By: Tom Harman, Contributing Author
As a part of the Superpave mix design system, gyratory
compactors are important in defining the mix characteristics to be achieved in
production. It is estimated that contractors and agencies have spent over $50
million in the purchase of these devices from different manufacturers. There
are currently eight approved models of gyratory compactors. It is important
that these devices provide consistent results from one to another, because
results from contractors' mix designs are used to provide construction control
and are subject to agency verification. A lack of consistency can result in
substantial and needless penalties and a mutual distrust between contractors,
testing firms and agencies.
Angle validation
When first implemented, the requirement for the gyratory
compaction procedure specified that an angle of 1.25° be maintained, and
this was relative to the outside of the sample mold. It has come to light that
different brands of gyratory compactors may yield different compaction results
in spite of having the required external gyratory angle. These compaction
differences may result in a design air void content difference of up to 1%, and
a change in asphalt content of up to 0.4%. The problem is that different brands
or models of compactors have different levels of compliance (give) in their
frames. This lack of consistency in compliance is responsible for a difference
in the internal angle (inside the mold) where the specimen is being compacted.
In order to address this, a Dynamic Angle Validator (DAV)
was developed by Test Quip Inc. for the Federal Highway Administration. The DAV
allows the internal angle of different machines to be checked and subsequently
adjusted. American Association of State Highway & Transportation Officials
(AASHTO) standards now allow a state agency to choose either external- or
internal-angle calibration. It is recommended that the calibration be done for
the internal angle if different brands or models of gyratory compactors are being
used. The cost of the DAV is around $9,000.
Dr. Kevin Hall of the University of Arkansas has led the
task group effort to examine the ruggedness of the method. At this time, it
appears that the ruggedness of the test method is not affected by temperature,
size of aggregate, position of the measuring device in the mold or number of
gyrations. It also appears that the method is independent of compactor brand.
Just as important as angle calibration is the issue of
gyratory compactor maintenance. Worn molds, bearings, etc., will result in a
gradual drift in results over time. Regularly scheduled maintenance is a
requirement of the AASHTO accreditation process.
The process for implementing the angle calibration procedure
will vary according to local agencies and contractors, but it is important that
both industry and agency representatives be involved. Detailed laboratory
procedures for calibration, logistics for carrying out inter-laboratory
comparisons and within- and between-laboratory tolerances all need to be
defined, using the AASHTO method as a starting point. It is useful to note that
since the state of Maine has instituted the use of the DAV there were disputes
on only 1.5% of 2,000 split-sample test results during 2002.
