Concrete – Part 2 - Understanding the basics – Carbonation

There are a number of different defects that will influence the durability and ultimately the structural integrity of a concrete component. Concrete defects are difficult to visually identify in their early stages and generally only become evident when staining, cracking or distortion, start to occur

Source: http://sussexropeaccess.co.uk/

The popularity of concrete as a construction material was discussed in last week’s article (Link) due to its many positive attributes, including; it’s strength in compression, it’s flexibility, as it can be poured into infinite shapes/forms and sizes, it can be applied in situ (on site in its wet form), or it can be cast in a factory and delivered site as a complete component (pre-fabricated), it has good fire resistant qualities and is very durable if constructed correctly and maintained well.  The imperfections in concrete were also introduced and in particular concrete’s weakness in tension, where steel reinforcement is introduced to address this deficiency. 

As also stated in last week’s article, when encapsulated in the very high alkaline environment of concrete, reinforced steel will passivate.  This means that the steel will be much less chemically active than it would normally be as the alkaline concrete is effectively protecting it.  A particular problem however is that concrete is porous allowing moisture and other contaminants to enter the concrete which can eventually lead to corrosion problems of the steel reinforcement.  This will then lead to associated degradation and spalling (pieces of concrete breaking and falling away), which can result in significant serious health & safety implications.

A number of years ago I can particularly remember undertaking an inspection of a large Secondary School in the West Midlands area.  One of the largest blocks within the school was a three storey 1970’s exposed concrete frame building with brick infill panels. As with many secondary schools around the UK much of the building stock was poorly maintained due to ongoing funding issues.  I noted severe horizontal cracking of the concrete in a number of locations at first and second storey level, to a point where large piece of concrete were hanging precariously over a walkway which was one of the main thoroughfares around the school.   Strangely, staff at the school seemed to be oblivious to the danger, possibly because the problem was not at ground level and therefore not obviously visible.  Needless to say, I spoke to the Head Teacher and contacted the Local Authority immediately and had the area cordoned off until emergency repairs had been carried out to make the area safe.  This emphasises how serious concrete defects can be if left untreated.

There are a number of different defects that will influence the durability and ultimately the structural integrity of a concrete component, similarly to the example discussed above. Concrete defects are difficult to visually identify in their early stages and generally only become evident when staining, cracking or distortion, start to occur.  It is therefore worth understanding some of the common defects which affect concrete, one of which is carbonation.

Carbonation – Newly installed untreated concrete especially where located externally is particularly vulnerable to carbonation.  Once concrete is exposed to the air, carbon dioxide which is present in the air in concentrations of approximately 0.3% by volume (www.answers.com) will dissolve in water which can be present within the pores of the concrete and will form a mildly carbolic acidic solution. This acidic solution reacts with the alkaline calcium hydroxide (which is one of the compounds in concrete) to form calcium carbonate. This results in a pH value drop from more than 12.5 to approximately 8.5, which significantly reduces the alkalinity of the concrete. The carbonation process progressively moves through the concrete, with the pH drop occurring through the concrete. When the carbonation reaches any reinforcing steel, the passive layer around the steel will deteriorate when the pH value falls below 10.5. The passive protection around the steel therefore disappears so that when the steel is now exposed to moisture and oxygen, it makes it vulnerable to corrosion. The image below (started at the top left hand corner), demonstrates the progress of carbonation through concrete.

Source: http://theconstructor.org/

Testing is necessary to confirm that carbonation is occurring, which is usually carried out by applying a phenolphthalein solution to the surface of a freshly fractured or freshly cut piece of concrete. When the solution is applied non-carbonated areas will turn red or purple while carbonated areas remain colourless. Phenolphthalein will change colour at a pH of 9.0 to 9.5. Non-carbonated concrete without any admixtures will achieve a pH value of 12.5 or slightly higher.  The image below shows that carbonation has occurred at the left hand side of the sample taken, whereas the right hand side remains un-carbonated.

Source: http://www.expin.it/

I have made reference the pH scale throughout this article. It is therefore worthwhile adding the image below for completeness;

Source: http://alissasohlovechemistry.wikispaces.com/

In next week’s article I will discuss a number of other defects that can occur in concrete including chloride attack, alkaline silica reaction, sulphate attack amongst others.

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