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Crack Repair - Corrosion Protection

Introduction

Concrete cracks. If it didn’t then the reinforcement would serve no practical purpose as it only takes load once the concrete has cracked. Proper design ensures the reinforcement distributes the cracks such that their widths are acceptable from an aesthetic and durability perspective. Hence, when concrete cracks corrosion is seldom a problem.
Instances where corrosion at cracks is possible are:

  1. in concrete immersed on one face below saline water with the other face atmospherically exposed. This occurs in tunnels, basement, and other underground structures.
  2. cracks that are wider than 0.3mm in any exposure where corrosion rate is not too low for corrosion to occur. 

Where such crack are not sealed shortly after they occur corrosion at the reinforcement around the crack may not be stopped by sealing alone. Local application of cathodic protection at the crack is one way of ensuring corrosion damage will not occur.

Which Cracks Need Treatment

Saline Water Retaining Elements

Where saline water is at pressure on one face of an element and the other face is open to air, water will flow under the pressure gradient through cracks that traverse from one face to the other. Such cracks typically occur due to early age thermal contraction. Where reinforcement is designed to control the cracks to a width that will self-heal in a relatively short time then the water that has penetrated along the crack will have deposited relatively little chloride in the concrete and corrosion will not be an issue.
Where saline water retaining elements crack and continue to leak they can become an issue due to corrosion of reinforcement on the air side adjacent to, and at the crack. Issues can also arise from concentration of salts at the surface as water evaporates off. Hence, all cracks in such elements should be sealed. The question is whether that sealing will prevent future corrosion.

The likelihood of corrosion at cracks at early ages is not high as:

  1. chloride concentration of the penetrating water is low relative to the chlorides that build up on the surface of concrete due to evaporative concentration
  2. there is no chloride replacement once the crack is sealed and the chlorides will diffuse away from the surface of the cracks reducing the concentration further
  3. concrete at early ages will complex a higher proportion of chlorides reducing their ability to diffuse and cause corrosion.
Figure 1 : Saline water penetrates down a crack and casues corrosion activation.  Figure 1 : Saline water penetrates down a crack and casues corrosion activation.
Figure 2 : Shows how water penetration on the water face prevents oxygen ingress and leads to a uniform electrical potential. Figure 2 : Shows how water penetration on the water face prevents oxygen ingress and leads to a uniform electrical potential.

However, where cracks have been leaking for some time the chlorides may have diffused into the concrete to a greater extent. Of more concern is that the penetrating saline water may have penetrated along the more porous interface with reinforcement.
Where chlorides have penetrated sufficiently to cause corrosion activation of reinforcement in the general area of cracks the small anodic area could be fuelled by a large cathodic area (Figure 1). The size of the cathode will depend on the resistivity of the concrete. The resistivity of a damp GP cement concrete might be only 3,000 ohm cm while a high performance concrete with fly ash, sag or silica fume is likely to be 10 times that. In the latter case the cathode will be small and the corrosion rate low.

Corrosion of reinforcement on the immersed face (Figure 2) is not an issue as:

  1. all the reinforcement quickly becomes saturated leading to no potential difference between reinforcement at the cracks and the other outer face reinforcement
  2. if there were cathodic areas on the reinforcement on the water face then corrosion rates would still be low due to the low oxygen concentration at the cathode and the low cathode to anode ratio
  3. high resistance path to cathodic reinforcement on opposite face.

The above suggests that if the cracks are sealed at an early age they are unlikely to give rise to corrosion issues on the air face and they will not give rise to corrosion on the immersed face even if not sealed. There is however a significant risk of corrosion occurring on the air face in the area of the cracks even after sealing if they are not sealed at an early stage.

It is not clear just how early cracks need to be sealed as it will vary from case to case.

Elements in Atmospheric Zones

Cracks in atmospheric zones do not lead to a corrosion risk if the width is less than 0.3mm (ref CIRIA C660)  at a cover of 40mm (wider cracks can be tolerated where the cover is higher). However at wider crack widths the risk of corrosion increases.

