Effects of Thermal Conditioning at High Temperatures on the Response of Concrete Elements Confined with a PBO-FRCM Composite System

Fiber-reinforced cementitious matrix (FRCM) materials are well-recognized as suitable for the strengthening of reinforced concrete (RC) and masonry existing structures. However, one of the main issues regarding these materials is the lack of knowledge about their behavior when subjected to elevated temperatures or fires. In this study, the effects of thermal actions on both the mechanical properties of the PBO (short of polypara-phenylene-benzo-bisthiazole)-FRCM materials (textile, mortar, and FRCM) and the structural response of unconfined and PBO-FRCM-confined concrete cylinders were analyzed. An experimental investigation was conducted on PBO fabrics, PBO-FRCM coupons, and unconfined and PBO-FRCM confined concrete cylinders. All specimens were thermally conditioned at temperatures ranging from 20°C to 200°C and then tested at room temperature (20°C). The thermal treatment consisted of five heating/cooling down cycles (one per day); in each cycle, the specimens were kept in an electric oven, heated at a predefined temperature over a period of 6 h, and then left to cool down at ambient temperature before the beginning of the next cycle. The test results evidenced that the thermal conditioning affected both the mechanical properties of the FRCM materials and the effectiveness of the concrete confinement. To predict the strength of the PBO-FRCM confined concrete cylinders after thermal conditioning, a semiempirical model defined through the analysis of a database of experimental results (those described in this paper and others available in the literature) was also proposed.