How to perform a compaction factor test on stiff concrete: Procedure and interpretation

The compaction factor test measures concrete workability by comparing the weight of partially compacted concrete (dropped under gravity alone) to the weight of the same concrete after full vibration or rodding. For stiff, low-slump mixes that barely react in a slump test, this method delivers precise, repeatable results that help you assess whether the fresh concrete will consolidate properly around reinforcement without voids.

Originally developed by the Road Research Laboratory in the United Kingdom, the compaction factor test is particularly suited to laboratory conditions and is commonly specified for mixes designed to be compacted by vibration. The formula is straightforward: compaction factor equals the weight of partially compacted concrete divided by the weight of fully compacted concrete, expressed to two decimal places. A value closer to 1.0 indicates higher workability.

What apparatus do you need?

The compaction factor apparatus consists of two conical hoppers mounted above a cylindrical mould. Each hopper has a trapdoor at the bottom. In addition, you require:

The method documented in BS 1881-103:1993 is now withdrawn; the current UK standard BS EN 12350-4:2019 measures degree of compactability through a different procedure using height readings rather than the two-hopper weight comparison. Always follow the method specified in your project or laboratory quality system. In India, reference IS 1199 (Part 2):2018 rather than the obsolete IS 1199:1959.

How do you perform the test step by step?

First, fill the upper hopper loosely with the concrete sample using the hand scoop, level with the rim. Do not tamp or compact at this stage; the fall distance provides the only compacting energy. Cover the cylinder at the base, then release the upper trapdoor so the concrete drops into the lower hopper. Use the tamping rod gently to dislodge any concrete clinging to the sides of the upper hopper.

Next, release the lower trapdoor and allow the concrete to fall into the cylinder. Strike off the excess level with the top using a trowel, wipe the outside clean, and weigh the cylinder plus concrete. Record this gross mass as W₁. Empty the cylinder, refill it with the same concrete in layers of roughly 5 cm, and compact each layer thoroughly—by vibration or rodding—until fully consolidated. Level the surface, then weigh again. Record this second gross mass as W₂. Finally, weigh the empty cylinder and record as W.

What does the formula tell you?

The compaction factor is calculated as (W₁ − W) ÷ (W₂ − W). The tare weight of the cylinder must be subtracted from both gross masses. For example, if the empty cylinder weighs 12.40 kg, the cylinder with partially compacted concrete weighs 29.18 kg, and the cylinder with fully compacted concrete weighs 31.05 kg, then the compaction factor is (29.18 − 12.40) ÷ (31.05 − 12.40) = 16.78 ÷ 18.65 = 0.90.

Typical ranges are:

Because cement hydration begins the moment mixing water is added, elapsed time matters. Test at the interval required by your method and record any deviation so results remain comparable between batches.

When should you choose compaction factor over slump?

The compaction factor test is more precise and sensitive than the slump test, especially for stiff mixes that yield slump readings below 50 mm or for concrete designed for vibration compaction. Very dry mixes show little to no slump but the compaction factor test can still distinguish meaningful differences in workability between batches. The method is primarily a laboratory tool, though it can be adapted for field use where precision is critical.

Key advantages and practical limits

Advantages include superior precision for low-workability concrete, the ability to quantify the internal energy required for full compaction, and suitability for mix design and quality control. Limitations are the need for laboratory-grade apparatus, longer test duration compared to a slump cone, and the requirement for trained personnel to ensure consistent handling and weighing. The test also demands careful cleaning between samples to prevent contamination.

If you regularly work with dry-cast or vibration-compacted concrete, particularly for precast elements or paving, the compaction factor test provides the numerical confidence that a simple slump measurement cannot deliver. Understand the apparatus, respect the timing, and record every detail—the compaction factor becomes a reliable fingerprint of batch-to-batch consistency on site.