Equipment and Applications
Two types of vibrators are common on building sites - immersion (poker) vibrators and surface vibrators. Each has its sphere of application, although on floors and other flatwork it is not uncommon for one to augment the other. A third type, for vibrators, are commonly used in precast work, and sometimes on building sites (this is not covered here).
Frequently referred to as 'poker' or 'spud vibrators', immersion vibrators consist essentially of a tubular housing which contains a rotating eccentric weight. The out-of-balance rotating weight causes the casing to vibrate and, when immersed in concrete, the concrete itself. Depending on the diameter of the casing, and on the frequency and amplitude of the vibration, an immersion vibrator may have a radius of action between 100 and 600mm (see Table 1)
Immersion vibrators may be driven by:
- A flexible shaft connected to a petrol, diesel, or electric motor
- An electric mortar situated within the tubular casing
- Compressed air
|Diameter of Head (mm)
||Recommended Frequency (Hz)
||Average Amplitude (mm)
||Radius of Action (mm)
||Rate of Concrete Placement (m³/h per vibrator)
|High slump concrete in very thin members and confined spaces. May be used to supplement larger vibrators where reinforcement or ducts cause congestion in forms
||Concrete 100-150mm slump in thin walls, columns beams, precast piles, thin slabs and along construction joints. May be used to supplement larger vibrators in confined areas.
||Concrete (less than 80mm slump) in normal construction, e.g. walls, floors, beams and columns in residential, commercial and industrial buildings.
||Mass and structural concrete of 0-50mm slump deposited in quantities of up to 3m³ in relatively open forms of heavy construction.
||Mass concrete in gravity dams, large piers and massive walls etc.
Table 1. Characteristics and Applications of Internal Vibrators
Surface vibrators are applied to the top surface of concrete and act downwards from there. They are very useful for compacting slabs, industrial floors, road pavements, and similar flat surfaces. They also aid in levelling and finishing the surface.
There is a number of types of surface vibrators including vibrating-roller screeds and pan-type vibrators which are used mainly on very specialised equipment such as road paving plant, but the most common type is the single or double vibrating-beam screed.
A vibrating-bean screed consists of either one or two beams, made from aluminium, steel or timber, to which is attached some form of vibrating unit. This may be a single unit, mounted centrally, or may consist of a series of eccentric weights on a shaft driven from a motor on one end and supported on a trussed frame. In general, the centrally mounted units have a maximum span of about 6m, but the trussed units may span up to 20m. The small units are normally pulled forward by hand whereas the larger units may be winched, towed or be self propelled.
The intensity of vibration and, hence, the amount of compaction achieved, decreases with depth because surface vibrators act from the top down. They are most effective, therefore, on slabs less than about 200mm thick. With slabs greater than 200mm in thickness, immersion vibrators should be used to supplement the surface vibration. A thick slab compacted by both immersion and surface vibrators will have a denser, more abrasion-resistant surface than one compacted by immersion vibrators alone.
Generally speaking, vibrating-beam screeds are not suitable for concretes with slumps greater than about 75mm, as an excessive amount of mortar may be brought to the surface. Ideally they should be used only on concretes with slumps between 25 and 50mm. Slabs 200mm or over in thickness should be compacted initially with immersion vibrators. Slabs of less than 200mm may also benefit from the use of immersion vibrators along their edges.