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Concrete Carbonation?

Why Should I Care?

Concrete Carbonation?
12 October 2021 Posted in , ,

Firstly, What is Carbonation?

Let’s hear from some of the experts in the concrete industry.
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.
The pH then drops from more than 12.5 to about 8.5. The carbonation process moves as a front through the concrete. When it reaches or nears the reinforcing the passive layer decays when the pH drops below 10.5 exposing the steel to corrosion.
Carbonation also occurs within the structure by entering through cracks and larger voids. Rapid breakdown can occur within areas of poorly compacted concrete making the re-bar more vulnerable to corrosion.
Carbonation is more rapid in urban and industrial areas, and alongside highly trafficked roads where CO2 is at higher levels.” – “Penetrating Hydrogels in Concrete Structures affected by Carbonation” 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.

“Carbonation of Concrete Structures.” The Constructor. N.p., n.d. Web. 12 Oct. 2021.
Source: “Carbonation of Concrete Structures.” The Constructor. N.p., n.d. Web. 12 Oct. 2021.

 

See a Common Theme?

As noted in each of these extracts, moisture content plays a significant part in the carbonation reaction.
In fact, later in Murray Gilbertson’s report, he says
“Removal of moisture within the pores of the concrete readily stops carbonation deterioration both within and on the concrete surface. It prevents the dissolution of CO2 and the formation of acid.” – ibidem.
 
So that looks like a window of opportunity for treating this issue, right? First, let’s look at the places where this effect is common, and why it needs attention.
 

Where is Carbonation Most Common?

As noted in one of the earlier extracts, carbonation is more rapid in urban and industrial areas, and alongside highly trafficked roads where CO2 is at higher levels.
 
From our own observations, we would add, any multi-storey or basement car park structure is a high-risk candidate. We have particularly seen the effect very particularly on exposed concrete roofs or soffits in these structures.
 

What are the Associated Risks

Firstly, thinking about the concrete itself – this is a slow-burning reaction that ultimately results in the corrosion of the steel. Not because of direct and immediate reaction with the steel, but because the resultant surface degradation and micro-cracking allow for the entrance of other contaminants, which in turn react with the steel. So that the carbonation mechanism will eventually lead to the deterioration and breakdown of the structure.
 
Caveat: note that expression ‘slow-burning’. For this reason, carbonation itself is not frequently regarded as a threat. However, when it comes to concrete deterioration, there is usually more than one thing happening at the same time – and carbonation may be part of a larger picture of premature deterioration.

Deterioration of a concrete ceiling due to carbonation - photo courtesy Structural Guide

BUT – and it’s a big BUT – there’s a half-hidden issue here, relating to health risks. As carbonation occurs, the surface of the concrete turns to dust and is often abraded away; or in the case of a ceiling, will slowly descend through the air. Step two – human beings can breathe this dust into their lungs. Important: this dust contains dry silica compounds, because that’s what concrete is made of. Ever heard of silicosis? It’s also slow-burning, and also a very real threat. In the context of a car park structure used by residents or workers every day, this is a serious concern.
 

How Can We Prevent or Arrest Concrete Carbonation?

Here’s where that point about the moisture arises. Specifically, we need to stop the moisture within the pores of the concrete – the moisture that moves about, stimulating the reaction. And yes – this moisture can be immobilised.
 
Spray-applied AQURON hydrogel is the answer, with its specialist catalytic action. The treatment penetrates reactively into the concrete, not dependent on gravity. This makes it perfect for ceilings and soffits, as well as vertical surfaces. The hydrogel treatment penetrates to about 150mm, using up and immobilising the moisture content. It also seals and hardens the concrete itself, enhancing durability and rescuing service life. Win-win!
 
Like to know more? Looking after a structure that needs some tender loving care? Get in touch!

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