Workability and methods to control it

Workability is the ease with which fresh concrete can be mixed, placed, compacted and finished without segregation.

Methods to control workability

1. Water content: increase/decrease the water, keeping w/c fixed by changing cement as well.
2. Cement paste content: more paste (cement + water) improves workability.
3. Aggregate grading, shape and size: well-graded, rounded and larger aggregates improve it.
4. Ratio of fine to coarse aggregate: adjust the sand percentage.
5. Admixtures: plasticisers, superplasticisers, air-entraining agents (increase), accelerators (reduce slump), mineral admixtures (fly ash increases).
6. Temperature control of the mix (cool ingredients).
7. Mixing time and method, and delivery time.

Explained: use of admixtures (plasticiser / superplasticiser)

The initial mix has a given w/c for the required strength, e.g. 0.45. A plasticiser is added (0.5-2% of cement mass) to raise the slump from 50 mm to 150 mm without adding water. The admixture molecules adsorb on cement particles, giving them the same charge, so they repel each other and the flocs break up, releasing trapped water. Thus the mix flows easily at the same w/c, strength and durability are kept, and cement is not wasted. The slump is checked at the site, and the dose is adjusted (the effect lasts 30-60 min). The alternative of adding water would only raise w/c and lower strength.