Introduction.
After concrete is placed at the desired location, the next step in the process of concrete production is its compaction. Compaction consolidates fresh concrete within the moulds or frameworks and around embedded parts and reinforcement steel.
Considerable quantity of air is entrapped in concrete during its production and there is possible partial segregation also. Both of these adversely affect the quality of concrete. Compaction of the concrete is the process to get rid of the entrapped air and voids, elimination of segregation occurred and to form a homogeneous dense mass. It has been found that 5 per cent voids in hardened concrete reduce the strength by over 30 per cent and 10 per cent voids reduce the strength by over 50 per cent.
Therefore, the density and consequently the strength and durability of concrete largely depend upon the degree of compaction. For maximum strength driest possible concrete should be compacted 100 per cent. The voids increase the permeability of concrete. Loss of impermeability creates easy passage of moisture, oxygen, chlorides, and other aggressive chemicals into the concrete. This causes rusting of steel and spalling (disintegration) of concrete i.e., loss of durability. Easy entry of sulphates from the environment causes expansive reaction with the tricalcium aluminate (C3A) present in cement.
This causes 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 other steel embedded in it. Once the carbonation depth exceeds the thickness of concrete cover to the embedded steel, steel becomes vulnerable to the attack of moisture. This expedites rusting of steel as the protective concrete cover remains no longer alkaline in nature. Voids also 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 with all such blemishes. Concrete with smooth and perfect, surface finish not only looks good but is also stronger and more durable. Compaction is achieved by imparting external work over the concrete to overcome the internal friction between the particles forming the concrete, between concrete and reinforcement and between concrete and forms and by reducing the air voids to a minimum. The compaction of concrete can be achieved by the following methods.
1.Hand Compaction.
This method of compaction is used for small and unimportant jobs. However, this method is extremely useful for thin elements such as slabs, and for members with congested reinforcements. This method can be used for mixes with any workability except for very fluid or very plastic mix. Hand compaction is achieved by rodding ramming, or tamping. Rodding is done with the help of 16 mm diameter, 2 m long steel rod to pack the concrete between the reinforcement, sharp corners and edges. Rodding is done continuously during concreting. Ramming is permitted only for unreinforced concrete constructions. The roof and floor slabs are usually tamped for achieving compaction. The tampers are 100 × 100 mm in section and about 1 m long. Tamping bars not only compact the concrete but also level the top surface. The limitation of this method is that a large water-cement ratio is required for full compaction.
2.Compaction by vibrators.
This is the most common and widely used method of compacting concrete for any structural element. The vibrations imparted to the fresh concrete reduce the internal friction between the particles of concrete by setting the particles in motion and thus produce a dense and compact mass. 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. Vibrations do not affect the strength of concrete but concrete of higher strength and better quality can be made with lesser water and given cement content. Therefore, with vibrations stiff concrete with low water cement ratio can also be well compacted. For full compaction, vibration can be considered to be sufficient when the air bubbles cease to appear and sufficient mortar appears to close the surface interstices and facilitate easy finishing operation. Vibration helps entrapped air to escape first from between the coarse aggregate particles and later from the 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 entraped air is driven out and that is the time when most of the consolidation takes place. Plastic mixes need less time of vibration than harsh or dry mixes.
The various types of vibrators in use are needle, formwork, table or platform, and surface vibrators.
1.Needle vibrators.
These are also knows as immersion, internal, or poker vibrator. Needle vibrator can be used for any type of concrete work. This consists of a steel tube, called poker, having an eccentric vibrating element inside it, a flexible shaft and a power unit. The needle diameter varies from 20 to 75 mm and its length from 250 to 900 mm. In places where the reinforcement is congested blades are used instead of needle. An average frequency of vibration used is 3,500 to 5,000 cycles per minute. The vibrator is immersed into concrete at a spacing of not more than 600 mm or 8 to 10 times the diameter of the poker. The vibrator is immersed at a place for about 30 seconds to 2 minutes. Location of the poker insertion should be staggered to ensure that every bit of concrete is compacted.The vibrator should be allowed to penetrate the concrete vertically (inclination 10°) under its own weight. For proper compaction in a heaped concrete the poker position should be changed gradually from the side of the heap and moving away from the heap all around. Poker should not be introduced in the center of the help .
2.Farmwork vibrators.
These are also known as external or shutter vibrators. These are generally used under the following circumstances:
1.Compaction of concrete is required to be done in a very thin or very densely congested reinforced section.
2. In addition to internal vibration, compaction is required to be done specially in the cover area where at times needle or poker vibrator is unable to do satisfactory compaction.
3. Compaction of very stiff concrete is required to be done because such concrete cannot be compacted by internal vibrators. Formwork vibrators are used for concreting columns, thin walls and precast units. These are rigidly clamped to the formwork, causing it to vibrate and consequently transfer the vibrations to concrete. 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 I-beam, which runs along the formwork touching the form stiffeners. The shuttering and the formwork have to be rigid, strong and watertight. The vibrators operate at a frequency of 3,000 to 9,000 cycles per minute. The formwork requires to be properly designed to transfer the vibrations to the concrete without itself getting displaced or opening up. These consume more power and are less efficient than the needle vibrators. 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 1000 mm below the top finished level of concrete. The top layer of concrete must be vibrated by a needle vibrator. Use of form vibrator 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.
Form vibrators if used in addition to internal vibrators help removing entrapped air along the concrete surface giving a much superior finish. Generally, tapping 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.
A special case of a formwork vibrator used in laboratories and factories is vibrating table. The vibrating table consists of a rigidly built steel platform mounted on flexible springs and driven by an electric motor. The average frequency of vibration is 4000 cycles per minute. The moulds are clamped on the platform. Vibrations are stopped as soon as the concrete in the moulds develops a level smooth surface. 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. The vibrating tables are very efficient in compacting stiff and harsh mixes required for precast elements.
Surface Vibrators.
These are also known as screed board vibrators. Surface vibrators are used for floor and roof slabs and pavement surfaces. These are effective only up to a thickness of 150 mm of concrete but can be used up to 250 mm. Surface vibrators cause movement of fine particles to the top and hence aid the finishing operation. The operating frequency is 4,000 cycles per minute.
Compaction by Spinning.
This method is also known as centrifugation and is used for producing circular elements such as pipes. The plastic concrete is spun into the horizontal mould at a very high speed. Water forced out of the mix during spinning flows out of the mould. The initial water cement ratio required for effective compaction is 0.35 to 0.4 which after spinning reduces to 0.3.
Compaction by Jolting.
In this method of compaction the mould containing dry concrete is subjected to jolt at a frequency of 100 to 150 jolts per minute. Jolting is a vibrating action of low frequency and high amplitude. The mould filled with concrete is raised by about 12 mm and then allowed to fall under gravity. This method is used for precast units such as hollow blocks, cavity blocks etc.
Compaction by Rolling.
In this method of compaction the soft and plastic concrete is fed continuously between rubber rollers under pressure up to 50 atmosphere. This forces out the excess water in concrete. Rolling is used to manufacture precast concrete products such as tiles.
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