Now almost all concrete work is accompanied by vibration using different type of vibrator. The vibrators used in this purpose are:
a. Internal vibrator
b. External vibrator
c. Vibrating table
d. Surface vibrator
e. Vibrating roller
The vibration produced from more or less identical sources, independent of vibration method; generally eccentric mass are rotated rapidly to generate vibration and using different mechanisms expected component of vibration is neutralized.
The external vibrator is generally designed to produce vibration of 3000 cycle/min~6000cycle/min. some vibrator can produce 9000 cycle/min.
In case of internal vibration it is usually expected to produce 3500 cycle/min~5000cycle/min. internal vibrator is also known as poker vibrator.
Vibrating table usually render (50~120) HZ and expected acceleration is about 4g~7g. It is found that different method of vibration use different range of vibration. Now question is-
What is the minimum value of acceleration, amplitude and frequency?
A minimum value of amplitude is 40 µm equivalents to 0.0015 in and an acceleration of 1.5g are believed satisfactory for compacting concrete. So why are we applying several times more acceleration for compaction?
With this minimum requirements operator have to apply vibration to concrete for long period and to save time the vibration parameters are changed. Depending on application method of vibration, the vibration parameters are also changed.
For example consider poker vibrator, where poker is immersed into fresh concrete while in case of external vibrators or vibrating tables, vibration is not directly transmitted to concrete.
When simple harmonic motion is applied by vibration:
For Amplitude = a
Frequency = f
The acceleration would be-
Acceleration = a (2πf) 2
In case of vibrating table frequency of vibration can be changed and different amplitudes are set for different size of concrete member.