Concrete vibrators are not used only to consolidate the concrete for maximum density, but they also internally blend the different lifts of concrete together into a single solid mass with few to no air pockets and no lift lines on the finished exposed surface. After deciding whether to use internal or external vibration, there are four things to consider in making an appropriate choice: frequency, amplitude, power, and size.

A flex-shaft vibrator can be used for on-site vibration of concrete for foundations, walls, columns, and slab work.
Wacker Neuson Corp. A flex-shaft vibrator can be used for on-site vibration of concrete for foundations, walls, columns, and slab work.

Making the choice

According to Perry Schnebelen, application engineer of Denver Concrete Vibrator in Denver, “Contractors overwhelmingly choose internal vibrators primarily because they are versatile enough to be used on many different applications.” The two most popular types of internal concrete vibrators are flexible-shaft and high-cycle. Driven by an electric motor or gasoline engine, flexible-shaft vibrators are designed for general construction. Typical applications include footings and stem walls, small slabs, driveways, and stairways. High-cycle vibrators are driven by a high-cycle generator that provides 180 Hz, three-phase power to an electric motor spinning an eccentric in the vibrator head. Both types of internal vibrators are designed to be submerged into the freshly placed concrete.

Internal vibrators operate at 8000 to 17,000 vibrations/minute. They should not be selected only on the basis of frequency. If the frequency is increased or decreased without changing power, it affects amplitude. The amplitude is the measured distance of the head, from side to side, as measured from its position of rest. When the amplitude is not constant, neither is the effective radius of action, resulting in uneven consolidation. According to Fred Paul, business development manager of Wacker Neuson Corp., Menomonee Falls, Wis., “The greatest advancement in concrete consolidation products over the past few years is the use of frequency inverters for speed control.” Generally powered by common, single-phase electricity, they convert the single-phase to a three-phase electricity, which results in a more stable vibrator speed under load.

Fred Oswald, president of Oztec Industries Inc., Port Washington, N.Y., says, “Internal vibrators can be used with any size, shape, or form material without any damage to the form.” The disadvantage of internal vibrators, however, is the potential damage to coated rebar. External vibrators, on the other hand, are attached to the outside of the form. An external vibrator does not come into contact with the concrete. Its vibration waves are transmitted through the form wall and into the concrete. External vibrators are very popular in the precast industry where conditions are the same from one pour to the next. Schnebelen says, “Though they operate at the same frequency range as internal vibrators, an external often will have a higher force output for the same application.” In addition to operating at a higher power level, external vibrators do not come into contact with rebar.

The size of the vibrator is an important factor. “With the continued development of higher strength concrete, the physical size of the columns and walls can be reduced,” says Oswald. “The amount of reinforcing is the same, if not more. This makes the insertion of adequately sized internal vibrator heads more difficult.” That has led to the development of slimmer vibrator heads with increased vibratory forces.

Though selecting an internal or external vibrator is determined by the application, there are times when a contractor will use both types. If a pour is too large for an external vibration to reach the center, a contractor may use an internal vibrator to consolidate the center portion and external vibration for the areas closer to the formwork. Schnebelen says, “The most common reason we see contractors use both is just as an added measure to ensure consolidation.” He adds that in such cases, it is important the internal vibrator be used first, and the externals are only turned on once the internals are either removed or at least a few feet above the level of the externals.

The basic methods of producing and using vibration for concrete consolidation haven't changed much over the years. The technology, however, continues to advance.