Graphical Construction Glossary >> Concrete and Concreting. >> Concrete Vibration

Concrete Vibration

A method of consolidating wet or plastic concrete by applying either internal or external vibration to the concrete to remove gross voids and entrapped air pockets

External Concrete Vibrators

Vibrating machines that are fixed to formwork or casting tables or as vibrating screeds to consolidate wet concrete.

Internal Vibrators

Machine driven oscillating probes that are placed into wet concrete to vibrate it. Commonly called "pokers".

Note! I almost said in the definition above that vibrators are used to remove entrained air, but of course some concrete products have air purposely entrained to make them lightweight so I'll use the terms gross voids and entrapped air pockets.

As a very rough rule of thumb it has been said that up to 20% of the material coming out of the back of a transit mixer is air, trapped in the concrete. We have to remove it and the way we do it is to apply vibration to it when it is placed in it's final position, while it is still in a liquid state.

Internal concrete vibration.Above is a petrol engined flexible shaft model that has an 1 1/4" diameter shaft. There are many makes and models of these but this type is very common on construction sites.

• There is 3/4" size for smaller jobs and pre-cast type projects that is usually driven by an electric motor.
• A larger size still is the 3" diameter shaft that is normally driven by compressed air and used for large jobs, mass footing etc.
• The shafts themselves come in different lengths.
• Newer types of flexible shaft vibrators, called high-cycle vibrators are driven by special electric motors giving a greater vibration output that are particularly useful in working with low slump concrete.

The use of these vibrators is very simple, but I will give a brief list of do's and don'ts

• Check the vibrator before ordering the concrete and on critical jobs always have two of them on hand.
• The probe is commonly called a poker and it should be lowered vertically into the concrete and left in position for at least 10-20 seconds before being slowly lifted out and repeating the process say about 12" away.
• Do not lay the poker flat and drag it through the concrete, this can cause segregation and lines of weakness in thin slabs that can cause cracking later.
• For the same reason do not use the vibrator as an aid in moving or dragging the concrete.
• In beams and the like operators are quite often seen wedging the poker between the rebars. While this seems to have a good effect by setting up a lot of vibration in the steel, it case easily pull the steel out of it's design position and it should not be done.
• Worse still is wedging the poker between the rebar and the formwork. Again this gives a very gratifying vibration of the form panels, but again, do not do it as it causes poker burns on the form ply, sometimes quite deep which will mar the face of the concrete and damage the ply.

External concrete vibration. These come in a few different varieties.

• The simplest type of course are simple hand tools the can be used in a last resort when something breaks down. Belting the outside of the formwork, particularly steel members with a hammer, dollying the concrete up and down with a shovel or rake, using a length of timber in wall or column forms to create the same effect of the concrete mass heaving up and down. Tamping the surface with screeds or timber, again in a rhythmic motion to compact thin slabs. It can and has been done many times but it is not recommended.
• Specialist mechanical devices that are built as part of large precasting machines or vibrating casting tables.
• Flexible shaft devices that can be clamped onto steel formwork.
• Vibrating screeds in the larger sizes perform both the function of a screed and a vibrator for concrete slabs.

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