Experimental Investigation of the Flexural Behavior of Preloaded and Precracked Unbonded Post-Tensioned I-Section Concrete Beams Strengthened with CFRP

This study presents an experimental investigation of the flexural response of preloaded, unbonded post-tensioned (UPT) I-section concrete beams strengthened with externally bonded carbon fibre-reinforced polymer (CFRP) composites. Seven beams with identical geometries and reinforcement configurations were tested under monotonic four-point bending, including one unstrengthened control beam. The remaining six beams were either strengthened in an undamaged condition or preloaded to 40% and 60% of the ultimate load of the control beam before strengthening. Two CFRP strengthening schemes were examined: Scheme 1, consisting of a longitudinal CFRP laminate, and Scheme 2, comprising the same laminate supplemented with U-shaped CFRP sheets added at the beam ends and midspan for enhanced anchorage. The results demonstrated that CFRP strengthening markedly improved the ultimate load capacity of the undamaged beams, with an increase of approximately 14.31% and 19.99% for S1 and S2, respectively, compared with the control beam. However, preloading-induced damage adversely affected the effectiveness of CFRP strengthening, with the ultimate load capacity decreasing by 3.75% and 7.98% for S1 and by 4.97% and 10.14% for S2 at preloading levels of 40% and 60%, respectively, relative to the corresponding undamaged strengthened beams. Scheme 2 increased the ultimate load capacity by 19.99%, 14.03%, and 7.82% compared with the unstrengthened control beam at preloading levels of 0%, 40%, and 60%, respectively. In general, Scheme 2 achieved ultimate capacities 4.98%, 3.65%, and 2.51% higher than those of Scheme 1 at the corresponding preloading levels. The additional U-shaped CFRP sheets further restrained laminate debonding and contributed to a more stable composite response. The results demonstrate that CFRP strengthening effectively enhances the flexural capacity of preloaded UPT beams; however, its performance depends on the severity of pre-existing flexural damage and the design of the anchorage system.