It is becoming increasingly popular to attach external form vibrators to formwork shutters. The benefit of these is to improve the concrete finish or to vibrate the concrete in area’s a traditional pencil vibrator cannot reach. Generally, external vibrators are only used on thin or heavily reinforced walls.

The issue with external form vibrators is that they re-liquify the concrete thus removing any setting affects you have allowed for in the pressure calculation.

To reduce concrete pressure the calculation assumes that after a set period, the concrete reaches initial set and the pressure in the lower part of the form does not increase. As shown in Figure 1 for this 5m wall we can assume the bottom 2733mm of pressure will not increase once we pour the last 2267mm of the wall.

If external form vibrators were to be installed on the full height of the wall, the whole wall would effectively be “shaken” and remove this initial set in the concrete and make the pressure fully hydrostatic (no setting) as can be seen in Figure 2, where the pressure is now 120KPa compared to 50KPa above. Full hydrostatic means the pressure is simply the density multiplied by the height and the pressure will be the greatest at the bottom then tapper linearly to the top where it will be zero pressure. Concrete weighs 24kn per cube so 24 x 5m = 120kpa.

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