Vibrators consolidate freshly placed concrete by helping entrapped air to escape. As the concrete subsides, large air voids between coarse aggregate particles fill with mortar. Finally, further vibration drives out most of the air, which is trapped in the mortar. Concrete doesn't move much during the second phase, but that's when most of consolidation takes place.
So as long as vibrators are operated properly and not used to move concrete laterally, consolidation enhances the concrete's performance and appearance. Additionally, the concrete's density, strength, and the bond with reinforcing steel, if applicable, are improved. A properly operated vibrator also can minimize surface blemishes such as bugholes.
Vibration Features
Almost all internal vibrators are the rotary type: Vibrating action is produced by the rotation of an unbalanced weight, called an eccentric, that's located inside of the vibrator head. As the eccentric rotates at high speeds, the vibrator head moves in an orbit.
Most internal vibrators are high frequency vibrators designed to operate at 12,000 to 17,000 vibrations per minute (vpm) Amplitudes range from 0.015 to 0.08 inches. Frequency is the number of complete orbits per unit of time. Amplitude is the maximum deviation from the point of rest.
When immersed in concrete, the vibrator's orbiting head strikes the concrete. The greater the amplitude, the stronger the impact. As such, the greater the frequency, the more impacts the concrete receives. Amplitude primarily affects movement of coarse aggregate particles while frequency mostly manipulates the mortar.
It is important to note that amplitude and frequency relate to each other. A vibrator with a high frequency, for example, generally has low amplitude because the vibrator head doesn't have the time to travel as far as it can at a lower frequency. Vibrators with higher frequency and lower amplitudes are best-suited for consolidating plastic mixes in thin sections because these mixes have higher mortar content. Vibrators with lower frequencies and higher amplitudes are well suited for stiff mixes in heavy sections due to the high quantity of coarse aggregate in these mixes.
The Options
Choosing an internal vibrator can be confusing. Although these tools share the same primary components, many different options are available.
The most important decisions to make concern:
- Power source: electric, gasoline, or pneumatic; hydraulic power is used primarily to operate vibrators in slip-form pavers
- Power unit location: inside or outside the vibrating head
- Shaft: length and type
- Head characteristics: diameter, length, and shape
Flexible-Shaft
Most of the internal vibrators used today can be classified as either flexible-shaft or motor-in-head. The primary difference between the two is the location of the vibrator's power unit. It can be located either inside or outside the vibrating head.
Flexible-shaft vibrators are the most commonly used in the concrete industry, simply because of their low price. A flexible-shaft vibrator's motor runs outside the head, connected by a flexible shaft that drives the eccentric. The most common models are powered by an electric motor, but some are driven by a gasoline engine (although they typically weigh much more than the electric models).
The shaft's critical component is a core made up of layers or wire and surrounded by a steel-reinforced casing—the core actually turns the eccentric.
Flexible-shaft vibrators can easily accommodate many different concrete construction-related applications. Various sizes of electric motors and gasoline engines are available and each will accept different shaft sizes.
Shafts come in a variety of lengths—from 3 to 30 feet. If a contractor was interested in increasing the length of the shaft, coupling systems or custom-length shafts can be ordered from the manufacturer. To speed the changing of the motors, shafts, and heads, quick-disconnect systems also are available as either standard or optional equipment, depending on the manufacturer.
Electric flexible-shaft vibrators are relatively lightweight and portable. They typically weigh less than motor-in-head models—an important consideration for crews that operate vibrators for hours at a time. However, heavier flexible-shaft units with larger motors can contribute to worker fatigue and they often require two workers to operate: one to direct the head and the other to hold the motor, control the cords, and move the generator, if needed. Two operators often are needed for the gasoline engine vibrator, one to operate the equipment's head and the other to move the engine around the jobsite. There are a few gasoline-based vibrators that are backpack units, however, they tend to have smaller engines.
Flexible-shaft models are stiffer, making it a bit easier for the operator to control the vibrator. A hard-shaft casing on some models permits no lateral movement, so the vibrator can be pulled out from the same spot it entered. Flexible-shaft models also are likely to be chosen for rebar-congested areas because it's easier for users to downsize the components to obtain a head that can fit between the reinforcing steel.
Depending on the design features, flexible-shaft vibrators can be high-maintenance tools. Both the electric motor and the gasoline engine are vulnerable to airborne dust and splattered concrete. The flexible shaft also is susceptible to strain and damage, especially if the motor is allowed to dangle as the operator consolidates the concrete.
A primary disadvantage of electric flexible-shaft vibrators is their loss of power in concrete. This power loss lowers their frequency and thus the ability to remove entrapped air from the mortar. Power loss can be caused by using a small-gauge extension cord, a cord that's too long, a shaft that's too long, or a vibrator head that's too big for the motor. Each of the previous problems is correctable. A more troublesome cause of power loss is low-slump concrete, which can't be altered after it's in the forms.
Motor-In-Head
Depending on whether electricity or compressed air is used as its power source, motor-in-head vibrators feature either an electric or pneumatic motor inside the vibrator head. Because the eccentric can be turned from inside the vibrator head, the need for a rotating drive shaft is eliminated. Instead, a hose typically leads into the head to connect the motor to the electrical source or compressor.
Because of their integral design, motor-in-head models may not provide users with as many options as the flexible-shaft models. The head size is fixed, and so are most of the hose lengths, unless an extension cable or hose-lengthening kit is used.
