Ordering Pre-mixed Concrete
How do I estimate the quantity I need?
Concrete is measured in cubic metres (m). Simply multiply the length and width of the area to be concreted measured in metres by the depth in metres. This figure gives the quantity of concrete required in cubic metres. Concrete may be ordered in multiples of 0.2 m, so when ordering the quantity required it should be rounded up to the next 0.2 of a m. For a typical garage site of length 7.5 m, width 3.5 m, depth 100 mm (0.1 m), 7.5 x 3.5 x 0.1 = 2.63 m will be required. Rounded up to the nearest 0.2 m= 2.8 m
Remember Never underestimate. Allow at least 10% extra concrete to compensate for uneven ground and small amounts of wastage. For instance, in the example
above: 2.63 + 0.26 = 2.89, ie order 3.0 m.
How do I order the quantity I need?
A standard concrete truck has the capacity to carry 5 m of concrete. Smaller mini-concrete trucks have a carrying capacity of around 1.8 m. Always check with the local pre-mixed concrete supplier as to the minimum order quantity, remembering that it is ordered in multiples of 0.2 m, ie 0.2,0.4,0.6 m, etc. Confirm how long the truck can be expected to stay to discharge the concrete.
Always nominate the use of the concrete (ie driveway, path, garage floor, garden shed floor, etc). Remember to give name and address, delivery time and date and agree on method of payment.
Check the delivery docket to confirm the details of the order before placement.
1 m of concrete = 1000 litres. If using a 50-litre capacity wheelbarrow to transport the concrete, it will take 20 wheelbarrow loads each weighing 130
kg. It is recommended that a conventional 50-litre wheelbarrow is only half filled to make transporting concrete more manageable, even though it will
mean extra cartage. Make sure there are several able bodied helpers on hand, with extra barrows, to assist.
Site access difficulties may be overcome by using a concrete pump. This will require a specially designed concrete so that it will pass through the
pump/pipelines properly. Always seek advice from the pre-mixed concrete supplier if a concrete pump is to be used. He will advise the grade of concrete best suited to particular construction needs.
What grade (strength) should I order?
Pre-mixed concrete is mixed and supplied in standard grades of 20, 25, 32, 40 and 50 megapascals (MPa) which is the strength the concrete should reach in 28 days after placement (known as the 28-day strength).
For most home projects, Grade 20 should be used, ordered as N20. As a general guide, concrete supporting a structure such as a foundation wall,
should be a minimum of Grade 20. Grade 25 is preferable for driveways, heavily trafficked paths and patios and any concrete paving near the sea. It is easier to finish.
Always inform the pre-mixed concrete supplier of the intended purpose and use for the concrete and the type of finish to be achieved - eg smooth steel
float finish or wood float finish.
What does Concrete Slump mean? Is it important?
Slump is important when ordering pre-mixed concrete. Slump is a measurement of the workability of the concrete. For example, concrete with a 120-mm slump is very workable while concrete with a 40-mm slump is fairly stiff and much harder to work. For most work around the home, order either 80-mm or 100-mm slump.
Do not add water to batched concrete as this will reduce its final strength.
Steps to Remember
- Estimate quantity: volume in m (length x width x depth + 10%)
- volume m (rounded up to nearest 0.2 cubic metre)
- grade (eg N20)
- slump (eg 100 mm)
- use (eg path, garage floor, etc)
- when using a concrete pump, seek advice as to the recommended . concrete grade.
If you are unsure of your estimated quantity, confirm measurements (length in metres x width in metres x depth in metres) with the pre-mixed supplier when placing your order.