Concrete carbonation causes embedded steel reinforcements to corrode, with the resulting expansion cracking and weakening the concrete.  It commences as soon as concrete is exposed to the atmosphere, and can advance at a rate of 1mm to 5mm per year, dependent on the concrete’s porosity and permeability. Carbonation is the most common cause of reinforcement corrosion in above ground structures but it can be prevented or halted with the application of a protective anti-carbonation coating system.

Concrete carbonation is the result of an electrochemical reaction between carbon dioxide, moisture and calcium hydroxide that is present in cement, producing calcium carbonate.  Calcium carbonate lowers the alkalinity of concrete from pH12 – 13 to around pH9.  This hardens the concrete and increases its compressive strength, but at this reduced pH level the protective passivation layer surrounding the reinforcing steel begins breaks down, leaving the steel vulnerable to corrosion.

Rust (Iron oxide) has up to 6 times larger volume than the original steel, so as the steel reinforcements rust, the expansion causes the concrete to crack and spall.

How is concrete carbonation assessed?

Concrete carbonation is assessed using an alkalinity indicator, usually Phenolphthalein, to establish the extent of the carbonation and its proximity to the reinforcing steel.  Carbonation induced corrosion is not generally a risk if the carbonation is further than 10mm from the steel.

However, until recently it has been widely accepted that loss of the passive protection of the steel reinforcement in concrete occurs at around pH9 -10.  Recent research suggests that the passive protection is lost at around pH11, rather higher than previously thought.

This presents a new challenge in the accurate detection of the extent of carbonation, as the conventional Phenolphthalein indicator provides a good indication of alkalinity up to around pH9.  The latest research found instances of corrosion (caused by carbonation) around 5 – 10mm ahead of the pH9 area detected by Phenolphthalein.  This indicates that carbonation induced corrosion commences at an estimated pH11.

How is concrete carbonation treated or prevented?

Carbonation can be prevented or halted with the application of a protective anti-carbonation coating system.  This should be specified for both new and existing structures.  There are several products available: epoxy coatings; acrylic sealers; and silane sealers.

It is important to specify a coating system that provides protection against the ingress of carbon dioxide, oxygen and water, whilst allowing damp substrates to breathe without blistering.

Contact Rooflock technical team today for assistance with our Flowlock range of Anti-Carbonation coating systems to protect new and existing concrete structures.