Stay cables maintenance

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.

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.

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.

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.

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.

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.

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.