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The degree of compactability test is described in BS EN 12350-4:2009 Testing fresh concrete: Part 4 Degree of compactability which replaces BS 1881: Part 103 Testing concrete. Method for determination of compacting factor.
The two tests, degree of compactability and compacting factor, are quite different.
For the degree of compactability concrete is carefully placed into a container, avoiding any compaction of the concrete. When the container is full, the surface is struck off. The concrete is then compacted by vibration (either by a vibrating table or a vibrating poker) and the distance from the top of the compacted concrete to the top of the container is measured.
The degree of compactability = internal height of the cylinder / height of the compacted concrete.
Acknowledgement: Concrete Society