When a bucket of low slump concrete is dumped into a form, the concrete is in a honeycombed condition. From 10 to 30 percent of the concrete is in the form of irregularly distributed entrapped air depending on the mix, size and shape of the form, amount of reinforcing steel, and method of depositing the concrete in the form. Contractors know they must overfill a form because concrete should shrink when it is compacted. The problem is to get this heaped up, honeycombed mass to subside into a dense concrete without the entrapped air. How does a vibrator fit into the picture? A vibrator causes a violent agitation of the particles in the mix. This eliminates the friction between the particles which enabled the concrete to support itself in a honeycombed condition. The mix now becomes unstable and starts to flow under the simultaneous effect of vibration and gravity. The concrete moves tightly against the form and around the reinforcing steel. The mix particles rearrange themselves. The mortar fills the voids between the coarse aggregate particles, and the entrapped air rises to the top of the layer. There are two basic ways to vibrate concrete: internally and externally. In internal vibration, the vibrator is immersed in the interior of the concrete and the vibratory waves carry outward to the surfaces. In external vibration, the vibration is applied to the surfaces and the vibratory waves penetrate the interior. On any construction job the vibrators should match reasonably well the concrete mix being vibrated. A 2 to 2 and one-half inch diameter vibrator is suitable for most building concrete, but larger vibrators are needed for stiff mixes and where the aggregate size is more than about 1 and one-half inches. Too often workmen are forced to struggle with vibrators poorly match to the concrete. In general, vibration should be carried out with the vibrator completely immersed in the concrete. If the vibrator protrudes through the top surface, some of the energy normally transmitted into the concrete will be lost. Additionally, the large force exerted at the surface causes a violent turbulence there. This results in segregation. Vibrators may also get overheated when they do not have the full benefit of the concrete's cooling effect.