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Any admixture that improves the workability of concrete is advantageous for enhancing the pumpability of concrete. Various chemical admixtures like air-entraining admixture, high-range water-reducing admixture, and finely divided admixture have been utilized to improve concrete pumpability.
The choice of the type of chemical admixture and its utilization in concrete depends on the mixture's characteristics.
Using air-entraining agents improves concrete cohesiveness and reduces segregation and bleeding. Mineral admixtures reduce the rate and amount of bleeding in concrete and improve its pumpability.
Types and Roles of Admixtures in Pumpable Concrete
Air-entraining admixture improves the cohesiveness and workability of concrete. As a result, the likelihood of segregation, which is a major concern in pumpable concrete, is reduced significantly. It also reduces the tendency of concrete to bleed.
The start-up of the pipeline after shutdown is easier with air-entrained concrete than with non-air-entrained concrete because of reduced bleeding. Bleeding in concrete reduces lubricant and increases the chance of pipeline blockage.
The recommended dosage of air-entraining admixture based on maximum aggregate size is presented in Table-1. These values are set based on the resistance of concrete against freezing and thawing cycles, but they should not be violated while pumpable concrete is produced.
Table-1: Recommended air contents for concrete resistant to freezing and thawing*
|Nominal maximum aggregate size, mm (in.)||Average air content for moderate exposure, %||Average air content for severe exposure, %|
Mineral admixtures are classified into three types:
- Relatively chemically inert materials such as ground limestone, ground quartz, and hydrated lime.
- Cementitious materials involve natural cement, ground granulated blast furnace slag, hydraulic lime, and slag cement.
- Pozzolan materials, for instance, class C and F fly ash, diatomaceous earth, volcanic glass, heated shales and clays, and silica fume.
The particle size of the above materials is mostly smaller than cement. The spherical particle shape and the smooth and dense surface texture of these materials improve the pumpability of a concrete mixture.
If the concrete mixture lacks an adequate amount of fines, the addition of finely divided mineral admixture enhances concrete workability and pumpability and reduces the rate and amount of bleeding.
Normal and High-Range Water-Reducing Admixtures
Water-reducing admixture increases the workability of concrete for a constant water-cement ratio or reduces the water-cement ratio at a constant slump.
Moreover, some water-reducing admixtures have the ability to entrain air which also improves concrete pumpability. If concrete relies on high-range water-reducing admixture (superplasticizers) for pumping, it should be discharged and placed before any loss in workability.
High-range water-reducing admixture is used to produce flowing concrete. Flowing concrete is suitable for pumping because it reduces pumping pressure and increases the rate and distance up to which concrete can be pumped.
Anti-washout admixtures increase the cohesiveness of concrete that is placed through pumps under water. Some examples of anti-washout admixtures are natural or synthetic gums and cellulose-based thickeners.
1. Improve workability
2. Reduce segregation and bleeding
3. Reduce the possibility of pipeline blockage
Small and stable bubbles, created by air-entraining admixture, work as a flexible ball bearing in the concrete mixture. They enable sand particles to move more freely, increasing mix plasticity without adding extra water. The air bubbles supplement sand grading, which is crucial for pumping concrete i.e. they reduce the gap grading effect.
Anti-washout admixtures increase the cohesiveness of concrete that is placed through pumps under water.
Retarding admixtures delays concrete set, which is important for concrete pumping in hot weather conditions.
The particle size of the above materials is mostly smaller than cement. The spherical particle shape and the smooth and dense surface texture of these materials improve the pumpability of a concrete mixture. If the concrete mixture lacks an adequate amount of fines, the addition of finely divided mineral admixture enhances concrete workability and pumpability and reduces the rate and amount of bleeding.
Factors affecting the performance of admixtures in concrete
Functions of admixtures in fresh and hardened state of concrete