Compaction of concrete is the process that entraps air from freshly placed concrete and packs the aggregate particles together to increase the density of the concrete.
The earliest means of achieving it, is with a rod or ramming or by vibrating.
Thus the aim of compacting the concrete is to acquire a dense mass without any voids, permitting the concrete to surround all reinforcements and fill all corners.
The compaction of concrete largely affects the strength, durability, imperviousness of concrete.
The experimental studies have shown that 1% air in concrete reduces strength by about 6%.
Purpose of Compaction of Concrete:
- To remove air bubbles trapped in concrete.
- To achieve high density.
- Also, to improve strength and durability.
- To eliminate honeycomb and other defects.
Types of compaction of concrete:
1. Hand compaction (Rodding or Tamping):
For smaller volumes of concrete and for narrow columns and crowded sections, these methods are suitable.
In this method, a tamping rod and steel rod of 10 to 16 mm with a bullet-nose or blunt-nose is used to compact the concrete by poking repeatedly into the concrete.
Rodding should be done rapidly.
2. Mechanical vibrators:
Concrete can be made compact by applying mechanical vibration through various types of vibrators.
Different vibrations are suitable for different conditions and different structural members.
These vibrators are discussed as follows:
Different types of vibrators:
It is the most commonly used vibrator called as internal vibrator, immersion vibrator or poker vibrator.
It consists of a power unit, a shaft and a needle, the power unit is electrically operated, the petrol driver or the air compressor driver.
Vibrations are caused by eccentric loads related to the rotor, shaft or any vibrating element.
The frequency of vibration varies up to 12000 per minute but generally, a frequency of 3500 to 5000 per minute is used.
The diameter of the needle varies from 20 mm to 75 mm and its length varies from 25 cm to 90 cm, . The bigger needle is used in mass concreting.
In crowded reinforcements, the needle can be replaced with a blade to facilitate vibration.
It is also known as a screw board vibrator.
A small vibrator placed on a screw board provides an effective way for compaction and level the thin concrete members such as floor slabs, roof slabs and road surfaces.
Mostly, floor and ceiling slabs are thin than a needle or any other vibrator cannot be used easily.
In such cases, surface vibrators can be used effectively.
It is a type of formwork vibrator, where the vibrator is fastened to a table any article placed on the table gets vibrated.
It has been mainly adopted in laboratories and in making small but precise prefabricated R.C.C. members.
3. Centrifugation of spinning:
This method is not used on site.
In this method, the concrete is subjected to high speed spinning so that the centrifugal force can achieve compaction.
This method is used for compaction of pre-cast concrete parts in pre-cast factories.
4. High pressure and shock:
This is also used in precast factories consists of jolting the formwork of precast elements and subjecting them to high air or steam pressure.
5. Vibro pressing:
It is also used for pre-cast elements, the outer pressure is applied to the open surface of the concrete in the mold and vibration is applied to the bottom of the mold.
This gives pre-cast parts of great density and very high quality.
Importance of Compaction of Concrete:
- It is very important absolutely compact the concrete as a result of air voids reduce the strength of the concrete.
- For each 1% of trapped air, the energy falls from somewhere between 5 and 7%.
- This means that concrete containing about 5% air voids as a consequence of incomplete compaction can lose as much as one-third of its strength.
- Air voids enhance the permeability of concrete which reduces its durability.
- If the concrete is not compact and impermeable, it will not be watertight and it will be less able to withstand aggressive fluids.
- Moisture and air reinforcement are more likely to penetrate, causing it to rust.
- Proper condensation also ensures that the formwork is completely filled i.e. it has no honeycomb material and the required finish on vertical surfaces.
Defects due to poor compaction of concrete under vibrations:
Highly entangled air vessels: Under vibration, a lot of entrapped air is released into the concrete, which reduces the strength.
Sand streaks: When heavy bleeding washes the mortar, a hard mixture left behind impairs its ability to work.
It is also caused by insufficient fine aggregate.
Placement lines: These are the dark lines between adjacent layers of concrete batches.
This occurs when the vibration did not penetrate through the underlying layers.
Separation: Heavy aggregates settle, while lighter aggregates rise.
Bleeding: Water is released to the surface due to excessive vibration.
Form damage: Over vibration can damage the formwork.
Care to be taken during Compaction of Concrete:
Following precautions should be taken during compaction of concrete as follows:
1. Selection of the correct method of compaction according to the situation.
2. Avoiding segregation of concrete.
3. Avoid bleeding of concrete due to over compaction.
4. To see that the concrete uniformly reaches all the nooks and corners of the formwork and reinforcement.
5. Reinforcement is not disturbed.
The purpose of compaction of concrete is to remove trapped air and get rid of all kinds of voids.
Concrete is finest compacted by vibration compaction improves concrete density, strength, bonding with reinforcement steel, minimizes surface blemishes and enormously improves the durability of reinforced concrete structures.