Specific Gravity, Water Absorption & Bulk Density — Part 3
Part 3 covers specific gravity, water absorption, bulk density and voids, and bulking of fine aggregate. Specific gravity is required by the IS 10262 absolute-volume mix design; water absorption corrects the effective mix water (SSD basis); bulk density converts volume↔mass; and bulking of damp sand (large volume increase at a few % moisture) wrecks volume batching if uncorrected.
Key Requirements
•Specific gravity (SSD/apparent) for IS 10262 absolute-volume mix design
•Water absorption to correct mix water to the saturated-surface-dry (SSD) basis
•Bulk density and voids (uses calibrated measures, IS 10079) for volume↔mass conversion
•Bulking of fine aggregate: damp sand bulks substantially — correct volume-batched sand or batch by mass
•Use a representative sample (IS 2430)
Formulas
free moisture = total moisture − water absorption (mix-water correction)
Effective mix water uses the SSD basis: surplus free moisture is deducted from added water
total moisture = aggregate moisture content (%)water absorption = absorption at SSD (%)
Practical Notes
✓Uncorrected aggregate moisture/absorption is the most common cause of an over- or under-watered batch — the mix W/C drifts silently.
✓Bulking: damp sand can bulk ~20–30% in volume at a few % moisture — volume-batched sand without a bulking correction badly mis-proportions the mix.
Common Mistakes
⚠Designing the mix without measured specific gravity (assuming a generic value).
⚠Not correcting mix water for aggregate moisture/absorption.
⚠Volume-batching damp sand without a bulking correction.