Compaction is where good concrete is either finished or ruined. The same mix can be dense and strong, or full of voids, depending on what happens in the minute after it is placed.
Why Compaction Matters
Freshly placed concrete can hold a significant amount of trapped air. That air weakens concrete sharply — entrapped air of just a few percent cuts strength markedly — and leaves voids that become honeycombing and paths for water to reach the steel.
How to Vibrate Properly
- Place concrete in layers of about 300–450 mm; do not fill a column in one lift.
- Insert vertically and let the needle sink under its own weight.
- Penetrate into the layer below by about 100–150 mm to knit layers together.
- Hold 5–15 seconds, until large bubbles stop and the surface around the needle looks glossy.
- Withdraw slowly, so the hole closes behind the needle.
- Space insertions so their zones overlap — commonly 300–450 mm apart.
- Work systematically across the pour so no area is missed.
Signs to Watch
| Sign | Meaning |
|---|---|
| Large bubbles still rising | Keep vibrating |
| Surface glossy, bubbles stopped | Done at this point |
| Water and paste pooling on top, stones sinking | Over-vibration or mix too wet |
| Honeycombs after stripping | Under-vibration or leaking formwork |
Common Mistakes
- Using the vibrator to move concrete sideways — causes segregation.
- Touching formwork — damages forms and marks the surface.
- Vibrating the steel to shake concrete through — displaces bars and cover.
- No standby vibrator — one breakdown mid-pour leaves part of the slab uncompacted.
- Skipping column bottoms and beam–column joints — the most congested and most important places.
The Short Version
Vertical, overlapping, into the layer below, until the bubbles stop — then out slowly. Keep a spare vibrator on site, and never use it to move concrete.
