What is Workability of Concrete:
Workability of concrete is the ease, with which concrete mixes can be mixed, transported, placed in formwork, and compacted completely as possible while using the lowest possible water/cement ratio. It is known as workability of concrete.
In the workable concrete mix, the function of water is to lubricate the concrete so that the concrete can be compacted with minimum effort at the site of work. The care should be taken that the concrete may not experience segregation, loss of homogeneity, compaction with a minimum amount of effort and finish easily; the presence of a certain quantity of water is of vital importance.
Workability is also the ability of fresh concrete to fill formwork properly with a sufficient amount of vibration and without reducing the concrete’s quality.
The fresh concrete workability mainly depends on water content, cement content, aggregate particle size distribution and amount of hydration and can be modified by adding chemical admixtures, like superplasticizer
Factors Affecting Workability:
- Water Content
- Mix Proportions
- Size of Aggregates
- Shape of Aggregates
- Surface Texture of Aggregate
- Grading of Aggregate
- Use of Admixtures.
1) Water content or Water Cement Ratio:
As we increase water-cement ratio workability increases simultaneously. But, the excessive water content may also result in bleeding of concrete mix. Moreover, high water content provides loose cement slurry which is capable of escape through the formwork joint.
As we increase water content, the strength of concrete gets reduced. To avoid this and to keep water-cement ratio constant add a sufficient amount of cement also while increasing water content in the concrete mix so that the strength remains the same.
Sometimes when concrete becomes too loose or high workability. Cement can be added to it to make concrete of good workability.
2) Mix Proportions:
When concrete mix contents higher amount of aggregate or aggregate/cement ratio is greater, less cement slurry is available to make concrete lubricant or workable. On another side, if the concrete is having lower aggregate/cement ratio, more paste is available to make the mix cohesive and fatty to give better workability
3) Size of Aggregate & Surface Texture:
The smaller the size of aggregate particle higher will the surface area of aggregate and which requires a greater amount of cement paste to lubricate them. So, smaller size aggregate needs more water-cement ratio to get good workability. On other hand bigger the size aggregate requires the less surface area and hence less amount of water is required for wetting the surface and less amount of cement paste require for reducing internal friction between them.
As the aggregate size is high low water requires lubricating it, hence the extra water is available for workability. In case of porous aggregate more water is required to lubricate them compare to nonporous aggregate for achieving the same degree of workability.
4) The shape of Aggregates:
It is a fact that concrete mix with rounded aggregate has greater workability with less water-cement ratio compare to angular, elongated or flaky aggregate. This shape of aggregate makes concrete harsh and difficult to handle.
The rounded aggregate offers better workability due to the fact that for the given volume or weight it has less surface area and fewer voids than the angular or flaky aggregate. Moreover, because of their rounded shape, the frictional resistance is also greatly reduced. This is a major reason for river sand and gravel cost more than crushed.
5) Grading of Aggregates:
Graded aggregates are those aggregate which has least amount voids in a given volume and that offers higher workability to concrete. As the number of voids reduced, the excess paste is available to lubricant the particles. With this excess amount of paste concrete mix becomes cohesive and fatty which prevents segregation of particles.
6) Use of Admixtures:
Workability of concrete can be increased by adding chemical admixture. As a chemical admixture air-entraining agent is used, which acts as ball-bearing between particles and increases workability, mobility and decreases bleeding, segregation. For better lubricating effect and more workability, pozzolanic materials can be used.
7) Weather Conditions:
The weather condition has a significant effect on the workability of concrete. As the temperature increase, the evaporation process becomes rapid and concrete loose water content quickly. As the water content in concrete reduces, workability also gets reduced. In an area, if the velocity of wind is high water in the concrete evaporates quickly and that results in lower workability of concrete.
Methods of Concrete Workability Measurement:
b) Compacting Factor Test
c) Flow Test
d) Vee Bee Consistometer Test