Pumpable concrete is a type of concrete transferred to a considerable height through pipes and used when a significant amount of concrete is required to be placed.
The pumpability of concrete is influenced by several factors like cementitious materials, aggregate characteristics, admixtures, and batching and mixing of concrete constituents.
Various cementitious materials such as fly ash and silica fumes have a positive effect on pumped concrete. Therefore, the maximum aggregate size should not exceed one-quarter of the pumping pipe diameter, and precautions should be taken before using a lightweight and heavy-weight aggregate in concrete.
In addition, batching and mixing should be carried out accurately and adequately; otherwise, the possibility of issues due to concrete pumping may arise. Â
What are the Factors Affecting Pumpability of Concrete?
1. Cementitious Materials Â
Generally, bleeding of pumped concrete is a major problem that needs to be avoided, and fine cement is used to remedy it. So, the finer the cement, the better.
The application of fly ash is considerably advantageous for pumped concrete because its spherical particles and glassy texture (see Figure-3) improve the cohesion of the mixture and decrease the amount of water needed for the mixture.
Silica fume is another admixture that can be employed to enhance the pumpability of concrete. In addition, it significantly increases the cohesion of the concrete mixture.
Moreover, ground granulated blast furnace slag may slightly improve the pumpability of concrete since the shape and texture of its particles are similar to that of Portland cement.Â
On the other hand, if a large amount of ground granulated blast furnace slag is used in the mixture that contains heavyweight aggregate, then concrete segregation is likely. Besides, the addition of ground granulated blast furnace slag postpones concrete setting time, which could be sometimes beneficial.
Similar to ground granulated blast furnace slag, metakaolin does not influence the pumpability of concrete in an obvious and effective way, but it is employed in low cohesion concrete.   Â
2. Aggregate Characteristics
The effect of different characteristics of aggregate such as shape, size, and density on pumpability of concrete will be explained in the following sections:
2.1 Aggregate Size
Usually, the maximum aggregate size employed to produce pumpable concrete should not be larger than the minimum pumping pipe diameter divided by four.
This is because the possibility of segregation would increase if the maximum aggregate size exceeds a quarter of the minimum pipe diameter.
Therefore, it is required to choose aggregate size cautiously to have a concrete mixture with the required cohesion.
2.2 Aggregate Grading
A good pumpable concrete mixture can be produced if the aggregate is well-graded. This type of grading permits the movement of water and fine materials toward the concrete-pipe interface. However, gap-graded aggregate is not suitable for pumpable concrete because the mixture could suffer severe segregation.
Well-graded aggregate, which is commonly acceptable to produce pumpable concrete, can be obtained by blending continuous coarse aggregate and sand with enough fine materials. This type of grading provides a smooth curve and reaches the particle packing theory conditions.
There are standards for aggregate blending that can be considerably beneficial for optimizing materials and obtaining well-graded aggregate. Finally, it is worth mentioning that it is important to maintain uniformity after the aggregate grading is specified.
2.3 Aggregate Shape
Generally, most aggregate shapes are appropriate for pumpable concrete if the minimum possible void ratio is maintained. It is desired to employ round-shaped aggregate for pumpable concrete because round gravels do not create friction force internally and with the pipe system. As a result, they can move easily through the pipes, especially at corners and tapered locations of a pumping system.
Angular aggregates should not be completely dismissed since the void ratio of angular shapes can be decreased, and eventually, pumpable concrete can be made.
Finally, if aggregate shapes are bad and undesirable, it is recommended to pay attention to sand proportion.
2.4 Aggregate Density
If the density of different particle sizes in a concrete mixture is different, it will exacerbate bleeding and segregation of pumped concrete. Moreover, when lightweight concrete is produced inappropriately, the lightweight aggregate may float out and most certainly cause hindrances during concrete placement.
Lightweight aggregate is porous and absorbs a significant amount of water in the concrete under pumping pressure. Eventually, pipeline blockage will occur. In addition, after the removal of pump pressure, the absorbed water comes out of the aggregate and leads to bleeding, especially during concrete slab placement.
So, necessary measures such as soaking lightweight aggregate in advance or introducing admixture to prevent water ingression into the aggregate need to be considered.
By and large, concrete made with lightweight aggregate cannot be pumped for long distances and considerable heights. In comparison, heavyweight aggregates may sink to the invert of horizontal pumping pipes and cause blockage.
It is required to dismantle and clean pipes after concreting; otherwise, it would become difficult to remove unsaturated concrete in the pipe invert. The pipe diameter should be slightly greater than the maximum aggregate size of the mixture to prevent possible blockage due to aggregates.
In addition, the hardened concrete stuck in the pipe would cause more problems for the next pumping operation because the concrete in the tube will be loose and block the pipe.
3. Admixtures
Generally, admixtures are used in most applications, and they certainly influence the pumpability of concrete. For example, water-reducing admixtures and high-range water-reducing admixtures are beneficial as they prevent excessive bleeding by reducing free water content and prevent the utilization of needless cement content.
Air-entraining admixture enhances concrete mixture cohesion and improves concrete pumpability. Nonetheless, if the cement content of the mixture is high, providing an air-entraining agent offers no benefits.
Retarder admixture is considered advantageous in hot environments and situations where unanticipated delays come up. It can increase concrete workability, and hence providing more time for concrete placement.
4. Material Batching and Mixing
Pumpable concrete is considerably affected by errors in batching, the quantity of mixing water, and other possible errors that may happen during material batching. Therefore, it is required to conduct concrete batching accurately.
For material mixing, pumpable concrete should be mixed adequately, and machines used for this purpose should be employed based on the manufacturer's recommendation. Lastly, it is advised to remix ready-mix concrete before loading it into the pump. Â
FAQs
Pumpable concrete is a type of concrete transferred to a considerable height through pipes and used when a significant amount of concrete is required to be placed.
The pumpability of concrete is influenced by several factors like cementitious materials, aggregate characteristics, admixtures, and batching and mixing of concrete constituents.
The factors that oblige concrete pumps are time and labor savings, enhanced scheduling, decreased site preparation, high-quality control, and the versatility of placing the concrete easily.
Concrete pumps can reach up to 150 m.
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