External post-tensioning for flexural strengthening of prestressed concrete girders with internal bonded tendons: experimental investigation and analytical interpretation

An increasing number of existing bridges are approaching―or have already exceeded―their design service life, calling for retrofit interventions to increase load-bearing capacity against magnified traffic loads and to meet recent requirements such as robustness and resilience. In the case of prestressed concrete (PC) bridges with low residual prestress (e.g., due to long-term prestress losses, creep, shrinkage, corrosion, or damage from service overloading), the deck can be characterised by higher deformability and cracking, producing higher likelihood of internal reinforcement’s corrosion and lower expected residual service life. Even if a number of passive strengthening techniques have been developed in the last decades, external post-tensioning (EPT) deserves higher consideration due to beneficial effects in terms of internal stress variation and upward deflection. This paper addresses the interaction between internal bonded and external unbonded tendons through experimental investigation and analytical study of the flexural bevahior of four 1:5 reduced scale PC bridge girders. Experimental results show that EPT girders returned comparable cracking load and a peak load approximately 45% higher than that related to the unstrengthened configurations with different internal prestress level. Considering contributions from both internal and external tendons, an in-depth sectional analysis was developed for the interpretation of the experimental response and a sensitivity analysis of the peak bending moment, confirming the EPT effectiveness to improve the structural performance of defective PC girders. When compared with experimental results, available code formulations for prediction of the EPT stress increase are affected by limited accuracy, hence requiring further studies to account for internal prestress.