Compaction of Concrete

When the fresh concrete has been placed in the desired area, it must be compacted. The process of compaction of concrete is essential because it consolidates the concrete in each corner and edge of the place where it is placed.

A considerable amount of air is entrapped in the concrete throughout the manufacturing and placing processes, causing the concrete to be adversely damaged. As a result, it is necessary to compact the concrete as soon as it is poured at the required place.

Concrete compaction removes entrapped air and voids, transforming the concrete into a dense mass. According to studies, just 5% of air voids in hardened concrete can lower its strength by 30% to 35%. This shows how critical it is to compact fresh concrete.

The voids in the concrete can quickly increase the permeability of the concrete, allowing moisture, oxygen, and other externally aggressive substances to get through. These substances can cause steel to rust, spalling of concrete, affect the properties of concrete, and result in a variety of concrete defects. As a result, whether the construction project is large or small, it is always recommended to compact the concrete.

Concrete can be compacted using a variety of techniques. For modest projects, hand compaction is usually used, whereas mechanical compaction is used for larger projects. We’ve gone over each of them in detail below.

Methods of Compaction of Concrete

The different methods of concrete compaction can be classified as follows:

• Hand Compaction
  • Tamping
  • Ramming
  • Rodding
• Rolling Compaction
• Jolting Compaction
• Spinning Compaction
• Vibrating Compaction
  • Needle Vibrator
  • Surface Vibrator
  • Formwork Vibrator
  • Table Vibrator

Also Read : Compaction Factor Test of Concrete
Also Read : Compaction and Consolidation Differences
Also Read : Types of Concrete

Hand Compaction.

One of the oldest methods of compacting concrete is by hand. For minor projects, this technique is still utilised. This type of compaction method is particularly helpful and cost-effective for compacting thin parts such as slabs.

Except for very fluid or very plastic mixes, this method can be used for any mix with any workability. Tamping, ramming, and rodding are all methods of hand compaction.

• Tamping : Tampers are similar to a thick square plate with a rod on it. Hand tampers are typically 10 cm x 10 cm in section and around 1 metre long. They’re great for compacting concrete and leveling off the upper surface.
• Ramming : Ramming is a type of equipment that is similar to tampers. However, this technique is only applicable to under-reinforced concrete works.
• Rodding : Rodding technique equipment has a 16 mm diameter plate at the bottom that is attached to a 2 metre steel rod, comparable to a hand rammer. This compaction procedure is usually carried out from the time the mix is poured until it is finished.

Rolling Compaction

The soft and plastic concrete is recirculated between rubber rollers under pressure up to 50 atmospheres in this method of compaction. This pushes the extra water out of the concrete.

Precast concrete items, such as tiles, are made by rolling compaction.

Jolting Compaction

In this method of compaction, the mould containing dry concrete is jolted at a frequency of 100 to 150 jolts per minute. Jolting is a low-frequency, high-amplitude vibrating movement.

The concrete-filled mould is lifted by about 12 mm and then allowed to collapse naturally. Precast units such as hollow blocks, cavity blocks, and other precast units are made using this process.

Spinning Compaction

This compaction procedure is used to make most circular parts, such as concrete pipes. Centrifugation is another name for it. In this procedure, a plastic concrete mix is spun into a horizontal mould at a high rate.

As a result, all of the excess water in the mix is forced out of the mould. The initial water cement ratio for successful compaction is 0.35 to 0.4, which decreases to 0.3 after spinning.

Vibrator Compaction

Compaction of concrete by vibrators is one of the most used and beneficial method from all the above mentioned methods of compaction. In this method vibrators are used for compacting concrete.

In this method of concrete compaction the internal friction between the concrete particles is reduced by setting them in motion. As a result of this dense and compact mass is produced.

This method of compaction is classified into four main types:

• Needle Vibrator
• Surface Vibrator
• Formwork Vibrator
• Table Vibrator

Needle Vibrator

Needle vibrators are more often used because of their compact size and flexibility in concrete applications. They’re also known as immersion vibrators, poker vibrators, or internal vibrators.

The needle vibrator is made up of a vibrating steel tube with a flexible shaft and a power unit. The needle diameter ranges from 20 to 75 mm, while the tube length ranges from 250 to 900 mm. The average vibrating frequency of this vibrator is 3500-5000 cycles per minute.

The distance between the two needle insertion locations should not exceed 500-600 mm. The time it takes to vibrate the concrete at a certain location should be between 30 seconds and 2 minutes.

When the reinforcement becomes too clogged for the needle vibrators to function correctly, the needles are replaced with blades.

Surface Vibrator

Surface vibrators, as the name implies, are used to vibrate flat surfaces such as floor slabs, flat roof slabs, and so on. Surface vibrators have a structure that is comparable to a shopping cart without wheels.

The vibrating machine is mounted on top of a flat, heavy square plate fastened at the bottom. Unlike needle vibrators, these vibrators remain on top of the concrete surface rather than being inserted into it.

Surface vibrators are recommended for compacting concrete with a thickness of less than 150 mm, but they are commonly employed on construction sites up to 250 mm.

However, it is usually recommended to keep the thickness value below 200 mm, because if you try to compact concrete with a thickness greater than 200 mm, the lower level concrete may not experience appropriate vibrations, leading to numerous concrete defects.

The typical frequency of vibration is up to 4000 cycles per minute.

Formwork Vibrator

Concrete columns, thin walls, and precast modules are all done with formwork vibrators. These are firmly secured to the formwork, forcing it to vibrate and, as a result, transmitting vibrations to the concrete.

When directly coupled to formwork, the performance of these vibrators is often poor. The vibrator is attached to a steel plate that runs along the formwork, touching the form stiffeners, and is attached to a channel or an I-beam.

The formwork and shuttering must be solid, sturdy, and watertight to use this method of compaction. The vibrators have a frequency ranging from 3,000 to 9,000 cycles per minute.

When utilised in combination with internal vibrators, form vibrators aid in the removal of trapped air along the concrete surface, resulting in a much better finish.

In general, pounding the exterior face of the formwork with a wooden mallet helps eliminate entrapped air, which blemishes the concrete surface despite appropriate internal vibration.

Table Vibrator

A table vibrator is just a vibrating table that is more commonly found in laboratories and industries than on construction sites. Typically, these are used to compact concrete specimens for quality testing.

The table vibrator’s structure is made out of a strong steel platform supported by flexible springs and powered by a powerful electric motor. They vibrate at a rate of 4000 cycles per minute on average.

The vibration should be halted when the concrete in the mould on the table acquires a smooth surface. In comparison to stiff mixes, medium workable mixtures require a higher frequency of vibration.

When employing this sort of vibrator, it has been discovered that raising the frequency and decreasing the amplitude as the vibration proceeds promotes consolidation. The vibrating tables are extremely effective at compacting stiff and abrasive mixes that are needed for precast parts.