Effects of Physical Hardening on Stress Relaxation in Asphalt Cements

This paper documents and discusses an investigation of how physical hardening phenomena during isothermal conditioning affect stress relaxation in asphalt cement. The importance of physical hardening is reviewed with data from trial sections and regular contracts. Seven asphalts were recovered from an Ontario, Canada, pavement trial. Tensile specimens were poured and conditioned for either 20 min or 72 h at low temperatures before being subjected to a stress relaxation test at −10°C. The extended conditioning period could more than double the residual thermal stress at the end of the test. The one asphalt to show no physical hardening came from the section that remained largely free of cracking. A second material that showed a moderate degree of physical hardening only recently started to crack by an appreciable amount. In contrast, the remaining five materials significantly hardened during the extended conditioning period and cracked prematurely and excessively in service. These findings clearly refute the hypotheses that physical hardening is not important and that stress relaxation can reverse the effect or reduce it to insignificance. Materials with an unstable colloidal structure were found most sensitive to physical hardening. The results of this study agree with earlier creep data obtained according to an extended bending beam rheometer test method. Hence, it is imperative that pavements be designed with criteria that take physical hardening effects into account to limit premature and excessive performance failures.