Grades 3–5 · Materials & Matter
What is the black material in a road?
A black road may look like one solid substance, but look closely and small stones appear. Asphalt pavement is a mixture. Its mineral pieces, called aggregate, include materials such as crushed stone and sand. A dark, sticky asphalt binder coats and joins them.
In the diagramAsphalt pavement joins stones with a sticky binderA road is a layered structureAsphalt surfaceStone baseGroundEnlarged mixtureExample by mass: 95 stone + 5 binder = 100Stones carry much of the load; binder holds them together.
The ingredients do different jobs. The hard aggregate pieces make up most of the mixture by mass and carry much of its load through their contacts. The binder helps the pieces stay together. In the drawing’s example, 95 mass units of aggregate join 5 mass units of binder. That is an illustration of the proportions, not a road-making recipe.
The black surface also rests on other pavement layers and the ground beneath. Together they spread and support loads from traffic. Asphalt binder softens when heated and becomes stiffer when cooled, so temperature matters to a road’s behavior. The road beneath a bicycle or bus is both a material mixture and a carefully supported structure.
About this explanation
Starting knowledge: Distinguish a mixture’s ingredients and compare parts out of one hundred.
Sources
- Federal Highway Administration — Warm Mix Asphalt FAQs: Mixture Ingredients
- Federal Highway Administration — Toward Sustainable Pavement Systems: A Reference Document