How to identify Asphalt Concrete

To correctly identify Asphalt Concrete (Bituminous Macadam), check each of these features in order. Many rocks and minerals look alike, so cross-reference multiple properties before deciding.

Step-by-step identification

1. 1

   Color

   Overall color and any zoning or banding

   Typically dark black to very dark gray, depending on the asphalt content and aggregate color.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to slightly greasy or waxy, especially on freshly fractured surfaces where the asphalt binder is visible.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Rough and granular due to the exposed aggregate particles, with a somewhat sticky or tacky feel if the asphalt binder is still fresh or warm. The texture can vary significantly based on the aggregate size and gradation.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Amorphous (asphalt binder) with angular to sub-rounded aggregate particles. No crystalline form.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None. Fractures irregularly through the aggregate and binder.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Not found in natural geological environments. It is an entirely anthropogenic material, found exclusively in constructed environments such as roads, parking lots, and industrial sites.

Key Facts

Hardness

Variable, depends on aggregate hardness and temperature. Generally soft to firm at ambient temperatures, becoming harder when cold and softer when hot. Not measurable on Mohs scale.

Specific Gravity

Typically 2.2 to 2.5 g/cm³ (for compacted mix), varying with aggregate density and asphalt content.

Crystal System

Amorphous (asphalt binder); aggregate components may be crystalline but are not relevant to the composite's overall crystal system.

Color

Dark black to dark gray.

Luster

Dull to greasy/waxy.

Transparency

Opaque.

Fracture

Irregular to conchoidal (in the asphalt binder component), breaking through aggregate particles.

Cleavage

None.

Composition

Hydrocarbon binder (asphalt/bitumen) and mineral aggregate (crushed stone, gravel, sand, mineral filler). Chemical formula for asphalt is highly complex and variable, primarily C, H, S, N, O.

Physical characteristics

• Crystal Habit: Not applicable; composite material with amorphous binder and granular aggregate.
• Cleavage Type: None.
• Fracture Type: Irregular to conchoidal (in binder), breaking through aggregate.
• Tenacity: Brittle when cold, ductile and plastic when warm.
• Luster Type: Dull to greasy/waxy.

Explore Asphalt Concrete