Vibrators are used when concrete is poured into formwork, so as to eliminate air bubbles trapped in the mixture, which weakens it. The vibration makes them come to the surface and thus makes the concrete more compact and durable.
Vibration can be transmitted in two ways:
by internal vibration, by plunging a vibrating poker into the concrete,
by external vibration, the vibrator being in this case fixed to the exterior of the formwork. This technique is used particularly in the pre-fabrication phase, and when the shape or thickness of the works does not permit the use of a poker vibrator.
Vibration also helps to distribute the concrete within the formwork more evenly. In fact, it avoids separation of the mixture, that is, separation of the elements (cement, gravel, additives etc), which make it up. It will thus be more compact, which will increase its strength and durability.
The handling of a vibrator requires real skill on the part of the workman in charge of it; he must pay attention both to the frequency of vibration, its duration, the distance between vibration points and the thickness of the concrete. Badly vibrated concrete will lose much of its quality. Experience in particular, enables the workman to stop at the right time, when the surface of the material has a milky appearance and the noise of his vibrator has changed.