For preparation of special products like precast concrete membranes, some special compaction methods are used, such as
• Air jets
Compacting in this method is effected by applying external pressure on top and vibrating from bottom. Table vibrators are usually made use of in this process. This process has an advantage of elimination of heterogeneous compaction as seen in only vibrations. A good durable concrete with water cement ratio a low as 0.3 can be obtained by the process and it has been successfully tried in production of flag stones for paving and concrete road kerbs.
In this method concrete in the mould is subjected to jolting action with the help of cams. Very dry mixes can be compacted by this method. Since the products can be immediately stocked after demoulding, this method is very successful for precast concrete products.
In this method the concrete is fed into the mould while it is being spun at a slow speed. The spinning speed is increased to a very high value after the pre-calculated amounts of concrete has been fed into the mould. High speed spinning forces out the water from the mix and the concrete finished by slowly reducing the spinning action and sprinkling dry cement on the surface in order to eliminate the risks of increased water cement ratio due to free water. Since the compaction is effected by centrifugal forces, it results in water tight products like pipes for water supply, storm water drains and sewage disposal pipes.
The compaction of concrete can also be done by air jets. This method is a recent invention. A 20 mm dia. Pipe about 1 m long taken. It is perforated in half of its length with 5 mm dia holes. The axis of the holes is inclined at 450 to the axis of the pipe. The pipe is fitted with a perforated end cap. This pipe is inserted in the concrete. Air under pressure of about 2 kg per sq cm is passed through the pipe. The air in coming out through the concrete helps its particles to come closer and thus affects its compaction. This method is suitable for wetter mixes only.