The paper describes the evaluation of the actual conditions, the repairing design criteria and procedures, and the final validation tests carried out on a severely damaged prestressed bridge in southern Italy. In the first phases several release tests have been performed on damaged and, for comparison, on sound beams, with the aim of evaluating the residual prestressing stress acting both on concrete and on tendons. In particular, the tests on concrete have been carried out by extracting cores which have been previously instrumented with strain-gages; in a similar way, the strands were tested by gluing a micro strain-gage on a singular wire of a strand and the cutting the wire itself. The test results were used for optimising the design of an adjunctive prestressing system, consisting in 6 external unbounded cables and in a continuous FRC reinforcing of the bottom wing. The tensioning procedure of the external cables of each beam was then monitored by means of 12 strain-gages installed on concrete at mid span and at the two support sections, and by measuring the vertical deflection using laser optical levels. The validation of the repairing works was finally performed by a static load acceptance test, performed using the same apparatus previously adopted for the tensioning monitoring.