Assessment of low-temperature embrittlement of asphalt binders using an acoustic emission approach

Thin films of paving grade asphalt binders moulded on granite substrates were stressed under rapid cooling and thermal restraint conditions at temperatures ranging from 15°C to -50°C. This temperature range encompasses typical pavement temperatures in colder climates. Differential thermal contraction between the granite substrates and asphalt binders induces progressively higher thermal stress levels in the binders as the temperature drops resulting in thermal crack formation, and the corresponding release of elastic energy in the form of transient stress waves. Using piezoelectric sensors (Digital Wave, Model B-1025), a four-channel acoustic emission (AE) system was used to record the acoustic emission activity during the binder/granite cooling process. It was observed that AE events of high energy clustered around a narrow temperature range. Assuming the cracking temperature (T CT ) to be the temperature at which the AE signal energy exceeds a pre-selected threshold energy level, this AE testing approach was found to be sensitive and repeatable for predicting cracking temperatures (T CT ) in four paving grade asphalt binders processed to two distinct ageing levels. The effect of binder type, ageing level and binder film thickness on cracking temperature is discussed. These AE-based T CT predictions showed strong correlations with predictions based on either the AASHTO TP1 or the MPIA protocols.