Oxidized Bitumen (Blown Bitumen Grades or Hard Bitumen) is blown from blowing hot air to Penetration Bitumen at the final phase of the refinement operation In this process, hot air with a temperature of 190 to 200 degrees Celsius is blown through a perforated tube into a compartment containing bitumen. Heated Penetration Grade Bitumen under controlled environment is blown with air which controls the oil content in the bitumen while it oxidized. So oxidized bitumen is hard, its softening point is much higher than regular bitumen. Also, its Penetration Index is higher than a regular one.
Also, This process gives the Oxidized Bitumen more rubbery properties than its original formula and they are simply harder bitumen.