening of fire damaged concrete cylinders using FRP confinement: Tests and analytical modelling

This study examines the effects of fiber-reinforced polymer (FRP) repair and strengthening on the axial stress–strain behavior of concrete columns after exposure to realistic fires. A total of 30 plain concrete cylinders, each measuring 150 × 300 mm, were cast for this investigation. Of these, three specimens were kept as reference at ambient temperature, while the remaining were exposed to ISO-834 standard fire for durations of 30, 60 or 90 min, with nine specimens in each duration group. After natural cooling, the heated specimens were categorized into three groups: i) three were left unconfined, ii) three were repaired and strengthened using two layers of carbon FRP sheets, and iii) three were repaired and strengthened with four layers of carbon FRP sheets. This study employs realistic ISO 834 fire scenarios and investigates CFRP confinement with up to four layers, addressing high confinement demands beyond current literature. The results showed that transverse confinement provided by carbon FRP sheets significantly improved axial strength and deformability for all specimens, while it did not fully restore the axial stiffness achieved before fire exposure. The effectiveness of FRP confinement increased with longer fire exposure durations. Additionally, two analytical models proposed previously for predicting the axial strength and ultimate strain of FRP confined fire-damaged concrete were evaluated in terms of their accuracy. The accuracy of the predictions was reduced with an increase in exposure temperatures for both models. Therefore, a new model is proposed within the scope of study, which shows good agreement with the novel test results.