Fire Resistance and Post-Fire Bond Integrity of Fabric-Reinforced Cementitious Matrix (FRCM) versus Epoxy-Bonded CFRP Strengthening Systems on RC Members: Experimental Assessment and Residual Capacity Quantification

Abstract

Fire safety is the most critical unresolved durability challenge for externally bonded FRP strengthening in buildings. The epoxy polymer matrices of conventional CFRP systems soften catastrophically above their glass transition temperature (Tg ≈ 55–70°C), causing complete bond loss and loss of composite action at temperatures routinely reached in building fires within the first 10–15 minutes of fire exposure. Fabric-Reinforced Cementitious Matrix (FRCM) systems, which substitute organic epoxy with inorganic cementitious mortar, offer a fundamentally different and potentially transformative solution to this problem. This paper presents a comprehensive experimental investigation comparing the fire resistance, post-fire residual bond strength, and structural performance of FRCM (carbon textile in hydraulic lime mortar) versus epoxy-bonded externally bonded CFRP (EBR) and near-surface mounted CFRP (NSM) on reinforced concrete members under four fire exposure conditions. Sixty pull-off bond specimens and thirty RC beams (150 × 250 × 2000 mm, fck = 30 MPa) were subjected to four conditioning protocols: ambient (23°C control), moisture immersion (40°C, 1000 h), thermal cycling (−20°C to +60°C, 100 cycles), and furnace fire exposure at four temperature levels (100°C, 200°C, 300°C, 400°C for 30-minute durations). Post-conditioning pull-off bond strength and residual flexural capacity were measured and analyzed against the environmental reduction factors CE of ACI 440.2R-17. Results demonstrate that EBR epoxy bond strength degrades catastrophically above 150°C (Tg threshold), retaining only 21% of ambient strength at 300°C and 8% at 400°C. NSM retains 38% at 300°C due to partial thermal protection by the concrete cover. FRCM retains 87% at 300°C and 74% at 400°C, confirming the fundamental thermal superiority of cementitious matrices. The implied fire-exposure environmental reduction factor CE,fire for epoxy EBR at 300°C is 0.21, compared to ACI 440.2R-17's most severe ambient exposure CE = 0.85 a critical and currently un-codified discrepancy of factor 4. Residual flexural capacity of fire-exposed beams mirrors the bond retention trends. FRCM beams exposed to 300°C retain 84% of ambient strengthened capacity versus only 23% for EBR beams. An empirical bond degradation model is proposed for all three systems as a function of exposure temperature, and revised post-fire CE factors are recommended for incorporation into ACI 440.2R-17.