The Long Term In-service Performance of Corrosion Protection to Prestressing Tendons in AGR Prestressed Concrete Pressure Vessels

The structural performance of prestressed concrete pressure vessels (PCPVs) and prestressed concrete containment vessels (PCCVs) is dependent on the maintenance of prestress levels within the concrete. In post-tensioned un-grouted systems the condition and performance of the tendons and the anchorage system at the end of the tendons is of prime importance. The tendons and anchorage components are normally protected from corrosion by a layer of thick viscous grease or wax which is applied at the time of installation. Little work has been done previously to investigate the performance of the corrosion protection medium for prestressing tendons in AGR (Advanced Gas-cooled Reactor) PCPVs in service. This paper reports the results of investigations carried out on samples of grease collected as tendons were withdrawn for examination under routine maintenance, inspection and test programmes on the PCPVs at the Torness and Hunterston B Nuclear Power Plants. The paper briefly describes the constituents of the grease and factors affecting its in-service performance as a corrosion protection medium in its long term operational environment. The method of testing is outlined, being based on the requirements of ASME XI but with modifications to allow the processing of relatively small test samples. The effect of ageing is considered and the changes in the material properties and ability to provide corrosion protection are described. The paper concludes that, although the grease has lost some of its lighter oily constituents with time, it is still capable of providing corrosion protection to the prestressing tendons