Internal Prestressing

Since the 1950s, the use of internal prestressing has become widespread across major infrastructure projects. As prestressing systems have evolved, many structures now require detailed assessment to understand the condition and residual tension levels of their internal prestressing cables.

With more than 20 years of dedicated experience, Sixense has developed unmatched expertise in the full range of prestressing systems used worldwide, along with the specialised tools needed for accurate cable diagnosis.

Our specialists support you in designing and implementing tailored monitoring programmes that reveal hidden defects, verify cable performance, and ensure the safe, long‑term management of your prestressed structures.

Our Auscultation Methods for Assessing Injection Quality and Cable Condition

Gammagraphy

Evaluation of grout filling in post‑tensioning ducts

Gammagraphy is a non‑destructive imaging technique widely used in the assessment of internal prestressing systems. It uses gamma rays emitted by a controlled radioactive source such as cobalt or iridium. By interpreting the resulting radiographic films, engineers can identify injection voids, poorly filled ducts, and even broken prestressing strands. Conducting a gammagraphic survey early in the diagnostic process allows investigations to be precisely targeted, reducing unnecessary openings and interventions.

Because this method involves regulated radioactive sources, Sixense works exclusively with authorized specialist partners to ensure safe, compliant execution and expert interpretation of results.

Recognition Windows & Video Endoscope Examination

Direct visual inspection of injection quality and cable condition

To accurately diagnose internal prestressing systems, inspection windows are opened to access the cables and their protective sheaths. This allows engineers to visually assess the condition of the grout, the presence of voids, and the state of the steel strands. Samples can also be taken for laboratory analysis when required.

If a void is detected, video endoscopy can be used to explore the duct further, providing detailed insight into the extent of the defect and the condition of the cable beyond the inspection window. This combination of techniques ensures a precise understanding of localized issues.

In‑Tense Crossbow Test

Measurement of residual tension in prestressing units

The Crossbow method measures the residual tension in reinforcement used for internal prestressing by analyzing how much force (F) is required to laterally deflect the cable by a known amount (w). The on‑site measurement curve is compared with a laboratory‑derived calibration curve specific to the reinforcement type, enabling accurate determination of remaining tension.

In‑Tense crossbows are regularly calibrated and supplied with a calibration certificate, ensuring reliable and traceable results. They are compatible with nearly all internal prestressing systems used worldwide since the early 1950s, making them a versatile tool for assessing aging infrastructure.

Curvature Measurement

Monitoring curvature variation under stress

This semi‑global method tracks how a structure’s curvature evolves under controlled loading or during normal operation. Governed by LPC Guide No. 82, curvature measurement—especially when combined with extensometric data—can be used to determine Navier deformation and assess structural behavior.

Key applications include:

Detecting section damage and identifying decompression moments
Evaluating the inertia of critical sections
Verifying the effectiveness of additional prestressing
Long‑term structural monitoring
Determining the actual position of the neutral axis

Slotstress

Residual stress measurement in concrete structures

Slotstress analysis reveals how loads are distributed within a concrete structure, helping engineers verify its current strength and anticipate how service loads may evolve over time. This method complements other internal prestressing assessment techniques by providing insight into long‑term stress redistribution as structures age or experience changing loads.

Our Instrumentation Methods for Detecting Wire or Strand Breaks

EverSense® Acoustics

Acoustic detection of prestressing strand breaks

When a wire or strand inside an internal prestressing cable fails, it releases a shock wave that travels through the surrounding concrete. EverSense® Acoustics uses a network of surface‑mounted sensors to detect this wave in real time.

The sensor grid is designed according to the prestressing layout, allowing engineers to pinpoint the exact location of the break through triangulation. This early‑warning capability is essential for managing risk in aging internal prestressing systems and preventing progressive failures.