Concrete basics for building: Estimating and ordering materials

How much cement, stone and sand do you need for each cubic metre of concrete?

What are the mix proportions for the correct workability and required strength? How does this translate into how much cement, stone and sand to order?

This section gives guidelines to answer the most commonly asked questions.

Remember that we use the term ‘cement’ to cover all branded, mark-bearing cements that comply with SABS EN 197-1/SANS 50197-1 strength class 32,5N.

Concrete Strength

Three grades of concrete are generally specified for general building work:

• Low strength concrete (15 MPa) is suitable for unreinforced foundations (single storey only), mass fill, infill concrete in masonry (only with 13,2 mm stone).
• Medium-strength concrete (25 MPa) is suitable for unreinforced foundations (single storey only), mass fill, infill concrete in masonry (only with 13,2 mm stone).
• High-strength concrete (30MPa) is suitable for reinforced concrete members, heavy duty floors and precast items such as concrete flagstones.

1. Mix proportions by volume

For smaller jobs and projects, concrete is generally batched by volume. Where more sophisticated equipment is available, batching by mass ensures greater accuracy.

Different mix proportions are given for different stone sizes. Use as little water as possible to achieve the required workability and finishability of the fresh concrete. The addition of too much water will reduce the concrete’s strength.

The mix proportions given in this section are conservative so that, even where workmanship and material quality is likely to be poor, the concrete should achieve the desired strength.

1.1 Initial estimate

The figures given in Tables 1 and 2 for the most commonly used stone sizes (26,5 and 19 mm, as well as 13,2 mm) are an initial estimate of proportions and quantities of each material required. Volumes of stone and sand may have to be adjusted on site to give better workability and finishability, depending on the quality, fineness, etc. of the aggregates.

Table 1: Material for a two-bag batch of concrete, using 26,5 or 19 mm stone

  Cement, 50 kg bags Sand Stone Yield, m3
Litres Wheelbarrows Litres Wheelbarrows
Low strength, 15 MPa 2 230 230 0,35
Medium strength, 25 MPa 2 160 160 0,26
High strength, 30 MPa 2 130 2 130 2 0,22

 

Table 2: Material for a two-bag batch of concrete, using 13,2 mm stone

  Cement, 50 kg bags Sand Stone Yield, m3
Litres Wheelbarrows Litres Wheelbarrows
Low strength, 15 MPa 2 230 160 0,35
Medium strength, 25 MPa 2 160 130 2 0,24
High strength, 30 MPa 2 130 2 100 0,20

Note that:

• Mix proportions are based on stone and sand batched in a loose state, i.e. poured into the batching container without being compacted.
• Yield is the volume of fresh concrete produced by mixing the dry materials with enough water to produce a workable mix (technically, 75 to 100 mm slump) for hand compaction.
• The amounts specified do not include allowance for wastage.

1.2 Adjusting mix proportions on site

The mix proportions for concrete refer to average materials, and an estimated 5% moisture in the sand.

In practice, the ratio of sand to stone may have to be adjusted if the mix is too stony or too sandy.

• Too stony: individual stones protrude from the top surface of the concrete. Reduce the stone content by 5%, and add the same volume (not percentage) of sand.
• Too sandy: scratch through the mortar on the surface with a nail; if the top mortar layer is thicker than a few millimetres above the stone, the mix is too sandy. Increase the stone content by 5%, and reduce the amount of sand by the volume of stone added.

In each case, the total volume of sand and stone remains the same.

2. Ordering materials

Before ordering cement, stone and sand, calculate how much concrete you need.

MEASURE the depth or height, length and width of each element in metres.

MULTIPLY height x length x width (measured in metres) = volume of concrete required in cubic meters (m3).

ADD the volumes for each element together, and use Table 3 or 4 to calculate amount of material to order.

The amounts given are for hand-compacted concrete, and do not include wastage.

Table 3: One cubic metre of concrete, using 26,5 or 19 mm stone

  Cement,
50 kg bags

Sand
m3

Stone
m3
Low strength, 15 MPa 5,8 0,65 0,65
Medium strength, 25 MPa 7,7 0,62 0,62
High strength, 30 MPa 9,2 0,60 0,60

 

Table 4: One cubic metre of concrete, using 13,2 mm stone

  Cement,
50 kg bags

Sand
m3

Stone
m3
Low strength, 15 MPa 6,7 0,75 0,54
Medium strength, 25 MPa 8,4 0,68 0,54
High strength, 30 MPa 10,0 0,65 0,50

 

 


Source: Cement & Concrete Institute. Photograph: Harmon Concrete.