United States Department of Transportation
i
Characterization of low temperature creep properties of crack sealants using bending beam rehometry.
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2009-11-01
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Abstract:Crack sealing has been widely used as a routine preventative maintenance practice. Given its proper installation, crack sealants can extend
pavement service life by three to five years. However, current specifications for the selection of crack sealants correlate poorly with field
performance. The purpose of this research was to develop performance guidelines for the selection of hot-poured bituminous crack sealants at
low temperature. In this part of the research, the creep behavior of crack sealant at low temperature is measured and performance criteria for
material selection were developed. Because various pavement and State agencies are well acquainted with and own the Bending Beam
Rheometer (BBR), which was developed during the Strategic Highway Research Program (SHRP), an attempt was made to utilize the same
setup to test hot-poured bituminous-based crack sealants. Testing conducted in this research project indicated that the standard BBR was
inappropriate for testing soft bituminous-based hot-poured crack sealant, even at -40°C. The measured deflection exceeded the BBR limit, for
some sealants, after only a few seconds of loading. To address this issue, the moment of inertia of the tested beam was increased by doubling
its thickness (from 6.35mm to 12.7mm). For the new beam dimensions, it was found that only 4% of the beam center deflection is due to shear, a
value deemed acceptable for sealant evaluation and comparison.
In an effort towards developing a robust testing procedure, 15 sealants from various manufacturers were included in the study and tested
between –4°C and –40°C. In addition, five sealants, which have known field performance, were tested to validate the laboratory results and
establish specification thresholds for the selection guidelines. Since stiffness calculation in the BBR test method requires that measurements be
made within the linear region of viscoelastic behavior, validation of this theory was conducted for crack sealants. This was found to be generally
the case with crack sealants, which allowed for the use of the time-temperature superposition. If the temperature-superposition principle is
applied, the stiffness at 240s at a given temperature can be used to predict the stiffness after 5hr of loading at a temperature that is 6°C lower.
With the assumption of linear viscoelastic behavior, sealants performance can be characterized through stiffness, average creep rate, and
dissipated energy ratio. Stiffness was found to be sensitive to temperature changes and could be used to differentiate between sealants. The
measurements of the average creep rate and the dissipated energy ratio were also found to be valuable in differentiating between sealants. In
addition, numerical modeling was used to simulate the mechanical response of crack sealants at low temperatures. Parameters that may be
used for evaluating crack sealant cohesive performance using the crack sealant BBR (CSBBR) are the stiffness at 240s, average creep rate, and
the dissipated energy ratio. For simplicity, the first two parameters, stiffness at 240s and average creep rate, are recommended for
implementation in the sealant performance grade. The recommended thresholds are maximum stiffness of 25MPa and minimum average creep
rate of 0.31.
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