Repair, upgrade, replace

Maintenance of stay cables

It is easy to appreciate that the cables on a cable-stayed bridge or other structure are critical to ensure their effective and safe performance.  The cables are particularly exposed to variable loading, climatic events and, more exceptionally, fire, ice and even malicious attack.

Backed by its long experience providing stay-cables for a vast array of new structures, Freyssinet has developed a comprehensive range of technical solutions and services for the maintenance and repair of all types of stay cables and suspension cable systems.

Stay cables maintenance - Pertuiset bridge France

Pathologies encountered

Most recent cable stay systems are highly engineered and well designed to be durable under all expected conditions over their long design life with appropriate regular maintenance.

However, there are many existing bridges that were built before modern stay cables systems were conceived and therefore less able to resist the severe conditions to which they can be exposed. Deterioration and damage to the cable systems can take many forms and be associated with all the components (strands, ducts, dampers, anchorages) making up the system.

Damages to strand cables can be by impact from vehicles, fire or lightning strikes or due to corrosion usually resulting from water ingress or corrosive environment (humidity, marine, chemical). Uncontrolled cable vibrations can also lead to severe damage (bending / fatigue).

Many cables are contained in an outer plastic sheath or duct which can degrade over time or be damaged by impact or vibration. Some of these duct systems were not always UV resistant and therefore the material may degrade and become brittle and severely deteriorated.  If inadequate allowance is made for thermal movement, this will cause problems in service especially where exposed to extreme temperatures.

The duct and anchorage systems will not always resist water ingress. The entry of water into the stays, particularly the lower / deck anchorages often causes corrosion of the steel elements. Damage can also be caused by freeze/thaw action. Aggressive environments such as marine / estuarine and proximity to industrial facilities / air pollution will accelerate deterioration.

Various forms of corrosion protection are employed on the anchorages of cable stays.  This can be paint systems, galvanizing, metallization or other forms of protective coating such as grease or impregnated tape.  When not adequately maintained, the breakdown of these systems results in corrosion and when this extends to the structural elements (strands / wedges / bearing plates etc.), structural performance and safety will be impacted.

The load in the cables can change over time for many reasons.  Loss of load in one cable can lead to unwanted increase in adjacent ones and the result can be local overstress in the supported members.

Long cables are often fitted with dampers to control vibration.  These dampers can be elastomeric, hydraulic or friction.  They can be damaged by impact or unexpected vibration loading, even under regular use if badly conceived, installed or maintained.  In most cases they are placed at or near deck level and therefore quite accessible.

Maintenance of stay cables

Tailor-made services

Our interventions are designed to eliminate defects through repair, upgrade or replacement. We also recommend and carry out maintenance in order to increase the structure’s service life.
Pictos Key figures 1
Maintenance
Early intervention for cable inspection and maintenance to avoid or reduce future deterioration
Pictos Key figures 2
Repair
Repair or upgrading to modern standards of specific identified defects in the cable system
Pictos Key figures 3
Replacement
Replacement of a complete cable or cables when condition makes repair impractical, uneconomic or impossible

1 - Inspection and maintenance

All critical structural elements of bridges and structures should be regularly inspected for signs of deterioration, failure or damage; and cable stays, of course, are no exception to this rule.

Specialist inspection

Freyssinet can offer a full service of specialist inspection including provision of access equipment or plant where necessary.

It is good practice to get within ‘arm’s length’ of all critical components to be able to observe directly.  For inaccessible areas, we utilize access equipment (mobile elevating working platforms, under-bridge units) or trained roped-access specialists.  More recently we can employ digital survey recording techniques including survey using drones or robots.  We conduct hammer tap surveys to map voids in cement grouted cables.

Mini-cameras or endoscopes are often useful to see inside anchorages or ducts to identify hidden defects.  Where appropriate we can open windows in the ducts and reinstate after inspection.  It is often necessary to remove anchorage caps and the filling materials inside them to inspect the critical area where the tensile element is anchored.

We have numerous technologies which are appropriate to investigate the condition of hidden elements of stay cables, including:

  • Acoustic emission surveillance – sensor placed strategically on the structure can listen for tell-tale sounds emitted by the energy release when a stressed element breaks. The location of the fracture can be calculated using the timing of the recording from adjacent sensors.
  • Magnetic flux leakage – defects, corrosion and section loss can be identified by magnetizing steel and measuring ‘leakage’ of the magnetic field.
  • Ultrasonic guided waves – can detect corrosion or wire breaks over a limited distance from the probe – usually limited to the critical area near the anchorage zone.
  • Force measurement– residual force in the tensile element(s) can often be reliably estimated by direct / indirect methods for individual strands / wires or complete tendons.

Maintenance

Regular or reactive maintenance of stay cables, usually targeted on the secondary components with the shortest service life, allows us to extend and guarantee optimum service life.  Such maintenance is carried out at agreed intervals or in response to extreme climatic events.

Partial re-tensioning or de-tensioning of the cables to correct identified changes in load or disparities between strands.

Removal of accumulated deposits on ducts and unblocking of anchorage / formwork tube drains.

