Pumpable concrete is that type of concrete which can be pushed through a pipeline for construction. It is made in such a manner that its friction at the inner wall of the pipeline does not become very high and that it does not wedge while flowing through the pipeline. It is very important to have a clear understanding of what happens to concrete when it is pumped through pipeline to any study of concrete pumping. Pumpable concrete emerging from a pipeline flows in the form of a plug which is separated from the pipe wall by a thin lubricating layer consisting of cement paste. The water in the paste is hydraulically linked with the interparticle water layer in the plug. Fig- 1 below shows the concrete flow under pressure. 
The pressure generated by the flow resistance must not be greater than the pump pressure rating for maintaining continuous plug movement. However, if the concrete is too saturated at higher w/c ratio, the concrete at certain pump pressures may be such that water is forced out of the mix, creating an increase in flow resistance and a possible blockage. Fig.2 illustrates such a condition. In other words, a very stiff concrete is not pumpable and also a concrete with high w/c ratio is also not pumpable. It is interesting to note that if a concrete is pumpable, it is implied that it is a good concrete.
