Prestressing in nuclear power plants anchor lift-off measuring system for 37 T 15 tendons

The Asco Nuclear Power Plant is composed of two 930 MW PWR units. The external wall of the Containment Building in each unit is made of prestressed concrete. The Pre-stressing System, a Freyssinet technology using 37 T 15 tendons, is of the non-adhering type (sheathing is filled with grease). This technique allows to verify the design conditions of the prestressing design directly through the tendons by means of periodical inspections carried out during the whole life of the structure. These inspections take place three times in the first five years and afterwards, once every five years. One of the objectives of these inspections is to measure the prestressing force that the type 37 T 15 tendons apply on the structure. The measurement of the force is made by the same KC 1000 jack used in the pre-tensioning with a capacity of 1000 ton which acts on the tendon anchorage, increasing the outside tendon tension progressively up to reaching the block anchorage lift-off from its support plate. The lift-off force applied by the jack is the internal tension developed by the tendon. The problem is to determine the exact moment when the tendon anchorage block 'lift-off' takes place. In the 1980's decade, the anchorage block lift-off was determined by inserting thin shims. The insertion of one of these shims between the support plate and the anchorage block indicated the separation. Simultaneously, the necessary force applied by the jack to overcome the tendon compressive action on the structure was registered. This force, extrapolated to a zero separation, was assumed to be acting on the anchorage. This procedure had the inconvenient of a lack of accuracy in determining the physical phenomena. The system used at the present to measure the lift-off force consists basically in taking continuous elongation and jacking force data by means of a computerized system. The present work describes this new system, resulting from 15 years of experience in measuring the lift-off forces in the prestressing system