Kumar IronUpdated
In this guide
What plum concrete is
It is plain cement concrete (PCC), with no steel, in which clean, hard boulders are embedded as the concrete is placed. The stones take up space that would otherwise need cement, sand and aggregate, so the overall cost per cubic metre drops.
It is meant for bulk, compression only work. It is not a substitute for reinforced concrete.
Typical mix and proportions
| Item | Typical value |
|---|---|
| Base concrete mix (cement : sand : aggregate) | 1:3:6 or 1:4:8 |
| Plum stones by volume | About 20 to 30 percent of the total |
| Stone size | Usually 150 to 300 mm, not more than about one third of the layer thickness |
| Gap between stones and to the face | At least 100 to 150 mm of concrete all round |
Your engineer or tender specification may fix different numbers. Follow them.
Where plum concrete is used
- Foundation filling and levelling under footings on uneven or deep ground.
- Gravity retaining walls and bund walls.
- Bridge abutments, culvert bases and dam work.
- Filling old wells, pits and depressions on a plot.
- Bases for heavy machines and boundary wall footings.
How to lay it correctly
- Use hard, sound stones like sandstone or granite boulders. No soft, cracked or flaky stone.
- Wash stones clean of mud and soak them so they do not suck water from the mix.
- Lay a bed of concrete first, then place stones by hand, not dropped from height.
- Keep stones apart. They must not touch each other or the shuttering.
- Fill and compact concrete around each stone, then repeat in layers.
- Cure for at least 7 days, longer with PPC or PSC cement.
Frequently asked questions
- What is the ratio of plum concrete?
- Usually a lean PCC mix of 1:3:6 or 1:4:8 with 20 to 30 percent plum stones by volume. The exact ratio comes from your drawing or specification.
- How big should plum stones be?
- Typically 150 to 300 mm, and not more than about a third of the thickness of the layer being cast.
- Can plum concrete be used for columns or slabs?
- No. It has no reinforcement and is only for mass, compression work like foundation filling and gravity walls.