A workable concrete exhibits very little internal friction between the particles forming the concrete and overcomes the frictional resistance offered by the internal surface of formwork as well as that by the reinforcement contained in the concrete with just the amount of compacting efforts forthcoming. The factors helping concrete to achieve these are as follows
Water Content: The fluidity of concrete increases with water content. At site the normal practice is to increase the water content to make the concrete workable which lowers strength.
In controlled concrete this cannot be resorted and even in uncontrolled concrete this should be the last choice. However, in case if more water is added due to any reason the cement content should be proportionately increased.
Mix Proportions: Aggregate-cement ratio influences the workability to a large extent. The higher the ratio leaner will be the concrete. In a lean concrete, paste available for lubrication of per unit surface area of aggregates will be less and hence the workability is reduced.
Aggregate Size: For big size aggregate the total surface area to be wetted is less, also less paste is required for lubricating the surface to reduce internal friction. For a given water content big size aggregate give high workability.
Shape of Aggregates: For a given water content, round and cubical shape aggregates are more workable than rough, angular or flaky aggregates, because the former type of aggregates requires less cement paste for lubrication as these have less surface area and lesser voids. In case of round aggregates frictional resistance is also small so less lubrication is required. For this reason river sand and gravel provide greater workability than crushed sand and aggregates.
Surface Texture: A rough surface aggregate will have more surface area than a smooth round textured aggregate. Hence, latter will be more workable for the reasons discussed above. Grading of Aggregates: Properly graded aggregates are more workable. It is so because such a mix will have least voids and thus excess cement paste will be available as lubricant. This also prevents segregation.
Admixtures: Air entrained concrete is more workable. It is so because air forms bubbles, on which the aggregates slide past each other increasing the workability. Another factor is that air entraining agents are surface active and they reduce the internal friction between the aggregates.
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