Raft concrete = raft slab + downstand beams + column thickenings. A 20 × 15 m raft 600 mm thick (180 m³) with 70 m of 600 × 400 mm ground beams (16.8 m³) and twelve 2 × 2 × 0.3 m thickenings (14.4 m³) needs 211 m³: 217.5 m³ with 3%, 28 trucks and about 5.4 hours on one pump.
Raft = L × W × t
Beams = total beam length × width × depth below raft
Thickenings = n × a × a × depth below raft
Pour time = order ÷ pump output
Worked Example
A 20 × 15 m raft, 600 mm thick, 70 m of 600 × 400 mm downstand beams, twelve 2 × 2 m thickenings 300 mm deep:
- Raft = 180.0 m³; beams = 16.8 m³; thickenings = 14.4 m³
- Total = 211.2 m³; with 3% = 217.5 m³
- Trucks = ⌈218 ÷ 8⌉ = 28
- At 40 m³/h: 5.4 hours
Pour Duration for 217.5 m³
| Pump output |
Duration |
| 30 m³/h |
7.3 h |
| 40 m³/h |
5.4 h |
| 60 m³/h |
3.6 h |
| 80 m³/h (two pumps) |
2.7 h |
Frequently Asked Questions
How do I calculate concrete for a raft foundation?
Raft length × width × thickness, plus downstand beams (total length × width × depth below the raft) and any local thickenings under columns or lift pits.
How long will a raft pour take?
Divide the order volume by the realistic pump output, often 30–60 m³ per hour per pump including truck changeovers. Large rafts use two pumps or are split by construction joints.
Do thick rafts need special care?
Rafts over about 1 m thick can generate enough heat to crack. Many specifications require a thermal control plan, temperature monitoring and sometimes low-heat mixes.
