ASTM D6648-01
(Test Method)Standard Test Method for Determining the Flexural Creep Stiffness of Asphalt Binder Using the Bending Beam Rheometer (BBR)
Standard Test Method for Determining the Flexural Creep Stiffness of Asphalt Binder Using the Bending Beam Rheometer (BBR)
SIGNIFICANCE AND USE
The temperatures for this test are related to the winter temperature experienced by the pavement in the geographical area for which the asphalt binder is intended.
The flexural creep stiffness or flexural creep compliance, determined from this test, describes the low-temperature stress-strain-time response of asphalt binder at the test temperature within the range of linear viscoelastic response.
The low-temperature thermal cracking performance of asphalt pavements is related to the creep stiffness and the m-value of the asphalt binder contained in the mix.
The creep stiffness and the m-value are used as performance-based specification criteria for asphalt binders in accordance with AASHTO Method of Practice MP1.
SCOPE
1.1 This test method covers the determination of the flexural-creep stiffness or compliance and m-value of asphalt binders by means of a bending-beam rheometer. It is applicable to material having flexural-creep stiffness values in the range of 20 MPa to 1 GPa (creep compliance values in the range of 50 nPa-1 to 1 nPa-1) and can be used with unaged material or with materials aged using aging procedures such as Test Method D2872, Test Method D1754, or Practice D6521. The test apparatus may be operated within the temperature range from -36oC to 0oC.
1.2 Test results are not valid for test specimens that deflect more than 4 mm or less than 0.08 mm when tested in accordance with this test method.
1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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Designation: D 6648 – 01
Standard Test Method for
Determining the Flexural Creep Stiffness of Asphalt Binder
Using the Bending Beam Rheometer (BBR)
This standard is issued under the fixed designation D 6648; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope 2.2 AASHTO Standard:
AASHTO MP1 Standard Specification for Performance
1.1 This test method covers the determination of the
Graded Binder
flexural-creep stiffness or compliance and m-value of asphalt
2.3 DIN Standard:
bindersbymeansofabending-beamrheometer.Itisapplicable
tomaterialhavingflexural-creepstiffnessvaluesintherangeof
20 MPa to 1 GPa (creep compliance values in the range of 50
3. Terminology
-1 -1
nPa to 1 nPa ) and can be used with unaged material or with
3.1 Definitions:
materials aged using aging procedures such as Test Method
3.1.1 asphalt binder, n—an asphalt-based cement that is
D 2872, Test Method D 1754, or Practice D 6521. The test
produced from petroleum residue either with or without the
apparatus may be operated within the temperature range from
addition of modifiers.
-36°C to 0°C.
3.1.2 physical hardening, n—a time-dependent, reversible
1.2 Test results are not valid for test specimens that deflect
stiffening of asphalt binder that typically occurs when the
more than 4 mm or less than 0.08 mm when tested in
binder is stored below room temperature.
accordance with this test method.
3.2 Definitions of Terms Specific to This Standard:
1.3 This standard does not purport to address all of the
3.2.1 contact load, n—the load, P , required to maintain
c
safety concerns, if any, associated with its use. It is the
positive contact between the test specimen, supports, and the
responsibility of the user of this standard to establish appro-
loading shaft; 35 6 10 mN.
priate safety and health practices and determine the applica-
3.2.2 flexural creep compliance, D(t), n—the ratio obtained
bility of regulatory limitations prior to use.
by dividing the maximum bending strain (see Eq X1.5) in a
2. Referenced Documents beam by the maximum bending stress (Eq X1.4). The flexural
creep stiffness is the inverse of the flexural creep compliance.
2.1 ASTM Standards:
3.2.3 flexural creep stiffness, S(t), n—the creep stiffness
C 802 Practice for Conducting an Interlaboratory Test Pro-
obtained by fitting a second order polynomial to the logarithm
gram to Determine the Precision of Test Methods for
of the measured stiffness at 8.0, 15.0, 30.0 60.0, 120.0, and
Construction Materials
4 240.0 s and the logarithm of time (see Eq 4, section 14.4).
D 140 Practice for Sampling Bituminous Materials
3.2.4 measured flexural creep stiffness, S (t), n—the ratio
m
D 1754 Test Method for Effect of Heat andAir onAsphaltic
4 (see Eq 3, section 14.2) obtained by dividing the measured
Materials (Thin-Film Oven Test)
maximum bending stress (see X1.4) by the measured maxi-
D 2872 Test Method for Effect of Heat andAir on a Moving
4 mum bending strain (see Eq X1.5). Flexural creep stiffness has
Film of Asphalt (Rolling Thin-Film Oven Test)
been used historically in asphalt technology while creep
D 6521 Practice for Accelerated Aging of Asphalt Binder
compliance is commonly used in studies of viscoelasticity.
