concrete form vibration

Concrete is the most widely used building material in the world, favored for its strength and versatility. Concrete is commonly applied by being poured into forms and left to harden. However, an important intermediate step must be completed before leaving the concrete to harden — vibration. Learn why vibration is so important, how to vibrate concrete and how to choose the best vibrators for your concrete form vibration application.

Why Getting Concrete Form Vibration Right is So Important

By its nature, concrete consists of cement, aggregate and water, which is mixed, then poured into a form to cure and bind into a solid. When the concrete is poured, it does not pour evenly, resulting in the entrapment of thousands of air bubbles into the mixture. The bubbles are called entrapped air. They can vary in size and shape. Depending on the type of mix and the placement method used, up to 20% of the total volume of the placed form can be entrapped air.

Trapped air may not seem like a significant problem, but it poses serious threats to the structural integrity and appearance of the final formed concrete piece:

  • Structural integrity: For every percent volume of trapped air, the compressive strength of a poured concrete product is reduced by 6%. The design itself also suffers when there is too much entrapped air. Every formed concrete product design assumes that the poured concrete will be dense and homogenous. When this isn’t the case, the design is inherently flawed and will not perform as anticipated.

  • Aesthetic appearance: On top of the functionality of the concrete, the aesthetic appearance of a concrete form is negatively impacted by trapped air. Air pockets can result in holes of varying sizes forming in the concrete, which may increase in size and appearance over time.

In the face of these issues, concrete vibration solutions have been developed as methods of removing air pockets from poured concrete. By simply vibrating a poured concrete mix, entrapped air can be displaced and forced to the surface, expelling it from the mix. While it is impossible to remove all entrapped air from a form, vibration can remove the majority of the air and displace any large air bubbles, resolving the issues mentioned above.

A Quick Summary of Terms

  • Concrete vibration: Also known as concrete de-aeration and concrete consolidation, concrete vibration is defined by the American Concrete Institute as “the process of reducing the volume of voids, air pockets, and entrapped air in a fresh cementitious mixture, usually accomplished by inputting mechanical energy.”
  • Concrete form: A concrete form is a mold that concrete is poured into to create the final product shape. Vibrators can be mounted on the outside of this form or placed directly in the mix inside the form to agitate the concrete mix.
  • Concrete vibrator: A concrete vibrator is a machine that applies mechanical energy to a concrete mixture by vibrating at a high frequency.
  • External concrete vibrator: External vibrators, also known as surface vibrators, are attached directly to the exterior of the concrete form. These vibrators work by vibrating the concrete mixture through the form. Though external vibrators are limited in the types of forms they can be used on, they result in a better surface appearance.
  • Internal concrete vibrator: Internal vibrators are the most common type of vibrator. Their popularity is attributed to their ease of use and ability to be handled by one person. Internal vibrators work by placing a vibrating head into the wet concrete then slowly withdrawing it while the vibrator agitates the concrete mix.

How Concrete Vibration Works

Concrete vibration works differently depending on the type of concrete vibration solution being used. As discussed above, there are two primary types of concrete vibrators — external concrete form vibrators and internal concrete vibrators.

External vibrators are attached to the outside of the form while internal vibrators are inserted directly into the concrete mix. Both vibrators work in essentially the same way by vibrating the concrete, but the source point and results of each vibrator type differ:

  • Source Point: External vibrators are attached to the outside of the form, causing the surface of the concrete mix to be vibrated most thoroughly. Internal vibrators create a vibration source point at the center of the form, causing the vibration to radiate out from the center so that the center is vibrated most thoroughly.
  • Results: Both types of vibrators cause the majority of air pockets to be dispersed and forced from the concrete mix. However, vibration doesn’t remove all air pockets, and the source of the vibration determines where the remaining air pockets tend to be located. For external vibrators, the surface of the concrete mix is most thoroughly agitated, resulting in great surface quality but trapped air pockets at the center of the form. The opposite is true for internal vibrators, which tend to result in poorer surface quality.
  • Applications: External vibrators can only be used on forms less than 6 inches thick, but are ideal for applications where surface quality is essential or the internal structure of the concrete piece contains rebar, which might entangle an internal vibrator. Internal vibrators are better suited to applications where a vibrator needs to be used across multiple forms.
  • Usage techniques: External vibrators are typically set to vibrate for a certain amount of time in order to thoroughly agitate the concrete inside the form. Internal vibrators require a bit more technique, however — the vibrator must be withdrawn from the mixture very slowly at a rate of around 1 inch per second for the best results. In both cases, if bubbles still emerge as the vibrator is turned off or withdrawn from the mixture, more vibration is needed.

