"To avoid cold joints, placing should be resumed before the surface hardens. For unusually long delays during concreting, the concrete should be kept live by periodically re-vibrating it. Concrete should be vibrated at approximately 15-minute intervals or less depending upon job conditions. However, concrete should not be overvibrated to the point of causing segregation. Furthermore, should the concrete approach time of initial setting, vibration should be discontinued and the concrete should be allowed to harden. A cold joint will result and suitable surface preparation measures should be applied."
American Concrete Institute
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