UFC 3-250-03
15 May 2001
phase angle, , low temperature stiffness, S, and creep rate, m. The criteria for G*/sin (the SHRP
rutting parameter) are designed to insure a minimum stiffness of the binder immediately after placement
to avoid "tender" mixtures and those mixtures with rutting potential early in the pavement life based on
an estimated high pavement temperature. The high pavement temperature is determined from the
mean 7-day high air temperatures to obtain an estimate for the pavement temperature. The maximum
for G*@sin (the SHRP fatigue parameter) helps to identify binders that may be susceptible to fatigue
damage as well as those exhibiting excessive embrittlement with age. Temperature data for
specification of a binder for a particular region can be obtained from local weather stations or from an
extensive database compiled by the FHWA (Federal Highway Administration). The database contains
information from thousands of locations in the U.S. and Canada and is available in the SHRPBIND
software. The SHRPBIND program is available at no cost from the FHWA. The software calculates the
7-day mean maximum pavement temperatures and the lowest yearly one-day pavement temperatures to
determine the SHRP PG for the area selected. The calculations are based on air temperatures, average
sunlight, and location. The software does not contain the low temperature modification equation above.
SHRP PG's are calculated by the SHRPBIND program along with the reliability for the given area. For
example, a PG64-22 binder refers to a material with the following properties: (1) a minimum flash point
of 230EC, (2) a maximum rotational viscosity of 3 Pa@sec at 135EC, (3) a minimum of 1000 and 2200 Pa
for G*/sin for the original (tank) and RTFOT-conditioned (Rolling Thin Film Oven Test) materials,
respectively, at a 10 radian/sec oscillatory shear and 64EC, (4) a maximum of 5 MPa for G*@sin for the
PAV-aged (Pressure Aging Vessel) material at 10 radian/sec oscillatory shear and 25EC, and (5) a
maximum stiffness of 300 MPa and a minimum creep slope of 0.3 at -12EC for the PAV-aged material.
This binder would be suitable in areas with a maximum pavement temperature of 64EC and a minimum
pavement temperature of -22EC.
(b) Use of performance graded binders on airfields. Performance graded binders can be
used on airfields with some restrictions. A typical airfield pavement experiences much higher loads than
a highway due to heavily-loaded cargo aircraft and high-performance fighter aircraft with high tire
pressures. To avoid rutting, higher loads require a more stable asphalt binder, especially at higher
pavement temperatures. In addition, most airfields suffer more from environmental distresses such as
low temperature cracking and oxidative hardening of the asphalt (which leads to cracking). Thus, for
airfields, use of SHRP performance graded asphalts above and below the recommendation for the given
region is warranted. For example, if an airfield that is subjected to heavy cargo aircraft traffic is located
in a region that requires a PG64-22, use of a PG 70-28 or 76-28 will provide additional insurance against
rutting and cracking. The added cost of producing an enhanced PG asphalt must be balanced against
expected benefits and life cycle costs.
(c) Polymer modification of asphalts. The addition of polymers to asphalts is a
burgeoning industry. Many polymers greatly improve the stiffness and flow characteristics of asphalts at
high temperatures and are being demonstrated in pavement applications to significantly reduce rutting
where this has been a problem in the past. The higher temperatures often required by the modification
can cause difficulty with mixing, placement, and compaction. New classes of polymers have become
available that are designed to improve the low temperature characteristics of an asphalt and should
improve the ability of the pavement to resist low temperature-induced cracking. The selection of a
modifier may be based on a range of factors that include availability, cost, properties, and familiarity with
the product. Due to differences in chemistry of asphalts, a particular polymer used with one asphalt
source will likely not yield the same physical properties as the same polymer with a different asphalt
source. Of primary importance in the selection of an appropriate polymer is the phase separation
characteristics of the asphalt/polymer combination during storage to ensure homogeneity of the binder
prior to mixing with aggregate and during testing of the material. Phase separation can occur in the
storage tank if the PMAB is not properly dispersed, leading to a heterogenous binder in the binder/
aggregate mix which may affect performance. The SHRP PG specification is currently the best utility
available for judging the expected performance of a PMAB. Although, not perfect, performance grading
2-12
Previous Page
