What are the reasons for blockage in pumping concrete?

Concrete pumping is commonly adopted in highly elevated locations for which access for concrete trucks is difficult. Construction works can be speeded up by using concrete pumping because a larger volume of pours can be achieved within a specified duration when compared with normal concrete placing methods.

Blockage may occur during pumping operation for the following two common reasons:

(i) For saturated concrete mixes, the pump pressures may force water out of the concrete resulting in bleeding. The flow resistance is then increased and may contribute to the blockage of pipelines.

(ii) If the cement content (or other components of concrete mixes that increase the frictional forces) is high, a higher frictional resistance to pumping may develop and the concrete may not be pumpable.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.