Safe de-tensioning of existing PT tendons

In recent years we have put more and more effort into the inspections and investigations of both cable-stays and post-tensioning (PT) tendons. The aim there is to determine the risk of corrosion and to find voids and broken wires of strands.

When you find deficiencies or high risks, the next topic to discuss with the client is to replace existing PT tendons. These discussions then lead to many items to consider such as structural assessment of the structure, additional PT, traffic loading and sequence, type of new tendons to add etcetera. The subject that I want to focus on in this article is how to safely de-tension tendons in existing structures.

De-tensioning existing tendons is actually a big challenge, as often tendons are not designed such that they can be de-tensioned with a hydraulic jack at the anchorages. The strand ends are typically cut behind the anchorages after the tendons have been stressed. You must realise that individual tendons hold very large forces. A typical 27 strands tendon is stressed at a force of 5.500 kN, which is equal to 550 Tons, or the weight of an Airbus A380 at take-off. The energy that is stored into one tendon is dependent of the length of the tendon, so the longer the tendon the larger the stored energy. Removing such tendon safely needs to be carefully studied.

We have developed two ways to safely de-tension tendons:

1.      Controlled de-tensioning by means of clamps.

This method is using a clamp around the tendon that picks up the force of the tendon. By using two clamps that are connected by bars, the tendon can be cut in the middle. The force is then released in a controlled manner by releasing the bars with jacks. The initial extension of the tendon is released by the stroke of the jacks. This method has been tested successfully in a real size mock-up and will soon be applied in a bridge. The challenge lies in designing a clamp that is safe and easy to install. We have found an innovative solution to do this.

2.      Controlled de-tensioning by cutting strand-by-strand in windows

This method is based on releasing energy of the tendon by cutting strand by strand in opening windows, which transfer the force of the tendon by the grout adherence to the remaining strands. These strands will plastify and undergo a large extension. This extension in a window will cause energy to be released. To gain confidence in this method we have modelled this behaviour in a computer model. For this we have partnered with a spin-off from a French university. They are specialized in analysis of car and plane collisions. One can image that this type of analysis is highly non-linear and covers special material laws, high speed dynamic behaviour and large displacements. To model the energy release strand by strand of a PT tendon similar analysis is required. After achieving the required de-tensioning behaviour in the computer analysis, we have calibrated the model with a mock-up de-tensioning. We are now confident that we can model any existing situation to show by analysis the safe method of de-tensioning.

A lot of work has gone into determining safe methods of de-tensioning existing tendons and we can safely advise to not try this at home. We will share more details in future, but feel free to contact me if you have any questions.