Marine Atmospheres

In marine atmospheres, where the cracks are likely to have a high moisture content, chlorides will be able to diffuse more rapidly down the crack and along the reinforcement. The potential difference that results between the chloride contaminated reinforcement and surrounding reinforcement may fuel a fast corrosion rate depending to the high cathode:anode ratio which is again resistance controlled.

If the cracks are sealed at an early stage in the cover zone then the path for chloride ingress is eliminated and the likelihood of corrosion is low. If not then the risk of corrosion could be high.

The potential risk of corrosion at leaking cracks in sweater retaining elements was identified by Chang on the Sydney Harbour Tunnel. Cracks in this immersed tube were tested by polarization resistance and the reinforcement was found to be actively corroding. Chang found that just sealing the cracks did not stop the corrosion and he developed a localized cathodic protection system to prevent corrosion. 

Carbonation
Figure 3 : Section of a precast panel after applying pH indicator. Carbonation down the crack and around the bar location is apparent. Figure 3 : Section of a precast panel after applying pH indicator. Carbonation down the crack and around the bar location is apparent.
Figure 4 : Application of a high output galvanic anodes provides cathodic protection to the active reinforcement and draws out chlorides. Figure 4 : Application of a high output galvanic anodes provides cathodic protection to the active reinforcement and draws out chlorides.
Figure 5 : ZLA applied on either side of the crack so that it doesnt matter if the crack is still leaking and the crack can be inspected in the future. Figure 5 : ZLA applied on either side of the crack so that it doesnt matter if the crack is still leaking and the crack can be inspected in the future.
Figure 6 : Roll Anodes applied either side of the crack and set so as not to draw chlorides towards the reinforcement.  Figure 6 : Roll Anodes applied either side of the crack and set so as not to draw chlorides towards the reinforcement.
Figure 7 : By the end of the anodes design life the reinforcement will have been repassivated and the chlorides removed.  Figure 7 : By the end of the anodes design life the reinforcement will have been repassivated and the chlorides removed.

Carbon dioxide can penetrate down a crack and around reinforcement causing localized depassivation of the reinforcement. Figure 3 shows such a crack from a survey undertaken by Papworth. Carbonation along the crack to the circular section where reinforcement was located is highlighted as white following spraying with phenolphalein.

Where cracks are narrow self-healing occurs along the crack, or at least around the cracks tip. However, where the crack is wider than 0.3mm the resistance to carbon dioxide penetration along the crack may be minimal. Following carbonation around the reinforcement corrosion of can ensue, during wet periods at least. 

Crack Sealing

There are various methods of sealing leaking cracks. In all case a material is injected to fill the crack and prevent further water penetration. This article is not intended to discuss the advantages and disadvantages of the different injection techniques but the materials used may influence the efficiency of cathodic protection and this is discussed for each approach below.

  1. Low Viscosity Epoxy – if the crack is live then it is likely to reopen and leak. Epoxies are generally not as low viscosity as other materials, i.e. less penetration.
  2. Polyurethane – if the water pressure is high the water may push its way through. The process gives excellent sealing as the expansive reaction with water in the cracks gives full selaing. PU’s are flexible so they seal moving cracks.
  3. Acrylic – These tend to combine flexibility with good resistance to water under pressure. Their low viscosity gives good crack penetration.
  4. Methyl-methacrylate – Low viscosity means high penetration. Low modulus types available for sealing live cracks.
  5. Micro-cement  - These are ionically conductive so do not block cathodic protection current. However they are brittle so live cracks may re-open.

Only micro cement grouts are ionically conductive and do not interfere with provision of cathodic protection. Other injection materials may isolate parts of the bar they touch. If they isolate the bar then this provides the corrosion protection however none of the materials are expected to provide full insulation but if they don’t then the CP will provide the corrosion protection.

Little research has been undertaken to validate the above but from a theoretical perspective all of the injection materials are likely to be suitable for use with galvanic anodes that provide true cathodic protection. High output anodes are preferred to help overcome inherent resistance of the injected concrete.

