Unless concrete is properly consolidated, voids reduce strength regardless of the water-cement ratio. Consolidating the concrete, usually by vibration, increases concrete strength by driving out entrapped air. It also improves bond strength and decreases concrete permeability.
VIBRATION IS A TWO-PART PROCESS
How does vibration consolidate concrete? It is a two-part process. A vibrator creates pressure waves that separate aggregate particles, reducing friction between them. Almost simultaneously, a second stage starts to occur as entrapped air bubbles rise to the surface. This deaeration process continues after the concrete has flattened out. Until both vibration stages are complete, the concrete isn't fully consolidated.
DIFFERENT VIBRATORS FOR DIFFERENT JOBS
The most common vibrator use is the electric, flexible shaft type. Other types include electric motor-in-head, pneumatic, and hydraulic. The type of vibrator must match the requirements of the concrete and the jobsite. The vibrator head must fit between the rebars and have a high enough frequency to quickly consolidate the concrete.
HOW TO USE AN INTERNAL VIBRATOR
Producing a dense concrete without segregation requires an experienced vibrator operator. The operator can judge whether or not vibration is complete by watching the concrete surface. When no more large air bubbles escape, consolidation is adequate. Skilled operators also listen to the pitch or tone of the vibrator motor.