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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.

 

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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.

 

ZAP Magnesite System

Flooring Corrosion prevention.

The chlorides in magnesite toppings can penetrate to the underlying floor reinforcement, which then corrodes causing concrete spalling and delamination. Conventional repairs can be expensive and unreliable as it is difficult to remove all the chlorides. corrPRE have developed two low cost effective electro chemical systems. One is to bond zinc sheet with Zinc Activator Paste (ZAPTM) to the concrete surface the other is to apply Zinc Layer Anode (ZLATM) to the soffit. With a connection to the reinforcement either system provides galvanic cathodic protection to the reinforcement to ensure corrosion is halted with minimal concrete and topping removal.

Pictured:

Spalling of slab reinforcement due to corrosion initiated due to chlorides from the Magnesite topping.

Magnesite Problem

Magnesite floor toppings were used extensively in the 60’s and 70’s to provide a smooth flat finish on rough finished slabs. The main component of these toppings was magnesia, fillers and magnesium chloride solution. After hardening the pores in the topping remained filled with the hygroscopic magnesium chloride solution. Over time the chlorides penetrate to the floor reinforcement and can cause corrosion.

As early as 1962 the corrosive effects were known and in Germany recommendations for use of Magnesite toppings included that the concrete remain dry. DIN 18560 1992 on screeds requires a waterproof barrier between the topping and slab. However these recommendations are often inadequately applied and Magnesite floor corrosion problems are common.

Corrosion typically occurs first at wetted areas as the chlorides diffuse through water in the pores and the water lowers the electrical resistance of the corrosion cell. However once damage is observed in wetted areas corrosion is likely to have commenced elsewhere and the whole floor must be considered in the repair solution.

Conventional Repair Issues

In discussions with corrPRE repair contractors involved in Magnesite floor repairs noted the following concerns with conventional patch repairs:

Corroding reinforcement is often repaired by breaking out the damaged concrete and patching. Where chlorides are involved it is universally agreed that the concrete behind the bar must be broken out to remove the chlorides. This undamaged concrete behind bars is difficult to remove.

If only damaged areas are broken out then incipient anodes will form unless some form of cathodic prevention is included for the areas around the patch (e.g. corrPRE’s GSC super anodes).

If only damaged concrete is repaired then it is likely other areas in the slab with slightly less chloride penetration will fail in the future unless other measures are taken, e.g. remove the entire Magnesite topping to remove the chloride reservoir and dry out the concrete and provide a water proof membrane to keep out any future dampness.

These measures are expensive and still leave a risk of failure. The contractors recognise that cathodic protection is the only assured way of stopping corrosion without the need for all of the above measures but noted practical systems for this application had not been developed.

Cathodic Protection

Cathodic Protection (CP) is where an anode is connected to the reinforcement and embedded in or adhered to the concrete. The anode is designed such that all the reinforcement becomes cathodic and cannot corrode. Anodes may either be impressed current (ICCP) or sacrificial (SACP).

ICCP requires extensive wiring and monitoring around twice a year to check functionality and are considered impractical for apartments, as maintenance costs are too high.

SACP systems have been available in Australia for 10 years but the anodes are generally too small to provide a practical means of applying global CP. Their main purpose is for use around the edges of repairs to reduce the risk of incipient anode failure.

By contrast ZLA or zinc sheet with ZAP provides a very high surface area capable of polarizing the reinforcement globally to give a true SACP system that negates the need to break out chloride contaminated toppings and concrete. The concept of CP is described in other SRCP data sheets on ZLA.

corrPRE’s ZAP Magnesite

corrPRE are the inventors of a series of sacrificial zinc anode systems for reinforced concrete. These systems have been applied in Europe for 10 years.Corrosion-prevention-and-protection-systems

On introducing these systems to Australia in 2011 various repair contractors asked if corrPRE’s ZLA system could be applied to prevent future corrosion without removal of Magnesite toppings and without extensive concrete breakout. corrPRE knew the ZLA system would provide adequate protection if it could be applied to soffits (see SRCP data sheet on ZLA for details of application) but the contractors advised this was often not possible.

The problem of using ZLA on tops of slabs is that the activation paste can get squeezed out. Hence corrPRE set about developing a system specifically for the tops of slabs that could be applied under an underlayment.

The system adopted is to use zinc sheet with site applied ZAP. ZAP dries to a relatively stiff paste that is less likely to squeeze out. But the system is also designed with supports under the underlayment to reduce the load applied to the ZAP.

For full compliance with Cathodic Protection codes monitoring of CP systems is required and this entails wiring of the system back to a junction box to permit intermittent the monitoring. This is important with ICCP systems as the current has to be adjusted over time. With SACP system there is no such adjustment and hence monitoring is not required.

The ZAP Magnesite system is based on checking for appropriate polarisation of the reinforcement and then sealing up the CP system.

Various steps in the sheet and ZAP system are shown in the figure above and a step by step guide is given below.

Application Guide

Break out surface to sound concrete. No need to remove sound but chloride contaminated concrete and no need to remove the Magnesite topping unless the thickness is an issue.

Make two connections to reinforcement in the top and bottom mat. Wire bottom mat connection back to points on the top surface (NB: If the design engineer is confident that the bottom mat is electrically continuous with the top mat then the connection to the bottom mat may be omitted)

Use standard concrete patch repair system suitable for use with cathodic protection to build floor to flat surface.

Cut 125mm wide 250 micron thick zinc sheet rolls to lengths equal to the width of the floor less 200mm (leaves 100mm tolerance / topping bonding strip at each end).

Bunch 20 mm holes along each edge of zinc strip at 200mm centres. These holes are to for plastic disc supports.

Apply ZAP to one side of a strip of zinc sheet. Turn sheet and press down onto concrete surface.

Press 30mm diameter x 2mm thick plastic discs over each punch hole pressing out any ZAP beneath. These discs transfer load from the topping to the leveled slab without compressing the zinc tape so the ZAP is not squeezed out.

Repeat process for each strip of zinc leaving 100mm between each strip or alternate strips depending on steel density and design life (preference is to walk on the 100mm strips but the zinc sheet can be walked on).

Electrically connect each end of zinc strips in series and incorporate rebar connections to top and bottom mats by brazing connections to zinc sheet. Make rebar connections so that they can be disconnected temporarily.

Leave to polarize reinforcement for seven days. Disconnect reinforcement connections. Undertake 4hr and 24hr depolarisation tests using surface half cells to check code CP protection criteria. Reconnect rebar connections permanently (i.e. soldered and insulated).

Apply Ardex P82 primer to all exposed concrete.

Apply Ardex K22 or K11 mortar over top of whole floor.

The depolarisation tests at 7 days may be omitted if the designer is satisfied that the reduced risk by undertaking the depolarisation tests does not warrant the 7 day delay.

Pre Application Inspection

There is little point in repairing the area of apparent damage only to find that other areas spall a few years later. Hence pre-repair testing is required unless the whole floor is to be protected.

To identify the extent of corrosion electrical potential measurement are taken over the surface of the floor. Beul 2003 assessed the active potential for Magnesite floors and found it was approximately -150mv vs CuSO4 cells. Where potentials are more positive than -150mv it suggests that chlorides have not penetrated to the rebar and if these areas of floor are kept dry there may be no future corrosion. Hence the ZAP Magnesite system is only applied to area where the potentials are more negative than -150m vs CuSO4.

References

Beul W. & Menzel K. 2003 “Corrosion damages caused by cast magnesite floor screeds” 

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