Q. Reasons for blockage in pumping concrete ?

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

Blockage can take place at the time of pumping operation for the subsequent two common reasons:

(i)  For saturated concrete mixes, pump pressures can force water out of the concrete leading in bleeding. Flow resistance is then increased and can contribute to blockage of pipelines. 

(ii) If cement content (or other components of concrete mixes which increase frictional forces) is high, a higher frictional resistance to pumping can develop and concrete can't be pump able.