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Learn A Bit More About Concrete

Estimating & Ordering Materials

This section gives guidelines to answer the most commonly asked questions, such as:

  • 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?

Concrete Strength

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.

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

  • Low strength concrete (15 MPa): suitable for unreinforced foundations (single storey only), mass fill, infill concrete in masonry (only with 13,2 mm stone)

  • Medium-strength concrete (25 MPa): suitable for unreinforced slabs or floors, reinforced slabs and foundations, footpaths and domestic driveways

  • High-strength concrete (30 MPa): suitable for reinforced concrete members, heavy duty floors and precast items such as concrete flagstones

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

The tables below give different mix proportions for different stone sizes. 

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

ALWAYS use as little water as possible. Use only enough to achieve the required workability and finishability of the fresh concrete. The addition of too much water will reduce the concrete's strength.

The cement must also comply with other physical and chemical requirements as specified in SABS EN 197-1 /SANS 50197-1 Cement - Part 1: Composition, specifications and conformity criteria for common cements.

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.

NOTE THAT the mix proportions are based on stone and sand batched in a loose state, i.e. poured into the batching container without being compacted.

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

TABLE 1

Cement

50 kg

bags

Sand

Stone

Yield

m3

Litres

Wheelbarrows

Litres

Wheelbarrows

Low strength, 15 MPa

2

230

3

230

3

0.35

Medium strength, 25 MPa

2

160

2

160

2

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

TABLE 2

Cement

50 kg

bags

Sand

Stone

Yield

m3

Litres

Wheelbarrows

Litres

Wheelbarrows

Low strength, 15 MPa

2

230

3

160

2

0,30

Medium strength, 25 MPa

2

160

2

130

2

0,24

High strength, 30 MPa

2

130

2

100

1

0,20

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.

Note: Filled flush, a wheelbarrow holds 65 litres.

The above mix proportions 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 millimeters 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.

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

MEASURE: the depth or height, length and width of each element for which you require concrete in metres

MULTIPLY: Height x length x width (measured in metres)

EQUALS: Volume of concrete required in cubic metres (m3)

USE: the tables above to help you order the required material. Or alternatively you could use Tables 3 & 4 below to assist you.

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

Cement

50kg Bags

Sand

M3

Stone

M3

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

Cement

50kg Bags

Sand

M3

Stone

M3

Low strength, 15 MPa

5,8

0,65

0,65

Low strength, 15 MPa

6,7

0,65

0,54

Medium strength, 25 MPa

7,7

0,62

0,62

Medium strength, 25 MPa

8,4

0,62

0,54

High strength, 30 MPa

9,2

0,60

0,60

High strength, 30 MPa

10,0

0,60

0,50

Dry mix materials:

A viable alternative for the home handyman is to buy materials from the hardware store or builder's merchants as dry mixes.

Bags of dry mixes for concrete, mortar and plaster contain all the materials needed for the application in the correct proportions. All you need do is just add water.

Now you are ready to start mixing... and placing, compacting and finishing the concrete - all of which are very important in ensuring that you produce good quality concrete. This applies to concrete cast in place, as well as to concrete for precast items such as decorative stepping stones, concrete sleepers, birdbaths and garden ornaments.

All of this is covered in the next section Working The Concrete.

Please navigate in the especially provided table at the top.

Warning: Safety requires a commitment from everyone involved. Although erecting a bolt-together pre-engineered steel building does not require specialized skills like carpentry or masonry and many of our customers do it themselves, construction work is dangerous, and larger buildings bring larger hazards.

Everyone at the site should wear appropriate protective clothing, including:

  • Hard hats

  • Gloves

  • Rubber-soled shoes

  • Eye-protection

On the up side, your erectors will have an outstanding set of clear common-sense drawings and can call on us for support if necessary.

 We seek to deliver consistent perfection and quality in the realm of all our DIY kits.

Try us: you may just experience enormous benefits.

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