More Practical Wheel Tracking Test for Rutting Resistance of Asphalt Mixtures

Wheel tracking tests have seen a vast increase in usage among various state department of transportations for measuring rutting resistance of mixtures that already meet volumetric property requirements. With the increase in using recycled materials and warm mix additives, it is clear that using volumetric properties alone to approve asphalt mixture designs is a risky approach. Wheel tracking tests are among the most widely used methods for evaluating rutting resistance, and the AASHTO T324 (Hamburg Wheel-Tracking [HWT]) is the most widely accepted and followed procedure used today in the U.S. However, there are challenges using the HWT, among which the most difficult are the poor repeatability, time required to complete the test, and the sample preparation details. This study reports on an alternative wheel tracking method called the Rotary Asphalt Wheel Tester (RWT) that can successfully address the challenges currently faced with using the HWT. The method requires no cutting of the gyratory samples, significantly reduces time to complete a sample, and appears to offer acceptable repeatability of the results. The method has existed for more than 15 years, available commercially, but used only in a few labs, and one acceptance criterion is already developed by one agency. The study includes evaluating an expanded set of mixtures tested at two temperatures, and two air voids. The results of the RWT are compared with the results of the HWT for numerous mixtures and they show that similar qualitative ranking can be achieved.

Thickness scaling of ferroelectricity in BiFeO3 by tomographic atomic force microscopy

Nanometer-scale 3D imaging of materials properties is critical for understanding equilibrium states in electronic materials, as well as for optimization of device performance and reliability, even though such capabilities remain a substantial experimental challenge. Tomographic atomic force microscopy (TAFM) is presented as a subtractive scanning probe technique for high-resolution, 3D ferroelectric property measurements. Volumetric property resolution below 315 nm3, as well as unit-cell-scale vertical material removal, are demonstrated. Specifically, TAFM is applied to investigate the size dependence of ferroelectricity in the room-temperature multiferroic BiFeO3 across two decades of thickness to below 1 nm. TAFM enables volumetric imaging of ferroelectric domains in BiFeO3 with a significant improvement in spatial resolution compared with existing domain tomography techniques. We additionally employ TAFM for direct, thickness-dependent measurements of the local spontaneous polarization and ferroelectric coercive field in BiFeO3. The thickness-resolved ferroelectric properties strongly correlate with cross-sectional transmission electron microscopy (TEM), Landau–Ginzburg–Devonshire phenomenological theory, and the semiempirical Kay–Dunn scaling law for ferroelectric coercive fields. These results provide an unambiguous determination of a stable and switchable polar state in BiFeO3 to thicknesses below 5 nm. The accuracy and utility of these findings on finite size effects in ferroelectric and multiferroic materials more broadly exemplifies the potential for novel insight into nanoscale 3D property measurements via other variations of TAFM.