Pea gravel concrete is concrete in which the coarse aggregate is not ordinary gravel but fine, rounded pea gravel of roughly 2 to 8 mm. Because the largest stone stays small, the mix can be placed in a much thinner layer than normal structural concrete without becoming coarse and crumbly.
Pea gravel is small, rounded, washed gravel. That fine aggregate keeps the mix workable in thin layers and around obstacles such as pipes and underfloor heating loops.
Mix proportions
A common mix, by weight:
| Ingredient | Parts |
|---|---|
| Portland cement | 1 |
| Concrete sand (0-4 mm) | 3 |
| Pea gravel (2-8 mm) | 1.2 |
Keep the mixing water as low as workability allows: too much water means shrinkage, cracking and a weaker surface.
Where it is used
- Composite floors on timber joists — profiled steel decking (dovetail sheeting) and pea gravel concrete act together as one floor plate, often at a total thickness of around 50 mm
- Underfloor heating over a timber floor — the thin, dense layer encases the pipes and spreads the heat evenly
- Thin screeds and levelling layers — where the build-up height is limited, for example when meeting an existing threshold or stair
- Small pours and repairs — where coarse gravel will not fit between reinforcement or into the formwork
Practical points
- Respect the thickness — follow the layer thickness the floor system supplier specifies; thinner means less load capacity and more risk of cracking
- Keep the edges free — fit a perimeter strip so the screed can shrink and impact sound does not bridge into the walls
- With underfloor heating — keep the cover over the pipes that the system supplier requires, and only start the commissioning heat-up once the layer has cured enough
- Cure it — thin layers dry out fast; covering or keeping the surface damp prevents shrinkage cracks
- Load on a timber floor — the layer adds weight; if in doubt, have the joists assessed before you pour
Related terms
- Screed
- Concrete mortar
- Joists
- Pea gravel
- Reinforcement
- Concrete floor
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