Factors Affecting Workability

Factors Affecting Workability

The factors helping concrete to have lubricating effect (workability) to reduce internal friction, for helping easy compaction are listed below;

(a)  Water content

(b)   Mix proportion

(c)  Size of aggregate

(d)   Shape of aggregate

(e)  Surface texture of aggregate

(f)   Grading of aggregate

(g)   Use of admixtures.

(a)   Water Content

The basic factor affecting the workability is water content which is expressed in litres of water per cubic meter of concrete. The higher the water content, higher will be the fluidity of concrete but subject to limitation of water -cement ratio. More water can be added, provided a correspondingly higher quantity of cement is also added to keep the w/c ratio same so that the strength of concrete remains unchanged.

(b)   Mix Proportion

Aggregate/cement ratio is the inversely proportion to the workability. With lower aggregate/cement ratio (rich concrete), more paste is available to make the mix cohesive and fatty to give better workability. On the other hand, the higher aggregate/cement ratio (leaner concrete), less quantity of paste is available to provide lubrication and thereby lowers workability.

(c)  Size of Aggregate

For the given quantity of water and paste, bigger size of aggregate will give higher workability because of reduction of total specific surface area and inter -particle frictional resistance.

(d)   Shape of Aggregate

The shape of aggregates influences the workability greatly. Angular/elongated/flaky aggregate makes the concrete very harsh whereas rounded aggregate enhances the workability in good measure because of reduction in total specific surface area and inter - particle frictional resistance for a given volume or weight. This explains the reason why river sand and gravel provide greater workability to concrete than crushed sand (crushed aggregate).

(e)  Surface Texture of Aggregate

Rough textured aggregate will exhibit poor workability whereas smooth or glossy textured aggregate will impart better workability because of reduction of inter -particle frictional resistance offered by smooth surface thereby contributing higher workability.

(f) Grading of Aggregate

A well graded aggregate has lesser amount of voids in a given volume. Other factors being constant, if the total voids are less, excess paste is available to give better lubricating effect. The mixture becomes cohesive and fatty preventing segregation of particles. Therefore, better is grading, less is the void content and higher is the workability.

(g)  Properties of Cement

The workability is also affected by the physical and chemical properties of cement, but to a much lesser extent. Rapid -hardening cement will have reduced workability as compared to ordinary portland cement because of higher specific surfaces.

(h)   Use of Admixtures

The presence and nature of admixtures affect the workability considerably. The use of the plasticizers and super plasticizers improve the workability manifolds. Use of air - entraining agents which are normally surface-active, reduces the internal friction between the particles. The air –entraining agents like resin, soap or chemicals introduce a large number of very minute air bubbles which act as rollers and increase workability. They decrease bleeding and segregation also.