Information for Ordering Concrete
When ordering pre-mixed concrete, you will need to tell the supplier:
- Name and address for delivery
- The use of the concrete (i.e. driveway, housing slab, commercial)
- The amount you need in cubic metres
- The Class of the concrete.
There are two classes that concrete can be supplied as: Normal Class and Special Class.
NORMAL CLASS CONCRETE
It has a strength grade of N20, N25, N32, N40 and N50 with the corresponding characteristic strength of 20, 25, 32, 40 and 50 MPa at 28 days. The slump at the point of delivery should be 20–120 mm and the maximum size of coarse aggregate should be 10, 14 or 20 mm. Normal class concrete is suitable for most purposes. For most domestic applications such as driveways and paths grade N20 and N25 are the common grades ordered.
SPECIAL CLASS CONCRETE
is specified when you have additional or alternative requirements to those for normal class concrete, eg lightweight aggregate, colour pigments, a non-standard strength grade. Special class concrete will not always be available from every concrete supplier.
- The slump in millimetres (mm).
- The maximum aggregate size (eg 20 mm).
- How you want to place the concrete (eg pump, shute, wheelbarrow, etc).
- Time of the first truckload and the time between truckloads. There must be enough time to place and compact one load before the next arrives.
- Any admixtures you may want in the concrete, though this is normally left to the pre-mixed concrete supplier.
- Always order more concrete (i.e 10%) than you need to allow for construction variations and/or some wastage. Concrete is ordered in 0.2-m3 increments. Ensure you round up when ordering.
IF MIXING YOUR OWN concrete: The cement powder is ordered in bags by weight (eg 20 kg) and type (eg Type GP). The coarse and fine aggregates are ordered in cubic metres by maximum size (eg 20 m3 of size 20 mm). Water is used by the litre or kilogram (1 litre of water = 1 kilogram).