Cathodic Protection

As noted impressed current cathodic protection was successfully applied local to crack on the Sydney Harbour Tunnel. However the need for control and monitoring systems for such local repairs is both costly in capital and maintenance. corrPRE’s Zinc Layer Anode is a 250micron thick zinc sheet with self-adhesive backing that is also designed as a zinc activator. When applied to a concrete surface and connected to the reinforcement it provides true cathodic protection to the underlying reinforcement. A schematic of a system applied to a crack is shown in Figure 4.

ZLA can be applied over the crack where leakage has stopped.  The adhesive of ZLA is water soluble so if the crack continues to leak the adhesive will break down. Alternatively ZLA can be applied on one or each side of the crack where the crack may still have some leakage or the ability to inspect the crack surface in the future is required.

ZLA may also be applied either side of cracks that are not sealed in order to provide cathodic protection (Figure 5) but in that case ZLA’s application should be well clear of the damp zone on each side of the crack. The resistance between the reinforcement and ZLA is needs consideration. ZLA has a strong current throw and can be placed up to 200mm from the crack and still provide protection to the reinforcement at the cracks.

An alternative to ZLA is to install corrPRE Roll Anodes along the side of the crack (Figure 6). Holes are drilled at approximately 400mm centres adjacent to the crack (typically staggered on either side of the crack) and the string connected to the reinforcement at each end.

Once the corrPRE anode is connected to the reinforcement a current flows such that:

  1. the reinforcement is cathodically protected. This means that for the life of the anode (typically 20 years) the reinforcement at the crack cannot corrode even if there are high chlorides present and high potential differences between the reinforcement around the crack and reinforcement elsewhere.
  2. chloride ions are drawn to the corrPRE anode. Chlorides are an integral part of the zinc activator paste and hence the extracted chlorides stimulates the anode functioning. Within a few years at least the chlorides will have been drawn away from the reinforcement.
  3. hydroxyl ions are generated at the reinforcement. This increases the pH at the bar and with no chlorides present the reinforcement is repassivated

This dual or hybrid action of cathodic protection and chloride extraction/ realkalisation is common with all CP systems. However the long term high current output of the corrPRE anodes means the hybrid performance is exceptional.

A key aspect to the hybrid performance is the removal of chlorides. In the case of ZLA all chlorides are drawn to the surface and away from all reinforcement. On completion of 20 years’ service a large proportion of the chlorides will come away with the removal of the anode residuals. The chloride extraction combined with the generation of hydroxyl ions at the reinforcement surface means that at the end of the anodes service life the reinforcement will no longer need to be protected and the anode will not need to be replaced (Figure 7).

In the case of Roll Anodes the chlorides are drawn to the anode rather which is not at the concrete surface. Care needs to be taken to ensure the Roll Anode location is such that they are not pulling the chlorides to a location that once the anode stops providing true CP (20 years) will mean the chlorides will cause problems to surrounding bars. This issue of where the chlorides end up is an issue for all anodes as they work in this hybrid fashion.

Summary

  1. To eliminate the risk of corrosion of the underlying reinforcement cracks should be should be sealed shortly after formation where saline water leaks through or where they are wider than 0.3mm in the atmospheric zone.
  2. There is a risk of corrosion at cracks where sealing has not been undertaken in accordance with 1)
  3. The risk of corrosion at cracks leading to damage can be assessed by corrosion rate testing of the reinforcement (e.g. use Corromap). Assessment of the results is not a simple or certain matter.
  4. The corrosion risk can be eliminated by application of high current output anodes. Impressed current CP can be applied locally but this is expensive and complex to install. Many galvanic anodes have low current output and may be inadequate in the cathodic protection, chloride extraction and realkalisation modes. However   corrPRE’s ZLA and Roll Anodes have proven high current output can provide  the output needed to give true CP and to provide a hybrid function over the anodes 20 year (typical) lives.
  5. Application of corrPRE anodes is so simple that in many cases it will be simpler to just apply the anodes and ensure protection rather than undertake corrosion rate assessments.
  6. ZLA can be applied over the top of sealed cracks or on each side of the crack if sealing is uncertain, or not undertaken, or the crack surface is to be inspected in the future,
  7. Roll Anodes can be installed along the side of the crack at approximately 300mm centres. The exact location of each anode should be selected to ensure it will not draw chlorides towards other reinforcement.

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