Compaction Factor Test: Procedure, Formula and IS 1199
What is the Compaction Factor Test?
The Compaction Factor Test is a more objective and sensitive measure of workability than the slump test, particularly for low and medium workability concretes (CF range 0.75–0.92) where the slump test may give zero or very small, non-discriminating results.
Developed by the Road Research Laboratory in the UK, the test is based on a fundamental definition of workability as “the amount of internal work needed to produce full compaction.” Instead of measuring how much concrete slumps, it measures what fraction of full compaction is achieved when concrete falls under its own weight through standardized hoppers into a cylinder.
The governing standard in India is IS 1199:1959. The test is particularly useful for road pavement concrete, precast products, mass concrete, and other applications where stiff mixes are required.
Apparatus and Dimensions
The Compaction Factor apparatus consists of three components mounted on a rigid frame, positioned vertically one above the other:
Upper Hopper (Hopper A)
- Top diameter: 250 mm
- Bottom diameter: 125 mm
- Height: 280 mm
- Has a hinged trap door at the bottom that can be opened quickly
Lower Hopper (Hopper B)
- Top diameter: 250 mm
- Bottom diameter: 125 mm
- Height: 230 mm
- Also has a hinged trap door at the bottom
- Positioned 200 mm below Hopper A
Cylinder
- Diameter: 150 mm
- Height: 300 mm
- Positioned 200 mm below Hopper B
- Total height of apparatus ≈ 1000 mm
Procedure (IS 1199)
- Weigh the empty cylinder (W₁). Record its weight.
- Fill the upper hopper (A) gently with fresh concrete — do not compact or press. Fill to the brim without disturbing or shaking.
- Open the trap door of Hopper A quickly and allow concrete to fall into Hopper B (lower hopper). Let all the concrete fall without interference.
- Open the trap door of Hopper B immediately (as soon as all concrete from A has fallen into B) and allow concrete to fall into the cylinder below.
- The cylinder will be more than full. Strike off the excess concrete flush with the top of the cylinder using a trowel.
- Weigh the cylinder with partially compacted concrete (W₂). Partially compacted = concrete compacted only by falling under its own weight.
- Now empty the cylinder and refill with fresh concrete from the same batch in layers, fully compacting each layer with a tamping rod (25 strokes/layer) or vibrator. Strike off flush.
- Weigh the cylinder with fully compacted concrete (W₃).
- Calculate the Compaction Factor.
Compaction Factor Formula
Where:
- W₁ = Mass of empty cylinder
- W₂ = Mass of cylinder + partially compacted concrete (fallen from hoppers)
- W₃ = Mass of cylinder + fully compacted concrete
CF always lies between 0 and 1. CF = 1 would mean full compaction was achieved by free fall alone (not possible in practice). The closer CF is to 1.0, the more workable the concrete. For practical purposes, CF ranges from 0.75 (very stiff) to 0.98 (highly workable).
Solved Numerical Example
Given:
- Mass of empty cylinder (W₁) = 4.00 kg
- Mass of cylinder + partially compacted concrete (W₂) = 14.62 kg
- Mass of cylinder + fully compacted concrete (W₃) = 15.66 kg
Solution:
CF = (W₂ – W₁) / (W₃ – W₁) = (14.62 – 4.00) / (15.66 – 4.00)
CF = 10.62 / 11.66
CF = 0.910
Interpretation: CF = 0.910 indicates medium workability, suitable for normal RCC work — beams, columns, slabs with mechanical vibration. This corresponds approximately to a slump of 50–100 mm.
Typical CF Values
| CF Value | Degree of Workability | Approximate Slump | Application |
|---|---|---|---|
| 0.75–0.80 | Very Low | 0–25 mm | Road pavements, dry lean |
| 0.80–0.85 | Low | 25–50 mm | Mass concrete, foundations |
| 0.85–0.92 | Medium | 50–100 mm | Normal RCC (vibrated) |
| 0.92–0.95 | High | 100–175 mm | Dense reinforcement |
| >0.95 | Very High | >175 mm | SCC, pumped concrete |
CF Test vs Slump Test
| Aspect | Slump Test | Compaction Factor Test |
|---|---|---|
| Best for | Medium to high workability | Low to medium workability |
| Sensitivity | Low for stiff mixes | High for stiff mixes |
| Equipment | Simple, portable | Bulky, less portable |
| Objectivity | Moderate | High (weight-based) |
| Time | 5 minutes | 10–15 minutes |
| Site Use | Very common | Less common (lab/pavement) |
🎯 Exam Tips (RTMNU)
- The CF formula (W₂–W₁)/(W₃–W₁) is a guaranteed numerical — practice with 2–3 examples.
- Hopper A dimensions (250 top, 125 bottom, 280 height) and Hopper B (250 top, 125 bottom, 230 height) and Cylinder (150 dia, 300 height) — all may be asked.
- CF = 0.910 = medium workability = normal RCC — tie a CF value to an application always.
- Why CF test is better than slump for low workability: CF can differentiate between CF=0.75 and CF=0.80 (both show zero slump); state this clearly.
- W₂ uses concrete compacted by falling (gravity); W₃ uses hand/vibrator — state the distinction in the formula explanation.
- Draw the apparatus diagram clearly — always include both hoppers and cylinder with labels.
✅ Key Takeaways
- CF test measures workability by comparing weight of self-compacted vs fully compacted concrete.
- Apparatus: Upper Hopper A (250/125/280), Lower Hopper B (250/125/230), Cylinder (150/300).
- CF = (W₂–W₁) / (W₃–W₁); range 0.75–0.98 in practice.
- CF = 0.75–0.80 (very low); 0.85–0.92 (medium); >0.95 (very high).
- More sensitive than slump for low-workability (stiff) mixes.
- Governed by IS 1199:1959.
📖 Related Reading: Slump Test: Procedure and Types | Vee-Bee Consistometer Test
🔗 External Reference: IS 1199:1959 – Methods of Sampling and Analysis of Concrete (BIS)
❓ FAQs
Q1. What does the Compaction Factor value represent?
The Compaction Factor (CF) is the ratio of the mass of concrete compacted by falling through the apparatus under its own weight to the mass of the same concrete when fully compacted. It represents the degree of compaction achieved by standardized gravitational work. CF closer to 1.0 = more workable concrete.
Q2. What is the CF value for normal RCC work?
For normal RCC work (beams, columns, slabs with mechanical vibration), the Compaction Factor is typically in the range of 0.85–0.92, corresponding to medium workability and an approximate slump of 50–100 mm.
Q3. Why is the CF test more sensitive than the slump test for stiff mixes?
For stiff mixes (CF 0.75–0.82), the slump test may give zero slump for all, making it impossible to distinguish between different workability levels. The CF test gives values of 0.75, 0.78, 0.82 etc. for these same mixes — a meaningful, measurable distinction — because it measures actual mass differences rather than a geometric collapse.
Q4. What is the sequence of operations in the CF test?
Fill Hopper A with concrete → Open trap door of A → Concrete falls into Hopper B → Open trap door of B → Concrete falls into Cylinder → Strike off flush → Weigh cylinder (W₂) → Empty and re-fill with fully compacted concrete → Weigh (W₃) → Calculate CF.
Q5. Can the CF test be used for very high workability concrete?
The CF test is not ideal for very high workability concretes (CF > 0.95) because the difference between partially and fully compacted concrete becomes very small, reducing sensitivity. For very high workability or SCC, the Slump Flow Test and Flow Table Test are preferred.