Lesson 01 · Aggregate Specific Gravities
It Depends What You
Mean by Volume
Gsb, Gse, and Gsa each measure a different "version" of an aggregate's volume — the difference comes down to which internal pores you decide to count.
Inside a Single Aggregate Particle
Hover over any pore or material to learn more
G
What is Specific Gravity?
Specific gravity is the ratio of a material's density to the density of water. For aggregates, particles contain internal pores — and Gsb, Gse, and Gsa differ only in how those pores are treated.
Permeable pores — connected to the surface. Water saturates them. Asphalt binder is absorbed into them during hot mixing.
Impermeable pores — completely sealed. Nothing penetrates them — not water, not binder.
Solid mineral — the crystalline rock material itself.
Each gravity value asks: "what volume does this particle occupy?" The three values give three different answers depending on which pores are included in that volume.
Gsb — Volume Used for Measurement
Hover pores to see what Gsb does with each type
Gsb
Bulk Specific Gravity
Uses the total external volume of the particle, measured on a Saturated Surface Dry (SSD) sample. Both permeable and impermeable pores fall inside this boundary and are counted in the volume denominator.
Gsb = A / (B − C)
A = oven-dry mass in air
B = SSD mass in air
C = submerged mass in water
B − C = total particle volume (all pores counted)
A = oven-dry mass in air
B = SSD mass in air
C = submerged mass in water
B − C = total particle volume (all pores counted)
Permeable pores — included
Impermeable pores — included (shown blue here — both pore types behave the same way in Gsb)
Gsb is always the lowest of the three values — largest volume in denominator, smallest ratio. Used to calculate VMA (Voids in the Mineral Aggregate) in Superpave mix design.
Test: ASTM C127 / C128
Used for: VMA
Gse — Volume Used for Measurement
Hover each pore — note the partially absorbed permeable pore
Gse
Effective Specific Gravity
During hot mixing, asphalt binder is absorbed into permeable pores — but not necessarily all the way. Some permeable pores are fully absorbed, some only partially. Gse excludes only the binder-filled portion of those pores. The unfilled portions remain as aggregate volume. This is why Gse sits between Gsb and Gsa — permeable pores are neither fully counted nor fully excluded.
Gse = (Pmm − Pb) / (Pmm/Gmm − Pb/Gb)
Back-calculated from Rice test (Gmm)
Pmm = 100% | Pb = binder content % | Gb ≈ 1.02–1.04
Back-calculated from Rice test (Gmm)
Pmm = 100% | Pb = binder content % | Gb ≈ 1.02–1.04
Permeable pores — partially or fully excluded depending on how much binder was absorbed during mixing
Impermeable pores — always included (binder cannot reach them)
Why Gse bridges the gap
Gsb includes the permeable pores within the aggregate particle as part of the aggregate volume.
Gsa ignores those pores entirely and considers only the solid portion of the aggregate.
Gse lies between the two because it excludes the pores that absorb asphalt binder.
Asphalt binder can penetrate some aggregate pores but not all of them. The pores that absorb binder effectively become part of the aggregate structure in the mix.
Because this absorbed binder cannot be measured directly, Gse is determined indirectly by back-calculating from the Rice (Gmm) test using the mixture properties.
Gsb includes the permeable pores within the aggregate particle as part of the aggregate volume.
Gsa ignores those pores entirely and considers only the solid portion of the aggregate.
Gse lies between the two because it excludes the pores that absorb asphalt binder.
Asphalt binder can penetrate some aggregate pores but not all of them. The pores that absorb binder effectively become part of the aggregate structure in the mix.
Because this absorbed binder cannot be measured directly, Gse is determined indirectly by back-calculating from the Rice (Gmm) test using the mixture properties.
Derived from: Rice test Gmm
Used for: Pa
Gsa — Volume Used for Measurement
Hover each pore to see what Gsa does with it
Gsa
Apparent Specific Gravity
Uses only the volume of solid mineral material plus sealed impermeable pores. Permeable pores are excluded because the measuring water fills them — they effectively disappear from the measurement.
Gsa = A / (A − C)
A = oven-dry mass in air
C = submerged mass in water (oven-dry sample)
A − C = Vsolid + Vimpermeable only
A = oven-dry mass in air
C = submerged mass in water (oven-dry sample)
A − C = Vsolid + Vimpermeable only
Permeable pores — excluded (water fills them)
Impermeable pores — included (water can't reach)
Gsa is always the highest of the three — smallest volume means largest ratio. Nearest approximation to the true mineralogical density, but rarely used directly in Superpave calculations.
