Asphalt oil or liquid asphalt is the binder that holds the aggregates together. Asphalt is the heaviest form of petroleum oil and is liquid at room temperature but has a very high viscosity (inability to flow). The warmer it gets the lower the viscosity. That is why asphalt concrete must be mixed at high temperatures so that it flows freely and can coat all the aggregates with the proper film thickness quickly. There are many different asphalts that are typically graded using a Performance Grading (PG) system. This system describes the oil due to its high and low working temperatures in degrees Celsius. For example, a “PG 64-28” asphalts working condition is 64 C to -28 C.