Material properties and structure of the wires and tendons change over time in a post tensioned design, and it is important to investigate the long-term performance. Since the beginning of the 1980s, Vattenfall Research and Development AB has performed laboratory testing of post tensioning wires, which has been removed from the containments of the reactors at Forsmark and Ringhals.
Additionally, a literature Review was carried out and together with on-site measurements of remaining tensioning force an analysis was made about what is to be expected in the future. Very little can be found in the literature concerning long term properties but by using data from previous measurements of the mechanical properties, it is found that the elongation limit of some of the tendons is decreasing.
When re-tensioning is carried out it may be a good idea to put spacings only on those tendons on which a low tensioning force has been measured. Over-tensioning may plasticise the wire and thereby impair the relaxation properties.
