(4)+Method of concrete compaction with hand and machine, type of vibrators

What is the compaction of concrete?

During manufacturing, transportation, and placing of concrete↗️ a considerable amount of air is entrapped in the mix. The process of expelling (removing) the entrapped air and achieving maximum density is known as compaction. If the entrapped air is not removed then it is difficult to correct segregation, the concrete may be non-homogeneous, concrete may be porous and attains reduced strength.

The process of compaction↗️ should not be underestimated. The strength of concrete is reduced by 30% by the presence of only 5% voids. If the workability of concrete is lower then it indicates that the amount of entrapped air is higher i.e., the stiff concrete mix has a high percentage of entrapped air, therefore, it would require higher compacting efforts.

why compaction is important?

The proper way of compaction they should be the concrete particle is properly bond each particle and mix together and the result as provide more strength and durability and reduce honeycomb. After removing shuttering the surface is more clean and smooth.  They also reduce the corrosion on the reinforcement bar and reduce crack from the concrete surface. The formwork corners remain proper fill and the surface is more clean and smooth.

Methods of compaction

The choice of a particular technique of compaction depends upon the type of structural element, water-cement ratio, rate of output, and desired properties of hardened concrete. The compaction of concrete can be achieved with the help of the following methods given below.

1. Hand Compaction
2. Machine Compaction

1. Hand Compaction

The process of ramming the concrete manually with the help of a steel rod is known as hand compaction. This technique of compaction is basically adopted in case of unimportant concrete work of a little bit magnitude. This method is also useful in situations where a large quantity of reinforcement is provided.

In this method, a steel rod is inserted vertically into a freshly placed concrete and is moved up and down to remove the entrapped air. The rodding action (also called tamping) is effective for a depth of concrete equal to five times the maximum size of aggregate and therefore the depth of each laver has to be restricted to this value. This method of compaction should be remaining continued until the cement mortar spreads properly on the surface of the concrete.

The method of hand tamping (compaction) is different for road slabs or floors. A tamping beam also known as screed is used to compact the concrete↗️. This process also helps in attaining a uniform level. The tamping and leveling are done simultaneously with the help of screed by holding two handles and bumped up and down and then moving forward.

2. Machine Compaction

The process of compacting the concrete with the help of machines is known as machine compaction. The equipment used for compaction of concrete by machine method is known as the vibrator. This method of compaction reduces the internal friction between the particles of concrete by providing oscillations to the particles thus creating a dense and compact mass.

Machine compaction is very useful for mixes having low water-cement ratios. Where the more amount high strength is required, it is necessary that stiff concrete, with low water/cement ratio, be used. The vibrated concrete with a low W/C ratio will have many advantages over the hand compacted concrete with a higher W/C ratio. Different types of vibrators used for compaction of concrete are given below.

i. Internal vibrator (Needle vibrator)

ii. Form vibrator (External vibrator)

iii. Screed vibrator (Surface vibrator)

I. Internal Vibrator 

This type of vibrator is the most commonly used method for compaction of concrete. This type of vibrator machine is also known as "Immersion Vibrator", "Needle Vibrator" or "Poker Vibrator”. In this method, the vibrator directly comes in contact with concrete and compact the concrete. An internal vibrator consists of a power unit, a flexible pipe, and a needle. The mechanical vibrators are operated with the help of an electric motor of a diesel engine. The vibrator is caused by an eccentrically loaded shaft. Nowadays the tendency is to use higher frequencies beyond 6000 rpm. (up to 15000 rpm). More the frequency of vibration higher is the degree of compaction.

The diameter of the needle varies from 20 mm - 75 mm and the length varies from 25 cm to -90 cm. The bigger sized needle is suitable for mass concreting work (such as in case of dams, retaining walls, etc.). The vibrating needle is immersed in the fresh concrete and as a result, it transmits vibrations. Due to induced vibrations, the entrapped air is forced to escape in the form of bubbles. Guidelines for using needle vibrators are given below.

• It is compact and can be carried from one place to another.
• The needle of the vibrator should be immersed vertically, in concrete, at points 0.45-0.75 m apart.
• The vibrator should be immersed and withdrawn slowly @ 30 mm/sec.
• The vibrator should be kept in one place for 5 - 15 seconds.
• They are best suitable for difficult positions and situations.
• They are used for columns, beams, etc.
• Vibrators should not touch the reinforcement.
• If concrete is placed in layers then the vibrator must go to the lower layer up to a certain depth for making the bond between two layers.
• The formwork should be strong and watertight to avoid leakage of mortar.
• Over-vibration may lead to the segregation of aggregates. 

II. Form vibrator 

This method of compacting the concrete is an external method that is why it is also known as an external vibrator. This type of vibrator is attached to the external surface of the formwork with the help of clamps. The vibrations provided to the surface of the form are transferred to the concrete. Much energy gets wasted because vibrations have to travel through formwork.

This technique is very useful and adopted where such as reinforcement, lateral ties, and spacer bars interfere in the process of the internal vibrator. They are best suitable for thin, densely reinforced and precast thin in-situ sections. These vibrators can compact up to 450 mm from the face but have to be moved from one place to another as concrete signs of progress. 

III. Screed Vibrators

This method is also known as surface vibrators. These types of vibrators are placed directly on the flat surfaces of concrete mass such as roads, floors, etc. These are best suitable for compacting concrete up to shallow depths (not more than 250 mm). Surface vibrators can be used for compacting the top layer of concrete in combination with needle vibrators for underneath concrete. Dry and stiff mixes can be easily and effectively compacted with surface vibrators.

• Surface vibrators cause movement of finer material to the top and hence helps in finishing operation.
• Other methods of vibrations are table vibrator, platform vibrator, compaction by pressure and jolting, compaction by spinning (or centrifugation), vibratory rollers, etc.

Selection of vibrators for different situations 

since the concrete contains particles of varying sizes, the most satisfactory compaction would perhaps be obtained by using different types of vibrators with varying speeds of vibration. The selection of suitable vibrator for different situations is mentioned below.

• Internal (Needle) vibrator - It more suitable for general concrete work with sufficient thickness and mass e.g., beams, columns, and mass concrete works like dams, retaining walls, etc.
• Form (external) vibrator - It may be the more suitable for a thin section like densely reinforced sections, thin precast sections, this type of vibrators more use for arches, tunnels lining, etc
• Screed (surface) vibrator - It may be generally more suitable for shallow concrete works like thin horizontal surface e.g., roads, floors, etc.

Difference between hand compaction and machine compaction

The main difference between hand and machine compaction is the given below.

Hand compaction

• The quality of the compaction is less satisfactory.
• The water-cement ratio required more in hand compaction.
• The voids in concrete more present.
• The impermeability in concrete is less.
• The surface finish is average.
• The bond between steel and concrete less strong.
• The formwork corners remain unfilled.

Machine compaction

• The quality of the compaction is more satisfactory.
• The water-cement ratio required less in hand compaction.
• The voids in concrete no present.
• The impermeability in concrete is more.
• The surface finish is more smooth and clean.
• The bond between steel and concrete more strong.
• The formwork corners remain proper fill.

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