Workability of concrete is measured in terms of the ease of mixing and placing of concrete. Highly workable concrete can easily be mixed, placed and transported. All the materials and processes involved in producing concrete affect the workability of concrete.
Factors Affecting Workability of Concrete
Following factors affect the workability of concrete.
- Water Content
- Mix Proportions
- Size of Aggregates
- The Shape of Aggregates
- Grading of Aggregates
- The 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.
Rich concrete mix (cement content is high) is more workable because due to sufficient cement aggregates will have proper lubrications for easy movement which means more workability.
Size of Aggregates
Finer particles require more water for a larger surface, hence aggregate with finer particles need more water to make it workable. On the other hand, bigger particles have less surface area, demand less water for wetting surface and require less amount of paste for lubricating. So bigger particles give higher workability for fixed water content. But maximum size of aggregate depends on some practical considerations like handling-mixing and placing equipment, concrete section, and spacing of reinforcement.
The 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 sub rounded 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
Well-graded aggregates tend to fill up voids and easily get workability. Less amount of water can make it workable. If grading is better, there will be fewer voids and excess paste will be available to give better lubricating effect. Due to excess paste, the mixture gets cohesive and prevent segregation. It also makes it get compacted easily i.e. increases the workability.
The 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 Admixtures
There are some admixtures which can improve workability. Some admixtures are mixed intentionally to increase workability and some admixtures increase workability as a side effect of its main purpose.
Use of Supplementary Cementitious Materials
There are many supplementary materials used for improving quality of fresh concrete. Some of these, like fly ash, improve workability and some of these like steel or synthetic fibers decrease workability.
Fresh concrete stiffens with time and loss workability though it is not exactly settling or getting strength at all. After mixing concrete, some water is absorbed by aggregate, some may be lost by evaporation and some may be spent for initial chemical reactions. 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.