A concrete is said to be workable if it is easily transported, placed, compacted and finished without any segregation. Workability is a property of freshly mixed concrete, and a concrete is a mixture of cement, aggregate, water & admixture. Due to this all the properties of concrete, whether in fresh state or hardened state, is affected by these ingredients and their proportions.
The single most important nature of workable concrete is its lubricating nature. If a concrete shows more lubricating nature, then it will have the following advantages, such as
It will exhibit little internal friction between particle and particle
It will overcome the frictional resistance offered by the surface of the formwork and reinforcement contained in the concrete
It can be consolidated with minimum compacting effort.
The factors which help concrete to have more lubricating effect to reduce internal friction for easy compaction are given below.
Size of aggregate
Shape of aggregate
Grading of aggregate
Surface texture of aggregate
Use of admixture
Let us now discuss each of the above mentioned factors in more detail
1. Water Content
Workability of concrete increases with increase in water content. Higher the water content per cubic meter of concrete, the higher will be the fluidity of concrete, which is one of the important factors affecting workability.
Adding more water to concrete also has some disadvantages as given below.
Increased quantity of water may cause bleeding in concrete.
Cement slurry also escapes through the joints of formwork
Strength of concrete may reduced.
2. Aggregate/Cement Ratio
The higher the aggregate/cement ratio, the leaner is the concrete. In lean concrete, less quantity of paste is available for providing lubrication, hence mobility of aggregate is reduced, resulting poor workability.
But in case of lower aggregate/cement ratio, the richer is the concrete. In rich concrete, more paste is available to make the mix cohesive and fatty to give better workability.
3. Size of Aggregate
Bigger size aggregates have following advantages as compared to smaller size aggregate.
It has less surface area
Requires less amount of water for wetting surface
Requires less amount of paste for lubricating the surface
So for a given water content & paste, bigger size aggregate will give higher workability.
Note: From the practical point of view, the maximum size of aggregate to be used will depend upon the handling, mixing and placing equipment, thickness of section and quantity of reinforcement.
4. Shape of Aggregate
Workability of aggregate is also affected by the shape of aggregate. Generally aggregate are found in variety of shapes, such as
Angular, flaky & elongated aggregate reduces the workability of concrete.
Rounded or sub rounded aggregate have following advantages
For a given volume or weight, it has less surface and less void, so excess paste is available to give better lubricating effect
Due to rounded shape it has less friction resistance
Because of the above mentioned reasons rounded aggregate shows a high workability as compared to angular, flaky or elongated aggregates.
Note: River sand & gravel provide greater workability to concrete than crushed sand.
5. Grading of Aggregate
Grading of aggregate have the maximum influence on workability. The better the grading, the less is the amount of void in it. When total void are less, excess paste is available to give better lubricating effect. With excess amount of paste the mixture becomes cohesive and fatty which prevents segregation of particles & least amount of compacting efforts is required to compact the concrete.
6. Surface Texture of Aggregate
Porous and non-saturated aggregate will require more water than a non-absorbant aggregate. For same degree of workability latter will require less water. On the whole this factor is only of secondary importance.
7. Use of Admixture
This is one of the commonly used methods to enhance workability of concrete.
Plasticizer and super plasticizers greatly improve the workability.
Air entraining agents are also used to increase the workability. Air entraining agents creates a large number of very minute air bubbles. These bubbles get distributed throughout the mass of concrete and acts as rollers and increases workability.
Pozzolanic materials are also used to improve workability of concrete.