What is Workability of Concrete ?
The term ‘Workability of Concrete’ is defined as the property of freshly mixed concrete which determines the ease and homogeneity with which it can be mixed placed, consolidated and finished.
An ideal water-cement ratio alone is not going to give maximum strength. 100% compaction of concrete is an important parameter for contributing to the maximum strength. Insufficient compaction will result in presence of air voids which has damaging effect on strength.
Generally to compact the concrete fully a higher water-cement ratio is adopted. The water function as a lubricant in the concrete which eases the compaction process. Thus water is required for
- Handling concrete without segregation.
- For placing without loss of homogeneity.
- Effort of compaction is required to finish.
The quality of concrete satisfying all these requirements is termed as workable concrete.
Factors Affecting Workability of Concrete
1. Water Content :
The amount of water content available in a volume of concrete will have a significant effect on the workability of concrete. The higher the water content per cubic meter of concrete the higher would be the fluidity of the concrete.
In general to increase the workability certain amount of excess water is added in concrete at site. However it should be noted that increasing the water content of concrete will have some effect on strength of the concrete. Besides, whenever more water is added a certain amount of cement is also added so that water-cement ratio remains constant.
2. Mix Proportions :
In general aggregate-cement ratio is also an important factor which influences the workability. If the aggregate-cement ratio is higher, the lean concrete is formed. In this case less quantity of paste is available for providing lubrication, hence workability is reduced.
Whereas if the aggregate-cement ratio is lower the rich mix is formed. In this case more paste is available to provide lubrication, hence better workability of concrete is observed.
3. Size of Aggregates :
The bigger the size of the aggregates the lesser is the surface area hence less amount of water is required for wetting the surface. Hence less paste is required for lubricating the concrete surface to reduce internal friction, thus we can say that the bigger size of aggregates will give higher workability. Reduction in size of aggregates will reduce the workability.
4. Shape of Aggregates :
The shape of aggregates also have considerable influence on the workability of concrete. Compared to rounded aggregates or cubical shaped aggregates the angular, elongated or flaky aggregates makes the concrete very harsh. Thus rounded shaped aggregates have better workability. This is due to rounded aggregate will have less surface area and less voids than an angular or flaky aggregate.
In addition, rounded aggregate has the reduced frictional resistance and this is a reason why river sand and aggregate provide greater workability of concrete than crushed sand and aggregate. Besides the shape of aggregates also has a significant effect on strength and performance of concrete.
In recent times usage of natural sand was restricted thus new machine was designed to yield well shaped and well graded aggregates.
5. Surface Texture of Aggregates :
The surface texture of the aggregate also has a significant effect on the workability of concrete. The total surface area of roughly textured aggregate is more than the surface area of a smoothly rounded aggregate of the same volume. Thus it can be inferred that roughly textured aggregate will show poor workability and smooth or glassy texture aggregate will give better workability of concrete.
The reduction of inter-particle frictional resistance offered by smooth aggregates also contributes to higher workability of concrete.
6. Grading of Aggregates :
This is an important factor which has maximum influence on the workability of concrete. A well graded aggregate has the least amount of voids in a given volume, thus when the total voids are less an excess paste is available to give better lubricating effect.
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With the excess amount of paste the mixture becomes cohesive and fatty which prevents segregation of particles. Thereby the aggregate particles will slightly pass each other with the least amount of compacting efforts. Thus the better the grading of aggregate the lesser would be void content and higher the workability of the concrete.
7. Use of Admixtures :
The use of admixtures also have a greater influence on the workability of concrete. In general the plasticizers and super plasticizers greatly improve the workability of the concrete. Use of air-entraining agent being surface active reduces the internal friction between the particles the air bubbles act as a sort of ball bearing between the particles and give easy mobility to the particles.
Similarly the use of fine glossy Pozzolanic materials increase the surface area and offer better lubricating effects for providing better workability of the concrete.
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