Regardless of the type of vibration solution used, it’s more common to under-vibrate than over-vibrate. When in doubt, it is generally safe to continue to vibrate the mixture for a reasonable amount of time. Though rare, over-vibration can result in the water and aggregates separating, degrading the strength and appearance of the final product. Typically, it is best to stop when no more air is escaping from the concrete or if the surface of the concrete develops a sheen.

Concrete Form Vibration Tips

There are a number of tools for form vibration, including external formwork or form vibrators and internal immersion vibrators. But they all work with precast concrete. The goal of the vibrator is to gently stimulate the concrete form so the bubbles all work their way out.  Here are some tips for vibrating concrete forms:

  • Use dry concrete mix: If your concrete is a dry mix, there will be fewer bubbles to start off with. 
  • Distance the vibrators on large pours: If you are doing extensive pours, spread the vibrators out. Three to six feet apart is a good distance.
  • Clamp vibrators down on formwork: For efficiency and effectiveness, securely clamp the vibrators down on the concrete framework. If using an internal vibrator, submerge it all the way into the concrete.
  • Use a vigorous setting: Certain types of concrete, such as low-slump, are a lot thicker and stiffer than other types. Don’t be afraid to use a more vigorous setting to eliminate all the air bubbles. If you find the aggregate separating from the mix, reduce the vibration intensity to resolve the issue.
  • Move the vibrator around: When you pull out the internal concrete vibrator, make sure the hole closes up. Then, move it to a different section. Don’t drag the vibrator from one spot to the next. Dragging will cause more air bubbles to be trapped and cause separation.
  • Move in a grid: Vibrate in a grid pattern where the circles overlap. You want to see the surface of the concrete liquefy and the air bubbles to cease. 
  • Watch the surface: The surface will indicate to you when you should stop the vibrating. As soon as the air bubbles are all out, the surface will have a nice sheen to it.
  • Use shallow layers: If you are doing compact layers on top of each other, make sure you push the vibrator down at least 4 inches into the lower layer. Do not try to compact a layer more than 2 feet thick with a smaller vibrator. Rather, do shallow layers, which give a more uniform look, as you have more control. 
  • Don’t set it for too long: If the concrete starts to set too far while the vibration is still on, the concrete will start to segregate. Great imagery of this is an omelet. If you stir the eggs so they froth and stop stirring, they will solidify into an omelet. If you keep mixing until the eggs are cooked, you’ll end up with scrambled eggs. It’s similar with concrete.

Best Vibrators for Concrete Form Vibration

When it comes to proper concrete vibration, choosing the best vibrator for your application is critical. Some factors to consider when choosing the best vibrators for concrete form vibration applications include:

  • Concrete vibrator style:Choose whether an external or internal vibrator suits your application best. If you’re seeking something easy and versatile for use on simple forms or don’t require perfect surface quality, then either an internal vibrator or an External Pneumatic High Frequency Roller Vibrator or Electric High Frequency Vibrator will suit your application well. If surface quality is important or if your application involves rebar, an external vibrator, like a vibration table may be more appropriate.
  • Frequency and amplitude: Vibrators have different frequencies and amplitudes, which are best suited for different types of concrete mixes. For example, high-frequency, low-amplitude vibrators work best for high mortar mixes, while low-frequency, high-amplitude vibrators work best for stiff concrete mixes.
  • Concrete vibrator size: Concrete vibrators come in multiple sizes, which plays a significant role in the radius of action for the vibrator. Typically, the larger the head of the concrete vibrator, the larger the radius of effect. For example, a 1-inch head will affect an 8 to 14-inch diameter, while a 2-inch head will affect an 18 to 26-inch diameter.

If you’re not sure what kind of vibrator would best suit your precast concrete vibration needs, speak with your concrete vibration solution provider to determine what options are available and which qualities you need for your application.

Contact Deca Vibrator for Concrete Vibration Solutions

If you are looking for top-quality concrete vibration solutions, Deca Vibrator is here to help.

Since 1980, Deca Vibrator has been providing quality material flow solutions for a variety of industries. We are recognized nationally and internationally for our experience and extensive knowledge in the field of industrial vibrators, serving clients across 6 continents.

Whether you need pneumatic or electric, internal or external vibrators, we have a range of options to meet your needs. Contact us today to learn more about our concrete vibrator options and how we can help you with your next construction project.