A new system for specifying asphalt materials has been developed. The system includes test equipment, test methods, and criteria. One portion of Superpave is a new asphalt binder specification. The document is intended to function equally well for modified as well as unmodified asphalts.
A new system for specifying asphalt materials has been developed. The system includes test equipment, test methods, and criteria. One portion of Superpave is a new asphalt binder specification. The document is intended to function equally well for modified as well as unmodified asphalts.
A new system for specifying asphalt materials has been developed. The system includes test equipment, test methods, and criteria. One portion of Superpave is a new asphalt binder specification. The document is intended to function equally well for modified as well as unmodified asphalts.
A new system for specifying asphalt materials has been developed. The system includes test equipment, test methods, and criteria. One portion of Superpave is a new asphalt binder specification. The document is intended to function equally well for modified as well as unmodified asphalts.
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I.
SUPERPAVE TO THE RESCUE
TTRODUCTION
In 1987, the Strategic Highway Research Program (SHRP) began developing a new
system for specifying asphalt materials. The final product of the SHRP asphalt research
program is a new system referred to as Superpave which stands for Superior Performing
Asphalt Pavements. Superpave software is a computer program that assists engineers in
materials selection and mix design. However, the term “Superpave” refers to more than
just the computer program. Most important, it represents an improved system for
specifying component materials, asphalt mixture design and analysis, and pavement
performance prediction. ‘The system includes test equipment, test methods, and criteria.
ASPHALT BINDERS.
One portion of Superpave is a new asphalt binder specification with a new set of tests to
match. The document is called a binder specification because it is intended to function
equally well for modified as well as unmodified asphalts. A portion of the asphalt binder
specification is shown in Appendix A.
‘The new system for specifying asphalt binders is unique in that itis a performance based
specification. It specifies binders on the basis of the climate and attendant pavement
‘temperatures in which the binder is expected to serve. Physical property requirements
remain the same, but the temperature at which the binder must attain the properties
changes. For example, the high temperature, unaged binder stiffness (G*/sin 8) is
required to be at least 1.00 kPa, But this requirement must be achieved at higher
temperatures if the binder is expected to serve in a hot climate,
Performance graded (PG) binders are graded such as PG 64-22. The first number, 64, is
often called the “high temperature grade.” This means that the binder would possess
20II, Superpave to the Rescue
adequate physical properties at least up to 64° C. This would be the high pavement
‘temperature corresponding to the climate in which the binder is actually expected to
serve. Likewise, the second number (-22) is often called the “low temperature grade” and
‘means that the binder would possess adequate physical properties in pavements at least
down to -22° C. Additional consideration is given to the time of loading (open highway,
city streets, intersections, etc.) and magnitude of loads (heavy trucks).
Another key feature to binder evaluation in the Superpave system is that physical
properties are measured on binders that have been laboratory aged to simulate their aged
condition in a real pavement. Some binder physical property measurements are
performed on unaged binder. Physical properties are also measured on binders that have
been aged in the rolling thin film oven (RTFO) to simulate oxidative hardening that
‘occurs during hot mixing and placing. A pressure aging vessel (PAV) is used to
laboratory age binder to simulate the severe aging that occurs after the binder has served
many years in a pavement (Figure II-1).
Rolling Thin Film Oven (RTFO) Pressure Aging Vessel (PAV)
Figure II-1. SHRP Binder Aging Techniques
Binder physical properties are measured using four devices:
+ dynamic shear rheometer,
+ rotational viscometer,
+ bending beam rheometer, and
= direct tension tester.I. Superpave to the Rescue
‘The dynamic shear rheometer (DSR) is used to characterize the visco-elastic properties of
the binder. It measures the complex shear modulus (G*) and phase angle (6) by
subjecting a small sample of binder to oscillatory shear stresses while sandwiched
between two parallel plates (Figure Il-2).
