Bond behaviour of GFRP bar in magnesium potassium phosphate cement mortar: effect of magnesia-to-phosphate ratio

Replacing reinforcing steel with glass fibre reinforced polymer (GFRP) bar has been increasingly recognised as an effective approach to addressing steel corrosion in marine structures. Since the highly alkaline environment of Portland cement (PC) may affect the long-term performance of GFRP bars, magnesium potassium phosphate cement (MKPC) has been considered a potential alternative,owing to its relatively low alkalinity and rapid early strength development.This study investigated the influence of the magnesia-to-phosphate (M/P)ratio,ranging from 1 to 13,on the bond behaviour of GFRP bars embedded in MKPC mortar at 7 days. The bond performance, including apparent transition stress, bond strength and cumulative bond energy, was characterised through pull-out tests.The results showed that these bond parameters generally increased and then decreased with increasing M/P ratio, following a trend closely associated with the corresponding variation in matrix compressive strength. The optimal performance was achieved at an M/P ratio of 9, which produced the highest bond strength and apparent transition stress of approximately 19.15 MPa and 9 MPa, respectively, together with the highest cumulative bond energy of approximately 241.3 Nmm/mm2 at a prescribed slip limit of 15 mm. The associated failure was mainly dominated by debonding at the resin-fibre interface, with partial peeling of resin and the fracture of ribs.Within the selected datasets,the MKPC-GFRP system showed higher bond strength than the PC-GFRP system at comparable matrix compressive strengths.