FACTORS AFFECTING WORKABILITY OF CONCRETE - REQUIREMENTS OF WORKABILITY

The main factors that affect the workability of concrete are explained briefly in the below section:


1. Water Content
2. Mix Proportion
3. Size of Aggregate
4. Shape of Aggregate
5. Surface Texture
6. Grading
7. Admixtures
8. Effect of Environmental Conditions
9. Effect of Time

Water Content

Higher the water content, more will be the fluidity of concrete mix, which will influence the workability. More water can be added, provided a corresponding higher quantity of cement also to be added to keep the water-cement ratio (w/c) on a constant.


Mix Proportions

Higher the aggregate-cement ratio, leaner will the concrete mix. In the rich mix (lower aggregate-cement(a/c) ratio) more paste is available in order to make the mix cohesive and faulty to give better workability. In a leaner mix, less quantity of cement is available for the lubrication per unit surface area of aggregate and hence the mobility of aggregate in the concrete mix is restrained.

Size of Aggregate

The bigger the size of the aggregate, the lesser is the surface area and hence lesser amount of water is required for wetting the surface and less paste is required for lubricating the surface to reduce the internal friction. For a given quantity of water and paste, bigger aggregates will give higher workability.

Shape of Aggregate

Angular, elongates or flaky aggregates make the concrete very harsh when compared to rounded aggregates. For rounded aggregates, the frictional resistance is reduced.

Surface Texture

The roughly textured aggregate will show poor workability and smoothly textured aggregates show good workability. The reduction of interparticle frictional resistance offered by the smooth aggregates also contributes to higher workability.

Grading

A well-graded aggregate has least voids in a given volume and a higher rate of workability. Better grading prevents segregation

Admixtures

The use of admixtures reduces the internal friction between the particles and improves workability.

Effect of Environmental Condition

The workability of concrete mix is also affected by the temperature of concrete and hence by the ambient temperature. On a hot day, it becomes necessary to increase the water content of the mix in order to maintain the desired workability. The amount of mixing water that is required to bring certain change in the workability increases with the temperature.

Effect of Time

The fresh concrete loses workability with time mainly because of lots of moisture due to evaporation. A part of mixing water is absorbed by aggregate or lost by evaporation in the presence of sun and wind, and part of it is utilized in the chemical reaction of hydration of the cement.

Loss of workability varies with the type of cement of concrete mix proportion, temperature, and the initial workability.


Requirements of Workability

The basic requirements of workability are:

Ø The workability of fresh concrete should be such that it can be placed in the formwork and compacted with minimum effort without bleeding and segregation.
Ø The choice of workability depends on the type of compacting equipment available, the size of the section and the concentration of the reinforcement.
Ø When compaction factor is less than 0.85, the compaction by hand using tamping and Roding is not possible.
Ø The insufficient workability may result in incomplete compaction thereby severely affecting the strength, durability and the surface finish.
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