Concrete Degradation – How Durable is Concrete?

Although the discovery and use of concrete – defined as a hardened mixture of aggregate, water, and a binder of some kind – can be traced all the way back to the Roman era, there are some distinct differences between ancient concrete and today’s modern concrete. The two major differences in favor of modern concrete are the use of tensile reinforcement and the ability to pour it into formwork – as opposed to layering it by hand.

Tensile reinforcement in the form of steel reinforcement bars – or rebar – plays a vital role in the strength and versatility of concrete structures today. Ancient Roman builders were limited in their use of concrete due to their inability to overcome this weakness. Having said that, even today’s more advanced concrete structures are some of the first to fall in large earthquakes.

As amazing and versatile a structural element concrete is, there are situations and conditions in which it is more likely to fail or exhibit structural defects. Needless to say, the durability of concrete is largely dependent on mixture and application, with service conditions often being a secondary cause of failure. Let’s Cracks in Concrete wallcover some of the basic and most important steps in concrete installation to ensure maximum service life and durability:

  1. Proper mixture. The idea is to get a homogenous and uniform mixture with as little water as possible. Concrete obviously needs water to hydrate, but after a sufficient amount is added to begin the chemical reaction, the less water there is the stronger the concrete will turn out. This needs to be balanced with workability as the drier the mixture is the harder it will be to spread and give shape. The amount of larger aggregate other than sand will vary significantly depending on whether the application is large or small. Smaller applications such as post stumps and fill-ins are best done with a 1:3:1 ratio – cement, sand, gravel, respectively. Larger applications such as foundations and structural elements are best done with larger gravel ratios – 1:3:5, cement, sand, gravel respectively.
  2. Sufficient and properly installed steel reinforcement. Steel rebar adds critical tensile strength to concrete without which it would not be very practical as a construction material. But equally as important is the correct placement of this rebar. If the rebar is too close to the surface, there’s risk of premature corrosion and spalling. Small cracks will always form to a certain extent, and it is through these cracks that water will enter. Repeated freeze/thaw cycles of this water will cause these cracks to get bigger, subsequently attacking the rebar if it is close enough to the surface.
  3. Proper curing. The ultimate strength of finished concrete has a lot to do with its curing process. The rule of thumb is to cure the concrete as slowly as Corroding rebar in concretepossible. This is why you might see concrete structural elements wrapped in plastic sheeting or sprayed with water regularly in dry or hot climates. Ponding is another method used to ensure proper curing where the curing concrete is a horizontal surface. Concrete will achieve 90% of its strength only after about 3 weeks, so you can see the importance of a slow curing process. Reduced strength, cracking, and spalling are all possible signs of improper curing.

As you can see, how durable concrete is or becomes depends on various factors. Aside from the above points, there are various additives you can mix into your concrete to make it more workable or resistant to certain elements. For example, you can add a plasticizer (1 to 2%) to make it not only easier to apply but also lessen the amount of water you’ll need to mix in – thus making it stronger by virtue of there being less water.

Or if you need it to be impermeable to water you can add a sealing agent (recommended for all outdoor applications). Other additives like accelerators and retarders can also be used to control the curing process for various situations. With these things in mind you can ensure your concrete job lasts as long as possible and accomplishes the purpose for which it was made. If done right, concrete is known to last hundreds or even thousands of years!

  • See Properties of Concrete for additional details and facts regarding the strength and properties of concrete.

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