Workability of Concrete:
Workability is the amount of work to produce full compaction.
The important facts in connection with workability are:
(i) If more water is added to attain the required degree of workmanship, it results in the concrete of low strength and poor durability.
(ii) If the strength of concrete is not to be affected, the degree of workability can be obtained:
By slightly changing the proportions of fine and coarse aggregates, in case the concrete the mixture is too wet; and
By adding a small quantity of water-cement paste in the proportion of the original mix, in case the concrete mixture is too dry.
(iii) The workability of concrete is also affected by the maximum size of the coarse aggregates to be
used in the mixture.
(iv) The workability of concrete is affected mainly by water content, water-cement ratio, and aggregate-
Factors Affecting Workability:
Following are the factors which affect the workability of concrete,
(a) Water content
(b) Mix proportions
(c) Size of aggregates
(d) Shape of aggregates
(e) Surface texture of aggregates
(f) Grading of aggregates
(g) Use of admixtures
(a) Water Content/Water Cement Ratio
- Water content in a given volume of concrete will have a significant influence on workability.
- The higher the water content per cubic meter of concrete, the higher will be the fluidity of concrete which is one of the important factors affecting workability.
- According to Abram’s law, the strength of workable concrete is only dependent on water-cement
- The volume of water in fresh concrete is related directly to the volume of empty pore space in hardened concrete. Similarly, the volume of cement in fresh concrete is related directly to the solid volume in hardened concrete. The water-cement ratio is therefore a measure of the void volume relative to the solid volume is hardened cement paste, and its strength goes up as the void volume goes down. So, the lower the water-cement ratio, the lower the void volume-solid volume ratio and the stronger the hardened concrete.
- In a hardened state concrete, strength is inversely proportional to the water/cement ratio as shown in the graph.
- The reason why the compressive strength concrete does not actually follow a hyperbolic curve at a lesser water-cement ratio is when the water to cement ratio is low in a fresh mix, then less water is available for the hydration of cement. Hence, some amount of cement paste remains un-hydrated.
- The strength of concrete much dependent on the following four factors:
1. Water to cement ratio
2. Cement to aggregate ratio
3. Maximum aggregate size
4. Physical properties of aggregates
NOTE: The factors (2, 3, and 4) are of lesser importance while factor (1), is the major influencing factor.
- The strength of concrete decreases with an increase in the percentage of air voids where air voids are formed by evaporation of water used in making concrete and by entrained air.
- As a cement age increases its strength reduces since it gets set partially by the absorption of moisture and forms small lumps.
- When the uniaxial compressive load is applied where the line of action of the load is perpendicular to the axis of the cube then load carrying capacity is assumed to be maximum.
- By increasing the cement to aggregate ratio the ultimate strength will increase up to some extent.
- The crushed aggregate gives maximum strength as it offers minimum voids in concrete.
- The rounded spherical or cubical-shaped aggregate when compacted contains fewer voids than an irregular and flaky aggregate of the same nominal size and they give more strength.
- The strength order according to the type of aggregate as:
- Crushed aggregate > 2. Cubical aggregate> Rounded aggregate > Flaky/irregular aggregate
- Inadequate compaction leading to air void contents of 5 percent and of 10 percent results in a loss of strength of 30 percent and 55 percent respectively.
(b) Mix Proportions/Aggregate Cement Ratio:
- The aggregate/cement ratio is an important factor influencing workability.
- The higher the aggregate-cement ratio, the leaner the concrete.
- In lean concrete, less quantity of paste is available for providing lubrication per unit surface area of aggregate and hence the mobility of aggregate is restrained.
- In the case of rich concrete with a lower aggregate-cement ratio, more paste is available to make the mix cohesive and fatty to give better workability.
(c) Size of Aggregate
- The bigger the size of the aggregate, the lesser the surface area, and hence less amount of water is required for wetting the surface, and less matrix or paste is required for lubricating the surface to reduce internal friction.
- For a given quantity of water and paste, a bigger size of aggregates will give higher workability.
(d) Shape of Aggregate
- The shape of aggregates influences workability to a large extent.
- Angular, elongated, or flaky aggregate makes the concrete very harsh when compared to rounded aggregates or cubical shape aggregates.
- Rounded aggregate will have less surface area and fewer voids than angular or flaky aggregate, not only that, but being round in shape, the frictional resistance is also greatly reduced.
- Because of the above reason, river sand and gravel provide greater workability to concrete than crushed aggregate and sand.
(e) Surface Texture
- The influence of surface texture on workability is again due to the fact that the total surface area of rough-textured aggregate is more than the surface area of a smooth, rounded aggregate of the same volume.
- Smooth or glassy textured aggregate will give better workability.
(f) Grading of Aggregates
- This is one of the factors which will have maximum influence on workability.
- A well-graded aggregate is the one that has the least amount of voids in a given volume
- Better the grading, the lesser the void content, and the higher the workability.
- In order to measure the workability of concrete mixture, various tests are developed. Workability tests such as flow test, Vee-Bee test, and compaction factor test are used to great extent in the laboratory. The slump test is commonly used in the field.