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External vibrators, consisting of an electrically or pneumatically operated motor with an out-of-balance component, are mounted on the formwork. They vibrate the formwork and the concrete in the immediate vicinity; hence they are only suitable for thin sections, probably less than 300 mm thick. External vibrators are mainly used in precast work but are sometimes used on site, especially where there is congested reinforcement.
Acknowledgement: The Concrete Society
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