When I work with precast concrete producers to help them properly size and apply external concrete form vibrators, there are many things to consider. Among them are:
Preferred Power Source
- Electricity
-
Compressed Air
Concrete Mix Characteristics
- Slump
- Aggregate Size
Permanent Mount or Portability
Spacing
How Long to Vibrate
Preferred Power Source
are commonly used for external form vibrators. These rotary vibrators can be powered by either electricity or compressed air.
Electric-powered vibrators usually have lower operating costs and require minimal maintenance. Suitable for controlled-noise environments, electric vibrators operate more quietly than equivalent pneumatics. They do have limited mobility since they are tied to a power source.
Compressed air is considered safer than electricity when used in damp conditions as there is no electrical shock risk. However, the cost to operate can be higher and comes with an increased noise level. Compressed air does require regular maintenance to ensure proper filtration and lubrication.
Concrete Mix Characteristics
Lower slump concrete is stiffer and generally not free flowing, responding better to slower speed rotary forces/higher amplitude. Higher slump mixes flow easily and can be pumped. This mix generally responds better to higher-frequency rotary force.
Permanent/Portable Mounts
We normally recommend permanently-mounted external form vibrators as the energy transfer is more predictable, efficient, and contributes to a longer life. Portable external vibrators require receiver bracketry and the frequent handling of the vibrators can contribute to an earlier demise.
Spacing
As a rule of thumb, rotary external form vibrators have a 3’ area of influence in all directions. We recommend spacing the rotary external form vibrators approximately 6’ apart for optimal coverage.
How Long To Vibrate
Vibrate until the concrete levels off in the form, the coarse aggregate particles become embedded, a thin film of mortar forms on the top surface, and the air bubbles stop percolating.
The vibrators should operate during the filling process to enable the entrapped air to move upward and escape faster than if the form was completely filled. A rule of thumb to consider is to vibrate approximately 1-2 seconds per 1-2 inches of form height (a 4’ high form = 48 to 96 seconds of vibration).
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