Improve, preserve, secure

External prestressing tendons

Many prestressed concrete structures were built last century with an over-reliance on the durability of reinforced concrete and bonded post-tensioning techniques. These technologies were not then as mature as they are today, in terms of design, quality and care taken in tendons installation (mainly injection).

The first objective of a prestressed structure’s inspection is to assess the risk associated with the operation of the structure, by verifying in particular that the effects of the prestressing are still present and that the condition of the prestressing tendons is satisfactory to ensure the durability of the prestressing over time.

Nowadays, proven solutions for upgrading or modernising the external prestressing tendons exist, which allow a prestressed structure to be brought back to the original level of structural resistance and safety.

Bridge - External post-tensioning

Pathologies encountered

The effects of prestressing are essential to the in-service behavior of a prestressed structure and as such, the external prestressing tendons are a major issue for the owner. The case of bridge decks made of prefabricated segments assembled by external prestressing is a striking example.

Inspections carried out on prestressed structures generally reveal the same types of defects, which are mainly voids in the cement grout filling the post-tensioning ducts and anchorage caps, water / moisture penetration or contamination by chlorides. The corrosion of the steel tensile elements ranges from surface oxidation to wire then strand failure, including stress corrosion in older generation prestressing steels.

Most likely at high-points of the tendon layout.

Bleed water accumulating during grout curing.

Poor detailing of joints, waterproofing or anchorage cappings.

Loss of section, wires breakage.

We carry out specialized maintenance work to extend the life of prestressed tendons, as well as emergency repairs to ensure the safety of structures following a cable break, or where remaining loading capacity is uncertain.

Our work focuses on the safety of users and workers and is part of a comprehensive approach that includes a preliminary inspection of the existing condition, as well as monitoring by instrumentation of the structure and safety measures in the event of a risk of failure (inside the structures for the workers mainly in case of repair works).

We plan our work to meet deadlines while minimising the impact on operations.

Why choose Freyssinet for your project

01. Expert analysis on pathologies

As experts in the prestressing tendons and kits, we can assist you in assessing the integrity of the structure. We help you to predict the functioning and performance of the tendons over time.

02. Monitoring program

We help you to define an inspection and testing programme adapted to your structural configuration.

03. Specific studies

Each structure generally has its own unique details, which require specific laboratory or field tests to accurately determine the extent and severity of a pathology.

04. Improvement solutions

Our teams are experienced in working on operational external prestressing cables, often corroded, to purge contaminated areas and treat weakened areas. We offer solutions to replace old generation cables with a more modern technology, safer for users.

Our range of solutions

Evaluation of cables condition

In addition to detailed inspections of prestressed tendons, we offer advanced investigations using state-of-the-art techniques, to detect cement grout injection voids, loss of prestressing steel section due to corrosion behind the anchors, force losses in prestressed tendons, etc.
  • Visual inspection of anchor heads
    and ultrasonic measurements
    to evaluate the corrosion
    of the prestressing steel
  • Inspection of accessible sections
    of the prestressing ducts
    by sonic testing and
    electromagnetic measurements.
  • Measuring the residual force
    in tendons (wires and strands)
    by a cross-bow test

Tendon acoustic monitoring

In the case of a tendon that falls into the category at risk, i.e. where corrosion of the prestressing steel (wires or strands) can lead to the overall cable failure, we offer to monitor the prestressing tendons with acoustic sensors, in order to detect in real time possible wire failures, to follow their kinetics (until or before tendon failure) and to facilitate decision making, as to the service level and its rehabilitation.
  • Detection of wire failure
    by accoustic sensors
  • Real time information by e-mail
  • Assistance to interpretate
    the kinetics of wire failure

Maintenance team safety

If corrosion of external prestressing tendons is suspected in a confined space, safety measures must be put in place to allow inspection and maintenance teams to work. We can help you assess the risks and select and implement protective measures.
  • Assessment of the impact of a traffic restriction or alternating traffic
    in order to ensure a satisfactory level of safety
  • Design and installation of safety straps to confine the energy release
    during cable breakage in case of repair operations.
  • Design and installation of safety straps to confine the energy release
    during cable breakage in case of repair operations.

Cable protection corrosion retrofitting

In existing structures, the prestressing cables were sometimes not properly injected or were injected with old products, or the sheaths were not sufficiently tightened. In order to restore the corrosion protection of the prestressing tendons, we propose preservation measures to repair the ducts (pipes) and reinject tendons.
  • Vacuum reinjection of voids in tendons
  • Reinstate the watertightness of ducts
    and anchor caps
  • Remove poor cement grout where feasible

Detensioning of external prestressing tendons

When the corrosion of an external prestressing tendons is too advanced, it may be necessary to de-tension the cable for replacement. We offer several detensioning techniques, proven on site, and chosen depending on the cable state, the condition of the structure and the risks associated with the sudden release of energy.
  • Cable detension by cutting
  • Controlled detension of tendons
  • Tendon detension with
    elasto-plastic dampers

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