The performance of near-surface mounted CFRP strengthened RC beam in fire

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Carbon fiber reinforced polymers (CFRP) have been widely used in the retrofitting and strengthening of concrete structures. However, the layouts of CFRP in externally bonded (EB) strengthening system and near surface mounted (NSM) system result in the vulnerability of fire resistance. To explore the fire resistance potential of NSM-CFRP strengthening system, 15 RC beams strengthened with CFRP and 2 reference beams were loaded to failure either under the ISO834 standard fire or at ambient temperature. In test, the following factors were considered: the strengthening method (near surface mounted technique/externally bonded technique), adhesive (epoxy and magnesium-oxychloride cement), fire protection materials (thin intumescent fire retardant coating and thick fire retardant coating) and location of fire protection (local patch protection/single-sided protection/U-shaped protection). With appropriate fire protection, the RC beams strengthened with NSM-CFRP resisted the standard fire for more than 3h at high load level. Besides the effects of the aforementioned factors, the friction stress at the CFRP/matrix interface when CFRP is in globally slipping is found playing a crucial role for the fire resistance of NSM-CFRP strengthening system. The experimental observation sheds some light on how the NSM-CFRP strips can retain the contribution at elevated temperatures and why NSM-CFRP strengthening outperforms EB-CFRP strengthening when exposed to fire.