The use of structural concrete has grown by leaps and bounds in recent years and this expansion has inevitably focused increased attention on problems related to the proper handling and placement of stiff mixes. The advent of prestressing and ultimate strength design has further emphasized these problems since greater demands are being placed on the concrete. Out of these needs arose the concept of vibration. The main functional characteristics of vibrators to consider when selecting such equipment is frequency, amplitude and force. A vibrator must be chosen that operates at a high enough frequency to consolidate the concrete thoroughly with a minimum of labor. Amplitude is the side to side distance that the head travels in its vibration cycle. It is of equal importance to frequency in gaging a machine's effectiveness. Four types of concrete vibrators are internal, form, surface, and table. Internal are highly advisable whenever possible because their output is transmitted directly to the concrete. Form vibrators are good for sections that are too deep and narrow or contain too much reinforcement. Surface vibrators work well on thin, horizontal sections such as roads. Vibration makes it possible to reduce the amount of cement in a mix without lowering its strength. Compensation for this reduction in cement content is made by using less water. In this manner cement requirements can often be lowered from 5 to 4 sacks in a typical mix.