How to Repair Carbonation-induced Corrosion?

Repair Carbonation-induced Corrosion

We talked about corrosion in another article, please read it first so you can understand what corrosion is and how damage occurs from the beginning.

What is Carbonation-induced Corrosion?

Briefly, when calcium hydroxide in concrete combines with carbon dioxide from the air to generate calcium carbonate and lower the pH of the concrete, this process is known as carbonation-induced corrosion. Because of the compromised protective oxide layer on the embedded steel reinforcement caused by this lower pH, the structure may become weaker and have a shorter lifespan.

Ok so now, you should know that carbonation does not occur uniformly over the entire structure. This means that carbonation is a variable process, and the depth of carbonated concrete will not be the same.

To get an idea on the penetration of carbonation, regular testing by the phenolphthalein spray
method, will be required.

Criteria to determine defected areas for repair

In the past, the following criteria have been used to determine which regions of concrete belong to each category:

  • Concrete spalling that is visible and you can see steel reinforcement.
  • Regions that produce a hollow sound when lightly struck with a club hammer (They are frequently seen near actual spalled locations, indicating that the issue is more serious than it appears.).
  • Regions where the steel reinforcement’s cover is thinner than 10 mm.
  • Regions where the reinforcement is only 5 mm from the carbonation front (this means that carbonation testing must be done roughly every two meters.).
  • Areas where the concrete is honeycombed.
  • regions where the half-cell potential values are more than -200 mV numerically (copper/
    copper sulfate). But be careful when applying this criterion—half cell potentials only indicate regions of active corrosion. As the concrete dries up in the summer, entire corrosion zones may stop. Using the half cell approach, the detection of corrosion caused by carbonation is not as accurate as that of corrosion caused by chloride. Half cell potential testing is not necessarily required if the aforementioned requirements have been met, however it can be a helpful supplementary diagnostic tool.

Steps to Repair Carbonation-induced Corrosion

Prior to cutting off or lightly grit blasting to remove surface dirt and deposits, the concrete surface is often water jetted. This process frequently exposes blowholes in the concrete surface.

You should know that all the areas of defect that were identified above will need breaking out behind the location of steel bars. The distance beyond the steel bars should be enough to allow one’s fingertips to pass behind the steel, making sure the breakout’s edge is cut squarely rather than “feather edged,” which would eventually lead to the repairs failing.

At all locations, the steel must be at very least wire brushed to remove all loose deposits or preferably grit blasted, or water jetted back to bright metal. If chlorides are present this is essential and will be dealt with in another article.

It can be essential to splice in extra or replacement reinforcement if the steel section loss exceeds a certain threshold. You as an engineer will make this decision. Usually, a loss of more than 30% is considered critical.

Repairs can start after the damaged area has been completely removed and cleared of any dust and dirt.

Typically, one of the top manufacturers will supply repair mortars that are specifically made for the job. In order to improve workability, permeability, or ease of application, they typically comprise one or more of the following: acrylic or other polymers, shrinkage-compensating additives, silica fume, slag, or a range of other chemical modifiers.

Although gauging sand and cement with, say, an SBR polymer emulsion has been employed and can work as long as the mortar is suitably batched and gauged with SBR in accordance with manufacturer’s specifications, simple sand/cement mortars alone should never be used.

Pre-batched proprietary repair mortars are likely to require a bonding agent of some type. They must be administered precisely as directed by the manufacturer, who frequently specifies that the repair must be done while the bonding agent is still wet. Bonding agents can also be very effective debonding agents if applied improperly!

In the event that no bonding agent is applied, the concrete’s broken-out surface must be completely dampened to break the suction, making sure the surface is both saturated and surface dry before the repair is applied.

We have been involved in the investigation of many failures where the substrate was not adequately wetted and water has been sucked from the interface of the repair and backing concrete, causing poor hydration of the cement and failure of the repair.

After that, the repair mortar will be rebuilt in accordance with any unique guidelines provided by the manufacturer. After that, repairs are typically made by hand, packing mortar onto the surface in layers no thicker than those advised and packing mortar under the bar using the fingers.

After the repair has been filled, it is leveled using a wooden rule and then floated with a steel float to a smooth finish.

Next, the repair needs to be cured following the manufacturer’s instructions. This may involve applying a wet hessian or, most frequently, a curing membrane sprayed on.

After the concrete is finished and hardened, it’s possible that there may be blowholes and surface defects, and the color of the patch will most likely differ from the surrounding parent concrete. Hence, to cover up any surface imperfections and conceal the patches themselves, a thin skim coat of a proprietary substance is typically applied. This coat is referred to as a “fairing coat.”

The next step is to apply a final layer of anti-carbonation paint, which has the twin functions of producing an even and beautiful color finish and preventing more carbonation in places that have not been fixed. It will be necessary to periodically update the coating, maybe every 10 to 15 years.

Safety First

Because cementitious repair mortars are highly alkaline and might result in burns or dermatitis, wearing appropriate protection clothes is imperative. Similarly, when mixing cement, dust masks need to be used.

Read Also:

How to Test Concrete for Carbonation?