Two types of electric motors are available for motor-in-head vibrators. One operates on a single-phase, 60-cycle electrical frequency using a universal motor. The second, called a high-cycle vibrator, operates on a three-phase, high-cycle electrical frequency using an induction motor.
The 60-cycle models can be operated using a commercial electrical frequency. In low-slump mixes, these models tend to lose power. Much like electric flexible-shaft models, which also use universal motors, 60-cycle vibrators are very load-dependent, which means that their mechanical frequency can be lowered by common jobsite conditions, such as stiff concrete or poorly sized extension cords and vibrator heads.
With a high-cycle vibrator, continuous high-vibration frequency can be provided even after being placed in stiff concrete. They maintain frequency because a three-phase motor uses three conductors to carry current instead of only one in a single-phase system. Therefore, one wire is always carrying current, keeping the mechanical frequency on the vibrator up to specification.
Because the connection hose for a motor-in-head vibrator protects only a power cord, not a drive shaft, the shafts are more pliant than those of flexible-shaft models. This pliability may be a disadvantage when the vibrator is used in areas congested with rebar. Because it moves laterally more easily, the head can sometimes get permanently stuck within the reinforcing cage.
Finally, pneumatic motor-in-head vibrators typically are used on large, high-volume concrete jobs where compressed air is required for other purposes.
The smaller models require an air compressor that can provide approximately 90 psi, while larger models require 120 psi. If other tools are run off the same compressor, the vibrators are likely to lose power, therefore a dedicated compressor is recommended.
There are two types of these air-powered motor-in-head vibrators, depending on what is used to connect the head to the compressor. Typically, this is a hose but occasionally a rigid shaft. The rigid-shaft model is used primarily for mass concrete, which is so stiff that workers have to thrust the vibrator into the concrete after it is deposited. For either type, the compressor simply forces air through the vibrator's inner hose or shaft until it turns the eccentric inside the vibrator head.
A pneumatic vibrators' bearing-less turbine assembly allows them to run at high speeds. Exhaust air is driven through the outer hose or shaft that's handled by the operator.
With their bearing-less design and few moving parts, pneumatic vibrators are less susceptible to motor damage than flexible-shaft or other motor-in-head models. With use, however, there may be some frequency loss caused by wear of moving parts. As the parts wear, air loss increases and vibration frequency decreases.
The Right Tool for The Job
Choosing a vibrator includes selecting the right head size, frequency, and amplitude for the project at hand. Each affects the vibrator's radius of action—the distance from the vibrator head within which consolidation occurs. A larger radius of action provides more efficient operation because fewer vibrator insertions are required.
Head-size is critical because it must be large enough to consolidate the concrete efficiently, but small enough to fit between rebar and do its job without segregating the mix or damaging the reinforcing steel.
Because rebar spacing, design slump, and form dimensions can vary from one placement to another, it's always a good idea to have more than one head diameter available onsite. Depending on jobsite conditions, additional performance variables include the vibrator head's shape, length, and coating.
For example, the traditional vibrator head is round-shaped, but some manufacturers provide square- or hexagonal-shaped heads. Round heads radiate vibrations in concentric patterns, uniformly away from the center of the eccentric. Square heads radiate vibrations from the head's flat surfaces, in flat waves rather than circles.
Case Study
Concrete structures designed to spend decades immersed in saltwater need to be constructed with special care. Take for example, the Hood Canal Floating Bridge in Kitsap County, Wash. Much of the mile-long highway bridge is supported by concrete pontoons, a number of which are being replaced. These heavily-reinforced pontoons require thorough consolidation to eliminate any voids in the concrete.
But the rebar and post-tensioning ducts make it difficult to use traditional internal vibrator heads for consolidation. Additionally, steel vibrator heads might chip the epoxy-coated rebar.
The contractor, Kiewit General, Poulsbo, Wash., looked to New York-based Oztec Industries to develop a vibrator head slim enough to fit between the forms and the reinforcement and that wouldn't damage the epoxy coating.
“It's a tight fit,” says Rich Whitlock, Kiewit General's concrete superintendent. “The space between the form walls and the rebar is only 1½ inches.”
Oztec came up with a 1 3/8 inch rubber-coated head that still could produce the amplitude, frequency and centrifugal force to move the concrete. It features 12,000 vpm, with 1/8-inch amplitude.
The job called for 6500-psi concrete that was designed to be particularly fluid, with a 20- to 22-inch spread. “This would normally be called a self-compacting slump, but because of the mix design and its cohesiveness, it still needs to be vibrated,” says Whitlock.
Making matters more difficult, Oztec also had to ensure that the vibrator head and shaft would not weave around and get stuck among the rebar in the side walls of the pontoon forms. The pontoons are very large concrete structures and getting the vibrator head stuck would be costly. Some of the pontoons are up to 360 feet in length, measuring 40 feet wide, 22 feet tall and consisting of up to 2000 cubic yards of concrete.
From setting the formwork, inserting the reinforcements, and placing concrete, the construction cycle for three or four pontoons takes up to eight months. The bottom slab is poured first, followed by the side walls, and then the top slab.
To keep this from happening, Oztec developed an inner rotating extension piece that would make the overall length of the head nearly 2 feet long, ensuring that it would not get stuck. To date, Whitlock's crews have completed 8 of the 14 pontoons without any incidents.
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This article is part of our October "Tools for the Concrete Pro" series. You can read other articles in the series by clicking the links below
