Concrete is an artificial stone that you can make yourself.
It has four ingredients that all contribute to the overall strength.
- Large stones (Aggregate) – The aggregate provides shear strength to the concrete. In other words the concrete has to crack around rather than through the stones. A good aggregate will have a mix of stone sizes rather than just one size of stone, this help the aggregate lock together
- Sand – acts to fill in the spaces between the aggregate and further helps lock it solidly.
- Cement powder – Is the basis for the ‘cement paste’ (glue) which coats the surface of the first two components and holds them together.
- Water – Reacts with the cement powder to form the cement paste
Just like baking a cake its important to get the proportions of the various components right for the best results.
Too Much Aggregate compared with the sand and the cement paste will be be just cement coated stones stuck together where they touch. (known as ‘Boney’)
Too Little Aggregate, or Too Much Sand and the mixis called ‘Fatty’ There will be too little aggregate to provide shear strength and much of the aggregate will sink to the bottom.
Too Much Cement will cost you money without adding to the strength of the concrete.
Too Much Water will dilute the cement paste meaning the ‘glue’ has less strength and will also cause increased shrinkage as the cement cures. (It is very unusual to use too little water as the concrete will be very dry and as a result be hard to place and finish)
If you are going to lay large areas of concrete its probably be best to get premixed concrete which should have the ingredients in the correct quantities.
For small quantities you can buy bags of dry concrete mix where you just add water. Make sure you follow the instructions.
If you have got a job that is of a reasonable size but too small for premixed concrete here is a suggested mix:
- Cement = 1 part.
- Sand = 2.5 parts.
- Stone or gravel = 3 parts.
This should be fine for paths, garden wall foundations, fence posts, washing poles, and driveways.
If you are going to order Ready Mix N25 (Normal 25) should be fine for a driveway, pathway or shed foundation.
For similar posts see Concreting