Why Vibration is Necessary for Concrete (Types and When to Use)

Vibrating concrete is a frequently overlooked but crucial step to achieving a great final product. It takes skill and knowledge to finesse the process of concrete pouring and setting, and there are many reasons not to rush the vibration procedure.

Vibration is a necessary procedure for concrete compaction to improve the strength, density, and durability of the concrete once it has set. This procedure improves the concrete’s bond to reinforcement steel and construction joints. Vibration removes entrapped air pockets which will also help you achieve a smoother concrete finish.

Keep reading to find out why vibration is necessary, the two main vibration methods, and which type of concrete vibrator to use for which project. 

Why Vibration is Necessary

Successful concrete consolidation and compaction require vibration. Consolidation is the process of bringing solid particles closer together by minimizing air pockets (source).

When concrete is still wet, vibration liquifies the concrete, enabling it to flow better into all the crevices of the form. Vibration also helps the concrete make the maximum amount of contact with the metal reinforcements. In this more liquid state, the larger air bubbles can rise to the surface more easily.

Poured concrete is made up of an average of 20% trapped air bubbles. Trapped air affects concrete strength and durability, among other things, which is why proper concrete compaction is such a vital part of a successful concrete project.

If you ignore the trapped air, you compromise the set concrete’s ability to withstand compression by 3 to 5% for every single percentage of air trapped in the mix (source).

The main purpose of compaction is to release as much trapped air from the concrete mix as possible. Depending on the density of the mix, more or less vibration will be necessary. A thick, more solid mix will require more vibration and a thinner, more fluid mix will require less vibration.

Effects of no Vibration or Under-Vibration

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Although small amounts of air are necessary for a concrete mix, referred to as micro-air, larger voids can have a significant negative impact on concrete strength and durability. Concrete vibration allows the larger air pockets to rise up to the surface while leaving the useful micro-air particles in the mix.

Air pockets larger than one millimeter create voids in the concrete, commonly referred to as honeycomb. The honeycomb inhibits concrete’s ability to properly bind with reinforcement. The less contact the cement mix has with the reinforcement, the less strength the dry concrete will exhibit. 

Not vibrating concrete enough leads to surface defects, and the most commonly occurring defects are blisters. 

When the surface layer of concrete has set, and air bubbles continue to rise out of the mix, resulting in blisters. They appear as fragile bulges of the surface layer that break apart, often leaving a small cavity underneath (source).

If the concrete is not consolidated properly in a frame, such as for a column or wall, the mix will not flow perfectly against the edges and have honeycomb areas within the mix. Honeycomb areas will not only cause structural weakness but also surface defects like cavities after removing the frames. 

The Two Stages of Compaction

When vibrating concrete, it is important to consider the two stages of compaction. Without considering compaction as a two-part process, one might cease the vibration of the concrete too soon and not allow enough time to expel all the air bubbles.

Stage 1: The Initial Level

The first stage of the compaction process is to apply vibration to the mix and liquefy the concrete. This stage allows the mix to slump down into the form to produce a level surface, often referred to as liquefaction. 

It is important not to assume the compaction is complete at this stage. Although the surface is level, there is still plenty of trapped air beneath.

Stage 2: Removing the Bubbles

The second stage involves continuing vibration until you expel all the unwanted air bubbles. This will be clear when bubbles have ceased to push through the surface for some time, and the surface is smooth.

Following through with this stage will result in much stronger, more durable concrete (source).

Which Vibrator to Use for Which Project

Depending on what type of project you are working on and the depth of your concrete pour, you will need to use a vibrator best suited to the job.

It is important to make sure you are using the vibrator with the right specifications for your project. Apart from the type of vibrator to use, there are other specifications to consider, like vibration frequency and size of the rod, for internal vibrators (source).

Internal Vibrator 

Internal vibrators, often referred to as needle or immersion vibrators, are the most commonly used (source).

Internal vibrators have a metal rod with vibration mechanics inside connected to a motor or engine. The metal rod is manually pushed under and held beneath the concrete surface.

Some internal vibrators need two people to operate the machine. Still, others have the motor or engine attached to a backpack, which is generally more favored, and they only need one person to control.

This vibrator is best suited for consolidating layers of concrete no thicker than 600mm; they use less electricity than other options but require manual operation. 

External Vibrator

You can refer to external concrete vibrators as shutter vibrators. There are two types, electric and pneumatic.

External vibrators mount to the sides of the concrete form approximately 6 inches apart. They can consolidate much larger areas at a time in comparison to an internal vibrator.

These are best used for smaller pours of concrete with thinner layers no thicker than 450 mm and have a much more powerful vibration.

Table Vibrator

A table vibrator is simply a robust steel table-like structure attached to four steel springs controlled by an electric motor.

Precast construction companies and factories use vibrating tables to consolidate dense concrete in molds and smaller precast concrete forms to make concrete products.

Surface Vibrator

Surface vibrators are placed directly on top of wet concrete and have either a vibrating pan or a screed.

These work best on concrete pours no deeper than 250mm and are most commonly used for pours under 150mm, for example, when repairing a concrete slab.

A surface vibrator works particularly well when trying to remove air bubbles from a shallow pour with a dry mix (source). 

How to Use a Concrete Vibrator

Compacting concrete takes patience and experience to get it right. Still, even if you lack in the experience department for this task, there are a few things to keep in mind for a good result.

Before starting, it is important to find out from co-workers which areas they have covered. There is such a thing as over-vibration, so it is best to know where to start. For our example, we are going to use an internal vibrator.

Once you know which area needs consolidating, push the rod into the concrete. Wait until the rod end is fully submerged before switching it on. Hold the rod in the same place until the area is compacted fully before moving on to the next area.

Watch the concrete and wait for the surface to level out, and then keep an eye on the air bubbles rising to the surface. When the air ceases to surface, and a fine layer of water separates and surfaces, compaction is complete.

Repeat this process across the whole surface of the concrete. Bear in mind slightly overlapping the sections will ensure no concrete is left unconsolidated.

Length of Time Needed

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There is no specific uniform amount of time needed for compaction; each mix and pour will be unique. However, in general, the length of time needed for this process to be complete is more or less the same, depending on a few factors.

In most cases keeping the rod of the vibrator submerged for between 5 and 15 seconds will suffice. Factors like the density of the mix, the depth of the pour, and the size of the vibrator used will all affect consolidation duration.

It is important to analyze the surface of the concrete during the consolidation process. If there are still air bubbles emerging upon slowly removing the vibrator, then the process is not complete. If this is the case, reinsert the rod and continue to vibrate the area (source). 

Tips To Improve The Vibration Procedure

Finessing the vibration procedure takes time and experience, but there are a few pointers that will help you avoid mistakes early on.

A common mistake made when using internal vibrators is to use the rod to move concrete around on the surface as one would do with a concrete placement tool. Doing this can damage the equipment and leave the surface looking rough.

When choosing a vibrator to use, choose one that will fit between the reinforcement gaps. You need to be able to push the vibrator rod needs down in between these gaps to ensure proper consolidation.

If the vibrator gets stuck on the way in, do not force it as it may have hit the reinforcement. Slowly remove it and try again to place it in between the gaps.

Lastly, a very important tip to make a note of, before starting your project, make sure you have all the vibrators ready before you pour the concrete. There is no time to get equipment when concrete is drying; consolidation needs to take place immediately.

It is wise to have spare vibration equipment on hand as well, in case you run into problems with the vibrator’s mechanics. 

Final Thoughts

Vibration is a process that cannot be rushed and is highly necessary to complete compaction. With patience and experience, the final result will ultimately be a success. Put this information to good use with your next concrete-related project. 

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