Last edited 25 Feb 2021

Concrete compaction

Contents

[edit] Introduction

Compaction is a process of expelling entrapped air. If we don’t expel this air, it will result into honeycombing and reduced strength. It has been found from the experimental studies that 1% air in concrete reduces the strength by approximately 6%.

There are two methods of compaction:

  • Hand compaction.
  • Mechanical compaction i.e vibration method
  • Compaction by pressure and jolting method
  • Compaction by a spinning method

[edit] Hand compaction

Hand compaction is used for simple, small structures. Workability should be decided in such a way that the chances of honeycombing is minimised.

Methods of hand compaction include:

  • Rodding: 'Poking' with a 2m long, 16 mm dia rod at sharp corners and edges. The thickness of layers for rodding should be 15 to 20 cm.
  • Ramming: Generally used for compaction on ground in plain concrete. It is not used either in RCC or on upper floors.
  • Tamping: The top surface is beaten by a wooden cross beam of cross section 10 cm x 10 cm. Both compaction and levelling are achieved simultaneously. It is mainly used for roof slabs and road pavements.

[edit] Mechanical compation

Vibration is imparted to the concrete by mechanical means. This causes temporary liquefaction so that air bubbles come to the top and are expelled.

Mechanical vibration techniques include:

  • Internal vibration: The most commonly used technique of concrete vibration. Vibration is achieved by eccentric weights attached to a 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.
  • External vibration: This is adopted where internal vibration can’t be used due to either thin sections or heavy reinforcement. External vibration is less effective and it consumes more power compared to the internal vibration. The formwork also has to be made extra strong when external vibration is used.
  • Table vibration: Mainly used for laboratories where concrete is put on the table.
  • Platform vibration: Similar to table vibrators but these are generally used on a very large scale.
  • Surface vibration: These are also called screed board vibrators. The action is similar to that of tamping. The vibrator is placed on screed board and vibration is given on the surface. It is mainly used for roof slabs, road pavements etc., but it is not effective beyond 15 cm depth.

[edit] Compaction by pressure and jolting method

  • In this method, stiff concrete is subjected to external pressure from the top and vibration from the bottom along with a series of jolting actions
  • Excess water added during mixing exits due to large pressure

[edit] Compaction by a spinning method

  • Compaction by Spinning is also called centrifugation
  • This method Compaction by Spinning is used in the manufacture of concrete pipes, concrete lamp posts etc
  • The initial w/c ratio of 0.35 to 0.40 reduced to about 0.30 after spinning

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