What is concrete compaction?
Compaction means packing the particles of a material more closely together, so trapped air and voids disappear. On site this mainly concerns fresh concrete and soil or sand. The aim is always a denser, stronger and more stable result. This article focuses on concrete; for ground, see soil compaction.
Compacting concrete
Freshly placed concrete contains air. Compacting sets it vibrating: it becomes temporarily more fluid, fills every corner of the formwork, wraps around the reinforcement, and the air bubbles rise out. The result is concrete with fewer voids, higher strength and better protection of the steel against corrosion.
Common methods:
- Poker vibrator: dipped into the concrete; the standard for walls, columns, beams and foundations.
- Vibrating screed beam: pulled across the surface of floors and slabs; compacts and levels at the same time.
- Formwork vibrator: clamped to the formwork, mostly in precast factories.
- Tamping or rodding: by hand, for small quantities and for tamped concrete.
Self-compacting concrete is designed to fill all spaces without vibration. It should not be vibrated.
Compacting soil
Soil and sand are compacted in layers with a plate compactor, rammer or roller. This raises bearing capacity and reduces settlement. More detail is in the soil compaction article.
Application
Compaction is part of every concrete pour, from a pad footing or garden wall foundation to a basement slab, and of every sand layer under paving, floors and foundations.
Tips and points of attention
- Pour in layers and push the poker into the layer below, so the layers bond into one.
- Stop in time: you are done when no more air bubbles rise and the surface looks even and glossy.
- Do not over-vibrate: too long in one spot causes segregation, with coarse aggregate sinking and water and fines rising.
- Too little vibration leaves honeycombing and voids, especially along the formwork and under the bars.
- Do not move concrete with the poker: place it as close as possible to its final position.
- Vibration is no cure for a poor mix: concrete that is too stiff or too wet does not improve. Choose the right consistence.
Related terms
- Poker vibrator
- Soil compaction
- Fresh concrete
- Honeycombing in concrete
- Segregation of concrete
- Degree of compactability
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