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When ordering premixed concrete, the following three specifications should be given as a minimum...
The usual order for concrete slabs is
20 MPA which means an indication of the strength of the concrete measured in the mega-pascals..
20 mm aggregate which means the coarse aggregate,( the stones) in the mix normally 20 mm maximum sized that is sieved through a 20 mm grid.
80mm slump which means a measure of the sloppiness of the mix determined by a slump test .
Premixed concrete can usually be ordered in an 0.2 cubic metre increments. Always pre-warn a concrete batching plant of the approximate volume of concrete required and asked for the trucks to be sent with suitable time intervals between 20 to 40 minutes to allow the emptying time of a previous truck. The emptying time can take from 10 to 20 minutes. If using a pump or shooting the concrete directly into the job, much more time may be required if the concrete is being moved into place using wheelbarrows. Some companies charge for waiting time if the truck is on-site for longer than 20 minutes.
The Slump Test.
It is important that the concrete is workable enough to enable it to form around the reinforcement and into the shape of the formwork during a concrete pour . On some jobs, this is critical, that a slump test is performed before in the concrete is used .A slump test is a measure of how workable a concrete mix is. Slump is measured in millimetres. Concrete, with a high slump is sloppy ,concrete with a low slump will be stiff and difficult to work with
Note
Do not add water on site, in an attempt to achieve a high slump, This weakens the concrete. Specify the appropriate slump, when ordering the concrete....
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