What is tamped concrete?

Tamped concrete is unreinforced concrete placed earth-moist, with very little water, and compacted in layers by tamping. It contains relatively little cement, so it is cheap but also weaker than structural concrete. A traditionally quoted mix is roughly 1 part cement, 3 parts sand and 5 parts gravel, with as little water as possible. In Dutch it is called stampbeton.

What it is used for

• Blinding under foundations and floor slabs: a thin, level layer a few centimetres thick to work on and to keep reinforcement clean.
• Sealing the ground in a crawl space against damp and vermin.
• Bedding and haunching kerbs and edgings so they do not shift.
• Light foundations, for example for a shed or summer house.

Historically, tamped concrete was also used for walls and foundations before reinforced concrete became common.

Application

1. Mix it earth-moist. Squeeze a handful: it should form a ball that does not drip and does not immediately crumble. Concrete that is too wet cannot be tamped.
2. Work in thin layers. Place the concrete layer by layer and tamp each one well with a hand tamper or plate compactor. Thick layers are not compacted inside.
3. Prepare a firm base. Compact the soil or sand beneath it first.
4. Keep it damp. Cover fresh concrete for a few days to protect it from drying out in sun and wind.
5. Structural work? Tamped concrete is not enough for the foundations of an extension, garage or house. Those need reinforced concrete designed by a structural engineer.

Common mistake: making the mix too wet because it is easier to work. It then shrinks more, ends up weaker and segregates, with the gravel sinking and a weak top.

Related terms

• Unreinforced concrete
• Blinding
• Rammed earth
• Soil compaction
• Structural engineer

Want to know more about building terms? Visit our knowledge base at fredsdiyplans.com.