Asphalt

A composite material commonly used to surface roads, parking lots, and airports. It consists of mineral aggregate bound together with bitumen.

Description of what it is like

It is a dark (usually black or grey) solid surface material. It is slightly flexible compared to concrete, which allows it to adapt somewhat to underlying soil movements, but it is also durable and waterproof.

Origin and where to find it

• Artificial: It is a manufactured material.
• Natural Asphalt: Some natural deposits contain a mix of bitumen and sand/rock (e.g., asphalt rock), which can be mined and used directly or enriched.

Minimum processing required

1. Preparation of Aggregates: Gravel and sand must be dried and heated (to prevent water from repelling the bitumen).
2. Heating Bitumen: Bitumen is heated until fluid (approx 150°C).
3. Mixing: Hot bitumen is mixed thoroughly with the hot aggregates.
4. Laying: The hot mixture is spread on a prepared base.
5. Compaction: It is pressed down with heavy rollers or tampers while still hot to remove air voids and create a solid mass.

Tools needed to work on it

• Heaters/Furnaces: To heat bitumen and aggregates.
• Mixers: To combine the ingredients.
• Shovels/Rakes: To spread the hot mix.
• Tampers/Rollers: To compact the surface.

Common forms of use

• Hot Mix Asphalt (HMA): Poured hot and compacted.
• Cold Patch: Uses additives to keep bitumen pliable at lower temperatures (less durable).

Possible substitutes

• Concrete: More rigid, lasts longer but harder to repair and creates a noisier ride.
• Cobblestones/Brick: Durable but labor-intensive and uneven.
• Macadam: Layers of compacted stone with a binder (tar or just water/dust).

Limitations and common failures

• Rutting: Deformation under heavy loads in hot weather.
• Cracking: Due to thermal expansion/contraction or base failure.
• Water Damage: If water penetrates (potholes).

Risks and safety

• Burns: The mix is very hot (150°C+).
• Fumes: Releases fumes during application.

Related materials

• Bitumen: The binder.
• Concrete: The alternative rigid pavement.
• Gravel: The main structural component.