Measures of workability; role of w/c ratio

Measures of workability (IS 1199)

• Slump test: slump in mm of a cone-moulded sample (25-125 mm).
• Compaction factor test: ratio of the weight of partially compacted concrete to that of fully compacted concrete (0.7-0.95).
• Vee-Bee test: time in seconds for the slump-cone concrete to remould into a cylinder under vibration (for dry mixes).
• Flow table test: spread diameter of concrete on a table after 15 drops (flowing concrete).
• Kelly ball penetration test, remoulding test, and for SCC: slump flow, L-box and V-funnel.

Role of the water-cement ratio

• Workability: a higher w/c gives a more fluid, workable mix. For a fixed cement content, slump changes directly with the water content (roughly 25 mm of slump for about 3% change in water).
• Strength: strength falls as w/c rises (Abrams' law, $f = A/B^{w/c}$), because excess water leaves capillary pores.
• Durability: a high w/c gives high permeability, so the ingress of chlorides, sulphates and water causes corrosion and frost damage (IS 456 Table 5 limits the w/c for RCC to 0.55, 0.50, 0.45, 0.45 and 0.40 for mild, moderate, severe, very severe and extreme exposure). Too low a w/c (below about 0.35) gives poor workability and incomplete compaction unless a superplasticiser is used.

So the w/c should be as low as full compaction permits.