About the research
The use of high recycled asphalt pavement (RAP) mixes is increasing due to both environmental and economical reasons. However, a high RAP mix can cause asphalt mixtures to stiffen due to the hardening effect of the aged binder in RAP materials. During the aging process of asphalt binders, the amount of maltenes is decreased and an increase in volume of asphaltenes as maltenes change to asphaltenes. Thus, there will be less maltenes to disperse asphaltenes throughout the aged binder structure. As a result, the aged binder would become harder and more viscous but less ductile, which could negatively affect the performance of the high RAP mixes in the field. A low temperature cracking potential is a primary concern with high RAP mixtures. To minimize a low temperature cracking, various rejuvenators have been utilized in the past instead of bumping down a PG grade of the specified virgin asphalt for high RAP mixes. However, in some cases, the premature failures such as rutting have been observed from the high RAP mixtures with rejuvenators. Rejuvenators can be made from lubricating oil extract or extender oil, which would be a great source of maltenes. However, rejuvenators may have certain amounts of saturates which may not be compatible with the asphaltenes in the aged binder. Therefore, two most critical issues associated with adding rejuvenators into the high RAP mixes would be: 1) the quality of diffusion of that rejuvenator into the aged binder and 2) the uniformity of rejuvenator dispersion within a high RAP mix and (Corbett 1975, Petersen 1984). A diffusion process of a rejuvenator into the aged binder would progress during mixing and construction process but it would stop after a certain period of time. The diffusion rate would be influenced by the viscosity of maltenes in the aged binder and it would increase by increasing amounts of diluent oil fractions at higher temperature (Oliver 1974). A mechanism of rejuvenators in high RAP mixtures could be defined as the following four steps (Carpenter and Wolosick 1980): (1) A very low viscosity layer is formed surrounding the RAP by a rejuvenator. (2) The rejuvenator begins to penetrate into the aged binder and makes it softer. (3) Penetration continues as the inner layer viscosity decreases whereas the outer layer viscosity increases. (4) Balance in viscosity is reached after a certain amount of time. The main objectives of this phase 3 study are to provide the Iowa Department of Transportation with: 1) a screening method for approving rejuvenator products in asphalt mixtures and 2) a method of field verification for hot mix asphalt (HMA) containing rejuvenators. To achieve these objectives, the following tasks will be performed: (1) evaluate the effectiveness of various rejuvenators to soften aged binders by employing analytical technologies to examine diffusion levels of various rejuvenators in the extracted asphalt, (2) perform rheological binder tests to determine the effects of rejuvenators on aged binder properties, (3) perform mechanistic mixture tests (HWT, DCT, dynamic modulus, flow number and fatigue test) to assess the effect of rejuvenators on high RAP mixtures, (4) build test sections with select rejuvenator(s) and perform laboratory tests of field loose mixtures and cores and (5) perform a condition survey of the test sections.