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Structural Repairs Bridge repair and protection
Cathodic protection offers an ideal solution for repairing and protecting reinforced concrete bridges, according to the Corrosion Prevention Association.
The use of cathodic protection (CP) for preventing corrosion of reinforced concrete structures was introduced to the UK in the mid-1980’s, when the Midland Links (better known as Spaghetti Junction) became the first UK bridge to use this innovative technique.
The advantages of using CP for bridges were explained using a range of project case studies at a recent Corrosion Protection Association (CPA) seminar, held at the Millennium Stadium in Cardiff.
HOW DOES IT WORK?
The seminar began with an overview of the corrosion process and how cathodic protection can be used to prevent it from occurring. Dr Chris Atkins of Mott MacDonald explained that in reinforced concrete structures, the steel usually sits in an alkaline environment and does not rust.
However, the presence of chlorides, either within the concrete, or coming in from outside, changes this environment causing the steel to rust. Firstly, part of the steel surface becomes an anode and loses metal. The anode then releases electrons that travel through the metal to another part of the surface, known as the cathole, where they react with oxygen and water.
Dr Atkins explained that cathodic protection (CP) works by ensuring that the steel which requires protection becomes a cathode, using one of two methods. Galvanic CP generates the electrons by using a material that corrodes more than steel, such as zinc; while impressed current CP uses a power supply (typically at less than 12V) to generate the electrons and an anode material that does not corrode. The methods can also be combined into a hybrid system that starts off using impressed current but is then left to operate in galvanic mode.
PROTECTION OF ROAD BRIDGES Dr Atkins outlined how CP is being used on the M4 at Chiswick. The road had a corrosion problem which affected 120 piers along its length. The drainage from the two joints above each pier often became blocked, allowing chlorides from the gritted road to penetrate the concrete.
CP offered an ideal solution since it would minimise the necessary concrete repairs to the replacement of damaged material only, saving on the cost of materials, duration of work, and minimising the need for temporary support.
However, the project was not so simple, since it was the upper section of the pier that was nearest to the source of chlorides and which had become heavily contaminated. This was not easily accessible and would have required either the closure of the M4, enabling the anode to be installed from the top, or extensive drilling up from the soffit.
After considering a number of options for drilling discrete vertical anodes, it was realised that there was an area with no reinforcement in the top section of each pier. The solution was to drill a single 18m horizontal hole of 50mm diameter through the top section, enabling the anodes to be placed where they were needed. This resulted in a reduction in the total length of drilling required, thus lowering the noise generated and the duration of the project.
The Silver Jubilee Bridge between Runcorn and Widnes in Cheshire is another project described by Dr Atkins. The Mott Hay and Anderson designed bridge was opened in 1961 and widened in 1977. A range of CP systems were developed and applied on the bridge and its approach viaducts in a series of phased repairs.
George Sergi of Vector Corrosion Technologies continued the discussion of corrosion protection for reinforced concrete highway structures, with a detailed description of how CP was
applied to the Midlands Links Motorway Viaducts (Spaghetti Junction).
Two different conductive paint anode systems were trialled along with a conductive polymer mesh anode with sprayed cementitious overlay; and an MMO coated titanium mesh in cementitious overlay. Sprayed zinc and discrete surface- mounted titanium oxide tile systems were also used. The trials investigated the physical condition, reviewed monitoring and performance data, secondary effects and compared the anodes. The use of CP systems meant that only identified physical defects needed to be repaired, allowing chloride contaminated concrete to remain in place. Full scale application of CP followed using conductive paint anodes initially but using a wider range of anodes subsequently.
The seminars was one of a series of free Corrosion Prevention Seminars offered by the CPA in locations throughout the UK.
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