Concrete Technology

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Jacob Sabu

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Compaction is the process which expels entrapped air from freshly
placed concrete and packs the aggregate particles together so as to increase
the density of concrete
WHAT IS COMPACTION?
Constituent Before Compaction After Compaction
Aggregates 60-75% 60-75%
Cement 25-40% 25-40%
Air 25-30% 1-2%
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 Increased ultimate strength of concrete
 Enhanced bond between concrete and reinforcement
 Increased abrasion resistance and general durability of concrete
 Decreased permeability of concrete, as moisture and air are more
likely to penetrate to the reinforcement causing it to rust.
 Minimisation of shrinkage characteristics
 Minimisation of other forms of cracking
Why compaction is required?
COMPACTION OF CONRETE

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COMPACTION OF CONRETE

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Methods of Compaction
Compaction can be done Manually and Mechanically.
 Manual / Hand Compaction
Hand compaction is used for ordinary and unimportant
structures.The various methods of hand compaction are
as given below.
 Rodding
 Ramming
 Tamping
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 External Vibration / Form vibrators
 Table Vibration
 Mechanical Compaction
 Internal Vibration
 Platform Vibration
 Surface Vibration
Vibration is imparted to the concrete by mechanical
means. It causes temporary liquefaction so that air
bubbles come on to the top and expelled ultimately.
Mechanical vibration can be of various types as given
under
 Compaction by pressure and jolting
 Compaction by spinning
 Vibratory rollers
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Manual / Hand Compaction
1. Rodding
It is a method of poking with 2m
long, 16 mm dia rod at sharp corners
and edges. The thickness of layers for
rodding should be 15 to 20 cm.
2. Ramming
It is generally used for compaction on
ground in plain concrete. It is not used
either in RCC or on upper floors.
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3. Tamping
It is a method in which the top surface is beaten
by wooden cross beam of cross section 10 cm x
10 cm. both compaction and leveling are
achieved simultaneously. It is mainly used for
roof slabs and road pavements.
Mechanical Compaction
1. Immersion/Internal Vibration
Frequently referred to as ‘poker’ or ‘needle ’ vibrators, immersion vibrators
consist essentially of a tubular housing which contains a rotating eccentric
weight.
The outof-balance rotating weight causes the casing to vibrate and, when
immersed in concrete, the concrete itself.
Depending on the diameter of the casing or head, and on the frequency and
the amplitude of the vibration, an immersion vibrator may have a radius of
action between 100 and 600 mm
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2 . External Vibration / Form vibrators
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 as compared to the internal vibration.
The form work also has to be made extra strong when
external vibration is used.
3. Table Vibration
It is mainly used for laboratories where
concrete is put on the table.
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4. Platform Vibration
It is similar to table vibrators but these
are generally used on a very large
scale.
5. 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.
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Advantages : Compaction by
Vibrations
 Efficient than Manual.
 Concrete can be placed and compacted in difficult
sections
 Slump requirements – 4 cm with vibrations & 12
cms with manual method
 Facilitates use of less water – thus
increased strength & durability
 In essence, method of removal of
entrapped air is different
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Compaction of Concrete by Pressure & Jolting
 Combined action of Vibrations – Pressure &
Jolting results in very good compaction &
dense concrete & dense concrete
 Very effective for very dry lean mixes
 Gen used for compacting hollow blocks,
cavity blocks, solid concrete blocks, railway
sleepers etc.
 Used for compacting concrete pipes
Compaction by spinning
 Recent technology
 Plastic concrete when spun at a very high
speed, gets well compacted by centrifugal
force.
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 Used for concrete spun pipes, hume pipes etc.
Vibratory rollers
 Recent development of concreting very dry and lean
concrete.
 Originated from Japan and US
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