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.
Followings are the factors affect the workability of concrete.
- Water Content
- Aggregate/Cement Ratio
- Mix Proportions
- Size of Aggregates
- Shape of Aggregates
- Grading of Aggregates
- Surface Texture of Aggregates
- Use of Admixtures
- Use of Supplementary Cementitious Materials
It is the most important factor of workability. Workability increases with the increase of water content (measured in kg or liter per cubic meter of concrete). We can express the relation in terms of water-cement ratio. If the water-cement ratio is small, it indicates high amount cement which is helpful for good strength. But the small water-cement ratio is responsible for lower workability. If proper compaction cannot be achieved, concrete will not be enough strong as desired. On the other hand, if the water-cement ratio is increased, workability and compaction problem will be solved but there may occur some other problems like bleeding and losing compressive strength. Hence an optimum water-cement ratio has to be maintained to balance workability and strength of concrete.
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.
Rich concrete mix (cement content is high) is more workable because due to sufficient cement aggregates will have proper lubrication for easy movement which means more workability.
Size of Aggregates
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.
Shape of Aggregates
Irregular shape and rougher texture of angular aggregate demand more water than the round shaped aggregate. For fixed volume or weight, rounded or subrounded particles have less surface area and less void and they have less friction resistance too. Hence round shaped aggregates show higher workability than angular, flaky or elongated aggregates.
Grading of Aggregates
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.
Surface Texture of Aggregates
Aggregates with smooth surfaces are more workable than roughly textured aggregates. Roughly textured aggregates show high friction and segregation tendency. Besides, nonabsorbent aggregates are more workable because porous and non-saturated aggregates demand more water than aggregates which are nonabsorbent.
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.
The loss in workability by time depends on various factors like:
- Initial workability: if initial workability is high, slump loss will be greater
- Property of cement: if alkali content is high and sulfate content is low, sump loss will be greater
- Moisture content of aggregate: dry aggregate will absorb more water and workability will decrease
High temperature reduces workability and increases slump loss. Slump loss is less influenced by temperature in stiff mixes because this type of mix is less affected by a change in water content.