Citation/Abstract

Asphalt concrete is a bonded granular material whose internal structure is anisotropic. Such anisotropy is due to anisotropic particles and void's structure, particle distribution and orientation, and restrain and force pattern used during compaction. Even with this knowledge asphalt concrete design is made considering asphalt concrete as an isotropic material. The biggest difference between an isotropic and anisotropic material is that, an isotropic material is characterized by two independent constants and an anisotropic material could be characterized by up to 81 constants. An extensive research to characterize and quantify anisotropy is only available for some composite materials. Based on the literature review, further study is necessary in the area of asphalt concrete pavements.

This study used an Ultrasonic Method to quantify anisotropy of asphalt concrete pavements (ACP). The method consisted of sending ultrasonic waves through asphalt specimens to measure the transit times in different directions, both in shear and compression. These transit times were used to calculate the velocities of propagation that were converted to the constants needed to fully characterize a transversely isotropic material.

A total of 45 specimens were tested. The experimental matrix consisted of three materials from three different quarries in El Paso (granite), San Antonio (soft limestone) and Brownwood (hard limestone). The specimens were then broken down into three different mixes; Superpave, CMHB, and PFC.

The Ultrasonic Method proved to be efficient in characterizing and quantifying the anisotropy of asphalt concrete specimens. It was shown that it was possible to obtain the five elastic constants for this type of material. The method was further validated by comparing the elastic constants obtained with the moduli determined using a V-Meter and a free-free resonant column device. The system worked well for the Superpave and the CMHB mixes. The coupling of the ultrasonic energy to the PFC samples was rather difficult.

The modulus values calculated by the Ultrasonic Method proposed in this study followed the same pattern of those calculated by the FREE-FREE. The highest modulus pertained to the SASuperpave and CMHB specimens. Followed by the BW Superpave and CMHB and the one with the lowest modulus was the EP-Superpave and CMHB. When in comparison to the V-meter and the FREE-FREE the values provided by the new Ultrasonic Method were found in between the high values of the V-meter and the low values of the FREE-FREE. This could be observed for all the three materials and the two different mixes.

The values for the EP-Superpave specimens varied from 13 to 15 GPa for E ₁ , 21 to 30 GPa for E ₂ , 7 to 8 GPa for G ₁₂ , 9 to 14 GPa for G23, 0.31 to 0.43 for V ₁₂ , and 0.03 to 0.09 for V ₂₃ . The values for the EP-CMHB specimens varied from 12 to 17 GPa for E ₁ , 22 to 29 GPa for E ₂ , 6 to 10 GPa for G ₁₂ , 10 to 14 GPa for G ₂₃ , 0.27 to 0.40 for V ₁₂ , and 0.01 to 0.09 for V ₂₃ . The values for the BW-Superpave specimens varied from 15 to 22 GPa for E ₁ , 12 to 34 GPa for E ₂ , 4 to 11 GPa for G ₁₂ , 5 to 17 GPa for G ₂₃ , 0.29 to 0.38 for V ₁₂ , and 0.01 to 0.15 for V ₂₃ . The values for the BW-CMHB specimens varied from 18 to 25 GPa for E ₁ , 25to 34 GPa for E ₂ , 7 to 8 GPa for G ₁₂ , 12 to 16GPa for G ₂₃ , 0.24 to 0.36 for V ₁₂ , and 0.05 to 0.09 for V ₂₃ . The values for the SA-Superpave specimens varied from 16 to 26 GPa for E ₁ , 23 to 35 GPa for E ₂ , 7 to 8 GPa for G ₁₂ , 12 to 17 GPa for G ₂₃ , 0.20 to 0.39 for V ₁₂ , and 0.01 to 0.03 for V ₂₃ . The values for the SA-CMHB specimens varied from 17 to 28 GPa for E ₁ , 18 to 28 GPa for E ₂ , 8 to 17 GPa for G ₁₂ , 10 to 15 GPa for G ₂₃ , 0.27 to 0.42 for V ₁₂ , and 0.01 to 0.09 for V ₂₃ .