Being one of the most important properties of fresh concrete its measurement is unfortunately not possible but a test namely Slump Test which ACI describes as a measure of Consistency, but the test is very useful in detecting variations in the uniformity of a mix of given nominal proportions.
The mould for the test is Frustum Cone of 300mm height (12 inch ) . A tamping rod is required of 16mm (5/8 inch) height.
Procedure :-
The Mould must be placed on a smooth surface with smaller opening at the top . The concrete must be filled in three layers. Each layer must be tamped 25 times by the rod . Immediately after filling to brim of the frustum it must be lifted slowly . The concrete will start to slump ( hence the name of the test) and this must be measured.
The decrease in the height of the slumped concrete is called slump, and is measured to the nearest 5 mm (in.). The decrease is measured to the highest point according to BS EN 12350-2 : 2009, but to the “displaced original center” according to ASTM C 143-10.
Standards :-
Description of Workability and Magnitude of Slump
Classification of Workability and Magnitude of Slump According to BS EN 206-1 : 2000
Here is the key point : There are standards for slump values pertaining to different structural elements but the site conditions may also play role to control them and it is the on site structural engineer who can reach a more conclusive value.
A thumb rule : 7.5 cm to 13 cm (3in. to 5 in.) is general value with 9-10 cm being the best. While as the slump value for slabs , columns can be really decided on site as the volume and site conditions will be key factors.
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