The strength of concrete varies with constituent proportions. When concrete is to be mixed on site rather than ordered from a concrete supplier, an estimate of the materials needed is necessary. Materials can be purchased as individual constituents with either the fine and coarse aggregate pre-blended (ballast/all-in aggregate) or separately.The quantities of cement, sand and coarse aggregate or cement and ´all-in aggregate´ for various concrete volumes are estimated in the Table below. The bags of cement are assumed to be 25kg and the largest size aggregate is assumed to be of nominal size 20mm.
Mix
Use
Materials
Separate
All in
Quantity
per m3
Foundations
Footings, blinding, fence posts
Cement
Sharp Sand
Coarse aggregate
All in aggregate
Water*
1
2.5
3.5
1
6
5 bags
500 kg
800 kg
1300 kg
100 litres
General use
Garage, shed base & domestic floors
Cement
Sharp Sand
Coarse aggregate
All in aggregate
Water*
1
2
3
1
5
6 bags
450 kg
850 kg
1300 kg
100 litres
Paving
Paths, drives & steps
Cement
Sharp Sand
Coarse aggregate
All in aggregate
Water*
1
1.5
2.5
1
4
8 bags
400 kg
800 kg
1200 kg
100 litres
Bedding mortar
For paving slabs
Cement
Sharp Sand
Water*
1
4
8 bags
1000 kg
130 litres
* Water should be added to obtain a workable but non-segregating mix, the total free water being roughly 170 litres per cubic metre of concrete, and 203 litres per cubic metre for bedding mortar. The water figures in the table for 1m3 have been adjusted to account for water in the aggregate.
The table is for batching by volume. A suitable gauge box or similar is therefore needed. A simple method is to use bucket, based on 1 bucket of cement. For a 1:2:3 general use mix it will be 1 bucket of cement with 2 buckets of sharp sand and 3 buckets of coarse aggregate, alternatively 5 buckets of all-in aggregate. The figures assume a general bulking factor for the materials and do not take into account any water in the sand or aggregate.
Please note: Use a separate shovel and bucket for the cement so that they do not get damp causing lumps in the cement or bucket.
See additional entries below for mixing small quantities by small hired concrete mixer or by hand. Another option is to use pre-batched bags of materials form builders merchants and just add water.
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