Compaction of Concrete – Importance – Under & Over vibration – PDF

Compaction of concrete is an important step of concreting that plays vital role in quality control. We ensure proper compaction of concrete after placing the freshly mixed concrete. Compaction is the process that expels air voids in freshly placed concrete. Different methods of compaction allows aggregate to come close together to achieve higher density. It also helps to increase bonding with reinforcement and overall strength of concrete.

For any type of concrete grade, there is a designed strength. We normally do compaction of concrete cylinders by rodding. To replicate that effect on field, contractors use different techniques to let freshly concrete mix reach the potential design strength.

Experimental studies shows that 1% void in concrete reduces strength of concrete by about 5% which goes on increasing with increase in percentage of voids. Therefore, compaction of concrete helps achieve higher strength, density, and low permeability.

Importance of compaction of concrete

Since concrete is an amalgam of various ingredients. In order to make the concrete matrix uniform and avoid degradation; compaction is crucial. It affects the durability, strength and imperviousness of concrete. So, compaction helps achieve following objectives:

  • It helps increase ultimate strength of concrete and bonding with reinforcement.
  • To increase abrasion resistant and durability of concrete.
  • Compaction decrease permeability and minimize chances of shrinkage and creep.
  • Formwork is completely filled with concrete and there are no chances of any honeycombing.
  • To achieve desired property of concrete.
Compaction of Concrete
Compaction of Concrete

Also Read: Compaction factor test for workability of concrete

Process of Compaction

Normal workable concretes consists of 15 to 20% air voids by volume. In that, aggregates are coated with mortar, but they have ability to slide or move against each other. However, internal friction of aggregates hinders slumping or compacting.

Compaction of concrete is two stage process. Liquification is a first process in which aggregate particles are set in motion and concrete is consolidated to fill the form and give top surface level. First stage is called the initial consolidation of concrete which is achieved quickly within 3 to 5 seconds. While second stage is expulsion of entrapped air from concrete which usually takes a little longer to rise to the surface and usually it takes 7 to 15 second. Compaction is prolonged until no air bubbles appear on the surface.

Also Read: Weight of concrete – Density- Concrete Slab – Driveways

Different Methods of Concrete Compaction

Concrete compaction is done either by hand or machine. Choice of compaction method depends upon many factors like workability of concrete, reinforcement, spacing in reinforcement, and complexity of formwork.

Hand Compaction Method

High workable and flow-able concrete is compacted by using hand compaction method. It is done for smaller volume of concrete for narrow columns. Concrete with low workability can also be compacted by this method if superplasticizer is added in concrete to decrease the slump and make concrete workable.

In this method, tamping rod or steel rod with diameter of 10 to 16mm is tamped repeatedly to achieve compaction in concrete. Then spade is used to give good appearance and entrap air bubbles from it completely.

Also Read: Advantages of Air-Entrained Concrete

Mechanical Compaction Method

This method is suitable for concrete mixtures having great amount of coarse aggregate content and low water to cement ratio in highly reinforced structures. There are following methods of mechanical compaction of concrete.

Shock or drop table

This method of concrete compaction is employed for low workable concrete for pre-casting job.

Centrifugation

This method of concrete compaction is done for construction of poles, pipes, or pile where moderate to high workable concrete is used. In this method centrifugal force is generated by giving circular motion to concrete which will help to achieve compaction.

Also Read: Different grades Of Concrete And Their Uses  – Concrete Mix Ratio

Vibro-pressing

This method is used for pre-casting of concrete. In this method pressure is applied on the surface of concrete in mold and then vibration is done to achieve higher density and impermeable concrete.

Vibration method of concrete compaction

It is the most extensively used method of compaction of concrete in which internal friction between aggregate is first removed for a short span because of which concrete mixture behaves like liquid and gravitational force holds in it which will enable mixture to settle in formwork and allow entrapped air to move upward. Internal friction will develop in concrete mixture again when vibration is completed. This method of concrete compaction is done by using following vibrators:

Internal Vibrator

Internal vibrators also called as spade or poker vibrator and use to compact concrete in beam, column, slab, and wall. Performance of the vibrator is affected by workability of concrete, frequency, amplitude, and head dimensions of vibrator. The time for which internal vibrator is used to achieve compaction of concrete depends upon workability of concrete, force of vibration, and nature of element that is being compacted.

Internal vibrators
Internal vibrators

Also Read: Fineness of Cement by Blaine –  Air Permeability test

External Vibrator

External vibrator includes major types of vibrators that are:

  • Table Vibrator or Surface Vibrator
  • Form Vibrator

Table or surface Vibrator

This type of vibrator is used for the compaction of floor or slab. It compacts slab up to thickness of 20mm. It is used for vibration of prefabricated RCC members.

Form Vibrator

This type of vibrator is properly attached to formwork to achieve desired objective otherwise it will result in poor vibration and loss in energy. It is used in the construction of masonry, pipes, and pre-casting concrete members.

External Form Vibrators
External Form Vibrators

Effect of under compaction of concrete

If concrete is poorly compacted, it will result in serious problems like honeycombing, permeability of concrete, lower bonding with reinforcement, cracks in structure, and poor finishing.

Vibration is crucial for low-slump concretes. They’re stiffer and difficult to work. So, be careful to under-vibrate low-slump concrete. Such concrete mixes require adequate vibration unless you are using self-consolidating concrete.

Effect of over vibration of concrete

In poor compaction there may be under-compaction or over-compaction. For compaction it holds true that excess of everything is bad. Excessive vibration or over-compaction can result in bleeding. With such, the amount of mortar or cement slurry can collect on the surface. Over-compaction can result in more shrinkage and moisture-loss from concrete.

Over-vibration can result if you keep it immersed in the concrete for too-long. So, to avoid under-strength of concrete; experts recommend adequate size of vibrator and suitable depth. According to Juan Rodriguez of TheSpruce, the recommended depth of immersion for vibrator is 6 inches. But in mass concrete, the vibrator should pass the underneath layer of concrete to have proper bond.

The bottom Line

 Compaction of concrete is an important process done to achieve higher strength, density, and impermeability values. Different methods of compaction includes manual methods like using tamping rod or spade, or by mechanical methods like using internal or external vibrators.

Mechanically vibrators are ideal for compaction of larger volume of concrete or concrete having low workability while hand method is suitable for high workable or lower volume of concrete.   

Also Read: Difference between compaction and consolidation   

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Saad Iqbal

Here I’m, Saad Iqbal – a civil engineering geek and a passionate blogger – with handy graphics designing skills. I’m in the online civil engineering community from over a decade now and have many fans, followers and friends throughout the world.

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