The important process of settling newly poured concrete is called compaction and a vibrating poker is a simple and effective hand held tool for doing the job. They are also known as internal vibrators because they are immersed in the concrete as opposed to external types, which may be attached to the form or float on top of the concrete.
What Problem Does A Vibrating Poker Solve?
Concrete that is poured into a form contains a large number of air bubbles that can make up as much as 20% of its volume depending on constituent factors of the mix. These bubbles must be removed before the concrete sets because they would degrade the strength and integrity of the hardened concrete. Bubbles prevent concrete from adhering to reinforcing structures, which negates their effectiveness and would mean that reinforced concrete is honeycombed with voids and not properly reinforced at all. It also risks permeability, whereby water could enter and damage the steel reinforcing structure over time.
How Does It Work?
It is called a poker because it does look like a fat poker, anything up to about 2 feet long. Inside is a rotating shaft with an eccentric weight, meaning it is unevenly centred around the shaft causing the poker to vibrate as the shaft rotates. The poker is plunged into newly poured concrete and the vibration travels through the semi liquefied mass near the poker head, moving the particles and enabling any air bubbles and excess water in the vicinity to surface.
That also means that the concrete forms into a homogenous texture of greater density and with better compressive strength. The poker is then extracted slowly at a rate of one inch per second and plunged in again a distance away that is equal to roughly four times its diameter, and so on until the entire mass has been compacted.
How Effective Is A Poker?
Speed of compaction, the work rate of a poker, is measured in cubic metres per hour. So a heavy duty 75mm diameter poker would be expected to compact at a rate of 20 cubic metres per hour in high volume pour such as in footings or foundations and less than that for denser concrete. The frequency of vibration is also a factor. Pokers typically vibrate at a speed in the range of 3,000 to 15,000 vibrations per minute (VPM). As it’s the vibration that liquefies the mix, the higher the VPM the greater the effect on stiff mixes. It is important to immerse the poker for the appropriate duration. If it is removed too quickly then some of the air will not have had time to reach the surface. Leave it immersed for too long and the cement will start to separate into its constituent parts.
Many sizes of poker heads are available, generally ranging in diameter from 25mm to 75mm. Each size is best suited to a specific job and consistency of concrete. Generally speaking, the bigger the head diameter, the greater the amplitude (distance it travels when rotated) and centrifugal force it generates. The diameter of the poker determines its effectiveness through different depths of concrete. For example:
- High volume pours, typical of foundations and footings, are best suited by a 75mm or greater diameter that should result in a speed of about 20 cubic metres per hour.
- For mass concrete pours, typical of bridges, structural supports, retaining walls and floor slabs, a dimension of 45mm to 75mm should result in a compaction speed of 10 to 35 cubic metres per hour.
- Beams and floor slabs count as low volume pours and a 35mm head should be good for 6 cubic metres per hour approximately.
- Other low volume pours, for example thin wall sections, thin beams, columns, are suited to a head dimension of 25mm to 35mm, giving a minimum of 1.5 cubic metres per hour.
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