Why vibrate concrete ?
Fresh concrete is a mix of cement, water, sand, gravel, additives and air bubbles. Vibration enhances aggregate compaction and expels air bubbles, ensuring that the moulds and formwork are well filled and that the rebars are coated. Vibration is the key element to achieve high-quality surface and optimal mechanical resistance.
When and how to vibrate?
Concrete must be vibrated as soon as it is poured, inserting quickly the poker into the concrete and slowly pulling it up (to ensure that the hole in the fresh concrete closes up when the vibrator is removed).
For vertical elements, panels, posts, etc., concrete is poured by layers of 50 to 60 cm. After pouring each layer, the vibrator must penetrate 10 to 15 cm into the previous concrete layer.
10cm poker penetration into the newly-poured concrete
In situations when an opening must be left such as the opening for a window, vibrate by layers of 50 cm, on one side until concrete appears on the other side avoiding the possibility of air pockets and ensuring the bottom section of the opening has been filled completely.
What to use to vibrate concrete ?
By using vibrating pokers or external vibrators. Vibrating pokers (also called immersion vibrators) are directly immersed in the concrete. External vibrators, less common, are attached to the mould or formwork. These are particulary suitable for dry concrete and/or high-density reinforcing rebars mainly used in the precasting factorys.
Three types of pokers/immersion vibrators
Depending on the energy source available onsite, there are three choices : pneumatic vibrating poker AIR model, electrical series such as AET, TNE, PEA ; or a mechanical RABBIT model. All of these vibrators operate by rotating an eccentric weight located in the poker and activated by:
- an air flow fed by a compressor (pneumatic)
- an electric motor incorporated in the poker, fed by a frequency and tension converter that transform the 230/400V-50 Hz into 42V-200Hz (Certain models such as single-phase EBA have an integrated converter housing which connects directly to the 230V network. (electrical)
- a flexible hose set in motion by an electrical motor or other types of drive unit. (mechanical)
How to choose a poker/immersion vibrator?
Select a poker according to its diameter and the volume to vibrate, considering that the pokers vibrate ten times its diameter. For example, a poker of 50mm in diameter has an action diameter of 50 cm.
There needs to be a correct overlap between zones of action in order to ensure all the concrete is properly vibrated.
For optimal results, it is recommended to select the largest possible diameter considering the reinforcement: the vibrator must fit between the rebars but must not touch them or push them apart.
Overlap between zones of action to avoid leaving non-vibrated areas
When to stop vibration?
Stop vibration as soon as:
- concrete does not settle anymore,
- the expulsion of air bubbles stops,
- the concrete milky sheen starts to appear on the surface
Precautions for use
- Never allow the vibrator to come into direct contact with formworks or rebars.
- Do not turn on the electrical vibrator until the tip is fully immersed into the concrete.
- For electrical pokers EBA and RABBIT models, directly connected to power, make sure that the power is protected by a circuit breaker of 30 mA and a ground wire. (This is not necessary for double insulation models)
- After use always clean the poker.
- Before storing, always lubricate the pneumatic poker by putting a little spoonful of oil in the admission hose letting it operate for a few seconds; then, hang it with the valve hose at the bottom, allowing the water to drain.
- Clean the electrical or mechanical models to avoid an accumulation of dry concrete on the devices.
Combined vibration and smoothing/planning
For slabs and roads, we use a vibrating screed. This device can be mechanical or pneumatic and provide a width of action of up to 6 meters. These vibrate the surface of the concrete upto few centimeters depth and ensure optimal flatness of the slab. Pre-vibration by vibrating poker may be required. There are heavy-duty models, a simple- or double-beam screed, well suited for certain civil engineering applications. There are lighter models (aluminum flat beam) used for thinner slabs, residential applications, garages or others industrials projects.