Specific Gravity and Absorption on a Porous Aggregate
A technician runs bulk specific gravity and absorption on a 1,000 g sample of lightweight/porous coarse aggregate per the standard procedure. The saturated surface-dry (SSD) mass is 1,050 g, oven-dry mass is 980 g, and the submerged (apparent) mass is 610 g. The technician computes bulk SSD specific gravity as 1,050/(1,050−610)=2.39 and absorption as (1,050−980)/980=7.1%. A second run gives absorption of 6.2%. The aggregate is intended for structural concrete where the specification caps absorption at 5%. The technician's towel-drying step took under 30 seconds and the surface still looked glossy in places.
1. Check the calculations and identify the likely source of run-to-run variability.
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Bulk SSD SG = B/(B−C) = 1050/(1050−610) = 1050/440 = 2.386, correct arithmetic. Absorption = (B−A)/A × 100 = (1050−980)/980 × 100 = 7.14%, correct. The variability (7.1% vs 6.2%) is most likely from improper SSD surface drying. A glossy surface means excess surface water remains, inflating B and thus absorption. The SSD condition requires a damp, matte surface with no visible film of water, achieved by rolling in an absorbent towel; porous/lightweight aggregate needs careful, consistent drying. Under 30 seconds with gloss indicates incomplete drying.
2. Does the aggregate meet the 5% absorption cap? What would you do before declaring failure?
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Both runs exceed 5% (7.1% and 6.2%), but the method's precision and the observed variability mean a single pair of runs is not enough to declare nonconformance, especially with a known SSD-drying problem. Repeat the test with proper SSD conditioning (roll in towel until matte, no gloss, consistent technique), run duplicates, and average. If the properly conditioned result still exceeds 5%, report nonconformance. Document the SSD method and any deviation. For porous aggregate, also consider whether the specification's absorption limit is appropriate for the intended use.
3. The technician also reports apparent specific gravity. How is it computed and why does it differ from bulk?
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Apparent SG = A/(A−C) = 980/(980−610) = 980/370 = 2.65. Bulk (oven-dry) SG = A/(B−C) = 980/440 = 2.23. Apparent SG excludes the permeable pores (uses oven-dry mass over oven-dry minus submerged mass), so it is higher than bulk SG, which includes permeable pore volume. The difference reflects the aggregate's porosity and is consistent with the high absorption.