  • Re-touching or renewing protective coatings on anchorage components and other metallic elements such as anti-vandalism tubes above the deck.
  • Re-tightening of ‘stuffing boxes’ (where present) to maintain anchorage water tightness.
  • Removal and replacement of anti-corrosion fillers such as in anchorage caps and where injected in the adjacent anchorage region and in limited cases in the free length of cable ducts.

such as bolts, fastenings, duct transition pieces, anchorage caps.

Certain ‘wear’ components need to be replaced and the dampers cleaned and re-sealed

Removal and (usually) replacement of an individual strand in order to test its current properties.  This is good practice after 10-20 years of service and by testing the strand sample, we can verify its integrity and check the residual fatigue life is in accordance with that expected.

2 - Repair and upgrading

After damage, deterioration or failure it is often possible to carry out a localised repair to the affected component. Similarly, where the as-installed elements are sub-standard and need to be upgraded to modern ‘state of the art’ standards, then we are ready to act.

Ducts

Where the ducts are locally damaged, such as by impact or fire, it is possible to replace sections of duct using in-situ welding, sometimes combined with proprietary couplers.

On older bridges, ducts were often simple plastic / HDPE pipes and were not well designed with regards to controlling cable vibrations or for UV resistance. We can post-weld helical fillets onto the ducts to reduce the effect of wind and rain induced vibration or wrap the ducts with special coverings / tape to resist UV degradation and improve aesthetic appearance.

Some bridges have cables consisting of individual strands / wires but with no overall duct encompassing the cable. We can retrofit such a duct to give greater protection to the strands and a better aerodynamic behaviour.

Water ingress

Often the ability of water from rain or condensation to penetrate formwork tubes / steel box anchorage zones, ducts and anchorage caps was not fully appreciated and therefore not well addressed. In some cases, the best solution is not to try to prevent the water ingress but to provide a drainage route for any accumulated moisture to escape under gravity.

Where water is entering and accumulating in / near the anchors, this needs attention. Often the weak point can be in the lower duct transition to the structure or the sealing and detailing of caps.  We can utilize replacement seals and retrofit flexible elastomeric ‘boots’ to improve the situation, often in conjunction with providing drainage where appropriate as a backup.

Corrosion protection is often provided by grease or wax that is injected into the anchors. If this injection was sub-standard leaving voids or the material unstable leading to deterioration or segregation in the long term, it is necessary to replace it.  The main difficulty can be to remove the old material and we have developed techniques to achieve good results.

Modern corrosion protection systems can be far superior to those used at time of construction. Most such systems have a limited life and will need to be reinstated periodically. With good material choices and professional application, the time between interventions can now be increased.

Dampers, deviators and strands

Longer cables or those on structures particularly susceptible to vibration are often fitted with dampers to control these effects. New developments can be used to improve the damping performance whether by modifying / replacing old dampers or retrofitting dampers to bridges exhibiting unforeseen and significant cable vibrations.

Where the control of geometric deviations due to installation tolerances or deflection of the structure was not envisaged or controlled by the original design then the bending stresses induced can be significant, particularly on larger diameter tendons. If analysis confirms that the stresses are large enough to affect the performance (fatigue life) of the cables, a solution can be to install a filter / deviator to localise and minimise these unwanted effects.

Some cables are subject to significant load variation and even load reversal. In these situations, it is possible that the strands can “slip” and lose some or all of their load when the strand / wedge / anchor block interface does not function as intended.  Remedies include restressing with new wedges, hydraulic re-seating (or ‘over-blocking’) and the addition of wedge retaining plates.

Where the individual cables are formed with multiple strands and damage or corrosion is limited, for example, to a limited number of strands then individual strand replacement is sometimes a cost-effective solution. The operation is invariably complex and Freyssinet has the experience and tools to carry out these procedures correctly and safely.

Protection devices

Cables can be damaged by fire, usually from an accidental situation where a vehicle catches fire on a bridge adjacent to the cables. Bridge owners are increasing acknowledging this risk and providing cables with fire protection over the lower section of the cables.  We have developed retrofit systems that can be added without the need to remove or replace the cables in their entirety.

Security from malicious or terrorist attack is a consideration for a limited number of critical or particularly vulnerable assets. We have experience in this field and can conceive bespoke solutions on request.

The susceptibility of cables in certain cold climates to ice accumulation is now better appreciated. Where temporary closure as a safety measure is not a palatable option, clients and owners are increasingly looking to proactive measures to deal with this phenomenon.  Unfortunately, the nature and frequency of ice accumulation leading to dangerous and disruptive ice falls is extremely variable and site specific.  Freyssinet is however able to liaise with our clients to understand their issue and propose suitable solutions.

Traditionally bridges with high pylons are fitted with lightning protection systems. In some cases, the system installed on the pylons does not protect the cables themselves from lightning strike(s).  There are examples of lightning causing local damage to the duct and, rarely, incidents have been serious enough to cause damage to, and even catastrophic failure of, the tensile elements themselves.  Freyssinet has intervened in these cases to repair or replace the cables and install lightning protection to avoid future reoccurrence.

3 - Cable replacement

Even if cable replacement is technically straightforward, there are usually site-specific issues to deal with such as working around the traffic, reducing disruption and adapting access systems to the situation in the field. Usually the challenge is more significant as the new cables require significant changes to the existing details and / or serious temporary works to ensure safety during all stages of our intervention.

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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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