The major problems that one encounters when pumping concrete are blockage of pipeline, and segregation and bleeding of concrete mix. Blockage is the failure of concrete to emerge at the end of the pipeline. It is characterized by an increase in pressure that can be observed on the pressure gauge.
Blockage can be caused by improper selection of pumping equipment like using an underpowered motor, coupling a small diameter pipe with a large diameter pipe, and attaching wide or sharp bends in the pipeline. Unskilled labor, improper maintenance of pumping equipment, and inappropriate concrete handling are also some critical factors resulting in blockage.
Moreover, bleeding and segregation in concrete are caused by incorrect mix design and poor-quality materials. This problem can be solved by increasing cement content or adding admixtures to increase the mixture's cohesion. Additionally, the use of well-graded aggregate in the mix would immensely help in concrete pumpability.
During the pumping process, the pump's pressure needs to overcome the friction between the concrete and the pipe in order to push concrete through the pipeline. The use of mortar at the beginning of the pipe would lubricate its internal walls and help the pump to overcome the friction force.
Major Problems in Concrete Pumping Works
1. Blockage of Concrete Pump
Blockage can occur due to numerous bends in the pipeline, too sharp or too wide turns, gap-graded aggregate, and failure to clean and maintain the pipeline after each concreting work.
High water-cement ratio and long waiting-time at construction site during which concrete loses its pumpability can also lead to blockage.
2. Segregation and Bleeding in Concrete Mix
Segregation is the separation of cement paste and aggregate during the handling and placement of concrete. Segregation occurs when pumping pressure overcomes the adhesion force in the concrete mix.
Bleeding is the escape of water from the concrete mix. It occurs due to bad proportioning of concrete mix. The use of gap-graded aggregate is the main reason for bleeding during pumping operation.
How to Avoid Concrete Pumping Problems?
1. Concrete Mixture
If the concrete mixture fails to retain a sufficient amount of water, the probability of a concrete pump getting blocked is high. Gap-graded aggregate allows water to bleed through the small channels formed due to voids, which may create blockage; therefore the gap-graded aggregate should be avoided.
In contrast, the use of well-graded aggregate tends to decrease the chance of water escaping from the mixture. It also makes the fresh concrete to have an adhesion force greater than pumping pressure, which would otherwise cause the concrete to segregate under pumping pressure.
If fine content is high, the pipe may create too much frictional resistance and possibly cause a blockage. Therefore, appropriate proportioning of concrete constituents is the easiest way to prevent the concrete mixture problems that may come up during the pumping process.
The cement-aggregate ratio should not be more than 1:6, and the w/c ratio should be between 0.5 and 0.65. Use a maximum aggregate size of 20 mm, and the optimum aggregate content should be between 35% and 40%.
The optimum quantity of fine aggregate should be between 35% to 40%, and that for material finer than 300 microns should be between 15 to 20%. The quantity of cements and other particles below 0.25 mm size should be between 350 to 400 kg/m2.
2. Handling Concrete
Improper mixing of concrete constituents leads to segregation in the mix, and hence in failed concrete pumping. Adequate mixing of concrete components ensures that aggregates have a full coating of cement grout to lubricate the mixture as it is being pumped.
When concrete mix remains in hot weather for a longer period of time, the concrete will stiffen and lose its pumpability to a great extent.
3. Labor Skills
Errors by unskilled labor may restrict the performance of equipment and possibly cause blockage. Improper handling of pump hose may create kinks that prevent the concrete mixture from flowing.
The pump hose may tear or rupture at the kink positions. A skilled pump operator would know how to set up the pump and eliminate the need to add more hose during the pumping process.
4. Selection of Pumping Equipment
The selection of improper pump equipment for the job under consideration can also cause blockage. For instance, the connection of a large diameter hose to a small diameter hose can create blockage.
Additionally, selecting an underpowered motor for a particular concrete task cannot produce enough pump velocity to force the concrete mixture through the pipeline system, which would result in blockage.
The diameter of the pipe should be evaluated to ensure its applicability for the job to be performed. Numerous bends, too sharp, and too wide bends need to be avoided; otherwise, higher pump pressure would be required to pump the concrete. The higher the required pump pressure, the higher the required horsepower of the motor.
Consider the use of pipe with a diameter of at least thrice the maximum aggregate size to avoid the risk of blockage near bend points. Prevent the use of aluminum pipes as it reacts with alkalis in cement and creates hydrogen gas, which generates voids in concrete and reduces the efficiency of the pump.
5. Use of Poor-Quality Materials
Another cause of problem while pumping concrete is the use of poor-quality materials. The low-quality materials produce concrete that is unqualified to be placed using pumping equipment as the unqualified pumping concrete may have too small or too high a slump value.
Consequently, the occurrence of blockages is highly likely. However, the use of the correct water-cement ratio would decrease the friction between coarse aggregate and pump pipes.
6. Selection of Improper Additives for Concrete
The use of improper admixtures such as retarders, pumping agents, filling agents can also be the reason for blockages during the pumping process.
7. Failure to Maintain Pump Pipeline
If the pipeline of pumping equipment is not cleaned and maintained properly, residual concrete inside the pipes would set and obstruct the flow of fresh concrete and ultimately causing blockage of pipeline.
Deteriorated or worn gaskets, couplings, and weld collars should be replaced to avoid grout loss. It is recommended to carry out regular maintenance works rather than fixing concrete blockages.
FAQs
The major problems associated with pumping are blockage of concrete pump pipeline and bleeding and segregation of concrete mix.
Concrete pumping problems can be prevented by carefully selecting concrete mix constituents, proper mix design, correct selection of pumping equipment, hiring skillful laborers, and performing regular maintenance of pumping equipment.
Blockage is the failure of fresh concrete to emerge from the end of the concrete pump pipeline. It occurs due to improper concrete mix design and incorrect selection of pump equipment, improper handling of equipment, and failure to maintain the pumping equipment.
The concrete pump can be cleaned with water or air pressure. The former is recommended because not only is it safe, but also more effective.
It is a type of concrete that can be pushed under pressure through a pipeline over a long horizontal distance or very high altitude to reach the pouring position. The pumpable concrete is commonly separated from the inside wall of the pipeline by a mortar paste.
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