How to Calculate Aggregate Specific Gravity
Comparing an aggregate's density to water's, and why it matters for mix design
What specific gravity means
Specific gravity — called relative density in current ASTM standards — is the ratio of an aggregate's mass to the mass of an equal volume of water. An aggregate with a specific gravity of 2.65 is 2.65 times as heavy as the same volume of water. Because it's a ratio, it has no units and doesn't change with the sample size.
It isn't the same as bulk density (unit weight). ASTM C127 distinguishes the density of the aggregate particles from the bulk density measured under ASTM C29, which includes the voids between particles. Specific gravity describes the particles; bulk density describes a measured volume of them.
Three specific gravities and absorption
A coarse aggregate test gives three specific gravity values and an absorption, all from three masses: A, the oven-dry mass; B, the saturated surface-dry (SSD) mass in air; and C, the mass of the SSD sample in water. B − C is the mass of water the sample displaces.
| Value | Formula | What counts as the particle |
|---|---|---|
| Bulk specific gravity (oven-dry) | A ÷ (B − C) | Solid plus permeable pores, dry mass — the lowest of the three |
| Bulk specific gravity (SSD) | B ÷ (B − C) | Solid plus permeable pores, mass including the water in them |
| Apparent specific gravity | A ÷ (A − C) | Solid material only, not the pores water can reach — the highest |
| Absorption | (B − A) ÷ A × 100 | Water held in the permeable pores, as % of the dry mass |
How the coarse aggregate test works
The coarse aggregate method (ASTM C127, AASHTO T 85) tests material retained on the No. 4 (4.75 mm) sieve. In outline, the washed sample is dried, soaked in water for the period the method sets, rolled in a large absorbent cloth until all visible films of water are removed, and weighed (B). It is then weighed in water in a wire basket (C), dried to constant mass and weighed again (A).
The WAQTC field procedure for AASHTO T 85 has larger particles wiped individually, and if the sample dries past SSD it is put back in water for 30 minutes before surface-drying resumes. B appears in three of the four formulas, so a sample left too wet or dried too far at this step shifts most of the results.
Fine aggregate is tested under a separate method, ASTM C128, using a pycnometer (a calibrated flask). The Aggregate Specific Gravity Calculator covers the coarse aggregate method only.
Which value to use
ASTM C127 says relative density is used to calculate the volume aggregate occupies in mixtures proportioned or analysed on an absolute-volume basis, including hydraulic cement concrete and bituminous concrete. The SSD value is used when the aggregate is saturated surface-dry — its absorption has been satisfied — and the oven-dry value when it is dry or assumed to be dry.
Absorption is used to calculate how much an aggregate's mass changes as its pores take up water. Stockpiled aggregate is rarely exactly SSD: C127 notes that aggregate mined from below the water table often holds more moisture than the test's absorption, and aggregate that hasn't been kept moist often holds less. That difference adds water to, or takes water from, a mix.
Typical values
Pavement Interactive, a secondary technical reference developed by a consortium of state DOTs, FHWA and the University of Washington, gives bulk specific gravities of about 2.400 to 3.000 for asphalt aggregates and absorption from just above zero to about 5%. Treat these as orientation only; the mix design uses your test result.
Worked Example
A coarse aggregate sample weighs 2,030 g saturated surface-dry in air, 1,265 g in water, and 2,000 g after oven-drying.
Water displaced: B − C = 2,030 − 1,265 = 765 g
Bulk specific gravity (oven-dry) = 2,000 ÷ 765 = 2.61
Bulk specific gravity (SSD) = 2,030 ÷ 765 = 2.65
Apparent specific gravity = 2,000 ÷ (2,000 − 1,265) = 2,000 ÷ 735 = 2.72
Absorption = (2,030 − 2,000) ÷ 2,000 × 100 = 1.50%
FAQ
- Why are there three specific gravity values?
- They treat the pores inside the particles differently. Apparent specific gravity leaves out the pores water can reach; the two bulk values include them, weighed oven-dry or saturated surface-dry. For any sample, bulk (oven-dry) < bulk (SSD) < apparent.
- Is specific gravity the same as unit weight?
- No. Specific gravity describes the particles themselves. Unit weight (bulk density, ASTM C29) is the mass of a measured volume of aggregate including the voids between particles, so it is lower than the particle density.
- What does saturated surface-dry mean?
- The pores inside the particles are filled with water but the particle surfaces are dry. It is the reference condition for absorption and for the SSD specific gravity.
Related Calculators
Related Guides
From the Blog
Sources
ASTM C127
Standard Test Method for Relative Density (Specific Gravity) and Absorption of Coarse Aggregate (current edition C127-25; significance and use read on the ASTM site).
ASTM C127 — referenceASTM C128
Standard Test Method for Relative Density (Specific Gravity) and Absorption of Fine Aggregate.
Pavement Interactive, Coarse Aggregate Specific Gravity (secondary reference)
Secondary technical reference, not a standard: definitions of the three specific gravities and typical values for asphalt aggregates.
Pavement Interactive, Coarse Aggregate Specific Gravity (secondary reference) — referenceWSDOT Materials Manual M 46-01, WAQTC FOP for AASHTO T 85
Field operating procedure for specific gravity and absorption of coarse aggregate: No. 4 sieve, drying, 15-to-19-hour soak, surface-drying with a large absorbent cloth, SSD mass B, mass in water C, oven-dry mass A.
WSDOT Materials Manual M 46-01, WAQTC FOP for AASHTO T 85 — reference