Metro Tunnels - the Risks of Concrete Carbonation

Carbonation in concrete isn’t safe for the concrete or the tunnel structure.

It causes a constant need for maintenance and cleaning, and eventually becomes a durability issue. It’s even a health hazard for people.

What is the problem with carbonation?

Why does it become a durability issue?

Why are regular sealers not sufficient to achieve a 100-year design life?

Concrete tunnels are being built with a 100-year design life. But the issue arises when no or limited protection is provided for the concrete. Sealers are used that need replacing years later, and the 100-year design life becomes ‘100 years depending on maintenance’.

In that time the concrete is attacked by chemical ingress, and durability is negatively impacted.

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Carbonation resistant sealing

To prevent the problem at the root, and not for the short-term, we need to drill in deeper.

Concrete carbonation – a close look.

“Carbonation occurs when CO? (carbon dioxide) from the air penetrates into the porous concrete and dissolves in free water to form a mildly acidic solution.

Unlike other acids that may chemically attack and etch the concrete surface, this acid forms within the pores of the concrete where the carbon dioxide dissolves in any moisture present. Here it reacts with the alkaline calcium hydroxide forming insoluble calcium carbonate.

Carbonation is more rapid in urban and industrial areas, and alongside highly trafficked roads where CO? is at higher levels.” 

Murray Gilbertson, G Group Consulting        

The cause: Moisture entering the concrete matrix.

This moisture brings all sorts of soluble contamination into the concrete – and along highways, road tunnels, and metros, the risk of this occurring is higher.

But what does carbonation do?

The initial reaction is to change the concrete's surface. The colour becomes black – have you ever noticed road barriers with black streaks? That’s a result of concrete carbonating, and it is very dusty – but that isn’t the only risk to the concrete.

The major damage is caused deeper down when the reaction lowers the pH in the concrete, reducing the protective protective oxide layer in steel reinforcing. The steel is then a lot more vulnerable to corrosion.

Corrosion is an expansive reaction. Cracks quickly form, and once the cover concrete has been breached the escalated result is concrete spalling ('breakout'), and reduced structural strength.

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Why does this happen?

Contaminants don’t stay at the surface. As in the image below, concrete is full of porosities that are formed by bleed water when the concrete cured – even high strength concrete.

The blue lines, indicating water migration, travel around the aggregates and sand. This moisture re-enters the concrete at a later date and eventually reaches the rebar, where the contaminants set to work reacting with and corroding the steel.

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The key is to immobilise the water.

Stop it from entering the concrete, but also prevent moisture moving around within.

This is done by spray-applying a colloidal silica to the surface.

By catalytic reaction with the alkaline moisture in concrete, the colloidal silica is drawn in deep, to a depth of up to 150mm.

Have you ever seen the gel test?

That’s what is happening inside the concrete when the treatment is spray-applied to the surface. It penetrates the concrete matrix and turns all available moisture into a solid state. Filling the porosities during the cure not only increases concrete’s impermeability, it also enhances curing. The system that MARKHAM uses achieves 90%+ moisture retention for effective curing as well as sealing the concrete segments.

This reaction takes moisture out of the equation, and effectively prevents any further internal reactions – corrosion, carbonation, and so on. As the gel eventually becomes part of the concrete itself (as it is formed by ingredients already found in concrete), the effect of the density increase is permanent.

From that point on, carbonation holds less threat to the concrete. Treating the surface with penetrating hydrogel treatment prevents the carbonation reaction taking place internally, and also protects the reinforcing steel.

The surface will be easier to clean, as the grime caused by carbonation cannot enter the concrete – neither can any other grime or contamination.

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What systems use penetrating hydrogels?

MARKHAM’s system that is tailored for any structure with exposed concrete is the INFRA-TECT system – there are various colloidal silica treatments offered by MARKHAM, but the right treatment is used according to the project’s needs.

And this system is a supply-and-apply model. MARKHAM’s own team carries out the applications, to ensure the treatment is applied correctly, and to provide QA and a performance-specific warranty for the project.

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Are you faced by other challenges in tunnel construction or design?

Preventing contamination ingress – increase the impermeability

Soft-water attack – prevent moisture-borne contaminants

Waterproofing the tunnel joints – closing the air gaps

Keeping the environment safe – eco-friendly waterproofing?

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Have a look at choosing the system for your own project!

Metro tunnel photo by Redd F on Unsplash