Slump is the standard measure of fresh-concrete workability — the vertical settlement (in millimetres) of a cone-moulded fresh concrete after the mould is lifted. It is performed per IS 1199 Part 2 using a 300 mm tall frustum cone with 100 mm top + 200 mm bottom diameter. The fresh concrete is filled in 3 layers, each compacted with 25 strokes of standard tamping rod (16 mm dia × 600 mm long), then the cone is lifted vertically + the slump measured. Higher slump means greater workability — easier to place, vibrate, finish — but may indicate excess water. Critical principle: **higher slump must be achieved through plasticizer / superplasticizer admixture, not through extra water**. Adding 1 litre of water per m³ raises slump by approximately 25 mm but reduces 28-day strength by 1-2 MPa per litre + increases shrinkage + cracking risk. The right slump depends on three factors: placement method (manual / pumped / tremie), member type (mass concrete / reinforced / heavily-reinforced), and rebar congestion. For each combination, an optimal slump range exists — too low means difficulty placing + honeycombing risk; too high means segregation + bleed water + low strength.
Notes
• Cardinal rule: NEVER increase slump by adding water at site. Use a plasticizer or superplasticizer admixture instead. Water-based slump increase compromises the w/c ratio + reduces design strength. Plasticizer-based slump increase achieves workability without strength loss + is the only correct method for low-slump concrete that's hard to place.
• Quantified strength penalty: adding 1 litre of water per m³ increases slump by approximately 25 mm but reduces 28-day strength by 1-2 MPa. So adding 4-5 litres to get an extra 100 mm slump can drop M30 concrete to M25 or below. This is a typical site mistake when concrete is hard to pump.
• Slump test procedure (IS 1199 Part 2): clean smooth surface for slump cone; fill in 3 equal layers (each 100 mm); compact each layer with exactly 25 strokes of 16 mm × 600 mm tamping rod, distributed evenly across the cross-section; do not penetrate the previous layer; level top; carefully lift cone vertically in 5-10 seconds; measure vertical settlement from cone top using ruler.
• Slump types: **True slump** — concrete settles evenly all around, mass-action; this is the valid result. **Shear slump** — one side falls or shears off; indicates poor mix design or insufficient cohesion; the test should be repeated with fresh sample. **Collapse slump** — concrete completely collapses or spreads; mix is too wet or has poor fines; should be rejected.
• Test frequency per IS 456 + IS 1199: minimum **one slump test per batch** of concrete (per truck for RMC); some specifications require one test per 2-5 m³; site QC plan should specify. For critical pours, test every truck on arrival before discharge.
• Hot weather impact (ambient > 30°C): concrete loses workability rapidly during transport + waiting time. Slump loss of 25-50 mm in 30 minutes is common. Counter-measures: target higher initial slump (add 25-50 mm); use retarder admixture; cool aggregate + water; transport in insulated trucks; minimise waiting time between batching + placement (placement within 30 minutes of batching per IS 7861).
• Retempering — adding water at site to fresh concrete to restore lost slump — is NOT permitted per IS 456 + IS 7861. It compromises strength + durability. The only permitted re-workability adjustment is via additional plasticizer (per concrete supplier's recommendation), not water.
• Slump test alternatives: **Compaction Factor Test** (IS 1199 Part 4) — for very low slump concrete (< 25 mm) where slump cone is insensitive. **Vee-Bee Time** (IS 1199 Part 5) — for similar dry mixes. **K-Slump Test** — for SCC. **Slump Flow Test** — for SCC (cone inverted, measure spread diameter).
• Self-Compacting Concrete (SCC) measurement is by **slump flow** — the cone is filled + lifted as in slump test, but the **diameter of spread** (not vertical settlement) is measured. SCC slump flow typically 650-800 mm. SCC also needs V-funnel test (5-15 sec) + L-box test (passing ratio > 0.8) for full characterisation per EFNARC guidelines.
• Slump tolerance per IS 456 Cl. 7.1.5: target slump ± tolerance. For slump < 25 mm: ±5 mm; 25-50 mm: ±10 mm; 50-100 mm: ±20 mm; 100-150 mm: ±25 mm; > 150 mm: ±30 mm. Out-of-tolerance batches require investigation and may be rejected.
• Slump test on each truck delivery is the contractor's primary tool to verify the consistency of the RMC supply. If multiple consecutive trucks show wider variation (e.g., 50, 110, 70, 130 mm), there is a batching plant control problem — either water dosing is variable, or aggregate moisture is changing rapidly without compensation.
• Pumping concrete: pump rate + line distance + member type dictate slump. Short pumps (< 30 m) can handle 75-100 mm slump; long pumps (> 100 m) need 125-150 mm slump + superplasticizer. Slump-flow test (rather than basic slump) is preferred for pumped concrete characterisation.
• For tremie concrete (underwater placement in piles, cofferdams), 150-200 mm slump is standard — high enough to flow + self-level without external vibration. Tremie concrete typically has higher cement content + uses superplasticizer + retarder.