Analysis of prestressing in old full-scale concrete members

Abstract

The effective prestressing force is a key parameter that affects the performance of prestressed concrete members (PCM). In fact, the accurate determination of the residual prestressing force is crucial to assess existing PCM since it influences load-carrying capacities, in-service structural properties and remaining service life. The aging of structures, with limited functional capacity, implies a major problem that must be addressed without delay. Progress in the promotion of innovative solutions to extend the service life of structures depends on: (a) the availability of reliable indirect methods, since old PCM were usually no instrumented during casting; (b) the formulation of adequate diagnostics of the residual stress–strain state of structures; and (c) the availability of highly refined predictive tools. The implementation of these elements in the management and maintenance systems of PCM will serve to support decision making associated with structural assessment and to gather information to establish criteria for the design of more resilient PCM.Among the destructive methods available to empirically assess the actual condition of PCM, it can be found: crack initiation, crack re-opening and tendon cutting. As destructive methods inevitably cause structural damage and are not suitable for application to in-service structures, non-destructive methods or with conditions that only require aesthetic restitution (e.g. exposed tendon deformation, hole-drilling and saw-cuts) appear as an alternative of increasing interest.As a first step to obtain feed-back from several experimental tests carried out on unmonitored full-scale PCM over the years, this paper examines different techniques and methods used to evaluate the residual prestressing force, and also presents the deviations and uncertainties regarding predictions of short- and long-term prestress losses from main code models used in design. It is concluded that it is interesting to follow a non-destructive indirect testing methodology to be able to keep in service and monitor the current structures. This would make it possible to draw up a maintenance plan based on the condition of the PCM.