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.
