Part 3 is the compacting factor test — the workability measure for low-slump, stiff and dry mixes where the slump test is insensitive (slump near zero tells you nothing). Concrete free-falls through two conical hoppers into a cylinder; CF = partially-compacted weight / fully-compacted weight (≈ 0.7 very low → ≈ 0.95 high). It measures the compaction effort needed — directly relevant to whether site vibration can consolidate the mix.
Key Requirements
•Use for low-workability / stiff / dry mixes where slump is insensitive (≈ 0)
•Apparatus: two conical hoppers above a cylinder; standard drop sequence
•CF = weight of partially-compacted concrete / weight of fully-compacted concrete
•Relate the acceptance CF to the actual site compaction capability
•Use for PQC, precast, low-W/C durable mixes (cf. IRC 58, IS 516)
Formulas
CF = W_partial / W_full
Compacting factor — ratio of partially- to fully-compacted weight in the cylinder
W_partial = weight of concrete partially compacted by the standard fallW_full = weight when fully compacted in the same cylinder
Practical Notes
✓A near-zero slump can't distinguish a placeable stiff mix from an unconsolidatable one — the compacting factor can; use it for dry/PQC mixes.
✓Tie the acceptance CF to the real site vibration available, not a lab ideal.
Common Mistakes
⚠Using slump for stiff/dry mixes (insensitive — use CF).
⚠No target CF related to the available site compaction.
⚠Treating CF and slump as interchangeable numbers.