Structural assessment of unreinforced and Basalt-FRP-reinforced glulam beams following long-term creep testing

The use of reinforcement in timber beams has been shown to enhance their short-term flexural performance; however, their long-term behaviour is more complex, particularly under variable climate conditions. Moreover, the ultimate behaviour of such elements remains insufficiently understood, as very long-term testing is rarely feasible. This study investigates 36 unreinforced and basalt fibre–reinforced polymer (BFRP) reinforced glulam beams subjected to four-point bending creep over a 431.2-week period under constant and variable climate conditions. First, three years of Moisture Content (MC) monitoring of six beams under fully-controlled variable conditions showed that creep behaviour is strongly moisture-driven. Unreinforced beams exhibited rapid early deformation, whereas reinforced beams showed a more gradual response. The coupling between moisture and creep was quantitively evaluated, with higher sensitivity observed in reinforced beams. Creep coefficient analysis over 431.2 weeks indicated that mechano-sorptive creep is 2–3 times higher than viscoelastic creep, while reinforcement consistently reduced deformation. Among eight calibrated models, the 5-Kelvin–Voigt formulation provided the best overall performance and reliable long-term predictions. The predicted values were found to lie between Services Classes SC2 and SC3 for beams under variable climate conditions, and to be close to SC1 for beams in constant climate. Finally, four-point bending destructive testing conducted after the creep phase indicated brittle tension failure in unreinforced beams, while reinforced beams exhibited combined tension and bond-related failures, with increased variability under variable climate conditions. An unexpected increase in the global modulus of elasticity (10–16%) between initial and final tests was observed.