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Causes of BlockagesThere 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 won't 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 can't 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.

Locating a BlockageConcrete pumping crews and the concrete pumper must be constantly aware of the possibility of a pump line blockage or rock jam, and be able to remove it promptly and safely. Variations in the mix, whether too rocky, wet or dry; foreign matter in the mix (such as old concrete that has broken away from mixer fins or unmixed clumps of concrete) and other mix anomalies are tip-offs that problems may have occurred or may be about to occur.

A rise in line resistance, as shown on the pump pressure gauge, indicates line blockage. The first suspect spot for blockage is the reducer, which connects the concrete pump to the pipeline system. A quick build-up in pressure prior to the jam indicates the blockage is most likely in the pump area. Slow pressure build-up is indicative of a jam further down the line nearer the delivery end.

The operator needs to examine the system, especially at the elbows or discharge hose. This can be done by tapping the hammer along the pipeline. Where concrete is jammed, the hammer will produce a dull thud as opposed to a more ringing sound where the line is clear.

All pipe joints should also be inspected for grout leakage as well, as this can be indicative of grout loss and subsequent blockage.

By carefully walking over or stepping on the discharge hose to depress it, a blockage may be located where the soft hose becomes firm, indicating jammed aggregate.

Clearing the BlockageBy alternately reversing the pump and resuming pumping for a few cycles, the pump operator may be able to break loose a minor rock jam. This should not be tried more than a couple of times, however, as it can jam the pipeline even tighter. If the reversal method doesn't work, the operator must locate the blockage then break back the line and clear it out.

Always make sure the line is no longer under pressure prior to clearing a blockage. Stand to one side of the line and remove the coupling nearest the jam. Let all the free-flowing concrete run out of the open end of the line by lifting the line, then bend the hose or tap on the pipeline in the area of the jam and shake out loose particles.

Important safety tip: When trying to clear a line blockage, NEVER use compressed air. If a greatly increased pump pressure won't move the blockage, compressed air won't be able to either. While using compressed air with proper safety precautions is OK for cleaning out unblocked sections of pipe, using it on blockages can cause all kinds of problems, including the need to relieve the built-up air pressure, residual air pockets, and additional blockages due to segregation.

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