Blown asphalt grades or Oxidized bitumen are produced by passing air through the penetration grades. This process gives the bitumen more rubbery properties than its original formula and they are simply harder bitumen.
The properties of asphalt can be modified by air blowing in batch and continuous processes. Blown asphalt has more and stronger molecular interactions than the original bitumen and is, therefore, more cohesive. Blowing causes the softening point to increase and the penetration to decrease. However, the increase in softening point is usually more than the decrease in penetration. This means that blowing reduces the temperature susceptibility of bitumen. The effectiveness of blowing depends largely on the original bitumen.