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The compacting factor test is described in BS 1881: Part 103 Testing concrete. Method for determination of compacting factor. This has now been withdrawn and replaced by BS EN 12350-4:2009 Testing fresh concrete: Part 4 Degree of compactability.
The two tests, compacting factor and degree of compactability are quite different.
For the compacting factor test, the top hopper is filled with concrete. The trap door is then opened to allow the concrete to fall into the lower hopper. The trap door of the lower hopper is then opened to allow the concrete to fall into the cylinder. The surface of the concrete in the cylinder is then struck and the cylinder full of concrete is weighed. The amount of concrete is then compared with the amount that fills the cylinder when the concrete is compacted in layers.
The compacting factor = mass of free-fall concrete / mass of compacted concrete.
Acknowledgement: Concrete Society