KNOW-HOW

Every road has a different destination.

Asphalt and bitumen

Asphalt is a mixture of mineral aggregate and bitumen. The content of bitumen ranges from 3 to 4% by mass. But what is bitumen? The DIN standard for bitumen defines it as a “viscous mixture of various organic substances won in the course of processing selected crude oils, the elasto-viscous behaviour of which changes with temperature”.

Plastic or elastic?

Asphalt mix is produced in asphalt mixing plants. Its properties can be specifically controlled depending on the requirements placed on the road to be built. Two examples:

• Natural sand, a fairly small percentage of chippings, small-sized mineral aggregate mix, a small percentage of filler, and a soft binding agent are used for pavements exposed to low loads like, for instance, roads in residential areas, foot or bicycle paths, or parking lots. The resulting mix is rather soft and prone to heaving, but highly suitable for compaction.

• Crushed sand, a high percentage of chippings, large-sized mineral aggregate mix, a high percentage of filler, and a hard binding agent are used for pavements that are subject to very high loads like, for instance, motorways and airports. The resulting mix is stable and highly resilient, but fairly averse to compaction.

What material to use for what purpose?

Selecting the right mix for the surface course depends on traffic load, environmental aspects, considerations in terms of traffic safety, and design guidelines. Most surface courses consist of asphalt concrete or stone mastic asphalt. Modern materials include, for instance, porous asphalt or low-temperature asphalt.

Asphalt concrete

Asphalt concrete is a densely graded mixture of high-quality chippings, high-quality crushed sand, natural sand, filler and road bitumen serving as binding agent, and is considered to be a type of rolled asphalt. Asphalt concrete is used for traffic surfaces with normal to medium loading like, for instance, city and country roads.

Stone mastic asphalt

Stone mastic asphalt is a special material for surfaces exposed to high loads, consisting of high-quality chippings with a particularly high percentage of maximum-size particles, a fairly low percentage of crushed sand, mineral dust, road bitumen and stabilizing additives. It is distinctive for its high stability even under extremely high loads like, for instance, frequent braking and accelerating, and is also resistant to high temperatures and exposure to sunlight.

Porous asphalt

Porous asphalt is an open-pored material, its composition ensuring that the compacted layer has a high void content. The large percentage of voids in the pavement reduces noise and prevents aquaplaning, as the water is immediately drained off through the road body.