Reinforced concrete (RC) structures may require strengthening for the various reasons. Considering the advantages of Fiber Reinforced Polymers (FRP) composites, one common method of strengthening RC elements is through FRP strengthening. One of the main obstacles to utilize a significant portion of FRP composites’ capacity in strengthening systems is the premature debonding of FRP sheets from the concrete surface. Therefore, it is necessary to present effective methods to prevent premature debonding of FRP from the concrete surface, such as the grooving method. This research includes a series of experimental and analytical studies on the behavior of FRP-strengthened reinforced concrete members under torsion. In the experimental phase, reinforced concrete specimens were constructed and strengthened using FRP composites in two groups. The influence of surface preparation methods (EBR and EBROG methods) and the installation of FRP anchors as a part of the strengthening systems were examined in the specimens of the first group. In the specimens of the second group, the effect of surface preparation methods (EBR and EBROG methods), the installation of FRP anchors in contact-critical strengthening systems were investigated. Digital Image Correlation (DIC) and torque-twist method were used to analyze the displacement and strain fields of the specimens. The failure mode and torque-angle curves in the strengthened specimens were also compared. In the analytical phase of this research, two analytical models, RA-STMT-FRP and CA-STM-FRP, were prepared to analyze the behavior of FRP-strengthened reinforced concrete members under pure torsion and the combination of loads, including torsion. The STMT-FRP design model was also presented for the strengthening of reinforced concrete members under torsion with FRP sheets, with the capability of accurately determining the section’s failure mode. The results obtained from the experiments on the reinforced concrete specimens showed that by changing the FRP installation method from EBR to EBROG in contact-critical strengthening systems, the FRP-to-concrete joint became stronger, and stresses were transferred to deeper concrete layers. The section’s failure mode also changed from premature debonding of FRP to longitudinal FRP rupture, resulting in a 54% and 25% increase in the torsional cracking and torsional capacity of the section, respectively. The test results of the first group indicated that simultaneous installation of transverse FRP strips using the EBROG method and FRP fans significantly increases the torsional capacity, torsional ductility, and deformability of the section. The test results of the second group also showed that by installing FRP fans in the section’s web, the torsional capacity and torsional ductility increased by 39% and 40%, respectively, compared to the control specimen. In the bond-critical strengthening systems, the installation of FRP fans changed the system’s behavior to contact-critical mode, resulting in a 63% increase in torsional ductility and a 43% increase in torsional capacity compared to the control specimen. In the specimens strengthened with complete wrapping patterns, the installation of FRP fans significantly increased the system’s deformability. To eva‎luate the accuracy of the analytical models, a set of experimental specimens from the previous studies was collected, and the analytical and experimental results related to these specimens were compared. The comparison of the torque-angle curve obtained from the CA-STM-FRP analytical model with the experimental results showed that the calculated torsional capacity by this analytical model had an average ratio of 81.0% to the reported torsional capacity from the laboratory studies for 13 specimens, with a standard deviation of 6%, indicating the reasonable accuracy of these two analytical models.

داود مستوفي نژاد , محمدرضا افتخار

عليرضا سلجوقيان , ابوالفضل اسلامي حسن‌آبادي