Trr No. 1323 Materials And Construction, pp. 70-76

Hardening occurs in asphalts during extraction and recovery, posing a serious problem for research into the properties of hot-mix and road-aged asphalts. Hardening appears to accur in all solvents and, to varying degrees, in all asphalts. The effects of light, oxygen, and temperature on this phenomenon were investigated. All are significant, and the data indicate that reflux methods of extraction should be avoided when the properties of the extracted material are to be investigated. The mechanism of the hardening is still unclear. Infrared spectra show definite chemical changes, but these are quite asphalt-dependent and do not relate clearly to changes in viscosity.