Asphalt pavement is formed by combining a tar known as bitumen with crushed rock, known as aggregate. The aggregate is what gives asphalt its structure, allowing for heavy loads. The bitumen is what holds the asphalt together and gives asphalt its color. Freshly paved asphalt is a thick, heat-absorbing black, but asphalt does not stay black forever. Over time, the bitumen can wear away and deteriorate, leaving gray aggregate—and therefore, gray pavement—behind.