Workability and its measurement

Workability is the property of freshly mixed concrete that determines the ease and homogeneity with which it can be mixed, placed, compacted and finished (IS 6461 Part 7). It combines mobility, stability (resistance to segregation and bleeding) and compactability. IS 456 cl. 7.1 gives the workability required for various placing conditions.

Methods of measurement (IS 1199)

1. Slump test (IS 1199; field and laboratory): a cone 300 mm high, with 100 mm top and 200 mm bottom diameter, is filled in four layers of equal height, each tamped with 25 strokes of a 16 mm rod. The cone is lifted vertically, and the slump is the settlement of the concrete from the top of the cone. A true slump is recorded; if a shear slump occurs, the test is repeated; a collapse indicates a very wet mix. Range: 25-125 mm. Not suitable for very dry or very wet mixes. IS 456: slump 25-75 mm for lightly reinforced sections, 50-100 mm for heavily reinforced sections.

2. Compaction factor test (laboratory; useful for low workability, CF 0.7-0.95): concrete falls through two hoppers into a cylinder; weigh the partially compacted concrete ($W_1$), then the fully compacted concrete ($W_2$):

$$\text{CF} = \frac{W_1}{W_2}$$

3. Vee-Bee test (very dry mixes): the time in seconds (Vebe seconds) needed to remould the slump-cone concrete into a cylinder under vibration; about 5-30 s.

4. Flow table test (IS 1199): a 760 mm table is lifted 12.5 mm and dropped 15 times; the average spread diameter gives the flow %; used for flowing concrete.

5. Kelly ball test: penetration of a 15 cm diameter hemisphere into fresh concrete (field).

6. Others: remoulding test, K-slump test, and for SCC the slump flow, L-box, V-funnel and J-ring tests.

Slump and Kelly ball are used at site, while the compaction factor, Vee-Bee and flow table tests are laboratory tests.