A Study of The Dynamic Behavior of ‎Asphalt Material Under ‎Milling Conditions

Diouri, Kaoutar

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A Study of The Dynamic Behavior of ‎Asphalt Material Under ‎Milling Conditions

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The success of the pavement industry relies on the successful design and construction ‎‎of resilient roadway infrastructure. Milling, which is the most widely used method for the ‎recovery of Reclaimed Asphalt ‎Pavement (RAP), involves the ‎forced removal of existing aged ‎material under high loading rates. Current milling ‎specifications are based on industry ‎guidelines, which do not take into account the potential ‎effect of the temperature, long-term ‎aging of the in-place mix, and the high strain rate on the ‎dynamic response of the asphalt ‎pavement during milling.‎ Thus, the objective of this research study was to investigate the ‎impact of these three key milling factors on the dynamic behavior and fragmentation of ‎asphalt material. ‎ The dynamic characteristics of asphalt mortar under high strain rates were investigated ‎through the Split Hopkinson Pressure Bar (SHPB) experiments. Asphalt mortar samples were ‎subjected to long-term oven aging and conditioned at three temperatures to simulate the ‎milling conditions within a laboratory environment. Fourier transform infrared spectroscopy ‎‎(FTIR) was conducted to quantify the changes in the functional groups related to asphalt ‎oxidation after long-term oven aging. The dynamic compressive stress-strain relationships of ‎asphalt mortar under high strain rates were characterized by a rate and temperature-dependent ‎viscoelastic constitutive damage model. Moreover, a numerical analysis of the effect of long-term aging and temperature on milling-induced stresses below the milling depth for two Hot ‎Mix Asphalt (HMA) materials was performed. Lastly, particle size analysis was carried out to ‎evaluate the effect of temperature and milling loading on the gradation characteristics of the ‎fragmented asphalt mortar samples collected after the SHPB experiments, and milled RAP ‎samples from the field. ‎ The outcomes of the SHPB experiments provided important insights into the dynamic ‎compressive properties of asphalt mortar under milling conditions. Asphalt mortar exhibited ‎greater compressive strength and energy absorption at higher temperature under high ‎strain rates. Statistical analysis indicated that the temperature has the most significant effect ‎on the dynamic strength, elastic modulus, and ultimate strain of asphalt mortar. The proposed ‎constitutive damage model was calibrated and validated using the results of the 72 SHPB ‎experiments. Therefore, it can serve as a preliminary theoretical framework for ‎characterizing ‎the dynamic compressive behavior of asphalt mortar under milling ‎loading conditions. The ‎results of the finite element simulations highlighted the crucial importance of selecting the ‎appropriate timing for ‎milling operations and conducting a thorough evaluation of the ‎properties of the underlying layers for each rehabilitation project.‎ Particle size analysis ‎revealed that the increase in the drum rotational speed from 100 RPM to 118 RPM results in a ‎greater uniformity coefficient of milled RAP samples. The statistical Weibull function was ‎successful in predicting the ‎gradations of the asphalt mortar and the milled RAP under ‎varying milling conditions. Thus, these findings provide valuable knowledge that can be ‎‎utilized for the effective control of the milling process, which, in turn, can significantly ‎contribute to ‎constructing sustainable and longer-lasting asphalt pavements.‎

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