What is Revibration in Concrete Compaction?
That vibration that is provided to the concrete after some time, at the time when the concrete gets placed and consolidated properly is called Revibration of concrete. When the concrete is placed in layers at a very deep structure then the vibration is done at the first layer.
So in many circumstances, the concrete does not get properly consolidated then the Re-vibration is done. Also, this process is quite beneficial because after the Re-vibration, aggregate particles are rearranged properly and the void is removed by air or water.
Effect of Revibration of Concrete.
The effect of Revibration is quite beneficial as follows:
- It provides strength to the concrete.
- It provides hardness to the surface of the concrete.
- The voids get removed by air or water to make the concrete consolidated properly.
- Revibration after approx 2hours when the concrete is properly placed then it gains maximum strength.
- Within the initial setting time caused an increased hardened density.
Also Read: Planning and Site Preparation for Concrete
What is the problem of over vibration of fresh concrete?
For the proper compaction, the vibrators (the vibrating part) are properly immersed in the concrete. Generally over vibration should be avoided during the compaction of concrete. Assuming the concrete mix is designed with low workability then over-vibration consumes more energy for the vibration which results in wastage of energy.
It also creates a problem which is segregation, the denser or heavier aggregate settles down and the light cement moves in an upward direction. The upper weak layer due to segregation of concrete leads to failure of concrete during operation.
Assuming the concrete is placed in a single lift for the roadways the abrasive strength is weak or poor for the surface of the carriageway. This leads to a crucial problem to the concrete carriageway which is constantly subjected to tearing by vehicular force. That is some problem that’s why Over vibration is avoided.
Difference between compaction and consolidation.
|1||The Compaction process is done quickly and it can be used for all types of soil.||It is a time taking process and it can be used for clay type of soil.|
|2||It is an artificial process.||It is a natural process|
|3||The dry density of soil has increased.||Dry density is increased by water content and will decrease.|
|4||Water concrete will remain the same.||In this process, the soil remains fully saturated.|
|5||Compaction is due to the compression of air in soil mass for the short period by moving or vibrator loads.||Static loading is applied for a long period in soil consolidation.|
|6||In this process, the soil gets fully saturated.||Consolidation is done due to the reduction of pore water from voids under steady and static load.|
Also Read: Complete Guide on Grades of Concrete
What is the Compaction of Concrete?
The process in which the solid particles are closely packed in freshly mixed concrete during the placing of the concrete by the reduction of volume of voids usually by tamping vibration, centrifugation, and subjecting concrete to high pressure for the compaction (consolidation) of the concrete.
The strength of concrete decreases if it contains voids after placing. If the voids are present in the concrete by 1% then it reduces the strength of the concrete by 5%. So to gain or attain enough strength of the concrete, voids are removed by compaction usually by some manual or mechanical means.
Method of Compaction of Concrete
There are two methods of compaction of concrete
Tamping is the process of consolidating concrete by ramming the concrete with the heavy glace faced type tools in case of dry concrete. In the case of plastic concrete mixes, a rod is inserted to the full depth of the concrete and moves in the upward and downward direction which increases the fluidity and air bubbles present in the concrete flow to the surface.
This type of tamping is effective for a depth of 5 times the maximum size of the aggregate and hence layer of concrete must be compacted, restrictions the depth to the limitations approximate depth of the layer a for hand compaction should be incased of mass concrete and 15cm in case of reinforced concrete.
Actually, a large amount of concrete can be compacted by hand but for effective compaction, we provide vibration by vibrators. In the case of vibration, compaction is affected by the reduction in the internal friction of the particles which results in the denser particle and compact mass of concrete.
Types of Vibrators
There are four types of vibrators generally used:
- Internal vibrator
- Form Vibrator
- Surface vibrator
- Vibrating Table
What is Internal Vibrator?
They are also known as needle vibrators. It has a power unit and a long flexible tube at the end where the vibrating head is attached. An electric motor, compressed air, or petrol engine is used to provide the power, the long tube rotates an eccentric weight inside the vibrating head. The frequency of the vibrators should be around 700 cycles per minute. The vibrating head is inserted in the concrete and this provides effective compaction to the concrete.
What is Form Vibrator?
They are also known as external vibrators. They are mounted on the formwork in the horizontal and vertical directions with a suitable spacing that does not exceed 90 cm in each direction.
They vibrate the concrete by the vibration and this much energy is wasted. They are only used in that condition where the internal vibrators can’t be practicable or used as in the case of thin and congested sections, arches, and tunnel lining.
What is Surface Vibrator?
They are also known as screed or pan vibrators. they are clamped to screed. They vibrate the surface of the concrete at the time of screening of the concrete. They are only effective if the depth of concrete is up to 20 cm.
If the depth of the concrete is more than 20cm then it can be used in combination with internal vibrators. They are used for pavements and slabs.
What is a Vibrating Table?
These are rigidly built steel platforms mounted on steel springs and driven by electric motors. The concrete is filled in the moulds which are mounted on the platforms and precautionary clamped to the vibrating table so that the mould and concrete vibrate are identical.
Vibrating tables are used for the compaction of stiff and harsh mixed concrete which is used in precast structural members in factories and laboratory apparatus.
Precaution for compaction of concrete with Vibration
When the concrete is compacted with vibrators the following precautions should be in mind to produce good quality concrete.
- The formwork must be strong and tight so that there must be leakproof from the mortar. Honeycombing is due to the leakage of mortar. A lining of plywood may be provided to prevent leakage if the surface finish is important.
- The thickness of the concrete layers is 15 cm to 60 cm. To avoid the trapping of air in the concrete.
- Vibrators are inserted vertically to provide uniform vibration on the overall portion of the concrete
- The vibrations must be controlled carefully. The internal vibrators should be inserted at a spacing of 45 cm to 60 cm for a time period from 5 sec to 15 sec.
- It leads to segregation if the vibrators are used for moving the concrete surface.
- The vibrator should be immersed into the entire depth of the freshly laid concrete or even into the layer below so it can be brought into the plastic condition.
- It will also prevent the formation of a weak joint between the two layers.
- The vibrator should not contact the surface of the formwork so that the formwork may not be damaged. The vibrator should be removed slowly from each location and should be operated continuously while being removed so that there is no hole on the surface of the concrete.
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