Effect of Sustained Load Level on Long-Term Deflections in GFRP and Steel-Reinforced Concrete Beams

Abstract

ACI 440.1R [1] applies an adjustment factor of 0.6 to the long-term deflection multiplier for steel-reinforced beams to reflect the experimentally observed differences between FRP and steel-reinforced members. The objective of this study is to evaluate the effect of the level of sustained load on the long-term multiplier in GFRP-reinforced beams. Five beams, including four GFRP-reinforced beams and one steel-reinforced control beam, were tested in four-point bending on a simply supported span with different temporary service loads (Ma = 1.80 Mcr–2.36 Mcr) and sustained load (Msus = 0.58 Ma–0.85 Ma) levels. Mid-span deflections were recorded twice weekly over a period of 100 days, and the long-term deflection multiplier, λΔ, was plotted vs. time. The results indicate that the temporary service load level, rather than the sustained load level, affects the long-term deflection multiplier with larger temporary service loads causing a smaller long-term multiplier. The current 0.6 multiplier on the time factor for FRP-reinforced beams also appears to underestimate long-term deflections. In addition, the multiplier to capture the effects of GFRP vs. steel-reinforcement may not be a constant value.