Physical Property Tests: Specific Gravity, Absorption, Unit Weight, Voids
Specific gravity and absorption
Specific gravity compares the mass of aggregate to the mass of an equal volume of water. Know three variants: bulk specific gravity (includes permeable pores), bulk specific gravity (SSD) (saturated surface-dry basis), and apparent specific gravity (excludes permeable pores). Absorption is the water absorbed into permeable pores, expressed as a percentage of dry mass. The test sequence: soak, determine SSD mass, determine submerged (buoyant) mass, then oven-dry and weigh. Traps include: not achieving a true saturated surface-dry condition (too wet or too dry), and mixing up which mass goes in the numerator of each formula.
Unit weight and voids
Unit weight (bulk density) is covered in the density domain, but here the link to specific gravity matters: voids content is derived from bulk density and bulk specific gravity. Candidates should know that voids reflect packing, not porosity within particles.
Worked example
A sample has dry mass 480 g, SSD mass 500 g, and submerged mass 310 g. Bulk SG = 480/(500 − 310) = 480/190 ≈ 2.53. Bulk SSD SG = 500/190 ≈ 2.63. Apparent SG = 480/(480 − 310) = 480/170 ≈ 2.82. Absorption = (500 − 480)/480 × 100 ≈ 4.2%.
Traps
Watch for: using the wrong mass in the denominator (the submerged-mass difference is the volume of water displaced), forgetting to correct for water temperature, and confusing absorption with moisture content. Absorption is a saturated property; moisture content is an as-received property.