What is Workability of Concrete? Factors Affecting Workability of Concrete

Hi friends, you are welcomed in the Civil Allied Gyan. The field engineers are always looking for how to improve the workability of concrete. Here I have explained about what is the workability of concrete and the factors affecting workability of concrete in detail. First we know about what is the workability of concrete.

What is workability of concrete?

The workability of concrete is the degree of ease which refers how easily the concrete mix can be moved and placed in the framework, consolidated and finished without any segregation.

What are the factors affecting the workability of concrete?

Cement, water, fine aggregates (sand) and coarse aggregates are the primary materials of concrete. Also some admixtures are used in concrete to enhance its properties. Therefore, properties of these materials and their content affect the workability of concrete.

The workability of concrete is affected by type of cement, water-cement ratio, mix design ratio, size shape and grading of aggregates. In addition to these, process of making concrete mix and the materials involved in concrete mixing also affects the workability of concrete.

These factors are briefly discussed below.

Factors Affecting Workability of Concrete

Factors Affecting Workability of Concrete

1. Composition of Cement

What cement is made of, directly affects the workability of concrete. The cement with more fineness requires more water for same workability compare to the cement with less fineness. The presence of aluminium oxide (Al₂O₃) and dicalcium silicate (C₂S) in large quantity increases the water demand to make concrete mix.

2. Cement Content of Concrete

The workability of concrete is also affected by cement content. Higher amounts of cement provide a greater amount of paste in the concrete mix that coats the surface of the aggregates and fills the voids between them. The more cement content helps to reduce friction between the aggregates. This also help in smooth movement of aggregates during mixing, moving, transporting, placing and compacting of concrete.

Also, for a given water-cement ratio, the increase in the cement content will also increases the required water content per unit volume of concrete and finally increases the workability of concrete.

Thus increase in cement content of concrete mix also increases the workability of concrete. While the less amount of cement in concrete mix reduce the workability of concrete.

3. Water Content of Concrete (Water-Cement Ratio)

The water-cement ratio is the most important factor which affects the workability of concrete. Generally, a water-cement ratio of 0.45 to 0.6 is used for good workable concrete mix (without the use of any admixture). As we mentioned above, increasing in the water-cement ratio increases the water content per unit volume of concrete and makes the concrete mix more workable.

Note that (only for nominal mixes):-

For manual concrete mixing, we take higher water-cement ratio to make the mixing process easier. The water-cement ratio can be reduced when the concrete is mixing by concrete mixer machine.

Note that (for designed mix concrete):-

For designed mix concrete, we take low water-cement ratio so that desired strength and durability of concrete can be achieved. The strength and durability of concrete is important for designed mix concrete. So water-cement ratio is mentioned with the mix design.

4. Mix Proportions of Concrete (Aggregate-Cement Ratio)

Mix proportion of concrete is the ratio of fine aggregates and coarse aggregates with respect to quantity of cement. Generally, this is called the aggregate-cement ratio of concrete.

Higher the quantity of aggregates w.r.t. quantity of cement nothing other than the less amount of cement. In this condition, very little paste is made for aggregate and so the workability of concrete mix reduces.

The use of less quantity of aggregates w.r.t. quantity of cement nothing other than the use of more cement. In this condition, a great amount of pest available for aggregates. So aggregates have proper lubrications the workability of concrete mix increases.

5. Size of Aggregates

Surface area of aggregates depends on their size. For the same volume of aggregates, the larger aggregates have less surface area compared to smaller size aggregates.

Lower size aggregates have larger surface area and so require more water and cement quantity to make a paste to cover up the entire surface of aggregates. While larger aggregates have less surface area and require less water and cement quantity to make a paste to cover up the entire surface of aggregates.

Hence, larger aggregates are more workable than the lower sizes of aggregates for fixed water content.

6. Shape of Aggregates

The shape of aggregates is also a factor that affects the workability of concrete. For the same volume or weight, the rounded and subrounded aggregates have less surface area compared to elongated or irregular shaped aggregates.

So it is easy to understand that rounded or subrounded aggregates require less quantity of water than elongated, angular and flaky aggregates for making concrete mix.

So, for fixed water content, rounded or subrounded aggregates are more workable than the elongated or irregular shaped aggregates.

