Proportioning materials.

Thursday, December 9, 2010

The materials used for mass concrete for foundations were often measured out by volume, the amount of sand and coarse aggregate being measured in wooden boxes constructed for the purpose. This is a crude method of measuring the materials because it is laborious to have to fill boxes and then empty them into mixers and no account is taken of the amount of water in the aggregate. The amount of water in aggregate affects the finished concrete in two ways: (a) if the aggregate is very wet the mix of concrete may be too weak, have an incorrect ratio of water to cement and not develop full strength and, (b) damp sand occupies a greater volume than dry. This increase in volume of wet sand is termed bulking.

The more accurate method of proportioning the materials for concrete is to measure them by weight. The materials used in reinforced concrete are commonly weighed and mixed in large concrete mixers. It is not economical for builders to employ expensive concrete mixing machinery for small buildings and the concrete for foundations, floors and lintels is usually delivered to site ready mixed, except for small batches that are mixed by hand or in a portable petrol driven mixer. The materials are measured out by volume and providing the concrete is thoroughly mixed, is not too wet and is properly consolidated the finished concrete is quite satisfactory.

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