Correct Answer - Option 2 : Both Statement (I) and Statement (II) are individually true but Statement (II) is not the correct explanation of Statement (I)
Concept:
Shutter Vibrators
A shutter vibrator is also called an external vibrator and is clamped rigidly to the form work at pre-determined points. This allows for the form and concrete to be vibrated.
They are more powerful for a given compaction in comparison to an internal vibrator.
These vibrators operate at a frequency of around 3000 to 9000 rpm.
The shutter vibrators are used more often for pre-casting of thin sections that do not have the thickness needed for internal vibrators.
Needle Vibration
It is most commonly used technique of concrete vibration. Vibration is achieved due to eccentric weights attached to the shaft. The needle diameter varies from 20 mm to 75 mm and its length varies from 25 cm to 90 cm. the frequency range adopted is normally 3500 to 5000 rpm.
The correct and incorrect methods of vibration using internal vibration needles are shown below.
Justifications for given statement:
In case of pre-stressed beams the depth requirement is less because of prestressing, so shutter vibrators are used for pre-stressed beam.
Needle vibrators are used where the depth is more, so it can be used for foundation concreting or RCC beam.
Needle vibrators are susceptible to get dysfunctional with leaking-in of cement slurry, but this will not happen in case of the shutter vibrator as shutter vibrator is an external vibrator whereas needle vibrator is an internal vibrator.
So here both the statements are correct.
