The Jetty was constructed in the early 1960’s to provide a safe berth for the importation of raw materials for the manufacture of aluminium. The roadway structure, leading to the Jetty head, is constructed on a series of circular steel piles set in the sea bed in pairs at fourteen metre centres along the length of the roadway with each pair of piles supporting a massive reinforced concrete cross beam. The deck of the roadway consists of an in- situ reinforced concrete slab cast onto pre-cast units that span between the cross beams on the piles.
The principal effort in this 26 week project was to repair damage to the reinforced concrete in the pre-cast deck units. This damage, which is shown to have arisen from corrosion of the embedded steel reinforcement, has been recorded over time and, following completion of an in-depth condition survey during 2007/8, the suspicion was confirmed that the salts from the marine environment had played a major role in the initiation and progression of an ongoing corrosion process on the surfaces of the embedded steel reinforcement. This process has lead to spalling and delamination of concrete at various locations around the structure where concrete material has either separated from the body of the structure and fallen into the sea or, is delaminated within the body of the concrete structure; this delamination takes the form of internal radial cracking around a point where the greatest stresses are exerted and ultimately lead to spalling. These damage patterns have been shown to have arisen from the internal stresses placed upon the concrete by the expansive nature of the corrosion process where rusting steel significantly expands in volume to a point that cannot be withstood by the concrete.
The concrete repairs on the structure were carried out in a traditional manner using cementitious materials applied by hand placed techniques. Due to environmental constraints, imposed by working over the sea in an environmentally sensitive area, removal of damaged and defective material was carried out using percussive means. The exposed steel reinforcement was subsequently blast cleaned and primed immediately before application of repair materials.
Due to the nature of damage and the association with high levels of external migratory chloride ions arising from contact with the sea-water, it was obvious that the only long-term remedial solution to the damage would be to install an electro- chemical system to negate the inevitable incipient anode effect that would ensue following completion of repairs, Following extensive discussions between the Client, the Consultant and the Contractor it was determined that the most appropriate solution in terms of best technical and economic performance over time would be the installation of a Galvanic Cathodic Protection system.
The system chosen is a revolutionary method that uses a wire comprised of an alloy of aluminium, zinc and indium; this material is designed as a sacrificial anode which is spray applied on to the surface of the concrete using a thermal arc spray system; to complete the circuit, an electrical connection is made between the steel reinforcement and the applied anode using a bolted connection to the steel and a zinc plate at the surface. This system has had extensive successful use in the USA and on a number of particularly large projects here in the UK.
The metallurgical and electro-chemical properties of this anode have been optimised to meet the requirements of cathodic protection in reinforced concrete. The unique alloy composition ensures:Ease of application and homogeneous coating composition Uniform sacrificial corrosion of the anode Minimal self protection and negligible wasted current capacity High anode efficiency.The anode is applied using thermal arc spray techniques involving the use of a power source, specialised application gun and air compressor fitted with suitable air drier. The anode wire is fed through the gun, melted with an electric arc and the molten metal droplets blasted onto the concrete surface. Typically the anode produces between 2mA/m2 and 20mA/m2 of concrete surface area depending on the electrical resistivity of the concrete in the range defined in BSEN 12696:2000 for the cathodic protection of reinforced concrete.
The corrosion products formed as a result of the anode performance are primarily harmless aluminium salts such as aluminium chlorides, commonly found in antiperspirants and insoluble aluminium hydroxide. As such, corrosion by-products from the anode have negligible environmental impact. The life expectancy of protection is determined by the thickness of anode applied to the concrete structure. Typically, an anode thickness of 200 microns is applied giving a theoretical life greater than 20 years.
One unique advantage of this product is that it can be applied under virtually all temperature and humidity conditions. This is a particular advantage in locations such as the Jetty where tidal and environmental constraints are of particular concern. But perhaps the most significant advantage is the fact that current distribution is even across the whole structure and, as the anode expires over time, the product can be re-applied without the trauma of having to break out and replace embedded anodes.
During the application various operation specific Health and Safety requirements were considered and implemented; including the use of appropriate personal protective equipment; and ventilation of the work area. The extensive concrete repairs, together with the design and installation of electro- chemical works were carried out by Quadriga Concepts Limited who were working in close association with both the Client and Consultant, Capita Symonds Limited.
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