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Some precast/prestressed structures require external vibration due to rebar congestion, a stiff mix or a relatively thin cross-section. And internally vibrating long prestressed products such as beams or girders is especially labor-intensive. Many producers use external vibrators in addition to internal vibrators to consolidate concrete more quickly. Regardless of mounting, select a vibrator with high-frequency, low-amplitude output to reduce form damage. (For material-movement purposes, i.e., installation on aggregate bin hoppers, a higher amplitude is acceptable.) Pneumatically powered vibrators' vibrations-per-minute (VPM) capabilities are greater than those of most electric vibrators. The hose diameter regulates the amount of compressed air entering the unit, which, in turn, regulates VPMs. It's important to match hose and fitting diameter to prevent air-flow restriction and insufficient vibration. Eccentric weight determines amplitude, a measurement of how far the vibrator moves from a set point. Amplitude produces centrifugal force. As a general rule of thumb, determine how many centrifugal force pounds you need by dividing the combined weight of form and concrete by three. For example, a 15,000-pound product and form requires a centrifugal force output of 5,000 pounds. More force may be necessary to vibrate low-slump concrete, so it's important to provide the manufacturer with the mix design. Also provide the amount of rebar in a given product; concrete slump and product wall thickness; the type, thickness and weight of form material; and length and height of form. For material-movement applications, divide the material weight by 10 to determine necessary force output. For example, to move 1,000 pounds of material along a sloped surface, you need 100 force pounds. External vibrators have an effective radius, as do internal vibrators. Use the product's effective radius, as provided by the manufacturer, as a guide for installing the equipment at the correct spacing. (Also get placement recommendations from the form manufacturer.) The effective radii of two vibrators should overlap by a few inches. Also, make sure vibrators are positioned symmetrically on the form to ensure equal consolidation. External vibrator mountings must be rigid. Use reinforcing scab plates and channel iron to stiffen mountings--this eliminates damage to both form and vibrator. During operation, you should be able to read the vibrator's name plate; this indicates a sufficiently rigid mounting. Do not install an external vibrator within 30 inches of the top of the form, especially if concrete isn't poured all the way to the top; this may cause an in-and-out movement that sucks air into the form instead of forcing it out. Internally vibrate concrete at the top of the form or use a vibratory screed. The article includes several available external vibrator models and accessories. KEYWORDS: external vibrator