For example, any seasoned concrete worker knows the problems associated with “honey-combing.” This occurs when trapped air bubbles in concrete do not properly filter out to the surface, causing gaps to appear. This is especially common around rebar within a concrete slab’s structure, causing that slab to be less structurally sound.
To prevent this, concrete must be properly vibrated as it is poured. Many contractors are unfamiliar with the precise process used to accomplish this, so we’ve created this step-by-step guide. Herein, you’ll learn a standard technique for vibrating concrete as well as a modified technique that utilizes a Sawzall.
Why Vibrate Concrete and Is It Necessary?
Often, contractors will question if vibrating is truly necessary for properly setting concrete. The answer here is…yes, it really is necessary.
This is because improperly vibrated concrete may still contain trapped air bubbles, which in turn can cause an undesirable “honey-combing” effect once the slab sets. Extensive honey-combing can even cause a slab to become unsound, causing cracks at best and requiring near-term replacement at worst.
So, if you want to get the job done right on the first try, vibrating is a necessity. This is doubly true if you are working with concrete slabs containing rebar. These slabs in particular are prone to honey-combing due to the existing air pockets around the rebar’s rods.
As such, all concrete construction teams should be prepared to vibrate their fresh slabs before performing other curing steps.
How to Vibrate Concrete
Now that you are convinced it’s necessary to do the process, below is a step-by-step guide to using a standard vibrator.
Step 1: Choose the Right Head Size
First, you’ll want to start by checking that your team’s concrete vibrator is equipped with a properly sized head. Though it’s difficult to tell just at a glance, differing vibrator heads can have a major impact on the vibrator’s rate of compaction.
As such, it’s important to choose a vibrator head based upon the context in which the concrete is being poured.
For example, a 20-30 mm head should be used to achieve the lowest level of concrete compaction. Such a head would be suitable for concrete poured into thin sections. From there, 35-40 mm head might be used for vibrating slender columns. Finally, you should choose 50-75 mm head for a maximum level of compaction in slabs that face no noteworthy restrictions.
If you don’t have a vibrator yet, check this list.
Step 2: Don Safety Gear and Place Vibrator
Once you’ve picked out the right head, you’re ready to start working with the wet concrete. To that end, you should suit up before jumping in with all concrete safety gear your team or work site requires. Most often, those working in wet concrete must wear special boots (see my recommendations), as well as mouth, eye, and hand protection at all times.
From there, you can turn the vibrator on and step up to your first insertion spot.
As concrete begins to fill that space, insert the vibrator and leave it in place for 5 to 15 seconds. After that, it can be withdrawn slowly and moved to the next spot. That next spot should be about 500 mm from the original insertion point. This process should continue until the entire slab has been properly vibrated.
Of note, you should never force your concrete vibrator into wet concrete. Instead, allow the unit’s weight to pull itself down. Otherwise, you risk damaging the unit’s head. If the head is not sinking down of its own accord, then that may be an indication that your concrete mixture is too thick for proper vibrating.
Step 3: Watch for Air Bubbles
During the 5-15 second insertion process, be sure to keep an eye out for air bubbles appearing on the concrete’s surface. These will signal that your vibrating process has been successful. To that end, you should continue vibrating that spot until no more bubbles appear for 3-5 seconds.
However, if no bubbles appear initially, that may be a sign that your concrete vibrator is not operating as intended. Be sure to double-check that you are following proper insertion procedures and ensure that your unit is being properly powered at its source.
Using a Sawzall or Other Reciprocating Saw to Vibrate Concrete: A Good Idea?
Interestingly enough, a purpose-made concrete vibrator is not the only tool that can be used to vibrate concrete. Some professionals today instead use a technique that utilizes a Sawzall (or any other reciprocating saw, for that matter).
This technique has its benefits, not least that it doesn’t require the acquisition of a separate, single-purpose tool. However, this method’s productivity is also notably limited because it cannot penetrate deeply into a slab’s structure.
To see this technique used in real-time by a real professional, check out the video below.
Mistakes to Avoid, Tips & Tricks
There are actually a fair number of common mistakes you’ll want to avoid when utilizing a standard concrete vibrator.
The first of these involves the process of removing the vibrator from a particular insertion point. Specifically, this should always be done as slowly as possible. Otherwise, you risk introducing new air pockets into the slab right after you shook out the old air pockets.
Also, be sure to never over-vibrate a single spot in a concrete slab. You’ll know this is occurring when a collection of fine particle matter begins to appear on the concrete’s surface. Those fine particles will need to be remixed in properly or else you risk modifying your concrete’s structure in and around that point.
Summary
If you take away anything from this guide, it’s that concrete vibration really is important. In fact, it’s just as important as any other step you take to ensure your fresh concrete slabs cure properly.
As such, utilizing a proper concrete vibration technique may be required for all of your future concrete pouring and shaping jobs. That proper technique isn’t too hard, though – just keep the steps and tips described above in mind and you should be good to go!