Compaction Factor Test of Concrete

The compactability of concrete is measured using the compaction factor test, which is an important feature of workability. The compaction factor is the weight difference between partially compacted and fully compacted concrete.

The compacting factor test was established in the United Kingdom and documented in BS 1881-103: 1993. It is suitable for aggregate sizes up to 40mm. This test operates on the principle of determining the amount of compaction achieved by a standard amount of effort done by letting the concrete to fall through a specified height.

It’s especially effective for concrete mixes with medium and low workabilities, which are often used when concrete is compacted by vibration; these dry concretes aren’t affected by the slump test.

The test is not acceptable for concrete with a very low workability of 0.7 or below, because this concrete cannot be thoroughly compacted for comparison in the manner stated in the test.

This test is more accurate than the slump test, especially for medium and low workability concrete mixtures. In laboratories, the compaction factor test is more commonly used to measure the workability of concrete mixes.

Apparatus Used in Compaction Factor Test

Two hoppers and one cylinder mould are stacked in three layers to make up the compacting factor device. The device stands between 1.2 and 1.4 metres tall. To release the concrete, the first two hoppers have small hinged doors at the bottom.

You’ll also require some additional equipment, such as a concrete mixing pan, steel rod, and weighing machine.

Procedure

• The concrete sample to be analysed is gently placed in the upper hopper.
• The hopper is loaded to the top with concrete, and the hinged door is opened to let the concrete fall into the lower hopper.
• Certain combinations have a natural tendency for clogging one or both hoppers.
• If this happens, the concrete can be pushed through by gently pushing the rod into the concrete from above.
• Trowels should be used to cover the cylinder throughout this process.
• The cylinder is uncovered, the trapdoor of the bottom hopper is opened, and the concrete is allowed to fall into the cylinder immediately after it has come to rest.
• The extra concrete that remains above the top of the cylinder is then cut away.

• The weight of partially compacted concrete is calculated by rounding the weight of the concrete in the cylinder to the nearest 10 g.
• The cylinder is replaced with concrete from the same sample in 50 mm layers, which are vigorously rammed or preferable vibrated to achieve complete compaction.
• The fully compacted concrete’s top surface is carefully struck off level with the cylinder’s top.
• The weight of partially compacted concrete divided by the weight of fully compacted concrete is known as the compacting factor.
• It is usually expressed to the second decimal place.
• The compaction factor ranges from 0.78 to 0.95 in most cases, while concrete with high fluidity has a greater compaction factor.

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Observation

• Unlike the slump test, changes in the workability of dry concrete are reflected in a considerable shift in the compacting factor, indicating that the test is more sensitive at the low end of the workability scale than at the high end.
• The ratio of the height of the cylinder while full of compacted concrete to the height of the compacted concrete after vibration determines the degree of compactibility.

Below table shows the different workability values :

Points to be Noted While Performing Compaction Factor Test of Concrete

• This test is sensitive enough to detect changes in workability resulting from the cement’s first hydration phase.
• If strictly comparable findings are to be obtained, each test is carried out at a fixed time interval after the mixing is done.
• After the concrete has finished mixing, 2 minutes has been determined to be a reasonable time to release it from the higher hopper.