PSI - Issue 78

Teklewoin Haile Fitwi et al. / Procedia Structural Integrity 78 (2026) 1775–1782

1776

1. Introduction The seismic vulnerability of existing reinforced concrete structures, particularly those constructed before the implementation of modern seismic design provisions, represents a significant concern. The vulnerability is especially pronounced in exterior beam-column joints, which play a crucial role in maintaining structural integrity during seismic events (Hertanto, 2005; Kiran and Genesio, 2014). The absence of adequate transverse reinforcement within the joint panel region, a common deficiency in existing pre-code RC structures, often results in brittle joint shear failure mechanisms that can cause progressive structural collapse. In response to this critical vulnerability, extensive research has focused on developing effective strengthening techniques, with initial efforts concentrating primarily on Fiber Reinforced Polymer (FRP) composites. Several studies have demonstrated the potential of FRP-based retrofitting approaches. It is worth mentioning the work by Antonopoulos and Triantafillou (2003) who tested 18 exterior RC joints and revealed the critical importance of mechanical anchorages in preventing premature debonding, while highlighting the influence of internal steel reinforcement and pre-existing damage on retrofit effectiveness. Del Vecchio et al. (2014) similarly demonstrated through testing of six full-scale RC corner joints that externally bonded FRP systems provide an effective strengthening solution to retrofit poorly detailed beam-column joints. Recently, the development of Fabric Reinforced Cementitious Matrix (FRCM) composites, which offer several advantages over conventional FRP systems, including superior fire resistance, compatibility with concrete substrates, and the ability to be applied in harsh environmental conditions, has emerged as a promising alternative for beam column joint retrofitting. Faleschini et al. (2019) experimentally investigated the effectiveness of FRP and FRCM composites to repair seismically damaged beam-column joints while Al-Salloum et al. (2011) compared the ability of the two composites to strengthen under-designed beam-column joints. Both studies showed that shear strength enhancement from FRCM retrofitted elements is comparable to that of FRPs-upgraded counterparts. While initial experimental investigations have demonstrated the potential of FRCM systems, the limited number of studies and the variability in testing configurations highlight the need for further comprehensive experimental validation to fully understand their effectiveness in joint strengthening applications. In this context, this study addresses the highlighted research gap by investigating through a full-scale experimental program the implementation of FRCM strengthening systems for seismically deficient, exterior beam-column joints. The performance of carbon and basalt FRCM systems is examined, focusing on their ability to enhance joint shear capacity, modify failure mechanisms, and improve energy dissipation characteristics. The investigation employs full scale specimens designed to represent typical existing Italian construction subjected to loading protocols that simulate seismic demands. 2. Material characterization This study presents the results of three investigated scenarios. One un-strengthened control specimen (specimen 1), tested to complete failure to establish baseline performance characteristics and two specimens (specimen 2 and 3) subjected to a damage-strengthening-retest protocol. Initially they were loaded up to 85% of their ultimate capacity to simulate seismic damage conditions, then unloaded and retrofitted. Retrofitted specimens were re-named as Specimen 2F (carbon FRCM-strengthened) and Specimen 3F (basalt FRCM-strengthened). After a curing period of 28 days retrofitted specimens were tested to failure to evaluate the effectiveness of their respective strengthening systems. Due to their real-scale dimensions specimens were cast using different concrete batches which results also in different concrete mechanical properties. Specimen 1 has a mean compressive strength of 16.30 MPa, tensile strength of 1.90 MPa, and elastic modulus of 22 GPa. Specimen 2, attained slightly higher values with 18.62 MPa compressive strength and 1.99 MPa tensile strength, while maintaining similar elastic properties. Specimen 3 has 35.40 MPa compressive strength, 3.40 MPa tensile strength, and an increased elastic modulus of 33 GPa. Concrete mechanical properties were obtained by tests on standard cylindrical specimens ( 150 x 300 mm ). Moreover, B450C-grade steel was used for the reinforcement, with average properties: ϕ8 mm stirrups have yield strength and ultimate strengths of 658.7 MPa and 674.7 MPa with 2.7% elongation; ϕ16 mm column bars exhibited 508 MPa of and 610 MPa of with 10.8% elongation; and ϕ20 mm beam bars exhibited 532.7 MPa of and 624.3 MPa of with