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Reinforcement Corrosion in Concrete

When corrosion of reinforcement in concrete first becomes apparent as isolated concrete cracks or spalls it is often an indication that environmental contaminants are more generally reaching the reinforcement.

 

If left cracking and spalling can become wide spread within a couple of years. It is important that apparent cracks and spalls are repaired and some form of protection applied to all concrete. Very often it will be too late to apply barrier coatings as the contaminants will have already penetrated the concrete. ‘Break out and patch’ is a commonly used repair method but it is often too expensive to be undertaken over all areas of contamination, and where it is it can be disturbing and create structural issues.

 

It has long been recognised that cathodic protection is the ideal repair method in that it can stop reinforcement corrosion without the need for breakout of contaminated concrete.

 

Impressed current CP is suited to major structures where the owner can afford the relatively high design, installation and maintenance cost and the deterioration must be halted over a long design life, e.g. 50 years. Sacrificial Anode CP is less expensive and requires less design and maintenance. It can be installed economically on small structures by relatively inexperienced installers but the anodes generally need to be replaced at 15-20 year intervals.

 

There are various impressed current and galvanic anode types available. All have their place and it is the suppliers responsibility to advise which anode type is most suited to a specific application.

 

Asset Inspection: Aerial Photography

Thanks to our close working relationship, SRCP now offers an aerial inspection service for the construction industry in Perth, via drone aerial video inspection from Pixel Pilot.

 
Pipe, Tank and Steelwork Corrosion

 

Hot Dipped Galvanising (HDG) is recognised as one of the most reliable corrosion protection systems available. Unfortunately once it starts to break down it cannot be re-instated - until now. Zinc Tape can be applied to the steel surface and provides better corrosion protection than the original HDG.

 

Repairing concrete columns with true cathodic protection

Concrete Columns – Structurally Difficult Repairs Made Easy

Cathodic protection of concrete columns during repair. ZLA applied to all faces of a large column exposed on all faces. Image above shows a view taken prior to the application of a coating.
Low cost true cathodic protection of concrete columns. Zinc Layer Anode (ZLA) is the lowest cost cathodic protection (CP) system.
Cathodic protection system for concrete where the surface is not suitable for Zinc Layer Anode. Roll Anodes used where the surface is not suited to Zinc Layer Anode (ZLA).
Cathodic protection system for concrete where there is a high amount of bending stress. Roll Anodes where there is high bending stress.
Cathodic protection system with a cement based coating. Columns inset in brickwork were protected by ZLA on the exposed face and then coated with a cement based coatings.

Columns are major structural elements that can carry high load. This makes for difficult and expensive structural repairs. When chlorides or carbonation lead to reinforcement corrosion the standard patch repair requires that the contaminated concrete is removed. This generally leads to significant part of the bar exposure and hence to ensure it is bonded back in the repair concrete has to go around the bar meaning breakout around the bar. These conventional patch repairs have the following:

  • extensive breakout is noisy and will disturb tenants
  • breakout may destabilise the residual unreinforced core
  • the load paths are difficult to assess. Complex calculations are necessary and repair material E-modulus must be appropriate
  • propping of the structure may be essential and expensive

These problems are largely eliminated if the breakout of the sound but contaminated concrete becomes unnecessary and the only way of achieving this is by the application of cathodic protection. Impressed current cathodic protection systems were developed precisely for this reason but they are generally not practical for isolated column repairs and most building contractors involved in repair do not have the capacity for design and installation of such systems.

Galvanic anode systems for concrete have been used for the last 15 years but they have lacked power and were not recommended for cathodic protection. By contrast corrPRE anodes have been uniquely designed to give high output and true cathodic protection.

ZLA – The Lowest Cost and Simplest CP System

Whether the column is built into a wall and exposed only over a limited area or is a free standing and exposed over its full circumference Zinc Layer Anode (ZLA) provides the lowest cost and simplest repair systems. Just remove the loose concrete and patch, apply the ZLA to the concrete surface, make a connection to the reinforcement and the reinforcement is protected by cathodic protection. A cementitious coating is applied for and aesthetic and waterproof finish.

Because the reinforcement is barely exposed there is very little breakout and no need to consider structural aspects. The load will be taken primarily by the original concrete which as the section is barely affected can easily carry the load.

Roll Anodes - Where Column Surface Not Suitable for ZLA

If the concrete surface is coated or if there is exposed aggregate finish ZLA is likely to be unsuitable. It is likely that installation of Roll Anodes will be a lower cost that preparing the surface of ZLA but coatings could be removed or exposed aggregate surface rendered to provide a surface suitable for ZLA.

Typically holes are drilled so that the Roll Anode will be located just below the concrete surface. The Roll Anodes are normally spaced at around 400mm centres but the number is determined by reinforcement density.

Roll Anodes – Columns with High Bending Stress

On some columns there may be high bending stresses at the extreme fibre. In these cases location of embedded anodes, including Roll Anodes, at the extreme fibre could lead to exceedance of the compressive strength at the extreme fibre. All zinc anodes have a soft surround that keeps them active and hence their strength cannot be counted in the columns strength. In these cases the anodes can be pushed a little further into the concrete and the hole filled with a grout of E-modulus similar to the parent concrete. Because the neutral axis moves to the compression face under bending the anode only has to be pushed in around 100mm to be at a point of low stress. Alternatively if bending only occurs across one axis the anodes can be drilled into the face so that they are parallel and close to the neutral axis.

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