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Internal prestressing

Tools and methods for diagnosing prestressed structures using pre- or post-tensioning

Since the 1950s, internal prestressing has become widely used, and many structures now require evaluation of the condition and residual tension of their internal prestressing cables. Over time, systems have evolved, leading to a wide variety of designs and materials that must be assessed with precision.

With more than 20 years of experience in this field, Sixense has unparalleled knowledge of global prestressing systems and the specialised tools needed for accurate cable diagnosis. Our expertise ensures reliable assessment of cable integrity, tension levels and potential deterioration.

Our specialists support asset owners and engineering teams in developing tailored monitoring programmes for internal tendons. This approach enables the early detection of hidden defects and ensures the safe, long‑term management of prestressed structures.

Our auscultation methods for assessing injection quality and cable condition

Gammagraphy

Evaluation of the filling level of post-tensioning grout in ducts

Gammagraphy is a non‑destructive imaging technique that uses gamma rays from a radioactive source—such as cobalt or iridium—to examine the internal condition of prestressing cables. By analysing the resulting radiographic films, engineers can identify injection voids and detect broken wires or strands. Performing a gammagraphic survey before detailed diagnosis helps target subsequent investigations more effectively.

Because this method involves regulated radioactive sources, Sixense partners with authorised specialists to safely conduct the inspections and interpret the results.

Recognition windows and video endoscope examination

Visual inspection of the injection quality and condition of the cable

In order to diagnose internal prestressing, it is necessary to open inspection windows in order to access the prestressing cables and their protective coating, so that they can be examined, tested or sampled for laboratory analysis. If a void is found, an additional video endoscopy examination may be carried out to assess its size and the condition of the cable beyond the window.

In-Tense Crossbow test

Measurement of residual tension in prestressing units

The Crossbow method is based on the principle that the greater the tension in the reinforcement, the greater the force (F) required to deflect it from its path by a given amount (w). Residual tension in the prestressing reinforcement can be obtained by comparing the on-site measurement curve (w = f(F)) with a calibration curve specific to the reinforcement type tested in the laboratory. In-Tense crossbows are regularly calibrated and therefore accompanied by a calibration report. They can be used to test almost all prestressing units that have been used for post-tensioning and pre-tensioning since the early 1950s.

Curvature measurement

Measurement of curvature variation under stress

This semi-global method involves monitoring how the curvature of a structure changes under controlled loading or during operation. It is governed by LPC guide no. 82. When combined with extensometric measurements, it can be used to determine Navier deformation.

Its main applications include detecting damage to a section and determining the decompression moment; evaluating the available inertia of critical sections; controlling the effectiveness of additional prestressing during implementation; long-term monitoring of structures; and verifying the actual position of the neutral axis.

Slotstress

Effective residual stress measurement in concrete structures

Slotstress analysis provides insight into the distribution of loads within a structure, enabling verification of its structural strength and the prospects for long-term changes in service loads. During its service life, the forces and stresses within a structure can change due to variations in loads and the structure’s ageing. These stresses may then become incompatible with the use criteria or pose a risk to the structure’s safety.
Slotstress therefore enables engineers to implement optimised maintenance or reinforcement strategies.

Our instrumentation methods for detecting the occurrence of wire or strand breaks

EverSense® Acoustics

Detecting breaks in internal prestressing strands using acoustic method

When a wire or strand in an internal prestressing cable breaks, it emits a shock wave that propagates through the surrounding concrete. Acoustic monitoring involves detecting this wave using a network of sensors positioned on the surface of the prestressed wall or beam. The sensor grid is designed according to the layout of the prestressing and allows the origin of the break to be located by triangulation.