One of the most important steps when pouring concrete is the consolidation or vibration of concrete. Concrete vibrators, if used properly, will help consolidate concrete and will reduce the number of air pockets inside the concrete mass. You can use internal vibrators or external vibrators depending on the application and where the concrete is being placed.
However, there are very important factors that you need to consider before buying a concrete vibrator: frequency, power, and size. Normally, a concrete vibrator will be acquired knowing that you can have multiple uses for it and that it can be used for a very long period of time. External vibrators shall be used to reach the center while the internal vibrator can be used to consolidate the center portion of the concrete mass.
Lack of consolidation can cause voids, rock pockets, honeycombing, and poor bonding with the rebar. In extreme cases, improper consolidation can affect the structural integrity of the walls. On the other hand, excessive vibration can create bulged walls and blowouts.
This is the most common used vibrator. It consists of a steel closed and round tip with an electric vibrating element inside of it. The tip is connected to an electric motor through a flexible hose. In order to have best results, the diameter of the tube shall be small enough so it can be inserted in between reinforcing bars. It is recommended to insert the vibrator into concrete at intervals of 24 inches or 8 times the diameter of the tube. When this vibrator is used, the insertion time shall vary from 30 seconds to 2 minutes. Concrete shall never be poured in layers thicker than 24 inches high.
The vibrator should be inserted vertically into the mix, and then slowly withdrawn. Vibrators work by allowing air to float up and out of the concrete, so the tip should not be withdrawn faster than the air can move upward, about 3 inches per second. If the wall is poured in multiple lifts, the vibrator tip should go deep enough to penetrate 3 to 6 inches in the previous layer. The tip of the vibrator is available in different radius, depending on how much rebar has been installed.
When to Use External Vibrators
External vibrators are attached directly to the formwork at certain points. The location of these points will consider the thickness of the concrete and where the points are located concerning the concrete mass. This type of vibrator will have a direct vibration effect on the formwork and the concrete, and by doing this, they will normally consume more power than other vibrators.
External vibrators will normally consolidate concrete in a radius of 18 inches away from the formwork. These vibrators can be found at casting yards and precasting facilities where immersion vibrators cannot be used and should be strong enough or located at specific locations so they can move all the formwork at once. External vibration is preferred in situations where columns or heavy concentrations of rebar could result in the tangling of an internal head. These tools have specific applications, such as small pours that require a minimal amount of vibration.
Surface vibrators are located on top of the concrete that has been poured. They are extraordinary when the concrete is less than 10 inches or in flat slabs. These are recommended on shallow structures or slabs and when used in combination with concrete having a low water-to-cement ratio. When you are pouring a slab or patching a surface, this is the type of vibrator that should be used. Don't use a surface vibrator when there are large volumes of concrete or when the slab is too thick, and the lower layer of concrete will not receive adequate vibration.
Rebar shaker is one good alternative to traditional vibration techniques. The technology is very similar to a regular vibrator, but in this case, the device is slipped over the top of the reinforcing bar and "shakes" or transmits the vibration into the concrete. It can be found in different diameters and can result in great savings in man hours and reduces clean-up activities. This tool will shorten the time it takes to pouring concrete into a cell or in a very tight space.