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:
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.
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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