Aging Conditions for Hot Mix Asphalt Cracking Test
Cracking, either due to reflection, fatigue, and/or low temperature, is one of the major distresses of asphalt pavements. Like many other agencies, the Montana Department of Transportation (MDT) is considering implementing laboratory cracking performance test(s) into the hot mix asphalt design process to ensure that the asphalt mix to be used for construction is cracking resistant. One of the key components of the cracking test(s) is the aging condition of the asphalt mix prior to the tests. The aging level greatly affects the cracking test results and, therefore, needs to be determined before an acceptance threshold of the cracking index is established as a specification. The aging level of asphalt mix in the laboratory needs to match that of asphalt materials in the field at the time of occurrence of pavement cracking. The 5-day aging at 95 Celsius (or 12 hours at 135 Celsius) simulates field aging of 7~10 years of asphalt surface layer. However, MDT often places chip seal within one year after the asphalt paving. The presence of chip seal significantly reduces the aging of underlying asphalt mix, based on previous studies. Therefore, determination of the appropriate aging condition of asphalt mix that is suitable for MDT paving practices and for climatic conditions in Montana is greatly needed before a specification of cracking test(s) can be developed.
The proposed study includes the collection of hot mix asphalt samples from pavements in Montana at the initial stage of cracking. The asphalt binders will be extracted and recovered, and performance grades of recovered asphalt binders will be determined and compared to original performance grades at the time of paving to determine the aging over the years. Laboratory-produced asphalt mixes will be aged in the laboratory to determine appropriate aging time and temperature that would render equivalent aging in the laboratory to that of asphalt mixes in the field. The aging conditions can then be included in the protocols of cracking test(s).