Concrete Vibrator Tips: How Fast, Frequent and Deep
Proper Concrete Vibration is a Must
How Concrete Vibrators Work?
There are many types of concrete vibrators, but do you know why they are needed? When the concrete is poured, it might have hundreds or even thousands of air-bubbles that substantially weaken it. The usual solution which, in many instances, is also a building code requirement is concrete vibration. Concrete vibrators will eliminate air bubbles by vigorously shaking the freshly poured concrete with a mechanical vibrator.
Three Different Ways Concrete Vibrators Work
The three common ways of using concrete vibrators are:
- Formwork Vibration: external electric or air-driven concrete vibrators are mounted to the exterior of the formwork. For larger pours, the external form vibrators are often spaced 6' apart. There are recommended for precast work.
- Surface Vibration: Large vibrators (sometimes called "jumpers") are directed manually onto the surface of the pour. They are limited in terms of pour depth -- often the consolidation process is ineffective when the slab is more than six inches deep -- but they also provide a smooth surface, which is especially desirable when appearance is an issue.
- Internal Vibration: Internal concrete vibrators are most often used. Many can be handled by a single operator. The process is relatively simple: the worker quickly jams the internal vibrator -- again, either electric or pneumatic -- down into the wet concrete, then slowly withdraws it.
How Much Time Do I Need to Spend Vibrating Concrete?
It's a truism in construction that most concrete is under-vibrated or improperly vibrated. The best technique with an internal vibrator is to withdraw the vibrator very slowly -- at a rate of around one inch per second. As long as bubbles still emerge as the concrete vibrator is withdrawn, more vibration is necessary. Contractors sometimes urge workers to complete this time-consuming task "efficiently," meaning quickly, but the result can be a structural failure after the concrete has cured. If you hold the vibrator for too much time in the concrete, the water and aggregates will separate causing other issues with the strength and aesthetics of the concrete.
A Few Concrete Vibration Best Practices
- It's prudent to have a spare concrete vibrator ready at hand as a backup in case the first vibrator fails
- If you are pouring low slump concrete be particularly careful not to under-vibrate it. ("slump" is a measure of the workability of concrete; because low slump concrete is stiffer, it requires more vibration)
- When pouring self-consolidating concrete, vibration can be eliminated.
- Be sure to penetrate previous lift or layers of concrete already placed to reduce cold joints. The vibrator should penetrate at least 6 inches into the previous layer. (A cold joint occurs when the newer layer of concrete is poured on an older layer that has set up and hardened enough to prevent the two layers from bonding).
- The concrete vibrator should be inserted close to the vertical to maximize its effect
- Do not over-bend the vibrator; it will fail.
- A frequent mistake is to use the vibrator as a concrete placement tool. If you do this, you could be creating an inconsistent surface and the concrete mix will be negatively affected.
- The vibrator should be held in the concrete mix at least 10 seconds
- The concrete vibrator should be pulled up at an average rate of no more than three inches per second; often one inch per second yields the best result
- The concrete vibrator head must be totally submerged into the concrete.
- Every time the vibrator is inserted, its radius of action should overlap the previous radius of action. A good rule of thumb is that the radius of action is four times the vibrator tip diameter.
- Verify that the area has not been vibrated already by other co-workers before proceeding.
- Stop vibration when air no longer escapes the concrete, not before to make sure all air has been removed.
- Don't turn on the vibrator until the tip is fully submerged.
- Stop vibration when the concrete surface takes on a sheen.
- For small jobs consider using a lightweight portable vibrator
- Do not force the vibrator into the concrete; it can be caught by rebar, the reinforcing steel.