In recent years, the high costs associated with designing, retrofitting, and rehabilitating aging and damaged structures have significantly increased the focus on structural strengthening among civil engineers. Reinforced concrete (RC) structures often require strengthening due to changes in structural usage, updates to code provisions, construction errors, natural hazards, or fire damage. Over the past few decades, fiber-reinforced polymer (FRP) composites have emerged as an effective method for strengthening RC structures, owing to their unique properties. These include high tensile strength, an excellent strength-to-weight ratio, ease of installation, and the ability to preserve the architectural appearance of structures. FRP composites are typically applied to concrete surfaces using two common techniques: external bonded reinforcement (EBR) and near-surface mounting (NSM). However, the primary factor limiting the full utilization of FRP composites is their premature debonding from the concrete surface. To address this issue, the grooving method has been introduced as a means to maximize the capacity of FRP composites, effectively delaying or even eliminating premature debonding. While most existing design guidelines and code provisions focus on structures exposed to room temperature (~25°C), many structures experience high temperatures during their service life. High temperatures alter the physical and chemical properties of concrete, leading to reduced compressive, flexural, and tensile strengths, as well as a decrease in elastic modulus. This degradation often continues for weeks after exposure. A comprehensive understanding of concrete's behavior at elevated temperatures is therefore essential for safe and effective structural design. Given these considerations, utilizing FRP composites to strengthen and reuse heat-exposed structures offers a logical, economical, and engineered solution. However, research on strengthening structures subjected to high temperatures is limited. Existing studies primarily use the conventional EBR method, with no research to date investigating the effects of grooving on structures exposed to high temperatures. Additionally, understanding the bond behavior between FRP composites and concrete is crucial for optimizing the capacity of FRP systems, which can be eva‎luated through direct shear testing. This study comprehensively investigates the bond behavior between FRP composites and concrete exposed to elevated temperatures. A total of 55 concrete prism specimens (150×150×350 mm3), 50 cylindrical specimens (200 mm height, 100 mm diameter), and 39 flexural specimens (50×50×350 mm3) were prepared and cured in water for 28 days. The specimens were subjected to thermal conditions in accordance with ASTM-E119 and ISO 834-1 standards and then strengthened using both EBR and externally bonded reinforcement on grooves (EBROG) methods. In the specimens strengthened using the EBROG method, two groove classifications were used: 20@10×10 mm2 and 15@5×5 mm2. In this study, direct shear tests were conducted to investigate the bond behavior of the FRP composite to concrete. The results of the direct shear test showed that increasing the temperature leads to an increase in the bond capacity between the FRP composite and the concrete. Specifically, in specimens strengthened immediately after being removed from the furnace and tested under direct shear, the bond capacity of a specimen exposed to 400°C increased by up to 30% compared to the control specimen. A comparison of bond capacities among the specimens in this study revealed that the grooving method increases the bond capacity of FRP composites by up to 50% compared to the EBR method. Furthermore, the results of the compressive strength test indicated that increasing the temperature causes a reduction in compressive strength, with the compressive strength decreasing by 80% at 600°C.

داود مستوفي نژاد , عليرضا سلجوقيان

محمدرضا افتخار , آلاء ترابيان اصفهاني