Compaction

• 
  After concrete is placed at the desired location, the compaction is the next step Compaction is necessary to remove entrapped air which is present in concrete. 
• Compaction is best done by vibration and is the most efficient method. There are internal vibrators and external vibrators. The internal  vibrators are the needle  vibrators or the poker vibrators which are immersion type. The external vibrators are form vibrators, vibrating tables, plate vibrators, surface screed  vibrators. 

Guidelines for Proper Compaction  using a Poker Vibrator 

• Concrete surface being compacted must be fully visible 
• The vibrator must be inserted quickly, hut withdrawal  must be slow so that the hole left by the poker is filled up as it is being withdrawn. 
• Locations of insertion of vibrator should be so staggered to ensure that whole concrete is compacted. 
• Do not touch  the formwork and reinforcement with vibrator 
• Vibrator should extend about 100 mm into the previous layer 
• Do not  use the vibrator continuously for a long  time. Keep enough spare vibrators. 
• Initially as the needle  vibrator is inserted in concrete, the  mix settles and the level  drops. If compaction is continued further, entrapped air bubbles rise to the surface and are clearly visible. As soon as the bubbles stop emerging it can be taken that compaction is complete. 
• When the needle vibrator's  sound becomes constant, it is an indication that concrete is compacted. This kind of expertise is developed by a skilled operator who can easily  make out from the sound emitted by the vibrator as to the level of compaction. 
• Another .indication of full compaction is when a thin shining film  appears on the concrete surface during  compaction. 
• For  slabs use plate vibrators/screed vibrators 
• For thin precast  concrete elements,  shutter vibrators are more effective.