Concrete should be thoroughly compacted during placing by vibration or other means and worked into corners of the formwork and round reinforcement, tendons, duct formers, embedded fixtures and the like to form a solid void-free mass having the required surface finish. Vibrators should be used in a manner that does not promote segregation, and vibration applied until the expulsion of air has practically ceased. Over-vibration should be avoided as this may cause the formation of a weak surface layer.
When external vibration is used, the formwork design and vibrator disposition must ensure efficient compaction and avoid surface blemishes.
Where permanent formwork is incorporated in the structure extra care is required, as full compaction of the concrete cannot be checked after the formwork is removed. The energy absorbed by the formwork should be taken into account when deciding on the method of vibration to be used.