Fresh concrete needs to flow into formwork, pack around reinforcement, and consolidate without leaving voids to avoid weakness. Engineers and concrete workers describe this behavior as workability, and the compaction factor test is one of the most reliable lab methods for measuring it (especially for stiff, low-workability mixes where the slump test falls short and is not suitable for application).
This guide walks through the apparatus, procedure, formula, and typical result range, based on the method set out in BS 1881-103:1993 (the equivalent Indian standard is IS 1199:1959).
What Is the Compaction Factor Test?
The compaction factor test measures workability by comparing the weight of concrete that compacts under its own weight (partial compaction) to the weight of the same concrete when it’s fully compacted by vibration or ramming.
The test was originally developed by the Road Research Laboratory in the United Kingdom. It’s particularly useful for concrete mixes that are too dry or stiff to give a meaningful reading on a slump test since a very low-workability mix will barely slump at all, but the compaction factor test can still pick up meaningful differences between batches.
Compaction factor = weight of partially compacted concrete ÷ weight of fully compacted concrete
A value closer to 1.0 indicates higher workability.
Apparatus Required
- Compaction factor apparatus: two conical hoppers (upper and lower) mounted above a cylindrical mould, each fitted with a trapdoor
- Hand scoop, approximately 15.2 cm long
- Tamping rod: steel or similar, 1.6 cm diameter, 61 cm long, rounded at one end
- Trowels
- Weighing balance, accurate to the nearest 10 g
Step-by-Step Procedure
- Fill the upper hopper. Using the hand scoop, place the concrete sample gently into the upper hopper up to its rim, and level the top. Avoid compacting it at this stage — the fall should be the only compacting action.
- Cover the cylinder at the base of the apparatus.
- Release the upper trapdoor so the concrete drops into the lower hopper. Use the tamping rod to gently push down any concrete sticking to the sides.
- Release the lower trapdoor and let the concrete fall from the lower hopper into the cylinder below.
- Strike off the excess concrete level with the top of the cylinder using a trowel.
- Clean the outside of the cylinder of any spilled concrete.
- Weigh the cylinder with concrete to the nearest 10 g. Record this as the weight of partially compacted concrete, W₁.
- Empty the cylinder, then refill it with the same concrete mix in layers of roughly 5 cm, compacting each layer thoroughly (by vibration or rodding) until fully compacted.
- Level the top surface of the fully compacted concrete.
- Weigh the cylinder with the fully compacted concrete. Record this as W₂.
- Weigh the empty cylinder on its own and record this as W.
Timing matters
Because cement hydration begins immediately after mixing, the test is sensitive to elapsed time. For results to be comparable between batches, carry out each test at a consistent interval after mixing commonly two minutes from the end of mixing to the release of concrete from the upper hopper.
Compaction Factor Formula
Compaction Factor Value= (W1-W) / (W2-W)
Where:
- W₁ = weight of cylinder + partially compacted concrete
- W₂ = weight of cylinder + fully compacted concrete
- W = weight of empty cylinder
The result is normally reported to two decimal places.
Typical Compaction Factor Values
| Workability | Compaction Factor |
|---|---|
| Very low | 0.70 – 0.78 |
| Low | 0.78 – 0.85 |
| Medium | 0.85 – 0.92 |
| High | 0.92 – 0.95 |
In general, compaction factor values for structural concrete fall between 0.7 and 0.95. A lower value means the mix needs more mechanical effort (vibration) to fully compact.
Compaction Factor Test vs Slump Test
| Aspect | Compaction Factor Test | Slump Test |
|---|---|---|
| Best suited for | Low and very low workability mixes | Medium to high workability mixes |
| Sensitivity | High: detects small differences | Low for stiff mixes |
| Apparatus | Two hoppers, cylinder, balance | Slump cone only |
| Field use | Difficult (bulky apparatus) | Easy, common on-site |
| Output | Numerical ratio (0–1 scale) | Slump in mm |
Advantages
- More sensitive than the slump test for dry, low-workability mixes
- Gives a numerical, repeatable value rather than a visual judgment
- Useful in mix design and quality control where vibration compaction is planned
Limitations
- Apparatus is large and better suited to laboratory use than site conditions
- More time-consuming than the slump test
- Requires careful, consistent timing after mixing to get comparable results between batches
Frequently Asked Questions
What does a compaction factor of 0.9 mean? A compaction factor around 0.9 indicates a mix with relatively high workability, needing less compaction effort to achieve full consolidation.
Which standard governs the compaction factor test? In the UK it’s specified under BS 1881-103:1993. The equivalent Indian standard is IS 1199:1959.
Why isn’t the slump test used for all concrete? Very stiff or dry mixes barely slump at all, so the slump test loses its ability to distinguish between batches. The compaction factor test remains sensitive across that range.
Read Also: