compaction of concrete

7/29/2019 Compaction of Concrete

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Compaction of concrete is the process that expels trapped air from the freshly placed

concrete and packs the aggregate particles together to increase the density of concrete.

Compaction of concrete is a two-stage process. First the aggregate particles are set in motion

and the slump to fill the from, giving a level top surface. In the second stage, trapped air is

expelled.

Why compacting concrete matters?

Air voids reduce the strength of the concrete. For every 1% of entrapped air, thestrength falls by somewhere between 5 and 7%.This means that concrete containing a

mere 5% air voids due to incomplete compaction can lose as much as one third of its

strength.

Air voids increase concrete's permeability. That in turn reduces its durability. If theconcrete is not dense and impermeable, it will not be watertight. It will be less able to

withstand aggressive liquids and its exposed surfaces will weather badly.

Moisture and air are more likely to penetrate to the reinforcement causing it to rust. Air voids impair contact between the mix and reinforcement. The required bond will

not be achieved and the reinforced member will not be as strong as it should be.

Air voids produce blemishes on struck surfaces. For instance, blowholes andhoneycombing might occur.

It increases significantly the ultimate strength of concrete and enhances thebond withreinforcement.

Increases the abrasion resistance and general durability of the concrete Decreases the permeability and helps to minimise its shrinkage-and-creep

characteristics.

Proper compaction also ensures that the formwork is completely filled ie there areno pockets of honeycombed material and that the required finish is obtained on

vertical surfaces


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Methods of Compaction

A. Manual Compaction by Hand Compactioni. Rodding

ii. Rammingiii. Tamping

B. Mechanical Compaction by Vibrationi. Internal Vibrators (Immersion Vibrators)

a. Poker Vibratorii. External Vibrators (Surface Vibrators)

a. Vibration plates and tampersb. Clamp vibratorsc. Slip formersd. Vibrating tables

Manual Compaction

Hand compacting is used only for small unimportant jobs that require compaction ofsmall amounts of concrete.

Also applied in such situations where a large quantity of reinforcement is used whichcannot be normally compacted by mechanical means.

The consistency of concrete ishould be maintained at a higher level The thickness of the layer of concrete is limited to about 15 to 20 cm

Rodding

Rodding is nothing but poking the concrete with about 2 metre long, 16 mm diameter rod to

pack the concrete between the reinforcement and sharp corners and edges. Rodding is done

continuously over the complete area to effectively pack the concrete and drive away

entrapped air.

Ramming

Light ramming can be permitted in unreinforced foundation concrete or in ground floor

construction.


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Ramming should not be permitted in case of reinforced concrete or in the upper floor

construction, where concrete is placed in the formwork supported on struts.

If ramming is adopted in the above case the position of the reinforcement may be disturbed

or the formwork may fail, particularly, if steel rammer is used.

Tamping

One of the usual methods adopted in compacting roof or floor slab or road pavements where

the thickness of concrete is comparatively less and the surface to be finished.

Tamping consists of beating the top surface by wooden cross beam of section about 10 cm x

10 cm. Since the tamping bar is sufficiently long it not only compacts, but also levels the top

surface across the entire length.

Mechanical Compaction by Vibration

The use of vibration may be essential for the production of good concrete where the

congestion of the reinforcement or the inaccessibility of the concrete in the formwork is such

that hand compaction methods are not practicable.

It is necessary that stiff concrete with low water/cement ratio be used.

Internal Vibrators

Needle Vibrator /Poker Vibrator

It consists essentially of a tubular housing, and a vibrating head that contains a rotating

eccentric weight.The out-of-balance rotating weight causes the casing to vibrate and, when

immersed in concrete,the concrete itself. Power is supplied by a small petrol, diesel, or

electric motor and in some cases by compressed air.

The effectiveness of an immersion vibrator is dependent on its frequency and amplitude, the

latter being dependent on the size of the head, the eccentric moment and the head weight

the larger the head, the larger the amplitude.


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Technical characteristics of internal vibrators

Method

The poker should be inserted vertically into the concrete as quickly as possible, penetrating

previous layers of freshly placed concrete (about 50-150mm) and then held stationary until

air bubbles cease to raise to the surface, usually in about 15-20 seconds. The vibrator should

then be slowly withdrawn and reinserted in a fresh position adjacent to the first.

The area vibrated at one time is called the Radius of Action. Poker movements should be in

a definite pattern so the radius of action overlaps and covers the whole area of the concrete.

compaction of concrete

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