Different ways have been developed over the years to improve the rheological properties of bitumen, one of the most important of which is temperature susceptibility. There are advantages and disadvantages associated with the various approaches:
- Air blowing was developed to increase the viscosity of vacuum tower residue but also has the advantage of improving temperature susceptibility. However not all bitumen production facilities are equipped with blowing towers.
- Polymer modification is a more recent development and can result in substantial improvements in rheological properties. However, certain polymer modified binders (PMBs) have shown substantial changes in properties during transport and storage.
- Chemical modification was first patented in 1973. It utilizes an acidic or basic modifier. Among the different acids, polyphosphoric acid (PPA) has been the most popular. However, there have been concerns that the bitumen PPA reaction may be reversed in an asphalt where lime is present.
The use of PG 76-22M asphalt cement has helped in the prevention of rutting and resistance to moisture damage. However, the increase in asphalt cement stiffness sometimes creates compatibility issues in the field. This study utilized the Sasobit® warm mix technology to reduce compaction temperatures, which allowed HMA mix temperatures to fall between 300°F and 325°F in lieu of the 350°F typically seen.
Conventional HMA production takes place between 250°F and 325°F, not to exceed 350°F, and placement and compaction occurs between 260°F and 300°F . Before mixing with hot liquid asphalt, fine and coarse aggregates are heated to high temperatures to drive off moisture, ease coating of the mineral aggregates with the liquid asphalt, and keep the complete mix fluid enough to be workable during placement. A number of new processes and products have become available that can reduce the temperature at which HMA is mixed and compacted.