Learn concrete vibration: how to vibrate and consolidate concrete
So you want to learn about concrete vibration and how to vibrate concrete? You’ve come to the right place! Everyone has different techniques for concrete consolidation, and there are so many variables in it, that eliminating honeycomb, surface air voids and cold joints becomes more of an art than a science. You have to watch, listen, and understand concrete, and know the tools and the correct process to properly consolidate concrete. In this post, we’ll cover the basics of how to consolidate concrete and the factors that affect consolidation.
*Note: the information contained in this post is a brief summary and overview of our free online video training course on how to consolidate concrete. To watch our FREE online video training course on how to consolidate concrete, click here. Although the science of concrete is more complex than we’ll get into in this article- we’ll show you how properly consolidate concrete – removing the air bubbles through proper vibration techniques.
What is consolidation?
Consolidation is the process of removing trapped air from freshly placed concrete. We know its impossible to remove all entrapped air but the task of consolidating (vibrating) concrete is to eliminate as many large air bubbles as possible. To get air bubbles to escape, you use an internal vibrator and drive it down into the poured concrete quickly, and lift it up slowly. Typically, under vibration is more common than over vibration, so be careful you’re not under vibrating.
Proper consolidation requires planning.
The key to proper concrete consolidation is planning. Before you jump in and start working with the concrete, create yourself a checklist to ensure you’ve planned for the following key items:
- Mix design, aggregate size, slump
- Correct truck spacing
- Forms sealed tight to prevent leaks
- Power source
- Quantity of vibrators
- Amount of form release
- Sufficient timing to vibrate
- Sufficient length of vibrator
- Manpower to cover the pour
- Backup equipment
Don’t start the job without having these things in mind, and accounted for.
What affects proper consolidation:
Before learning the method to proper consolidation, you need to know the factors that affect proper consolidation:
- Mix design: here we’re talking about plasticizer vs non-plasticizer. A mix with plasticizer will require less effort than non-plasticized mix. SCC does not need vibration. The stiffer the concrete, the greater the risk of honeycomb. Note, that you have to work a stiff mix more than a fluid mix. On the other hand, if the mix is to wet and becomes unstable, then segregation can happen causing honeycombing, excessive bleeding, and sand streaking on the surface. An example of honeycombing below:
- Slump / spread: What you want to look for as far as mix, is work-ability: when you pull the vibrator out, does it leave holes? What you want is when you pull out the vibrator, that the concrete seals up behind it and doesn’t leave a big hole. Make sure your mix is creamy and workable, not too stiff, not too wet.
- Form release: there are three types of form release oil: 1) Barrier 2) Reactive, or 3) Combination of barrier and reactive. Typically, you don’t see barrier anymore. To determine the correct one for you, start with your specs: what is it calling for? Then submit based on your specs. Once you get approved, review your manufacturer’s recommendations, which will help you determine the right one for your application.
How to consolidate concrete: vibration
There are three general ways to consolidate concrete: form vibrations or form shakers, surface vibrators (vibratory screeds), and internal vibrators.
(or skip ahead and watch the FREE video course)
External vibrators are electric or pneumatic concrete vibrators that mount to the exterior side of form work and cover a large area, and usually you can space them 6’ apart. The use for external vibrators include floor to ceiling walls, beam, and one-sided blind walls. However, SCC is being used more and more each day for heavily congested areas, blind side walls, floor to
ceiling walls and hard to get to areas thus eliminating the use of external form vibration in these applications. Should you decide to use a form vibrator: if you have a pour and you are using both an internal and external vibrator, you shouldn’t use both at the same time. When running the external vibrators, watch from above and be sure to run the vibrator long enough- watch for the air bubbles to escape. Run the vibrator long enough until there’s no more air bubbles releasing from the surface.
These are typically vibratory screeds, leaving you with a honeycomb free surface. It’ll scrape off the top surface and give you a nice creamy top and can consolidate up to 6″ slabs, and provides the finishers with a nice working surface.
These are the most commonly used for consolidating concrete. There are two different types that we’ll point out: high-cycle and flexible shaft. High-cycle has the motor inside the head, whereas the flexible shaft has the motor in the end. In this category are backpack vibrators, which field crew love- they have high versatility, they don’t need power cords, and don’t require an extra crew member on the motor.
Choosing vibrators for concrete consolidation
You need to know the frequency and amplitude of the vibrator. High frequency, low amplitude vibrators work best for high mortar mixes, whereas low frequency and high amplitude works best for stiff concrete mixes. So you need to determine not only the right type of vibrator style, but the output of frequency and amplitude for the job you’re working.
How long to vibrate concrete:
After you’ve determined the vibration method and tools you are going to use, the next thing you need to know is how long to vibrate the concrete to get it to release air. Time is crucial when you’re vibrating, and to some degree it’s a matter of experience and getting a feel for it. As you bring the vibrator up slowly, watch the air bubbles rise, then stop when a thin film of paste is covering the surface. Coming up slowly is the key to proper consolidation, at a recommended rate of about 1″ per second (though you’ll get advice on anywhere from 1″-3″ per second). Insert again quickly, and remove slowly, watching the bubbles rise and diminish. You may think that you’re coming up too slow, but start slow and watch how the concrete reacts, and see how it looks at the end. Generally, the slower the better. Watch the video below for more detail on vibrating the concrete, how to estimate how long it will take you per wall or per job, and proper length of time:
Spacing and head size
You need to make sure to use the proper size of vibrator head for the appropriate application. There are lots of variables when it comes to finding the radius of action your vibrators will put out: slump, configuration (wall, footing, beams, etc.) frequency, and shape of equipment. But here is some general info based on experience in the field:
- ¾” Head= 6”-8” Diameter
- 1” Head= 8”-14” Diameter
- 1-1/4” Head= 10”-16” Diameter
- 1-1/2” Head= 16”-22” Diameter
- 2” Head= 18”-26” Diameter
- 3” Head= 20”- 32” Diameter
See the example of correct, and incorrect vibrator spacing below:
Overlap and form face distance
How much overlap do you need? How do you know where to set your depths for your vibrator? If you get the vibrator right up to the form face you could get sand streaking or damage the bar. Most manufacturers will recommend 2″ away from the wall face. Watch the video below for more explanation and to see an example of how to figure out your overlap and form face distance when vibrating:
This has been a quick overview of some of the information taught in our free course on how to consolidate concrete, available free of charge at Construct-Ed.com. Browse our course library to search all online construction courses.