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| Nov 03, 2014
Pouring concrete with heavy rebar ain't easy.
Pouring concrete with heavy rebar is difficult, challenging work. It's a pain in the neck. The size, the weight, coordinating the construction, keeping the full structure together. But the biggest pain of all is getting concrete properly consolidated around it.
We’ve been to hundreds of jobsites, and talked to workers, supervisors and foremen in person and on the phone for over 50 years. In our experience, the biggest headache for concrete contractors is getting high quality consolidation and a great finish. Many times, finish is sacrificed to ensure concrete strength and durability. The end product has bugholes that ruin the finish, and surface cracks often appear only a few years after the job.
Internal Concrete Vibration and Heavy Rebar
Internal vibration is using a dip-style internal concrete vibrator to vibrate concrete. A dip-style vibrator is inserted into the concrete, activated, and moved up and down throughout the poured concrete. Internal vibration is very much dependent on the skill and attention of the operator. A skilled worker can get great results. However, more often than not, there is little consistency between operators.
The vibration forces the entrained air and free water to move up and out toward the walls of the concrete form. Often, this means that the air bubbles are trapped against the form walls leaving behind bugholes and honeycombing.
The concrete is consolidated – but the visible finish isn't great.
While internal vibration is frequently used in heavy rebar conditions, there are consequences. Too often, the concrete is not properly consolidated because the internal vibrator hits against the rebar, can't fit between the rebar, or the operator has not been trained for the heavy rebar conditions. To correctly use an internal vibrator:
- Insert the vibrator directly downward into the concrete and directly up with as little movement left or right as possible.
- Proper vertical penetration is important because too much movement to the left or right will cause the vibrator to make contact with the rebar and damage the vibrator or alter its area of influence. Both of which can lead to poorly consolidated concrete
- Avoid dipping and pulling the vibrator back at angles.
- Avoid inserting the vibrator in a spot too far or too close from the last spot it was inserted in. Doing so can reduce the area of consolidation that occurs.
External Vibration and Heavy Rebar
External vibration means never having to struggle to vibrate inside a heavy rebar cage. External vibrators mount to the outside of the form and shake the form. Air bubbles are forced to the surface of the concrete rather than against the walls of the form. Operators move the vibrators vertically up the exterior of the form and activate them as the concrete level rises. Vibrator brackets come in clamp-style, weld-on style, and suction style. Talk to a VIBCO Concrete Vibrator guy to determine the right bracket for your form style.
Using Concrete Vibrators is easy, but there are key points to consider. Read the top 5 things to know about external vibrators, making sure your vibrator is running properly, and review VIBCO's Concrete guides detailing how to VIBCO concrete vibrators for concrete walls, columns, precast forms, countertops, and more. This information will help you to understand the science and technique of external concrete vibrator so you get perfect consolidation every time.
We supplied a jobsite in New York City where the rebar looked like this (left):
Yes – that is double wrapped rebar support. Heavy, heavy, cumbersome stuff to deal with. But the job went on without a hitch because the supervisor believed in using our SVRC 5500 external vibrators.
Heck – the supervisor even came up with a new way for mounting these vibrators that we hadn’t thought of before.
On the jobsite and as you prepare to use concrete vibrators, there are some key points to understand:
- Areas of influence and position. This is the single most critical point to consider. All concrete vibrators have a defined area of influence. By understanding the Area of Influence, you will be able to calculate the number of vibrators to use, placement of vibrators (movement) up the concrete form, and how they should be spaced on the form.
- Run time. A handful of phone calls will come through the office every month with a furious customer on the other line. Their frustration? Their vibrator died. When we ask what happened. They tell us how long they ran it for and at what power level. Turns out, they overran and overloaded their vibrator and blew it out. At the New York City job, run time was certainly something we explained.
- Mounting. How the vibrator actually sits on the form and how tightly it’s pressed against the form can make or break your vibrator – literally. We tell jobsite hands all the time how important it is to make sure that your vibrator is firmly fastened to the concrete form. If it’s not mounted properly the vibration from the vibrator won’t transfer properly into the form and into the concrete. Moreover, if the vibrator has any space between it and the form, the vibrator can shake itself to pieces by banging against the form.
The Bottom Line on Vibration and Heavy Rebar
If you’re working with heavy, tight rebar, getting properly consolidated concrete is a challenge that is efficiently solved with external concrete vibrators.
If you intend to use internal vibration, be sure to use trained operators who understand good vibration technique:
Avoid dipping at angles, or "dragging" and moving the vibrator while inserted in concrete.
- Dip straight down and straight up
- Measure out the distance between insert points to make sure you’re not missing space between one point and another
If a smooth, professional finish is what you’re after, external vibration your best choice. Talk to an Expert Concrete Vibrator Guy to learn about proper mounting, run time and the area of influence of the vibrator. Our experts have worked on hundreds of jobsites and will advise you how to get fully consolidated concrete with a great finish.