Strengthening Efficacy of External FRP Laminates on Aged Prestressed Beams with Unbonded Strands

As prestressed concrete (PC) structures age, long-term effects, e.g., creep, shrinkage, and prestress losses, compromise their structural performance. Strengthening these aged PC beams has become a crucial matter. One effective solution is to use externally bonded fiber-reinforced polymer (FRP) sheets; however, limited research has been done on aged PC beams using the FRP, especially for beams with unbonded prestressing strands (UPC beams). Therefore, this research investigates the fiexural strengthening efficacy of external FRP sheets on aged UPC beams with unbonded tendons. Aging minimally affected the failure modes of UPC beams, with nonstrengthened beams showing ffexural failure via rebar yielding and concrete crushing, and FRP-strengthened beams failing due to FRP debonding and tensile reinforcement yielding, though tendons in the aged beams did not yield due to prestress losses, unlike the new beams. The U-wrap anchor curbed widespread debonding, leading to tensile reinforcement yielding and FRP rupture. Aging hastened crack growth and stiffness loss, increasing deffections and reducing load resistance, but FRP reinforcement mitigated these effects, enhancing cracking resistance by 14% over the unstrengthened aged beams and 7% over the new beams while boosting ultimate resistance by 9% above the non-strengthened new beams. Compared to the new FRP-strengthened beams, the aged counterparts had lower cracking resistance, stiffness and capacity—showing 20% higher deffections, 7–9% lowerserviceability loads, 7%–17% reduced ultimate strength and 17% less deformability—due to prestress losses and premature FRP debonding.