06-09-2022, 12:07 PM
EXTERNAL VIBRATORS
Eternal Vibrators can be form vibrators, vibrating tables or surface screed vibrators, plate vibrators, vibratory rollers, vibratory hand floats or trowels.
FORM VIBRATORS
Form vibrators (See Figure Nos.B and 9) are to be securely attached to the outside of the formwork. They are generally used under the following circumstances:
Compaction of concrete is required to be done in a very thin or very densely congested reinforced sections.
In addition to internal vibration, compaction is required to be done specially in the cover area where at times the needle or poker vibrator is unable to do compaction satisfactorily.
Compaction of very stiff concrete is required to be done because such concrete cannot be compacted by internal vibrators. 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 1-beam which runs along the formwork touching the form stiffeners.
The formwork requires to be properly designed to transfer the vibrations to the concrete without itself getting displaced or opening up. The spacing of the form vibrators has to be such that intensity of vibration is uniformly applied over the concrete within the formwork. 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 1 000 mm below the top finished level of concrete. The top layer of concrete must be vibrated by a needle vibrator. Use of form vibrators 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.
In a heavily reinforced section wherein an internal vibrator cannot penetrate it is sometimes helpful to vibrate reinforcing bars by attaching form vibrators to exposed or projecting bars. This helps entrapped air underneath the rei nforcing bars to get released and increase the bond between the reinforcement and the surrounding concrete.
Form vibrators if used in addition to internal vibrators help in removing entrapped air along the concrete surface giving a much superior fi nish. Generally, trapping 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.
VIBRATING TABLES
They are mainly used in precasting plants or concrete testing laboratory . They are equipped with controls to vary frequency and amplitude according to the size
of the element to be compacted or the concrete consistency. In this type of compaction method formwork is clamped to the vibrator instead of other way round as in case of form vibrators.
However, as in case of form vibrators in this case also the form and the concrete are both vibrated together. 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.
SURFACE VIBRATORS
This type of vibrator is used to consolidate concrete in floors and other types of flat works. The surface vibrators apply vibration through a flat plate or abeam directly on the top surface of the concrete. It is recommended not to use these vibrators when concrete slump is in excess of 75 mm as it cause excess accumulation of mortar at the top causing loss of strength on the surface and subsequent loss of wear resistance. Vibrating screeds give positive control of the strike off operation and result in a great deal of saving in labour for levelling and finishing. However, surface vibrators must not be operated after the concrete has been adequately consolidated.
Surface vibration is least effective along the edges and corners hence a poker vibrator should compact the concrete along the entire form edge immediately before the vibrations by surface vibrator starts.
Screed vibrators are generally recommended for consolidation of slabs and floors upto 150 mm thickness if they are reinforced or upto 200 mm thickness if they are not reinforced.
Internal vibration is necessary for all slabs over 200 mm thick and for all slabs of lesser thickness with reinforcing steel, conduits or other embedments .
Eternal Vibrators can be form vibrators, vibrating tables or surface screed vibrators, plate vibrators, vibratory rollers, vibratory hand floats or trowels.
FORM VIBRATORS
Form vibrators (See Figure Nos.B and 9) are to be securely attached to the outside of the formwork. They are generally used under the following circumstances:
Compaction of concrete is required to be done in a very thin or very densely congested reinforced sections.
In addition to internal vibration, compaction is required to be done specially in the cover area where at times the needle or poker vibrator is unable to do compaction satisfactorily.
Compaction of very stiff concrete is required to be done because such concrete cannot be compacted by internal vibrators. 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 1-beam which runs along the formwork touching the form stiffeners.
The formwork requires to be properly designed to transfer the vibrations to the concrete without itself getting displaced or opening up. The spacing of the form vibrators has to be such that intensity of vibration is uniformly applied over the concrete within the formwork. 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 1 000 mm below the top finished level of concrete. The top layer of concrete must be vibrated by a needle vibrator. Use of form vibrators 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.
In a heavily reinforced section wherein an internal vibrator cannot penetrate it is sometimes helpful to vibrate reinforcing bars by attaching form vibrators to exposed or projecting bars. This helps entrapped air underneath the rei nforcing bars to get released and increase the bond between the reinforcement and the surrounding concrete.
Form vibrators if used in addition to internal vibrators help in removing entrapped air along the concrete surface giving a much superior fi nish. Generally, trapping 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.
VIBRATING TABLES
They are mainly used in precasting plants or concrete testing laboratory . They are equipped with controls to vary frequency and amplitude according to the size
of the element to be compacted or the concrete consistency. In this type of compaction method formwork is clamped to the vibrator instead of other way round as in case of form vibrators.
However, as in case of form vibrators in this case also the form and the concrete are both vibrated together. 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.
SURFACE VIBRATORS
This type of vibrator is used to consolidate concrete in floors and other types of flat works. The surface vibrators apply vibration through a flat plate or abeam directly on the top surface of the concrete. It is recommended not to use these vibrators when concrete slump is in excess of 75 mm as it cause excess accumulation of mortar at the top causing loss of strength on the surface and subsequent loss of wear resistance. Vibrating screeds give positive control of the strike off operation and result in a great deal of saving in labour for levelling and finishing. However, surface vibrators must not be operated after the concrete has been adequately consolidated.
Surface vibration is least effective along the edges and corners hence a poker vibrator should compact the concrete along the entire form edge immediately before the vibrations by surface vibrator starts.
Screed vibrators are generally recommended for consolidation of slabs and floors upto 150 mm thickness if they are reinforced or upto 200 mm thickness if they are not reinforced.
Internal vibration is necessary for all slabs over 200 mm thick and for all slabs of lesser thickness with reinforcing steel, conduits or other embedments .