Unreinforced concrete — also called plain concrete — is concrete with no reinforcement, or with a negligible amount, that is structurally loaded in compression only. Concrete is strong in compression but weak in tension: its tensile strength is only a fraction of its compressive strength. As soon as tensile stresses arise, unreinforced concrete cracks. Its use is therefore limited to situations where those stresses do not occur.
Difference from reinforced concrete
| Unreinforced concrete | Reinforced concrete | |
|---|---|---|
| Compression | Carried by the concrete | Carried by the concrete |
| Tension | Barely — it cracks | Carried by the steel |
| Spanning | Effectively none | Possible, as a beam or slab |
| Failure | Brittle, without warning | Greater deformation capacity |
That brittle failure mode is the main reason to use unreinforced concrete only where it is genuinely appropriate: there is no visible warning beforehand.
Where it is used
- Blinding — a thin layer of lean concrete beneath a foundation, giving a clean level surface for fixing reinforcement and pouring
- Mass fill — filling voids, old cellars or spaces beneath a structure
- Massive foundation blocks and pads where the load arrives essentially vertically
- Kerb bedding, haunching and edge restraints
- Rammed concrete in historic building — compacted in layers, widely used in cellars, walls and foundations into the early twentieth century
- Ground-bearing slabs on a well-compacted subgrade with limited loading; in practice steel fibre or mesh reinforcement is usually added anyway to control shrinkage cracking
Points to watch
- Unreinforced does not mean “no design”. An unreinforced element still has to be checked; on foundation work, have this done by a structural engineer
- Control the shrinkage. Concrete shrinks as it cures. Without reinforcement, shrinkage cracks form wherever they happen to; saw cuts or induced joints let you decide where
- Curing is essential. Keep the concrete damp or covered for at least several days after placing — drying out too fast gives a weak surface and shrinkage cracking
- Avoid point loads and one-sided bearing; these induce bending and therefore tension
- Choose the right strength and exposure class. Concrete used externally must be frost resistant; the exposure class governs the mix
Related terms
- Reinforced concrete
- Reinforcement
- Rammed concrete
- Blinding
- Shrinkage crack
- Concrete strength class
- Shallow foundation
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