Workability is a parameter that a mix designer is required to specify in the mix design process. A concrete which has high consistency and which is more mobile need not be of right workability for a particular job.

Every construction work of concrete requires a particular workability. A concrete that is considered workable when used in the thick section is not workable when considered its use in thin section. Workability of concrete depends on the:

  • Ø Thickness of the section
  • Ø Extent of reinforcement
  • Ø Mode of compaction

Different concrete mixes with same consistency can have different workability characteristics. The presence of excess mixing water, undermixing and overmixing are common causes of many problems that are related to the physical properties of the concrete in its fresh state and the hardened state.
According to ASTM, workability can be defined as the “Property determining the effort that is required to manipulate a freshly mixed quantity of concrete with minimum loss of homogeneity”. Workability is necessary to compact concrete to the maximum possible density.

Need For Workability in Concrete

The strength of Partially compacted concrete compared to that of fully compacted concrete in terms of strength ratio.

RStrength = fck of partially compacted concrete / fck of fully compacted concrete

In terms of Density Ratio

Rdensity = Density of partially compacted concrete/density of Fully compacted concrete

With the reduction in the density, the voids will increase.

The figure below shows the relation between the density ratio and the strength ratio. From the figure, it is clear that the strength ratio decreases with the decrease in the density ratio.This indirectly shows that the strength reduction is due to the increase in the voids (lower density). The presence of voids about 5% may reduce the strength by as much as 30%.

Fig.1. Relationship between the Strength Ratio and the density Ratio 

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