How much concrete a foundation needs

Concrete arrives in 5–9 m³ mixers. Under-ordering is expensive: a second truck for the shortfall costs almost as much as a separate delivery, and the joint between layers weakens the structure.

Strip foundation

Volume = strip length × width × height.

Example: a 10×8 m perimeter plus one 8 m internal wall. Strip length = 36 + 8 = 44 m. Width 0.4 m, height 0.8 m. Volume = 44 × 0.4 × 0.8 = 14.1 m³.

Raft foundation

Volume = slab area × thickness. For a 10×8 m slab 25 cm thick: 80 × 0.25 = 20 m³. If the raft has stiffening ribs, calculate their volume separately and add it.

Pile cap

Volume of one pile = π × R² × depth. For a ⌀300 mm pile 2 m deep: 3.14 × 0.15² × 2 = 0.141 m³. Twenty piles come to 2.8 m³, and the cap is calculated like a strip.

Which concrete class

Spare

Allow 5–7% above the calculated figure. The reasons never change: an uneven trench bottom, formwork spreading under the pressure of the concrete, losses during placement. On 14 m³ that is almost a full cubic metre — and it is exactly the one that runs short.

We deliver by mixer at an agreed time. A large pour must be planned for a single day: a break of more than two hours between layers creates a cold joint and the slab loses its monolithic strength. Tell the manager the volume and timing and we will schedule the trucks.

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