How to work out the concrete quantity for a foundation as pure geometry, and which classes are determined by the designer.
Introduction
The amount of concrete for a foundation is first and foremost a volume. When the cross-section is rectangular, it is simple: multiply length, width and height together. The hard part is not the maths, but getting the right measurements and the right concrete class.
The volume formula
If the cross-section steps (e.g. a thickening, a base course or a stair step), split the pour into several boxes, calculate each separately and add them together. Add a wastage allowance for an uneven base and over-pouring — that is your own judgement.
Example
A strip footing of 10 m, 0.40 m wide and 0.60 m deep: V = 10 · 0.40 · 0.60 = 2.40 m³. With 5 % waste, order 2.40 · 1.05 = 2.52 m³.
What the calculator does NOT decide
- 01Strength class and exposure class — selected by the designer in accordance with DS/EN 206 and DS 2426.
- 02Reinforcement and cover — shown on the structural drawing.
- 03Frost-free depth and bearing capacity of the base — depends on the soil and the project.
Frost-free depth — why the bottom must go down
A foundation must be taken down below the frost line, so frost in the soil cannot lift and settle the building (frost heave). Under heated buildings, the frost-free depth in Denmark is normally calculated to at least 0.9 m below finished ground level; under freestanding, unheated foundations (e.g. a garage structure or a freestanding wall) normally at least 1.2 m. Frost-free depth does depend on the soil type, and alternatively effective frost insulation (edge insulation) can be used.
Terrain covering's layer structure
The foundation and ground floor are connected. Under the floor, the layers are built from bottom to top, so moisture, radon and thermal bridges are handled in one cut:
- 01Capillary break layer — a layer of washed, coarse aggregate (e.g. 16/32 mm shingle/ballast, often approx. 150 mm) or lightweight clinker that breaks capillary uptake of groundwater.
- 02Radon and moisture membrane — a dense membrane (PE film or equivalent) that runs unbroken up into the plinth and connects with the foundation's moisture barrier so radon and moisture do not penetrate.
- 03Insulation — compression-resistant insulation under the concrete floor that breaks the cold bridge between warm floor and cold ground.
- 04Reinforcement mesh and concrete deck — the loadbearing ground deck is cast on top of the insulation, often with a rebar reinforcement mesh according to the structural drawing.
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