Concrete
One of the most common building materials used in structures as well as decorative additions is concrete. Although we are well accustomed to seeing it, concrete is not found naturally the way we find iron or aluminium. Concrete is made from cement, aggregate or rock, and water. The aggregate combines both fine aggregate like sand, and coarse aggregate like gravel.
People often incorrectly refer to concrete as cement, but cement is only an ingredient of concrete. Cement alone is not as strong is concrete, and it is much more expensive. Asphaltic concrete is used for making airfields in the UK and in worldwide road construction. It is made from asphalt cement which is a petroleum bi-product, as well as coarse aggregate.
Properties:
Most people think that concrete dries after mixing with the water and aggregate, and that is how it is formed. In actual fact, the concrete does not solidify. The water evaporates, causing the cement to hydrate, and thus gluing all the mixture together and making a substance similar to stone.
Due to this process and qualities of the mixture, concrete has a relatively high compressive strength, but much lower tensile strength.
For a quality concrete product, you should meet these criteria:
- Resistance to freezing/thawing/deicing chemicals
- Low permeability
- Resistance to wear
- Strength
Creating a quality concrete product relies on the water to cement ratio. Also, reducing the amount of cement used will reduce the price and make it a more cost-effective product. The mixture needs water to be workable, but conversely the less workable it is, the stronger the concrete will be.
To be considered ‘workable’, the fresh concrete must be able to fill its proposed form or mould without reducing the quality. Workability relies on the water content, amount of cement, the shape and size distribution of the aggregate, and the age. The age of the mix will affect its hydration. Additional water or a plasticizer will increase the workability, but an excess can result in water loss or segregation of the mixture.
Typical mixture composition:
- Cement: 7-15%
- Water: 14-21%
- Aggregate: 60-80%
To reduce the amounts of water and cement required, (and therefore keep costs down), you should use the stiffest mixture that you can make workable, use the largest aggregate appropriate for the job, and balance the fine to coarse aggregate ratio effectively. For example, concrete composed of smooth pebbles is not as strong as that made with rough broken rock.
Aggregate is the element that makes the difference between hardened cement and the structural concrete. The aggregate will strengthen the concrete and makes the mixture more effective. In short, you need a low water/cement ratio and a strong aggregate.
Uses:
Home Construction – Concrete is a useful material for building homes because it offers good heat and sound insulation, as well as being quick to build and easy to maintain. There is also a wide range of finishing that you can use to complete the house.
Driveways/Paving – You can use a ready mix concrete for these, which is beneficial because it lays well to support traffic and loads and endures all weather. It is equally durable against cracking, and can be designed in many ways and colours.
To construct your own concrete path or drive:
Mark a boundary around the area in which you wish to lay your concrete with wood pieces about 7 inches deep into the ground, and then prepare your hardcore. The hardcore lies below the concrete to provide a solid foundation, and consist of broken bricks or stones crushed into a solid depth of about 4 inches. Ballast can be used to fill in the gaps, and it should then be crushed firmly again with a garden roller or hired equipment.
To mix the concrete you should follow:
- 1 x Cement
- 2 x Fine Aggregate (Sand)
- 4 x Coarse Aggregate (Gravel/Rocks)
If all these are dry, a water to cement ratio of around 0.55 is required. So therefore if you use 25kg of cement that will equal 13.75 litres of water. However, frequently parts of your mix like the sand will be damp, and so you will have to carefully modify your mix so that it is not sloppy.
Always work immediately after the concrete is mixed and aim to use it all within two hours to prevent cracking.
Lay your concrete paving 3 inches thick, pushing the mixture against the wooden edges. With the back of a rake drag the concrete across the surface, making sure all edges are filled and that there are no air pockets.
Surface finishing must only be attempted when the concrete has lost its sheen and hardened enough so as to allow the steel finishing trowel to pass over it without causing damage or disturbing the cement. Kneel on a flat board to reduce surface pressure, and compact the concrete by chopping the surface as you finish. Shovel in and work on any areas that require more mixture, as concrete does not wear well in thin sections.
Do not remove the wooden boundary markers until the concrete is set, and you can lay polythene sheeting over the top for slower solidification. It will take around 4-10 days to harden.
Repairs:
If you discover a crack in your concrete, most repairs can be easily remedied using a diamond-shaped pointing trowel. Small cracks can be fixed with PVA adhesive mixed with sand and cement.
If the crack is simple with sharp edges the filling itself should be sufficient. If the edge is worn away it may lead to the new mix crumbling. To prevent this, chop into the old damaged concrete with a cold chisel and create sharp angles.
Scrape out the loose debris and make a mixture of cement and sharp sand at 1:3, and brush water into the crack. Force the mixture in as far as possible until full, and then level.