Practical problems dealing with concrete pumping

For placing large quantities of fresh concrete, piston pumps are generally used [50]. Concrete is pushed alternately by two pistons acting in cylinders. The first problem to be solved is to fill the total length of the pumping network without creating a blockage. Blockage may occur due to leakage (generally at the joint between several pipes, where cement paste may flow out, provoking an accumulation of aggregate) or segregation. Also, after a stop in the pumping process, concrete setting may begin. To avoid blockage, a number of precautions must be taken:

• the network must be concrete tight;
  
• fresh concrete should have a minimal susceptibility to segregation;
  
• a sufficient volume of cement grout has to be pumped before fresh concrete, so that the coarse aggregate particles which are expelled out of concrete by the pumping strokes remain in a suspending fluid;
  
• the time of practical use of the concrete, during which the consistency is soft enough, should be higher than the time necessary to pump and to place it.

However, even when these conditions are met, the pump may be unable to convey the concrete up to the end of the pumping network at the required flow rate. This emphasizes the need for a thorough understanding and application of rheological principles.