Aging evaluation of asphalt samples with Low Field Nuclear Magnetic Resonance

In this study, a recently developed methodology to assess aging of pavements by using Low Field Nuclear Magnetic Resonance (LF-NMR) was applied to asphalt mix samples at different aging states. The ultimate objective of this method is to monitor the long-term aging of asphalt binder in mixture cores without binder extraction. The asphalt samples were long-term aged at 70 °C for different durations. At the end of this process, specimens were heated again to 70 °C and measured in the LF-NMR while cooling to room temperature. Lower cumulative amplitudes were found in samples that were aged for longer durations. An NMR probe that can detect shorter transverse relaxation times T₂ was used in order to increase the accuracy of the method. The unaged asphalt binder was measured as temperature dropped from 80 to 40 °C and it was observed that the cumulative amplitude was fairly constant throughout the process. The corresponding change in the binder viscosity was expressed by the T₂ shifts to shorter times. This means that the faster NMR probe was capable to minimize the NMR signal losses and therefore it is a suitable tool to investigate asphalt samples. Asphalt binders were extracted from the aged samples and subjected to LF-NMR measurements with the fast NMR probe. It was possible to detect aging trends in the binders, which were in agreement with viscosity trends measured with the Dynamic Shear Rheometer (DSR). The differences in the cumulative amplitude among binders with different aging states mainly depend on the hydrogen nuclei to weight ratio (chemistry) and less on the temperature and measurable limit of the NMR device. This improves the accuracy of the method in the estimation of the binder aging state. In addition, the employed aggregates were separately investigated using LF-NMR.