Paving materials, like asphalt, are manufacturered in a plant. An asphalt plant has many parts. There are areas to store and weigh the construction materials, machinery to make the asphalt and a laboratory to test samples for strength and durability.

Asphalt is a combination of "aggregate" and "binder". Aggregates are textured rocks and sand-like materials. Aggregates come in different sizes and textures: coarse, fine or very fine. Most aggregates come from nature: crushed rock or gravel for coarse aggregates; natural sand or finely crushed rocks for fine aggregates. Very fine aggregates are called "fillers". Common fillers are limestone dust or cement.

Aggregates - Notice The Different Size And Textures Of Rocks

The rocks and other aggregates are stored in stockpiles (large piles) at the asphalt plant. The stockpiles are located near conveyors. The conveyors look like little roller coasters or monorail systems. There are small bins in the conveyor and the rocks or aggregate are scooped into the bins. The conveyor transports the bins around the asphalt plant where the materials are needed.

Stockpile and Conveyor System

The binder is the material that holds (binds) the mixture together. In the past tar was used. Today a substance called bitumen is used as the binder. When the binder is heated to 300^oF, it turns into a liquid. When it cools it turns into a hard solid mass. Rocks or aggregate are added to the binder to make the asphalt stronger.

Engineers select and calculate the correct quantities of each rock size needed to produce a strong asphalt pavement. Calculated percentages of the different sizes of rocks are combined to determine the appropriate blend of rock materials. The mixture of rocks and asphalt binder are then compacted and put through a series of tests which smash, stretch, and freeze the pavement to determine the best blend of rocks to use in a certain climate.

Different measuring techniques are used in the field than in the laboratory. In the field engineers use huge quantities of each rock size and weigh them on scales as large as a garage. In the laboratory, much smaller quantities of each material are needed and ordinary measuring utensils are used.

The drum mixer at the asphalt plant tumbles all of the ingredients until they are well coated with the asphalt binder. The tumbler works like a clothes dryer. Mixing the ingredients in the bowls is a similar process. As the cookie mixture cools while it is being stirred, it becomes stiffer and starts to stick together. Asphalt behaves in the same manner.

In the field, the pavement is spread with the paver and then rolled into a thin mat with a roller. The roller is very heavy and smashes all of the air out of the pavement which helps to make the asphalt very strong. You can still see the different materials in the asphalt. Immediately after pavement is rolled out it is still very hot.