Compaction of Concrete

Compaction of concrete is a very important site operation that allows the fresh concrete to reach its potential design strength, density and low permeability. When done correctly, it ensures that concrete completely surrounds and protects the reinforcement, tendons and cast-in inserts. It also has a direct impact on achieving the specified surface finish.

To make your structure strong and long lasting for its designed life, it is very important that your structure is nonporous or impermeable. Porous concrete can lead to various problems like leakages, corrosion etc. These can happen if proper compaction is not done during the placing of concrete.

Requirement of compaction of concrete

Consolidation/ Compaction is the process of inducing a closer arrangement of the solid particles in freshly mixed concrete or mortar during placement by the reduction of voids, usually by vibration, rodding or tamping or some combinations of these actions

There are several reasons why compaction of concrete is needed, some of the main points are given below.

• Freshly placed unconsolidated concrete contains excessive and detrimental entrapped air
• If allowed to harden in this condition, the concrete will be porous and poorly bonded to the reinforcement
• It will have low strength, high permeability, and deduced durability
• Hence, compaction is necessary to remove entrapped air and eliminate all types of voids that may cause reduction in strength as well as durability

Purpose of Compaction of concreteand the best method to be adopted

The main purpose of compaction of concrete is to get a dense mass without voids, to get the concrete to surround all reinforcement and to fill all corners. Given below is the process of concrete compaction and method adopted.

• Concrete is best compacted by vibration
• 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, concrete gets denser
• Rodding, spading and tamping are all ways of removing air from concrete and to compact it, however the best and most efficient method is vibration

Strength of compaction of concrete and types

Elimination of entrapped air voids increases the strength of concrete. The strength is increased about 5% for every 1% of entrapped air removed during compaction

There are different types of vibrators used for compacting the concrete namely,

• Internal vibrators
• Form vibrators
• Vibrating tables
• Surface vibrators

Compaction of concrete; Does over vibration cause segregation?

• Well proportioned normal density concretes that have adequate consistency are not readily susceptible to segregation resulting from over vibration.
• A mixture with excess mortar is prone to segregation when over vibrated
• It is better to have a little extra vibration than not enough vibration

When the rising entrapped air bubbles stop breaking to the surface of the concrete, vibration can be discontinued

The guidelines for compaction of concrete while using the internal/needle vibrator

Given below are a few guidelines that needs to be kept in mind while using internal/needle vibrator.

• Operate the needle vibrator in a vertical position
• Allow the vibrator to penetrate as deeply as possible under its own weight through the entire depth of the freshly deposited concrete and into the layer below, if this is still plastic(before initial set of concrete) or can be brought again to a plastic condition. Thus the plane of weakness at the junction of two layers can be avoided and monolithic concrete obtained
• After vibration, withdraw the vibrator slowly to ensure closing of the hole created from the insertion
• Vibrate concrete at the rate of 5 to 15 seconds per insertion
• The needle should not be used to make concrete flow through long distance
• When the lift of concrete is greater than about 50 cms(2 feet), the vibrator may  not be fully effective in expelling air from the lower part of the layer
• Do not switch off the vibrator until it is withdrawn completely clear of the surface of the concrete

Compaction of concrete; Can concrete be revibrated?

In order to ensure good bond between lifts, the upper part of the underlying lift should be revibrated, provided the lower lift can still regain a plastic state. This will eliminate settlement cracks and the internal effects of bleeding and reduces the number of surface voids(“bugholes”). If applied too late, vibration can damage the concrete

Is it is possible to produce a concrete without Compaction of concrete?

Yes. Some uses of concrete do not allow adequate access for vibration due to severe congestion of reinforcement to affect compaction.During such circumstances, Self Compacting Concrete(SCC) can be produced

• SCC contains a high range water-reducing admixture and mineral admixture, viz, fly ash and flows under the action of gravity
• In the case of Self Compacting Concrete(SCC), nearly all the entrapped air escapes without compaction effort
• Rather than slump, the measurement used for this concrete is slump flow, which is the diameter of spread

Conclusion

The purpose of compaction of concrete is to remove entrapped air and eliminate all types of voids. Concrete is best compacted by vibration compaction improves concrete density, strength, bonding with reinforcement steel, minimizes surface blemishes and greatly improves the durability of reinforced concrete structures

Author

About Author – Author’s areas of research include Concrete Technology, Non- Destructive testing of RC structures and Repair and Rehabilitation of RC structures. He is involved in more than 150 consultancy projects for various Government, Public Sector and Private agencies, including Defense and Nuclear departments.