An internal vibrator, also called a poker or immersion vibrator, is a rod-shaped device inserted directly into freshly placed concrete to achieve mechanical consolidation. The vibrating head generates oscillations that reduce internal friction between aggregate particles, allowing entrapped air and excess water to rise to the surface and the mix to settle densely around reinforcement and formwork.
Design and Drive Types
The unit consists of a vibrating head (poker), a flexible shaft or cable, and a drive unit. Poker head diameters typically range from 25 mm to 150 mm, selected according to placement depth, reinforcement density, and aggregate size. Drive units are available as electric (single- or three-phase motors), pneumatic, hydraulic, or petrol-driven converters. High-frequency electric converters (42 V or 48 V, 200 Hz) are common on sites where low-voltage safety requirements apply.
Vibration Parameters
Standard internal vibrators operate at frequencies of approximately 150 to 200 Hz (9,000 to 12,000 vibrations per minute). High-frequency models reach up to 300 Hz for smaller poker diameters, used in thin sections or heavily congested reinforcement. The radius of action, the zone within which the concrete is effectively consolidated, is roughly seven to ten times the poker diameter, depending on mix consistency and slump.
Application Technique
The poker is inserted vertically at regular intervals, spaced at about 1.5 times the radius of action, and withdrawn slowly to prevent voids from forming behind the head. Each insertion should penetrate 100 mm to 150 mm into the previously placed and still plastic layer below to achieve a monolithic bond between lifts. Vibration duration per insertion generally ranges from 5 to 15 seconds, continued until mortar appears at the surface and the escape of large air bubbles ceases. Excessive vibration time or spacing can cause segregation or aggregate settlement and should be avoided.
Effect on Fresh Concrete
Before consolidation, entrapped air in freshly placed, unvibrated concrete typically ranges from 5% to 20% by volume, depending on mix design, aggregate grading, and placement method. Proper use of an internal vibrator reduces this entrapped air to approximately 1% to 2% by volume in conventional structural concrete, improving density, strength development, and surface finish. Intentionally entrained air for freeze-thaw resistance, introduced via air-entraining admixtures, is unaffected by this process and remains within its specified target range, commonly 4% to 7% depending on exposure class.
Application Limits
Internal vibrators are unsuitable for self-consolidating concrete (SCC), which achieves compaction through its own flowability without mechanical vibration. They are also inappropriate for very thin slabs or toppings below approximately 100 mm, where surface vibrators or vibrating screeds are the standard tool. Guidance on consolidation procedures, insertion spacing, and vibration duration is provided in ACI 309R, Guide to Consolidation of Concrete.