Bitumen is primarily used to pave roads. It is a brown or black mixture of hydrocarbons, which is derived from petroleum. The consistency of bitumen varies from viscous to solid, and in its solid form, it is commonly known as asphalt. Bitumen is a crucial component of asphalt the most widely used material for constructing and maintaining roads in the world. There are over 4,000 hot mix asphalt plants in Europe alone, producing some 300 million tonnes of asphalt per year. Asphalt is typically a mixture of approximately 95% aggregate particles and sand, and 5% bitumen, which acts as the binder, or glue. The viscous nature of the bitumen allows the asphalt to sustain significant flexibility, creating a very durable surface material. There are many different types of asphalt, each with its own combination of different amounts and type of bituminous binder and mineral aggregate, and each type of asphalt has performance characteristics appropriate for specific applications. Thus, for each application there is a suitable asphalt mixture available. Asphalt is totally recyclable and recycling has increased significantly in recent years. Quantities recycled directly back into road surfaces vary from country to country, but can be as high as 70%. Asphalt is routinely milled and re-laid along with fresh materials, saving money and preserving non-renewable natural resources.