Applied Stress
or Strain Poston of
Oscillating Pate
Oscillating CG 2
Plate Fixed Plate
A
Asphalt A A =
ic
Leycle
=e
Figure 1-2. Dynamic Shear Rheometer
‘The DSR measures G* and 6 by measuring the shear strain response of the specimen to a
fixed torque as shown in Figure II-3. In this figure, the shear strain response of a binder
specimen is “out of phase” with the applied stress by a certain time interval At. This time
interval represents the time lag in strain response. Phase lag is normally reported in
angular measurement by simply multiplying the time lag (At) by the angular frequency
(@) to arrive at a phase angle (6). For totally elastic materials there is no lag between
applied shear stress and shear strain response and 6 equals zero degrees. For totally
‘viscous materials, strain response is completely out of phase with applied stress and 6 is
90 degrees. Viscoelastic materials like asphalt binders posses phase angles between zero
and 90 degrees, depending on test temperature. At high temperatures, 8 approaches 90
degrees while at low temperatures 6 is nearly zero degrees. The binder specification uses
either G*/sin 6 at high temperatures (> 46° C) or Gtsin 8 at intermediate temperatures
(between 7° and 34° C) as a means of controlling asphalt stiffness.II Superpave to the Rescue
Viscoelastic: 0<8 < 90°
%
Angi : complex shear modulus
oo rey)
| G* 7,
> At
oman ONT goexan
st ‘
Tne i angular frequency
\_/ phase angle
Figure II-3. Computation of G* and 6
By controlling stiffness at high temperatures, the binder specification ensures that asphalt
provides its fair share of the overall shear strength of the mixture in terms of high
temperature elasticity. Likewise, the specification ensures that the binder does not
contribute to fatigue cracking by limiting its stiffness at intermediate temperatures.
‘The rotational viscometer (RTV) characterizes the stiffness of the asphalt at 135°
where it acts almost entirely as a viscous fluid. It is a rotational coaxial cylinder
viscometer that measures viscosity by the torque required to rotate a spindle submerged
in a sample of hot asphalt (Figure II-4) at a constant speed. The binder specification
requires that binders have a viscosity of less than 3 Pa-s. This ensures that the binder can
be pumped and otherwise handled during HMA manufacturing.
applied torque
tanner
spindle
asphalt
sample
sample chamber
Figure II-4. Rotational Viscometer
The bending beam rheometer (BBR) is used to characterize the low temperature stiffness
properties of binders. It measures the creep stiffness (S) and logarithmic creep rate (m).
24HI, Superpave to the Rescue
These properties are determined by measuring the response of a small binder beam
specimen to a creep load at low temperatures (Figure II-5). By knowing the load applied
to the beam and the deflection at any time during the test, the creep stiffness can be
calculated using engineering beam mechanics. ‘The binder specification places limits on
creep stiffness and m-value depending on the climate in which the binder will serve
Binders that have a low creep stiffness will not crack in cold weather. Likewise, binders
with high m-values are more effective in shedding stresses that build in asphalt
pavements as temperatures drop, again, ensuring that low temperature cracking will be
minimized.
Constant (Creep) Load
deflection
Load Deflection
|
Figure II-5. Bending Beam Rheometer
Some binders, particularly some polymer-modified asphalts, may exhibit a higher than
desired creep stiffness at low temperatures. However, may not crack because they retain
their ability to stretch without fracture at low temperatures. Consequently, the binder
specification allows a higher creep stiffness if it can be shown through the direct tension.
test (DTT) that binders are sufficiently ductile at low temperatures. The output of the
DTT is tensile failure strain, which is measured on a small dog bone shaped specimen
that is stretched at low temperatures until it breaks (Figure II-6). As with the BBR, the
DTT ensures that the binder's resistance to low temperature cracking is maximized.II, Superpave to the Rescue
6 =
°
So
i
°
v
strain
Figure 11-6. Direct Tension Tester
MINERAL AGGREGATES
SHRP researchers also blieved that mineral aggregates played a key role in HMA
performance. While they did not develop any new aggregate test procedures, they refined
existing procedures to fit within the Superpave system. Two types of aggregate
properties are specified in the Superpave system: consensus properties and source
properties.
Consensus properties are those which the SHRP researchers believed were critical in
achieving high performance HMA. These properties must be met at various levels
depending on traffic level and position within the pavement. High traffic levels and
surface mixtures (j.e., shallow pavement posi
on) require more strict values for
consensus properti
; Many agencies already use these properties as quality requirements
for aggregates used in HMA. These prop.
+ coarse aggregate angularity,
+ fine aggregate angularity,
+ flat, elongated particles, and
+ clay content.
By specifying coarse and fine angularity, SHRP researchers were seeking to achieve
HMA with a high degree of internal friction and thus, high shear strength for rutting