Trial Concrete Mixes:proportions & quantities for ordering

Notes

• Recommended concrete strengths for various uses are shown in the table below.
  
  
• Mix proportions in the table overleaf are based on the assumption that a CEM IIA 32,5N cement will be used. CEM I 42,5N or higher cements will give a stronger concrete but may be less economical. Cements with higher extender contents (eg CEM IIB or CEM III) will develop strength more slowly and will require particular care with curing. Masonry cements complying with SANS 50413-1 are not recommended for use in concrete.
  
  
• The amount of water required is not given in the table. The mix should contain enough water to achieve the required consistence. Consistence may be assessed by eye or measured by carrying out the slump test (SANS 5862-1:1994). Recommended slumps are:
  50–100 mm for compaction by mechanical vibration
  100–150 mm for compaction by hand
  
  
• 0,001 m3 = 1 litre
  The capacity of a builder’s wheelbarrow is 65 litres.
  
  
• A mix made according to this table, and to the required consistence, should be assessed for stone content before being used on a large scale. This can be done by compacting some of the concrete in a container, eg a bucket, by the means (vibration or hand tamping) to be used on the job.
  
  If stones protrude from the surface, stone content is too high.
  
  If not, scratch the surface of the compacted concrete (before it hardens) with a nail or screwdriver. If the stone content is right, stones should be found two or three millimetres below the surface. If they are deeper than this, the stone content is too low.
  
  If stone content is too high, reduce it by 10% and increase sand content by the same amount, ie volume or mass. Then reassess.
  
  If stone content is too low, increase it by 10% and reduce sand content by the same amount, ie volume or mass. Then reassess.
  
  
• The mix proportions given in the table overleaf are conservative. If the quantity of concrete to be made exceeds about 100 m3, it is probably possible to save costs by selecting materials and having a mix designed. For information on the choice of materials consult the C&CI.
  

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Published by the Cement & Concrete Institute, Midrand, 1997, reprinted 1999, 2001, 2003, 2006.
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