Sunday, November 2, 2014

Concrete – Part 1 - Understanding the basics

If installed correctly, newly installed reinforced concrete should last for many years with a minimal amount of maintenance; however it can be vulnerable in certain locations/uses to a number of possible defects, especially where used externally 

Concrete is an extremely popular material for construction and can be found in most parts of the World in one form or another.  In many countries, along with the use of steel, concrete is the primary material used for buildings/structures of all shapes and sizes, because of its many positive attributes. These include; being extremely strong in compression, which means that it can sustain large loads which are applied, before it will start to deteriorate or fail. It is extremely flexible 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 very good fire resistant qualities and is durable if constructed correctly and maintained well.  A significant disadvantage however is that concrete is extremely weak in tension, which in basic terms means that it will break up very easily when forces are applied that try to push or pull it apart.

Compressive and Tensile forces - To understand compression and tension forces let us think about a spring as an example.  If an even ‘push’ force is applied to each end of the spring at the same time then the spring will compress and shorten.  This action is adding compressive forces into the spring.  When a load is applied to the top of a concrete component in a building then exactly the same forces are being introduced, however concrete has the ability to withstand high levels of compressive forces, particularly when certain mixes are used or when steel reinforcement is added. Tension or tensile forces are effectively the opposite of what is described for compression.  Using our spring again as an example, instead of applying an equal ‘push’ force at either end, let us now apply a ‘pull’ force. This will lengthen the spring and add tensile forces within it.  This is what will happen toward the bottom of a concrete beam.  Whereas the compressive forces applied at the top of a concrete beam will apply load in a downward direction and compress the beam, this can also result in tensile forces appearing near the bottom of the beam which will have a tendency to want to ‘pull apart’, as identified in the image below:

Concrete’s weakness in tension is therefore mitigated by introducing steel reinforcement (which is strong in tension) at the position in the concrete which is weakest in tension, which is near the bottom of the beam.  The result is a complete component which is strong in both compression and tension and capable of withstanding extremely large loads/forces, which is ideal for building and construction.

If installed with the correct materials/mix and good workmanship, newly installed reinforced concrete should last for many years with a minimal amount of maintenance; however it can be vulnerable in certain locations/uses to a number of possible defects, especially where used externally.  Some of the more serious concrete defects are a result of deterioration of the concrete which results in the reinforcement being exposed and starting to corrode.  Concrete is a very alkaline material, typically 12.5 to 13 on the PH scale.  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.  If corrosion to the reinforcing steel occurs this will result in the build-up of corrosion generating internal stresses and subsequent cracking and spalling (breaking and falling away) of the concrete. This is demonstrated in the image below.
As explained above, when first installed the reinforcement in the concrete does not corrode because the concrete provides a protective alkaline environment due to the presence of large quantities of calcium hydroxide which is produced as Portland Cement hydrates and cures (hardens). (Portland Cement is the most common form of cement used in concrete for general purposes, which is produced from firing a mixture of clay or shale, and limestone or chalk.  The clinker that is produced in the kiln, as a result of the firing process is ground to the fine light grey powder which most people will be familiar with). However, when moisture and other contaminants enter the concrete an environment for a range of different concrete defects is created.

Over the next two weeks I will consider a number of different concrete defects including Carbonation, Chloride Attack, Alkaline Silica Reaction and Sulphate Attack.  Specific concrete defects are difficult to identify from a purely visual inspection, however, armed with the information discussed above and a little knowledge of what to look for it is possible arrive at a reasonable prognosis, which can be later confirmed with sampling and testing of the concrete.

Concrete is a very dense/heavy material and when it starts to exhibit defects that can result in cracking and spalling it can be very serious from a structural perspective as well as a health & safety perspective. The images below provide some examples of what can happen when concrete starts to exhibit defects, some of which I will discuss in more detail next week.

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  1. Thanks for posting this article Gary...very interesting.

  2. Very useful Gary, perhaps you would consider contacting Michelle Tobias at pro conferences. I think we would all benefit from hearing this as a presentation at a CPD event

  3. Thank you for such a clear explanation especially the diagrams.