Concrete Pump Blockage - Causes and Remedy

Pumping concrete is the most efficient, economical and reliable way to place concrete at the construction site. But, the process occasionally encounters with blockage or rock jams or in some situations blow out or hose whip.

Concrete pump blockages are annoying and can delay the concreting process drastically. Concrete pumping can be performed smoothly only if we plan properly the complete stages of the process i.e. the concrete mix, the pressure of pumping, the type of pump, the experience of operator etc. and be aware about the possible blockages and the ways to locate and clear it.

In this article, we will discuss the main causes of concrete plump blockage, how to locate it and ways to clear it.

Causes of Concrete Pump Blockage

The three main causes of concrete pump blockage are:

1. Wrong Concrete Mix
2. Inappropriate Concrete Pumping Pipeline
3. Operator Error

No Time To Read??? Major Problems & Causes of Concrete Pumping Blockage 

1. Wrong Concrete Mix Causing Concrete Pump Blockage

A pumpable concrete must move through the pipeline in the form of a cylinder or slug that is separated from the walls of the pipeline using the lubricating layer of water, cement and fine sand. A good concrete mix must  pass through the reducers, bends and hoses in a particular pipe setup.

For proper pumping of concrete, we need a concrete mix that have aggregates that are fully coated with cement paste and a mix that have the ability to retain water content. In contradictory to these conditions, if water is not retained properly, i.e. bleeding of concrete, or when there is segregation of concrete mix and early setting of concrete inside the pump, then it results in blockage of concrete pump.  

Bleeding of concrete will result in the pump to choke water, this is mainly due to poorly graded aggregates. Insufficient mixing of aggregates causes segregation of concrete mix. A concrete mix designed without considering the climatic conditions can cause premature setting of concrete that makes stiff concrete mix that is tough to pump as it does not fill the pumping cylinders, that would create excessive pumping pressures.

2. Inappropriate Concrete Pumping Pipeline

Given the various types of concrete pumps, a wrong choice of pumping system for a given concrete mix adversely affect the concrete pumping operation. 

• A wrong choice of pump capacity and motor horsepower to move the specific concrete mix can affect the smooth pumping of concrete, throughout the length of the pipeline.
• A pipe system that is not cleaned for removing old concrete can cause blockage, which in turn result in bleeding and segregation. 
• Defective couplings, gaskets, or weld collars can result in loss of grout.
• Bends in pipeline that are too short, sharp or numerous can increase concrete pumping pressures.
• The junction where two pipeline hose diameters are coupled cause blockage or rock jams, as the concrete moving from larger diameter to smaller diameter cause difficulty and delay.

Fig.2. An abrupt reduction in the line diameter can cause blockages due to the lose of lubricating grout

3. Operator Error Causing Concrete Pumping Blockage

The common errors that cause concrete pumping blockage are:

1. Improper setting up of pumping system by inexperienced operators. This means a proper pumping system setup for a particular job won't demand the addition or removal of pipe or hose in between the concrete pumping process. If a newly dry pipe is added, the new dry conditions within it can cause blockage.
2. Kinking or twisting or curling can occur when the hose is not handled properly. Kinking results in a rock jam. When kinking occurs, the diameter of the hose is reduced which restrains the passage of aggregates and lets the lubricating grout pass. This is called rock jam.
3. Kinking also results in premature localized wear of the hose, and eventual rupture of the hose at that point.

Fig.3. Localized wear cause due to bending of hose too sharply-reducing its inside diameter causing wear to the hose wall and possible blockage.

How to Locate Blockage in Concrete Pumping Line?

Being aware of blockage or rock jams while pumping concrete helps to remove them promptly and safely. 

Mix variations ( too dry or wet or rocky concrete mix), foreign matters in concrete, and other mix anomalies can cause blockage while pumping concrete.

Some tips to locate the blockage in concrete pump lines are:

1. Pump and pump gauges are the two important tools for evaluating the concrete quality of an operator. If the concrete mix is not of pumpable quality, no amount of pumper expertise would work. 
2. If the pump pressure gauge shows a rise in the line resistance, it is an indication of blockage.
3. The first suspect spot for blockage is the reducer area. The reducer is the area that connects the concrete pump to the pipeline system.
4. A blockage in the pump area is indicated by a quick build-up in pressure prior to the jam.
5. A slow pressure build-up is indicative of a jam that is further down the line nearer the delivery end.
6. Checking the elbows or discharge hose to examine any concrete blockage is one way of locating the operator. It is done by tapping the hammer along the pipeline. A hammer tapping causes a thud sound in the pipeline area with blocked concrete and a ringing sound, where the line is clear.
7. Checking for grout leakage is one way to prevent concrete blockage in pipelines. 
8. Carefully walking over or stepping over the discharge pipe and finding an area where the soft hose becomes firm is an indication of jammed aggregate or blockage.

How to Clear Concrete Pumping Blockage?

1. Alternatively reversing the pump and resuming the pumping for a few cycles can help loosen or break the rock jam formed. Do it a couple of times.
2. If reversing and resuming does not work, locate the blockage and clear it out, by removing the line.
3. Before clearing the blockage, make sure that the line is not under pressure. 
4. Staying on one side of the line, remove the coupling nearest the jam until all the free-flowing concrete runs out of the open, by lifting the line. After which bend the hose or tap on the pipeline in the areas of the jam and shake out the loose particles.
5. Never use compressed air to remove the block as it won't help.
6. Cleaning unblocked areas with compressed air are fine, but blocked areas create all kinds of problems. 
7. Once the blockage is identified, set up an exclusion zone, at the end of the hose, in the line of fire. NO ONE is permitted in the exclusion zone during the releasing method.   
8. Another important risk associated while clearing the blockage or during the concreting process is the "Hose Whip". Hose whip is the uncontrolled and rapid motion of the flexible rubber hose on the end of a concrete placing boom or other concrete delivery lines. This becomes violent when the air enters the line and becomes pressurized due to an obstruction further down the line and the obstruction becomes dislodged.
9. Hose whip can be avoided by using suitable plants and safe systems to work.

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