CALCULATING QUANTITIES OF MATERIALS FOR CONCRETE

As a general ‘rule of thumb’, one cubic metre (1m ) of compacted and finished concrete weighs 2400kg. It is easy, therefore, to calculate the quantity of concrete based on the weight of a concrete slab or foundation. From this, the quantity of each component part of the mix can be calculated based on the mix proportions used.

For example, for a foundation slab 10m long x 4m wide x 0.25m thick, using a concrete mix of 1:2:5 (cement:fine aggregate:coarse aggregate), the calculations would be as follows:

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Volume of concrete = 10m x 4m x 0.25m = 10m @ 1:2:5 (total of 8 component parts).

Weight of concrete = 2400kg/m3 x 10m3 =

24000kg.

Add an amount for waste (typically 10 per cent) =

24000kg x 10 per cent = 26400kg @ 1:2:5.

So the quantities are:

• Cement = (26400kg + 8 parts) x 1 = 3300kg

• Fine agg’ = (26400kg + 8 parts) x 2 = 6600kg

• Coarse agg’ = (26400kg + 8 parts) x 5 = 16500kg

Water content is typically 50 per cent of the cement content by weight; 1kg of water = 1 litre, so the amount of water required = 3300kg x 50 per cent = 1650kg = 1650 litres.

Note: when calculating quantities by weight, there is no need to take account of volume shrinkage on mixing!

Good quality concrete must:

• have the correct mix proportions;

• use well-graded aggregates;

• use clean and dry (whenever possible) aggregates;

• have the correct water:cement ratio;

• have adequate but not excessive workability for the circumstances;

• be properly and sufficiently mixed;

• not segregate during mixing or transportation;

• be well compacted;

• be adequately cured;

• be adequately protected from the elements;

• be free from shrinkage cracks;

• be durable and weather resistant;

• be of sufficient strength and fit for purpose.