How To Work Out The Concrete Mix For Your Mould

For ease of calculation, we work by volume and not weight. Thus 250ml of oxide would be 1 cup of powder, or 5ml would be one teaspoon.  If using 1 Litre of cement, use an old 1 litre bottle and fill that.  Once you have measured the volume you can use any container marked with what you need to use. For instance, you could use a 10 Litre bucket for the stone.

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• Assume mould size is 100mm x 100mm x 50mm (9-cavity).
• Therefore 10cm x 10cm x 5cm x 9 (cavities) = 4 500 cubic centimetres (or 4,5 litres) total volume.
• Assume the mix ratio required is 1:2:3 (one part cement to two parts sand to three parts stone – six parts in total. (all mixes are printed on a pocket of cement).
• Now divide the six parts into 4500ml:
  750ml x 1 = 750ml Cement
  750ml x 2 = 1 500ml Sand
  750ml x 3 = 2 250ml Stone
  Total = 4 500ml (if you have more than one mould, simply multiply by that number)
• Increase each of your parts by about 10% because volume is reduced when mixing in the water. Otherwise the mould will not be filled to the brim as convergence takes place.
• The volume of water required varies a lot depending on size and type of sand, stone and cement used. Your material might also be damp. Your mix should be roughly the same consistency of thick porridge. (too much water will reduce concrete strength and cause excess shrinkage.)
• Pour mix into mould, (ensure that it has already been prepared with release agent) and shake mould to remove air bubbles. Now leave to set.
• After concrete has set, de-mould by inverting and distorting the mould slightly and pressing in the centre back of the mould.
• Your mould can now be cleaned and prepared for the next casting.
• Cure the concrete by keeping moist for 7 days (concrete reaches full strength after 28 days).
• The laying of the stones can take place as soon as enough strength has been attained. However, driveway stones should not be driven on for 28 days.