• Evaluation of Macrotexture and Friction of Alternative Asphalt Surface Course Materials

    NCDOT Research Project Number: 2024-12

Executive Summary

  • Low macrotexture may contribute to reduced pavement skid resistance. Revising the existing North Carolina DOT (NCDOT) asphalt mixture categories to address these problems has some practical constraints due to contractor practices, familiarity with mixture designs, and unintended consequences to durability. On the other hand, a preliminary evaluation of the surface mixture guidelines used in neighboring states shows important differences with the NCDOT current practice. 

    This study was instigated to evaluate and understand the effectiveness of these alternative surface course materials, namely stone matrix asphalt (SMA) and microsurfacing, but others as well. There are two primary goals of the research reported herein; 1) characterizing friction and texture performance of alternative surface mixtures designs from other states and 2) identifying alternative structural mixture designs that could be used in North Carolina. 

    A set of pavements with different traffic volumes, ages, and spatial locations were selected from two neighboring states. On each pavement, friction and texture observations were collected and these observations were used to update the model coefficients of the performance models proposed in the FHWA/NC/2022-05 project. 

    Additionally, the crash history of each pavement was collected and a ‘before-after’ safety evaluation was conducted to compare the safety performance of the alternative surface course materials with respect to the performance of pavements using only dense-graded surface course materials. For this evaluation, the pavements selected have the same surface type in the ‘before’ and ‘after’ periods. The ‘after’ period crash frequency was compared with the frequency in the ‘before’ period by computing the %change, defined as the percent difference in the number of crashes in the ‘after’ period with respect to the crashes in the ‘before’ period. The surfaces that were evaluated in this work seem to experience lower crash frequencies than the pavements with a normal dense-graded surface mixture. 

    Finally, if one considers the traffic exposure, then the surface type with the lowest crash rates are the SMA mixtures. However, the microsurfacing was the surface type with the lowest ‘after’ crash rate. A performance evaluation of SMAs used in two neighboring states compared to North Carolina’s dense-graded mixtures was conducted through linear viscoelastic characterization and pavement performance simulations. 

    The results showed that SMAs have cracking performance most similar to North Carolina’s B and C mixes but are softer and more viscous (higher phase angle) than the D mixes. They also exhibit superior resistance to permanent deformation compared to B and C designs, but lower resistance than D designs. Lastly, a life-cycle cost analysis was conducted. The investment was defined as the additional cost required to build a specific surface treatment compared to constructing a pavement with a regular dense-graded surface. The crash cost reductions expected from the reduction in crash rates were computed and the analysis indicated that crash cost reductions of the alternative surface studied were greater than the investments. 

    While this analysis provides a preliminary estimate of the viability of the alternative surface course materials, there are many factors, including engineering familiarity with the techniques, user cost impacts, material supply limitations, and others that the cost analysis could not fully account for. ​​

  
B. Shane Underwood
Researchers
  
B. Shane Underwood; Cassie A. Castorena
  
Matthew Hilderbran
  
Mustan Kadibhai, PE, CPM

Report Period

  • August 1, 2023 – July 31, 2025

Status

  • Complete

Category

  • Pavement, Materials and Maintenance

Sub Category

  • Pavement Performance

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