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Causes of Blockages

There are basically three main causes of pump line blockages: a deficiency in the mix design; problems with the pipeline itself; and the human factor, or operator error.

The Wrong Mix

The most common mix problem is concrete that does not retain its mixing water. Concrete can bleed due to poorly graded sand that allows water to bleed through the small channels formed due to voids in the sand, or if the concrete is too wet.

Insufficient mixing can cause segregation in the mix. For successful pumping, aggregate must have a full coating of cement grout to lubricate the mix as it is being pumped.

A delay in placing the concrete due to traffic or job site problems, as well as hot weather conditions, may cause the concrete to begin to set prematurely. This creates a mix that may be too stiff to pump, because it wont fill the pumping cylinders, causing excessive pumping pressures.

Problems with the Pipeline

The entire pumping system must be evaluated for the job it is to perform. Considerations include a properly sized system including pump capacity and motor horsepower to move the concrete through the full length of the pipeline.

Pipes that have been improperly cleaned may cause blockages where old concrete has set, and may cause bleeding and segregation. Defective couplings, gaskets, or weld collars also can result in the loss of grout.

Another thing to look for are bends that are too short, too sharp, or too numerous, all of which increase concrete pumping pressure. Variations of pipeline diameter, such as when a larger diameter hose is coupled with a smaller one, may cause blockages or rock jams because the concrete cant flow as quickly through the smaller diameter pipeline.

Operator Error

The most common error from inexperienced operators is setting up the pumping system improperly. Operators must know to set up each job so that pipe or hose only needs to be removed, not added on. This is because if the placing crew has to add hose once the pour is in progress, the dry conditions inside the added hose is likely to cause a blockage.

Careless handling of flexible rubber discharge hoses can also be a problem, since kinking can occur. A rock jam is likely to be the end result of a kinked hose, as the inside hose diameter is reduced, which restrains the aggregate in the line while the lubricating grout is allowed to pass. Premature localized wear of the hose, and eventual rupture of the hose, may also occur at the point where the hose is kinked.

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