How to Vibrate Concrete: Compaction Techniques for Slabs & Columns
Vibrate concrete for 5–15 seconds per insertion point. Insert the poker at regular intervals and penetrate 50 mm into the lower layer when pouring in lifts. Stop when large air bubbles stop appearing at the surface and the texture becomes smooth.
Why Compaction Matters
Compaction removes entrapped air that would otherwise weaken concrete. It also reduces the number and size of air pockets at the surface of off-form finished walls and columns.
Every 1% of entrapped air reduces concrete strength by approximately 5 MPa. Five percent trapped air equals a 25 MPa loss — effectively turning N40 into N15.
For most concrete (except SCC), compaction by internal or external vibration is required.
How to Use a Poker Vibrator
Insert at regular intervals
Work systematically across the pour. Insert the vibrator at regular spacing (typically 300–500 mm apart) rather than randomly.
Vibrate for 5–15 seconds
The correct duration can be assessed by: surface movement and texture change, sound change of vibrator to a constant tone, and ‘feel’ change in your hands.
Penetrate 50 mm into lower layer
When pouring in multiple lifts, the vibrator must penetrate 50 mm into the lower layer to bond the layers together. Haphazard penetration at different angles and depths does not create a monolithic structure.
Withdraw slowly
Withdraw the vibrator slowly (about 100 mm/second) to allow concrete to close behind it without leaving a void.
Never transport concrete horizontally
Do not use the vibrator to move concrete along a horizontal surface — this causes segregation (stones separate from paste).
Source: Boral Australian Concrete Guide (2023 Edition 2.1), Chapter 4.
Vibrator Types
| Type | Best for | Frequency | Notes |
|---|---|---|---|
| Internal poker vibrator (50–75 mm head) | Slabs, footings, most pours | 3,000–9,000 vpm | Most common on residential sites |
| Internal poker (25–40 mm head) | Columns, walls, congested reinforcement | 9,000–15,000 vpm | Smaller radius of action; insert more frequently |
| Screed vibrator (surface) | Thin slabs, paths | — | Limited to 150 mm depth; less effective |
| External form vibrator | Precast, congested forms | Variable | Attached to formwork; not practical on-site |
Over-Vibration: What to Avoid
Signs of over-vibration:
- Aggregate sinks to bottom (segregation)
- Paste and water rise to top
- Surface becomes watery and weak
- Off-form surface may show aggregate-poor zones
Over-vibration occurs when vibrating for too long (>30 seconds in one spot) or using too large a poker for the mix. With the correct mix design and poker size, 5–15 seconds is sufficient.
Self-Compacting Concrete — No Vibration Required
Self-compacting concrete (SCC) does not require compaction from internal or external vibration. SCC is designed with high flowability to fill forms and compact under its own weight. It is only suitable for some applications — consult your ready-mix supplier.
Frequently Asked Questions
How long should you vibrate concrete?
How far apart should I insert the concrete vibrator?
Can I use the vibrator to move concrete sideways?
Does all concrete need to be vibrated?
What is the radius of action of a concrete vibrator?
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