[C M Dordi]

Concrete Compaction

After concrete is placed at the desired location concrete compaction is the next step in the process of concrete production. This step has a very significant contribution to the durability, strength, finish and appearance of the concrete structure.

We have in our earlier publications already covered various steps of concrete production. Even though concrete placing and compacting are done more or less simultaneously, it is felt that these should be treated separately specially because of their individual importance to the quality of concrete.

Concrete must be compacted at the same pace as it is being placed. Compacting process consolidates fresh concrete within the moulds or formworks and around embedded parts and reinforcement steel. Compaction is necessary to remove entrapped air which is present in concrete after it is mixed, transported and placed.

Compacting also helps eliminate stone pockets and thereby eliminate all types of voids that may possibly be left in the concrete, causing reduction in strength as well as durability.

Compaction of concrete is required to get rid of entrapped air and voids as much as possible, down to less than one percent in normal concrete. The amount of entrapped air is directly related to the workability of concrete. Lower the workability, higher is the percentage of entrapped air. In other words stiff concrete mix has high percentage of entrapped air and therefore would need higher compaction effort than highly workable mixes.

The importance of removal or entrapped air or voids from concrete can be best emphasized by the following :

Voids reduce the strength of concrete. With every one percent entrapped air the strength is reduced by about 5% to 6%. Five percent entrapped air mean 30% loss of strength.

Voids increase the permeability of concrete. Loss of impermeability creates easy passage of moisture, oxygen, chlorides, and other aggressive chemicals into the concrete. Th is causes rusting of steel and spalling (disintegration) of concrete i.e. loss of durability.

Easy entry of sulphate from the environment causes expansive reaction with the tricalcium aluminate (C3A) present in cement. This cause disintegration of concrete and loss of durability.

Entry of carbon dioxide causes carbonation of concrete i.e. loss of alkalinity of concrete or loss of the protective power that concrete gives to the reinforcement or concrete cover into the embedded steel, steel becomes vulnerable to the attack of moisture. This expedites rusting of steel as the protective concrete cover is no longer alkaline in nature.

Voids reduce the contact between embedded steel and concrete.  This results in loss of bond strength of reinforced concrete member and thus the member loses strength.

Voids such as honeycombs and blowholes on the exposed surface produce visual blemish. Concrete surface is not good to look at with such blemishes. Concrete with smooth and perfect surface not only looks good but is also stronger and more durable.

For more details visit or download

concrete compaction fd.pdf Size: 1.92 MB  Downloads: 0

[Manish Jain]

COMPACTION BY VIBRATION

Compaction is best done by vibration. Rodding, spading and tamping are all ways of removing air from concrete and to compact it, however the best and most efficient
method is vibration. Rodding is an age old method being used. However, this method requires very sincere manual effort, good supervision and high workability. Under
the present circumstances it is not possible to achieve this and hence compaction by mechanical means is always preferred.

On Vibration, the concrete mix gets fluidized and the internal friction between the aggregate particles reduces, resulting in entrapped air to rise to the surface. On
losing entrapped air the concrete gets denser.

Vibration helps entrapped air to escape first from between the coarse aggregate particles and later from the mortar. As the concrete subsides, large air voids between
the coarse aggregates get filled with mortar. When vibration continues some more entrapped air from the mortar is driven out. However, during this second phase,
concrete does not show any movement but it is in this phase that maximum - ~trapped air is driven out and that is the time when most of the consolidation takes
place.

For a stiff concrete mix greater effort is required for fluidisation and hence compaction has to be for a longer period. For a lean or over wet mix, compaction effort required
is less. On vibration of an over wet concrete mix the coarser aggregates settle down and a weak layer of fine materials (cement paste with finer fines of fine aggregates) will rise on the surface . With properly designed cohesive mix the formation of weak laitance layer is prevented. It is recommended that at any time if weak laitance layer (layer of cement paste and finer fines of fine aggregate) is formed it must be removed. 

As long as vibrators are operated properly and not used to move concrete laterally, compaction enhances concrete performance and appearance . Compaction improves concrete density, strength, bonding with reinforcement steel, minimizes surface blemishes and also to a great extent helps in improving the durability of the structure. 

