8.6 : Vibrating Concrete

Mechanical vibrators are instrumental in compacting newly poured concrete within formwork and around reinforcements. This process is essential to eliminate trapped air pockets and establish a dense concrete mass. One widely used method is vibrating by internal vibrators, often referred to as a poker vibrator or immersion vibrator. It is rapidly inserted through the full depth of the freshly laid concrete and slightly extends into the layer below it (which remains in a plastic state). Consistent vibrations are applied across the concrete surface at intervals of every 2 to 3 feet for a period of 5 seconds to 2 minutes, depending on the consistency of the concrete mix. As the vibrator is slowly withdrawn, the hole left behind is naturally sealed, preventing the formation of voids.

Another method employs external vibrators that are attached to the formwork to vibrate the formwork and the concrete simultaneously. This is used for precast elements or thinner sections of concrete. In contrast to internal vibrators that directly vibrate the concrete, external vibrators do additional work by vibrating the formworks, making them less efficient than the internal vibrators. Occasionally, external vibrators have to be shifted from place to place depending on the placement of the concrete. For precast concrete units, a vibrating table is used, wherein the formwork is fixed directly onto the platform to achieve a uniform vibration throughout the unit.