Always: Gsb ≤ Gse ≤ Gsa
Typical Gsb Values by Rock Type — Canadian Context
| Rock / Aggregate Type | Typical Gsb | Absorption (%) | Common Regions | Notes for Mix Design |
|---|---|---|---|---|
| Limestone | 2.60 – 2.70 | 0.5 – 2.0 | Ontario, Manitoba, Alberta, PEI | Most common Canadian aggregate; absorption varies with formation porosity |
| Dolomite | 2.68 – 2.85 | 0.3 – 1.5 | Ontario, Quebec, Prairies | Denser than pure limestone; low absorption; excellent resistance to polishing |
| Granite / Gneiss | 2.62 – 2.72 | 0.2 – 0.8 | BC, Atlantic Canada, Shield regions | Low absorption; good angularity from crushing; widely used in surface courses |
| Basalt / Trap Rock | 2.85 – 3.10 | 0.2 – 0.5 | BC, parts of Ontario | Highest gravity; dense volcanic origin; premium skid resistance |
| Quartzite / Sandstone | 2.55 – 2.65 | 1.0 – 3.5 | Prairies, BC Interior | Moderate absorption; check for clay contamination and freeze-thaw durability |
| Slag (Air-cooled ACBFS) | 2.20 – 2.55 | 1.0 – 5.0 | Ontario (steel-producing regions) | Lower gravity; high absorption → significant binder absorption; adjust Pb |
| Recycled Concrete (RCA) | 2.20 – 2.50 | 3.0 – 8.0 | Urban centres, all provinces | Low gravity, high absorption; mortar content drives variability; use cautiously in HMA |
Why Gsb affects VMA directly: A lower Gsb (e.g. slag at 2.35) makes the divisor in the VMA formula smaller, which increases the subtracted term and decreases calculated VMA. In practice, this means a mix may appear to meet the VMA minimum when it does not — the low Gsb flatters the result. A denser aggregate (basalt at 2.95) produces a higher calculated VMA. Always confirm Gsb from the specific quarry source.
High absorption and binder demand: Aggregates like slag and RCA absorb binder into permeable pores during mixing. This absorbed binder (Pba) does not contribute to film thickness or bonding. Failing to account for absorption means your in-place mix will be binder-starved — brittle, permeable, and prone to ravelling.
Same Particle — Three Different Volume Answers
Hover any pore shape in the diagram above
| Symbol | Name | Permeable Pores | Impermeable Pores | Relative Value | Primary Use in Superpave |
|---|---|---|---|---|---|
| Gsb | Bulk | ✓ Included | ✓ Included | Lowest Largest V | VMA calculation; mixture proportioning |
| Gse | Effective | ~ Partial (unabsorbed portion remains; absorbed portion excluded) | ✓ Included | Middle Intermediate V | Pa (air voids); back-calculated from Rice test |
| Gsa | Apparent | ✗ Excluded (water fills them) | ✓ Included (water can't reach; displaces water) | Highest Smallest V | Mineralogy reference; rarely used directly |
The cardinal rule: Gsb ≤ Gse ≤ Gsa
This inequality must always hold. If your back-calculated Gse falls below Gsb, something is wrong — check your Rice test, binder content input, or binder specific gravity. The ordering is your built-in quality check.
This inequality must always hold. If your back-calculated Gse falls below Gsb, something is wrong — check your Rice test, binder content input, or binder specific gravity. The ordering is your built-in quality check.
Worked Example — Calculating Gse, VMA, and Pa from Lab Data
A Superpave mix design uses the following measured values. Walk through each calculation step by step.
Given — Lab Measurements
| Gsb (blended agg.) | = | 2.648 |
| Gmm (Rice test) | = | 2.512 |
| Gmb (compacted spec.) | = | 2.370 |
| Gb (binder SG) | = | 1.030 |
| Pb (binder content) | = | 5.2 % |
| Pmm | = | 100 % |
Step 1 — Back-calculate Gse
Gse = (Pmm − Pb) / (Pmm/Gmm − Pb/Gb)
= (100 − 5.2) / (100/2.512 − 5.2/1.030)
= 94.8 / (39.809 − 5.049)
= 94.8 / 34.760
Gse = 2.727 (Gsb 2.648 < Gse 2.727 ✓)
= (100 − 5.2) / (100/2.512 − 5.2/1.030)
= 94.8 / (39.809 − 5.049)
= 94.8 / 34.760
Gse = 2.727 (Gsb 2.648 < Gse 2.727 ✓)
Step 2 — Absorbed Binder Pba
Pba = 100 × Gb × (Gse − Gsb) / (Gsb × Gse)
= 100 × 1.030 × (2.727 − 2.648) / (2.648 × 2.727)
= 100 × 1.030 × 0.079 / 7.222
Pba = 1.13 % absorbed binder
= 100 × 1.030 × (2.727 − 2.648) / (2.648 × 2.727)
= 100 × 1.030 × 0.079 / 7.222
Pba = 1.13 % absorbed binder
Step 3 — Calculate VMA
VMA = 100 − (Gmb × Ps) / Gsb
Ps = 100 − 5.2 = 94.8 %
VMA = 100 − (2.370 × 94.8) / 2.648
= 100 − 224.676 / 2.648
VMA = 15.16 % (min. target ≈ 14–15 %)
Ps = 100 − 5.2 = 94.8 %
VMA = 100 − (2.370 × 94.8) / 2.648
= 100 − 224.676 / 2.648
VMA = 15.16 % (min. target ≈ 14–15 %)
Step 4 — Calculate Pa (Air Voids)
Pa = 100 × (1 − Gmb / Gmm) = 100 × (1 − 2.370 / 2.512) = 100 × 0.0565
Pa = 5.65 % (Superpave design target at Ndes = 4.0 %)
Pa = 5.65 % (Superpave design target at Ndes = 4.0 %)