Structural Coefficient for High Polymer Modified Asphalt Mixes
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Structural Coefficient for High Polymer Modified Asphalt Mixes

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      Final Report, March 2017–June 2019
    • Abstract:
      Mixes This report summarizes the information and findings from this study aiming to determine the structural coefficient for high polymer (HP) asphalt concrete (AC) mixes. In this research, HP AC mixes are defined as asphalt mixtures manufactured using asphalt binders modified with Styrene-Butadiene-Styrene (SBS) at the approximate rate of 7.5% by weight of binder. The following activities and analyses were completed to achieve the overall goal of this research study: Reviewed and summarized current and past literatures on the performance of HP asphalt binders and AC mixes. In summary, while several previous studies highlighted the positive impact of the HP modification of asphalt binders and mixes, there was a lack of understanding of the HP AC mix structural capacity as expressed through the structural coefficient for the AASHTO 1993 guide. A total of eight polymer modified (PMA) and eight HP AC mixes were designed and evaluated in the laboratory for their engineering properties and performance characteristics. The 16 mixes were used in a full mechanistic analysis using the 3D-Move model. Overall, HP AC mixes showed better performance characteristics when compared with their respective PMA AC mixes. The estimated initial fatigue-based structural coefficients ranged from 0.33 to 1.32. Using advanced statistical analyses an initial fatigue-based structural coefficient for HP AC mixes of 0.54 was determined and verified for other distress modes (e.g., rutting, reflective cracking). The determined structural coefficient was verified using full-scale testing. Two extensively instrumented experiments were conducted in the PaveBox facility at University of Nevada; experiment No. 1 evaluated a flexible pavement with PMA AC layer and experiment No. 2 evaluated a flexible pavement with a reduced thickness of HP AC layer. In general, the reduced thickness of the HP AC layer resulted in similar vertical surface deflections except under the center of the applied surface load (higher deflections), higher vertical stresses at the middle of the base layer, and similar vertical stresses at 6 inch and 24 inch below the subgrade surface. In addition, the mechanistic analysis showed that the HP pavement will result in better fatigue and rutting performance in the AC layer, slightly higher rut depths in the unbound layers but similar total rut depths. In general, the overall results of the laboratory evaluation, pavement modeling, and full-scale testing support the use of a structural coefficient of 0.54 for HP AC mixes in Florida. A testing plan for the Florida Department of Transportation (FDOT) Accelerated Pavement Testing (APT) has been recommended to further validate the recommended structural coefficient. The main thrust of the APT plan is to identify unique cases where additional rutting may occur in the crushed aggregate base (CAB) layer under the reduced thickness of the HP AC layer.
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