A residual strength model for deteriorating post-tensioned concrete bridges

Inspection of UK’s entire stock of post-tensioned concrete bridges of the internally grouted duct type has revealed a variety of defects. These include the presence of grout voids in incompletely grouted ducts, and chloride-induced tendon corrosion ranging from slight surface pitting to extensive localised corrosion and fractures. These defects may have a detrimental effect on the load carrying capacity of such bridges but, to date, there have been only limited attempts to provide satisfactory analytical methods to assess their residual strength. The lack of appropriate analytical tools may lead to unnecessary strengthening and repair, or even costly replacement. This paper describes the development of an analytical model to assess the residual strength of deteriorating post-tensioned bridges. The model is a useful tool in the investigation of the effects of tendon failure and regions of grout voids. A non-linear method of analysis is proposed, based on the concept of strain compatibility, but suitably modified to accommodate the behaviour of unbonded tendons in regions of incomplete grouting in the ducts. Also incorporated in the model is the contribution of failed tendons after re-anchoring into the grout. This uses the established phenomenon of re-anchoring to estimate the distribution of residual prestress after tendon failure, where both the distribution of grout voids along the beam and the quality of the grout are considered. The provision of such an analytical tool should be of considerable value to bridge engineers involved in the assessment of corrosion-damaged post-tensioned bridges, and will assist in the development of strategies for extending the lives of such structures through repair and strengthening.