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Wharf Repair

Wharves

Wharfs are in one of the most severe exposures and through the last third of the last century concrete durability design was often not up to the task. Hence concrete repair for chloride induced corrosion is very common in wharfs, jetties, piers and other marine structures older than 20 years. In high chloride concrete repairs it is virtually essential to incorporate cathodic protection in order to obtain a reasonable assurance of a successful outcome.

Where corrosion damage has occurred but the residual life is short then use of high output sacrificial anode may provide the most economic form of cathodic protection. However marine structures often have a long life requirement and high reinforcement density making impressed current cathodic protection more suitable.

 

Wharf Concrete Repair

Marine structures, such as jetties, wharves, dolphins and offshore platforms are often more problematic than other structures because of the variations in exposure. Some areas will be in extremely severe exposure with differing levels of splashing and inundation, while others will be in atmospheric spray zones of varying chloride intensity. Like other structure variations in steel density and extent of corrosion must also be assessed as part of the concrete repair process. This presents different issues for sacrificial anode cathodic protection and impressed current cathodic protection systems.

Sacrificial Anode Cathodic Protection

As the steel density increases so does the density of anodes required. Where the steel surface area that will draw current is over about 1.5m2 per m2 of concrete surface through which the anodes can be applied then the only sacrificial anodes that are likely to provide a life of 20 years are the 0.5mm thick sheet version of Zinc Layer Anode (ZLA) or multiple 250mm long Roll Anodes where the section is thick. The former has been used on massive beams and the latter on pile caps. The rate of zinc consumption in the anode due to the high steel current density and surface area will lead to early anode consumption otherwise.

Wharf piles may traverse tidal, splash and atmospheric exposures and will have to be divided into zones for SACP protection: i.e.:

  • Lower tidal zone – The frequent inundation means the concrete can be protected by in water anodes. Application of anodes in the concrete in this area is unlikely to be practical.
  • Upper Tidal Zone and Splash Zone  - Zinc Layer Anode cannot be used as its adhesive is too susceptible to water damage. Roll anodes are used where there is no concrete damage and high output GSC Super Anodes within patches where there is damage. They are protected from water damage by being fully encased.
  • Atmospheric Zone – Zinc Layer Anode is often the lowest cost solution although where application of the waterproof coating is difficult or on piles the combination of GSC Super Anodes and Roll Anodes may still be the optimum solution.

Impressed Current Cathodic Protection

In water anodes are still likely to be the best means of protecting lower tidal zones. The following addresses anodes for the upper tidal, splash and atmospheric zones.

Impressed current cathodic protection systems can be designed for high current outputs per unit area. For example MMO Titanium Nets can provide current densities of up to 45mA/m2 of concrete. This is sufficient to deal with the highest density of steel surface area in the severest exposure zone. Concrete repair using net anodes is particularly suited to flat surfaces like wharf walls where shotcrete can be applied over the net to complete the electrochemical continuity. However application of a quality cementitious overlay in the limited time window available can be problematic in splash and tidal zones.

MMO Titanium Ribbon Mesh anodes were popular in the past for splash and tidal zones. Although expensive because of the time involved in cutting slots in which to embed the ribbon the limited volume of fast setting mortar required to fill over the ribbon meant they could be practically applied in the short application window. However, these have recently fallen into disrepute for use in tidal and splash zones. Their common failure in under 20 years due to current dumping in low resistivity areas leading to acidification and mortar failure has led to some expensive re-repair.

MMO Titanium Discrete Anodes are now often the anode of choice for tidal and splash zones as they can be deeply embedded in the concrete. Where steel densities are high some designers are opting for multiple bladed anodes, made from ribbon mesh welded to a central titanium rod, located in a large diameter hole (35+mm) and grouted up . Others believe that packing so much mesh in a single hole circumvents improperly the anode current limits in codes and standards and believe this will lead to acidification issues in the future.

Where steel densities are not so high the clear choice of discrete anodes is often to use durAnode4 as this anode is relatively low cost, only requires a 12mm diameter hole and has many proprietary features making a quality installation quick and simple installation.

In the upper splash and atmospheric zones choices widen. Many of the above anodes could be used but a standout for marine structures is the Cassette system. In this system the ribbon anode is mounted in an inert casing that is bolted to the concrete surface.  The phenomenal speed and simplicity of installation, limited preparation and moderate materials cost means this is often the lowest cost installed system by far but it still has a design life of 50 years. Although not attractive this is not an issue on many marine structures.

      

 

 

 

 

Cassette installed on Jubilee Bridge has a 10 year track record.  

Although ideal for buildings due to its aesthetic finish Zebra conductive coating is also a sound contender for use in atmospheric zones. It is a low cost installation and materials option but the coatings typically has a replacement interval of 20 years.

 

Conclusion

There are a wide range of anode systems avaialble for wharf structures. In order to ensure a system that closely meets the owners needs is developed the formal process developed by BCRC for selection of anode systems is recommended. This process involves discussion with the owner on what features are important to the owner in terms of cost, noise, aesthetics, installation difficulties, access, performance risk, reliability, maintenace, life and relaiability and formally evaluating each anode system avaialble against these criteria.

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