Workability and its impact on quality

Workability is the ease with which a concrete mix can be mixed, transported, placed, compacted and finished with the minimum loss of homogeneity (IS 6461 Part 7). It includes three components: mobility (flow), stability (resistance to segregation and bleeding) and compactability (ability to be fully compacted). It depends on water, cement paste, aggregate grading, shape and size, and admixtures; it is measured by slump, compaction factor or Vee-Bee tests (IS 1199).

Impact on the quality of concrete

• Low workability (too stiff): the mix is difficult to compact, entrapped air and voids remain, causing honeycombing, poor bond to the steel and low strength (1% of voids reduces strength by about 5-6%). Congested reinforcement is not filled.
• Too high workability (excess water): causes segregation and bleeding, laitance, a high w/c ratio with lower strength, higher permeability, more shrinkage and cracks, and poor durability.
• Suitable workability gives full compaction, uniform density and homogeneity, a good surface finish, good bond with reinforcement and the strength designed for, with the lowest labour and energy cost.

 Quality
   |        ____
   |      /      \
   |     /        \   <- optimum workability
   |    /          \
   +----------------> workability
  honeycombs   segregation

Therefore the right workability for the type of structure (IS 456 cl. 7.1; e.g. slump 25-75 mm for lightly reinforced sections, 50-100 mm for heavily reinforced sections) should be provided, using admixtures instead of extra water.