Metro tunnels – How to prevent Concrete Carbonation

A concrete deterioration mechanism.

If I was stationed in a cold, dark tunnel, with trains regularly hurtling past shaking the surroundings and pounding my heart into my throat, I would probably carbonate too.


However, 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.


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 these periods of time where maintenance been neglected, the concrete is attacked by chemical ingress, and durability is negatively impacted.


The preventative measure is either keep up the maintenance and give the contamination no opportunity to penetrate the concrete… Or, at design stage, choose a carbonation resistant sealer that needs no reapplication.


Carbonation resistant sealing

Then the question is – What does permanent sealing look like?

We need to drill down to the problem.

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 carbonation – a close look.

“Carbonation occurs when CO2 (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 CO2 is at higher levels.” Murray Gilbertson, G Group Consulting

“The most favourable condition for the carbonation reaction is when there is sufficient moisture for the reaction but not enough to act as a barrier.” – “Carbonation of Concrete.” Concrete Org UK. N.p., n.d. Web. 12 Oct. 2021.

“Carbonation is a measure of the degradation in steel-reinforced concrete as it ages and is especially relevant to concrete structures exposed to moisture.” – “Deep Carbonation in Concrete Cracks.” ALS Global. N.p., n.d. Web. 12 Oct. 2021.


The common theme here: Moisture entering the concrete matrix.

This moisture brings all sorts of soluble contamination into the concrete – and where the level of carbon dioxide (or carbon monoxide) is higher from exhaust fumes, like highways, road tunnels, and metros, the risk of this occurring is higher.

But what does carbonation do?

The initial reaction is to change the state of the concrete surface. The colour slowly turns to a black concrete face – have you ever noticed road barriers with black streaks? That’s a result of concrete carbonating. That black surface is dusty – but that isn’t the only risk to the concrete.

Surface dust slowly wears the concrete away, but the major damage is caused deeper down when corrosion forms and opens up cracks in the concrete.

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


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.


Concrete is full of capillaries that are highways for moisture.

The blue lines, indicating water migration, travel around the aggregates and sand which is connected by CSH, the strength of concrete. 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.


The key is to immobilise the water

Stop it from entering, but also prevent any that is inside the concrete from moving around.

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.

Now, we have to remember, this is at microscopic level. A typical concrete pore or capillary is around 50 nanometres, and colloidal silica is in the range of 5-20 nanometres in size. For scale, a human hair is 100,000 nanometres.

The silica reacts with moisture, and turns it into a gel, filling the porosities, stopping that moisture from moving, and preventing any more from entering the concrete matrix.

Have you ever seen the gel test? That’s what is happening inside the concrete when the treatment is spray-applied to the surface and penetrates the concrete matrix. 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.

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 increased density is permanent.


From that point on, carbonation holds no more high 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 penetrate into the concrete – neither can any other grime or contamination.


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.


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 


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


Photo by Redd F on Unsplash

Let us keep you Updated.

Subscribe to our ON.CRETE. updates to receive our latest content, plus news and views from the world of concrete.

Scroll to Top