In last few
months, North Carolina Department of Transportation (NCDOT) has encountered
recently paved sections exhibiting early distress throughout the state which is
a major concern. NCDOT conducted an
internal investigation on the causes of early distresses. And this investigation did not yield any conclusive
results, hence; NCDOT recognized the need for a more in-depth investigation.
To address
these early distresses seen on various projects throughout the state, an
investigation was performed to evaluate mixes from projects that have seen early
distress, as well as mixes from projects that have not. Various mix design
factors may be contributing to the early distresses, but it is largely unknown
as to the relative magnitude of the contributions of these factors. These factors include but are not limited to
– source aggregate properties, overall mix gradation, percentage of each
stockpile used (natural sand content, screening content, coarse aggregate
content, recycled asphalt pavement (RAP) content, recycled asphalt shingles
(RAS) content), binder content, recycled binder content, virgin binder content,
virgin binder grade, mixture voids in mineral aggregates (VMA) & voids
filled with asphalt (VFA), and design gyration level.
In this
study, four contractor mixes were replicated in the lab. Three mixes from three different contractors
which vary in natural sand content, screening content, RAP content, RAS
content, binder content, binder grade, & gyration level, were selected for
testing. Additionally, a fourth mix was
designed in the lab by substituting fine RAP for the RAS in the RAS mixture,
changing to a PG 64-22 binder from PG 58-28 binder, and adjusting the binder
content as required to meet the required volumetric properties. This study used
component materials (aggregate, RAP, RAS, and binder) sampled from the
contractor’s stockpile.
These four
mixes were then subjected to the following tests - Hamburg, IDEAL-CT (using
Indirect Tensile Strength Testing), Semi-Circular Bend (SCB) Test, and Asphalt
Pavement Analyzer (APA) Test. The results
from these tests were examined to determine if variations of the various
factors explain the results seen, or if they can be ruled out as a significant
variable affecting the results.
The data from this study will provide a better
understanding of the important variables (volumetric parameters) affecting the
laboratory performance and eventually predict mixtures which might show early
distresses in the field.
