Reliability-based evaluation of the prestress level in concrete containments with unbonded tendons

It has been a common practice both in Sweden and worldwide to enclose nuclear reactors with prestressed concrete structures. The prestress level decreases with time from its initial value due to various degradation mechanisms. To ensure that the prestress level is sufficient the tendon force is measured at regular in-service inspections. The intention with this paper is to present a reliability-based procedure to evaluate the prestress level on the basis of data from in-service inspections. Existing approaches to evaluate the prestress level do not take into account the variability in the measured prestress. It is not possible to achieve a complete assurance concerning the prestress level. However, by using a probabilistic model, involving the variability in the measuring result and the structural behaviour, the prestress level could be confirmed in a more stringent way. Both the time dependent loss of prestress and the possibility of tendons being broken (due to defects as corrosion) are considered. To avoid through-wall cracks in the concrete it is required that the prestress shall counterbalances the tensile stresses expected at an internal accident. The factor of interest is the prestress level in the concrete and not the force in individual tendons. Several tendons influence the prestress level in a specific part of the containment. The required prestress level shall be fulfilled in all parts of the containment where each part is influenced by a number of individual tendons. It is suggested in this paper that this problem can be analysed as a structural reliability problem idealized as a series of correlated parallel subsystems. (C) 2006 Elsevier Ltd. All rights reserved.