Workability
The workability is defined as the case in which fresh concerete can be placed and compacted fully without segregation.
Workability should be considered as an important physical property of fresh concerete alone without reference to method of compaction and particular type of construction.
Factor affecting workability
The factor affecting the workability of concerete are as follows.
(1) Water Content
(2) Shape of Aggregates
(3) Size of Aggregates
(4) Mix Proportions
(5) Surface Texture of Aggregates
(6) Grading of Aggregates
(7) Use of Admixtures
(8) Time
(9) Temperature
(1) Water content
Adding more water to the concrete increases workability but more water also increases the potential for segregation (settling of coarse aggregate particles), increased bleeding, drying shrinkage and cracking in addition to decreasing the strength and durability.
(2) Shape of aggregate
Rounded and half rounded shape aggregate are more workable than rough, angular or flaky aggregate, because the Rounded and half rounded type of aggregate requires less cement paste for the lubrication as these have less surface area and lesser voids .
(3) Size of aggregate
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) Mix proportion
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
(5) Surface texture
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.
(6) Grading of aggregate
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.
(7) Admixture
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.
(8) Time
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:
(a) Initial workability: if initial workability is high, slump loss will be greater
(b) Property of cement: if alkali content is high and sulfate content is low, sump loss will be greater
(c) Moisture content of aggregate: dry aggregate will absorb more water and workability will decrease
(9)Temperature
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
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