On 25 September 1992 the Department of Transport (DoT) issued a press notice stating that it would not be commissioning any new grouted duct post-tensioned bridges in England. The decision was taken due to fears that existing grouted duct post-tensioned concrete bridges were badly corroded and could be in a state of imminent collapse. The press notice also announced that existing grouted duct post-tensioned bridges were to undergo detailed inspection. Non-destructive techniques needed to be developed to allow detailed investigation of these structures. The results of these investigations had to be accurate to a high level of confidence as decisions on repair, renovation or destruction would be made on the findings of the investigation. This thesis will give the reasons for the DoT's decision followed by an overview of possible non-destructive techniques available at the time of issue. The main body of work carried out investigates the use of the Sonic Impact-Echo method of non-destructive testing. This involves the development of suitable testing equipment and preliminary laboratory and field investigation. Detailed numerical simulations were carried out using the Finite Element Method in order to quantify the probable limits of the Sonic Impact-Echo method. Final laboratory investigations were carried out on a model with known defects. Detailed field testing was carried out on test beams manufactured by the Transport Research Laboratory in Crowthorne and by TBV Stanger at Elstree.