This paper presents an experimental investigation of a 64-year-old, full-scale, simply supported, prestressed bridge girder retrofitted with external posttensioning. The specific retrofit can be used to increase the flexural strength of corrosion-damaged prestressed beams. The test specimen, taken from a bridge located in Virginia, was subjected to monotonically increasing loads until the occurrence of a diagonal compression failure in the vicinity of one of the external tendon anchor blocks. The experimental test was accompanied by finite-element analyses, aimed to elucidate the failure mode and quantify the impact of the repair on the capacity of damaged beams. Simplified calculations based on strut-and-tie models and design code guidelines are found to give satisfactory estimates of the failure load, while also describing the principal aspects of the force transfer between the external tendons and the beam and the stress state in the immediate vicinity of the anchor blocks.