7. Grading of Aggregates

Grading of aggregates is also the most important factor that affects the workability of concrete. Well graded aggregates require less amount of water-cement paste to fill up voids and easily get workability.

If all the sizes are available in the appropriate percentage in the aggregate, then it is called well graded-aggregates. A well graded-aggregate has fewer voids in it. And excess amount of paste will be available to give a better lubricating effect. Thus the better grading of aggregates increases the workability of concrete.

8. Surface Texture of Aggregates

Surface texture of aggregates is another factor that affects the workability of concrete in the same way as the shape of aggregates.

Aggregates with smooth surfaces have less surface area than the aggregates of same volume with rough texture. Roughly textured aggregates show high friction and segregation tendency. Thus the concrete mix with smooth surfaced aggregates is more workable than with rough textured aggregates.

9. Use of Admixtures in Concrete

There are many types of admixtures that are used in concrete for enhancing its properties. There are some admixtures that are used to increase the workability of concrete.

Plasticizers and superplasticizers are such type of admixtures which increases the workability of concrete even with low water/cement ratio. These admixtures reduce the quantity of water required for same value of slump. So they are called as water reducing concrete admixtures.

Also air entraining concrete admixtures are used in concrete mix to increase its workability. It reduces the friction between aggregates by the use of small air bubbles which acts as the ball bearings between the aggregate particles.

10. Use of Supplementary Cementitious Materials

There are many supplementary cementitious materials which are used with cement to improve the quality of fresh concrete. Fly ash, fibers, silica fume and slag cements are commonly used supplementary cementitious materials.

Fly ash is a supplementary cementitious material that is used to improve the workability of concrete by reducing the water content required for same degree of workability or slump value. While synthetic fibers or steel are used to reduce the workability of concrete as it makes the movement of aggregates harder by reducing the lubricating effect of cement paste.

The use of silica fume can increase or reduce the workability of concrete on the basis of its quantity. The use of silica fume in concrete at a low replacement rate can improve the workability of concrete. But it can reduce workability of concrete when added at higher replacement rates. Silica fume are used as pumping aid for concrete mix when it is used as 2 to 3% by mass of cement.

The use of slag cement also improves the workability of concrete but it's effect depends on the characteristics of the concrete mixture in which it is used.

11. Time

Fresh concrete stiffens and loses its workability as time passes though it is not exactly getting set and strength at all. After preparing concrete mix, some water is absorbed by aggregates, some may be lost by evaporation and some may be spent for initial chemical reactions. The loss in workability of concrete by time depends on various factors like:

Initial workability: If initial workability of concrete is high, slump loss will be greater.

Property of cement: If alkali content is high and sulfate content is low in cement, slump loss will be greater.

Moisture content of aggregate: Dry aggregates absorb more water than usual and so workability of concrete mix decreases.

12. Temperature

Temperature also affects the workability of concrete. High temperature causes to reduce workability of concrete and to increase 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.

The workability of concrete is also affected by compaction type (by hand or machine) and humidity of surrounding.

Frequently Asked Questions (FaQs) 

Q. What is the importance of workable concrete in construction?

Ans. For making concrete structures, there is a need for concrete mixes that can be easily mixed, moved, placed, compacted within the formwork of any shape or size and finished with minimal loss of homogeneity. Concrete that meets all these standards is called workable concrete. An unworkable or poorly workable concrete mix cannot easily take the desired shape or size of structure. So it is very important to use the highly workable concrete mix in construction.

Q. How to increase workability of concrete?

Ans. Even after reading this, you must have known about the factors affecting the workability of concrete. Now we will know how we can improve the workability of concrete.

These days, the most common method to improve the workability of concrete is the use of chemical admixtures. The use of supplementary cementitious materials materials like slag cement has better effects on workability of concrete. But, the positivity of using admixtures like air entraining is incomparable.

It is noted that workability improvement method depends on the type of structure and particular requirement of the concrete performance.

Q. What is unworkable concrete?

Ans. The concrete mix with too little or too much water content is called unworkable or poor-workable concrete. It is very difficult to make a homogeneous mass of concrete with such a paste.

Note that:- Workability and strength of concrete are inversely proportional to each other. i.e. increasing the workability of concrete decreases the strength of concrete, which affects the durability of concrete.

Q. What type of concrete should one use?

Ans. The concrete mix has enough workability (Neither too nor poor) and has a good strength should be used in making concrete structures.

 

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