No-fines concrete is concrete with a coarse aggregate and a deliberately gap-graded particle distribution, leaving open voids of roughly 10 to 30 mm between the particles. Because the fine fraction — the sand — is largely or entirely absent, the aggregate particles do not fill each other’s spaces. The cement forms only a thin skin bonding the particles at their contact points. It is also known as pervious concrete or single-sized aggregate concrete.
Properties
- Water permeable — rainwater passes straight through the open structure instead of running off the surface.
- Lightweight — the density is considerably lower than that of normal gravel concrete, simply because a large part of the volume is air.
- Limited compressive strength — the contact area between particles is small. No-fines concrete is not a structural concrete, and its open structure offers no cover to reinforcement.
- A key to good adhesion — the rough, open surface is an advantage where render is applied over it.
Applications
- Drainage layers and permeable pavements — wherever water has to be able to escape: beneath surfacings, behind retaining walls, in infiltration systems.
- Permeable paving sub-base — a base course that lets rainwater through rather than sending it to the drain.
- Insulating variants — where the aggregate consists of expanded clay pellets, the result is a light, moderately insulating concrete used for levelling layers and fills.
No-fines concrete also has a place in post-war building history: during reconstruction after the Second World War, crushed rubble from destroyed buildings was used as the aggregate at a time when gravel and cement were scarce.
Points to watch
- Do not compact it like ordinary concrete. Vibrating closes up the voids and removes exactly the drainage function you wanted. Light tamping is enough.
- Keep the water content tight. Too much water washes the cement paste off the particles and lets it collect at the bottom; too little and the particles do not bond.
- Clogging over time. A drainage layer loses its function once silt, sand or leaf litter gets into it. Separate it from the surrounding ground with a geotextile.
- Not frost resistant when saturated. Make sure the water can actually drain away.
Related terms
- Concrete
- Particle size distribution
- Aggregate size fraction
- Gravel
- Expanded clay pellets
- Concrete mix
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