g PSW- Compaction of concrete is required to be done in a very thin or very densely congested
reinforced section. - In addition to internal vibration, compaction is required to be done specially in the cover
area where at times needle or poker vibrator is unable to do satisfactory compaction. - Compaction of very stiff concrete is required to be done because such concrete cannot be
compacted by internal vibrators.
Formwork vibrators are used for concreting columns, thin walls and precast units. These are
rigidly clamped to the formwork, causing it to vibrate and consequently transfer the vibrations
to concrete. The performance of these vibrators when directly attached to formwork is not
generally satisfactory. The vibrator is mounted on a steel plate, which is attached to a channel
or the I-beam, which runs along the formwork touching the form stiffeners. The shuttering and
the formwork have to be rigid, strong and watertight. The vibrators operate at a frequency of
3,000 to 9,000 cycles per minute. The formwork requires to be properly designed to transfer the
vibrations to the concrete without itself getting displaced or opening up. These consume more
power and are less efficient than the needle vibrators.
The compaction time of form vibrators is generally between 1 to 2 minutes. Form vibrators
should not be used on top of the vertical formwork. Generally they should be fixed 1000 mm
below the top finished level of concrete. The top layer of concrete must be vibrated by a needle
vibrator. Use of form vibrator at top locations will generally cause separation of concrete from
the formwork. Due to formwork being inadequately stiffened at top it results in, in and out
movement causing the separation.
Form vibrators if used in addition to internal vibrators help removing entrapped air along
the concrete surface giving a much superior finish. Generally, tapping with a wooden mallet on
the external face of the formwork also helps remove entrapped air which generally blemishes
the concrete surface inspite of adequate internal vibration.
A special case of a formwork vibrator used in laboratories and factories is vibrating table.
The vibrating table consists of a rigidly built steel platform mounted on flexible springs and
driven by an electric motor. The average frequency of vibration is 4000 cycles per minute. The
moulds are clamped on the platform. Vibrations are stopped as soon as the concrete in the
moulds develops a level smooth surface. Medium workable mixes generally require higher
frequency of vibration than stiff mixes. It is observed that increasing the frequency and decreasing
the amplitude as vibration progresses improves consolidation by using this type of vibrator.
The vibrating tables are very efficient in compacting stiff and harsh mixes required for precast
elements.
'
2 X These are also known as screed board vibrators. Surface vibrators are used
for floor and roof slabs and pavement surfaces. These are effective only up to a thickness of 150
mm of concrete but can be used up to 250 mm. Surface vibrators cause movement of fine
particles to the top and hence aid the finishing operation. The operating frequency is 4,000
cycles per minute.
g
2 2
This method is also known as centrifugation and is used for producing circular elements such
as pipes. The plastic concrete is spun into the horizontal mould at a very high speed. Water
forced out of the mix during spinning flows out of the mould. The initial water cement ratio
required for effective compaction is 0.35 to 0.4 which after spinning reduces to 0.3.