The efficiency of compaction is related to the workability and characteristics of the vibrator.

For more details visit or download

 concrete compaction fd.pdf

[C M Dordi]

TYPES OF VIBRATORS

Broadly vibrators can be classified into two 

Internal Vibrators
External Vibrators

Generally, mechanical compaction is done using internal vibrators i.e. using immersion or poker vibrators. This compaction method is preferred because the poker works directly on the concrete and can be moved from position to position very easily and quickly.

INTERNAL VIBRATORS

The Internal immersion type vibrators are of two types :
The head contains only vibratory mechanism and a flexible Shaft connects it to the drive which is operated by an electric motor or petrol or diesel engine. They are portable and convenient to use along with a separate motor.

The motor and vibratory mechanisms are both in the head. These types of vibrators are operated electrically or by compressed air.

Both these vibrators equally match each other in their performance, the only primary difference between the two is the location of the vibrator's power unit either inside or outside the vibrating head.

Indigenously made vibrators do not have drive inside the head. This may be mainly due to economic reasons. Immersion vibrators with a separate motor are quite commonly used. Generally, electric motor drives are preferred whenever electric power supply is easily available. Petrol or diesel run motors are only used in cases where electric power is not easily available.

The flexible shaft vibrators with separate drive motor have better applicability for different requirements. They are lighter in weight and easier to handle than motor in-head type of vibrators.

For proper compaction it is essential that the operator does not get tired due to heavy weight of the needle and the flexible shaft. It is often essential to change the operators after some time or to have one person handling the needle and another the motor.

The main disadvantage of electric flexible shaft vibrators is their loss of power under load (in concrete). This power loss lowers the frequency and the ability to remove entrapped air from the motor. Power loss problems can be avoided to a certain extent by using correct length of shaft or vibrator head to match the capacity of the motor. However, power loss due to low slump of concrete is unavoidable as concrete workability cannot be altered once it is in the form. 

The flexible shaft is also particularly susceptible to strain and damage due to bad and rough handling. The flexible shaft vibrator can be a high  maintenance tool specially if it is repeatedly handled roughly or the needle repeatedly hits the reinforcement formwork during compaction.

The motor-in-head vibrators are initially more expensive. However, due to their design features, they are less vulnerable to damage and have fewer maintenance problems. Therefore their life cycle cost may work out to be nearly the same as flexible shaft vibrators. However, the connecting hose between the needle and power supply source protects only the power cable and not the flexible drive shaft and hence it is more delicate and gets damaged more often than the tougher flexible hose housing the flexible shaft.

For more details visit or download

 concrete compaction fd.pdf

[Manish Jain]

please also explain about the external vibrators

[C M Dordi]

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 .

[Manish Jain]

OVER VIBRATION

A good cohesive concrete mix will withstand over vibration but a lean concrete mix will segregate and result in concrete of non-uniform strength, quality and durability. Concrete will be weaker at the top and would develop cracks and will not perform to the extent desired.Over vibration results in excess laitance· on the top surface which in case of columns and wall tops can be easily removed by chipping while preparing the construction joint.

In case of slabs, however, this removal is impossible. Therefore excessive laitance formation must be avoided either by using a cohesive mix not likely to bleed and I or not overworking the surface with the poker vibrator. However, in cases this is not completely possible, after the compaction is over the top surface can be scooped
up once again using a fork (panja) and coarse aggregate brought on to the surface and then levelled and compacted with a float.

[Manish Jain]

REVIBRATION

As long as the concrete is still workable (still not achieved its initial set) there is no harm if it is revibrated. Tests have shown increase in strength due to revibration. n columns and walls where surface finish is of importance, there is sometimes a tendency for blowholes to occur in the top 600 mm of a lift. The reason is that, unlike the lower layers, this top layer does not have the advantage or the weight of additional concrete on top to increase the compaction. It can be advantageous to
revibrate the top 600 mm of concrete after the initial compaction but before it achieves the initial set.

It is also observed that in hot weather concreting shrinkage cracks are reduced if the concrete pour in slabs are lightly vibrated initially and then revibrated after a
lapse of time. Revibration and refinishing helps reduce shrinkage cracks.