Using a Pressurized Aging Vessel (PAV)
3.2.5 m-value, n—the absolute value of the slope of the
E 77 Test Method for Inspection and Verification of Ther-
logarithm of the stiffness curve versus the logarithm of time
mometers
(see Eq 5, section 14.5).
3.2.6 test load, n—the load, P, of 240-s duration used to
t
1 determine the stiffness of the asphalt binder being tested; 980
This test method is under the jurisdiction of ASTM Committee D04 on Road
6 50 mN.
and Paving Materials and is the direct responsibility of Subcommittee D04.44 on
Rheological Tests.
Current edition approved June 10, 2001. Published August 2001.
This standard is based on SHRP Product 1002 and AASHTO TPI.
Available from theAmericanAssociation of State Highway and Transportation
Annual Book of ASTM Standards, Vol 04.02. Officials, 444 N. Capitol St. NW, Washington, DC 20001.
4 7
Annual Book of ASTM Standards, Vol 04.03. Deutsches Institut fuer Normung (German Standards Institute), Beuth Verlag
Annual Book of ASTM Standards, Vol 14.03. GmbH, Burggrafenstrasse 6, 1000 Berlin 30, Germany.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
D 6648
4. Summary of Test Method 6.2 Measurements for which the mid-point deflection of the
test specimen is less than 0.08 mm are suspect. When the
4.1 The bending beam rheometer is used to measure the
mid-point deflection is less than 0.08 mm, the test system
mid-point deflection of a simply supported prismatic beam of
resolution may not be sufficient to produce reliable test results.
asphalt binder subjected to a constant load applied to the
mid-point of the test specimen.The device operates only in the
7. Apparatus
loading mode; recovery measurements cannot be obtained with
7.1 A bending beam rheometer (BBR) test system consist-
the bending beam rheometer.
ing of the following: (1) a loading frame with test specimen
4.2 A prismatic test specimen is placed in the controlled
supports, (2) a controlled temperature liquid bate which main-
temperature fluid bath and loaded with a constant test load for
tains the test specimen at the test temperature and provides a
240.0 s. The test load (980 6 50 mN) and the mid-point
buoyant force to counterbalance the force resulting from the
deflection of the test specimen are monitored versus time using
mass of the test specimen, (3) a computer-controlled data
a computerized data acquisition system.
acquisition system, (4) test specimen molds, and (5) items for
4.3 The maximum bending stress at the midpoint of the test
verifying and calibrating the system.
specimen is calculated from the dimensions of the test speci-
7.2 Loading Frame—A frame consisting of a set of sample
men, the span length, and the load applied to the test specimen
supports, a blunt-nosed shaft to apply the load to the midpoint
for loading times of 8.0, 15.0, 30.0, 60.0, 120.0, and 240.0 s.
of the test specimen, a load cell mounted in line with the
The maximum bending strain in the test specimen is calculated
loading shaft, a means for zeroing the load applied to the test
from the dimensions of the test specimen and the deflection for
specimen, a means for applying a constant load to the test
the same loading times. The stiffness of the test specimen for
specimen and a deflection measuring transducer attached to the
the specific loading times is calculated by dividing the maxi-
loading shaft. A schematic of the device is shown in Fig. 1.
mum bending stress by the maximum strain.
7.3 Loading System—A loading system that is capable of
5. Significance and Use
applyingacontactloadof35 610mNtothetestspecimenand
maintaining a test load of 980 6 50 mN within 6 10 mN.
5.1 The temperatures for this test are related to the winter
7.3.1 Loading System Requirements—The rise time for the
temperature experienced by the pavement in the geographical
test load shall be less than 0.5 s. The rise time is the time
area for which the asphalt binder is intended.
required for the load to rise from the 35 6 10 mN contact load
5.2 Theflexuralcreepstiffnessorflexuralcreepcompliance,
to the 980 6 50 mN test load. During the rise time the system
determinedfromthistest,describesthelow-temperaturestress-
shall dampen the test load to 980 6 50 mN. Between 0.5 and
strain-time response of asphalt binder at the test temperature
5.0 s, the test load shall be within 6 50 mN of the average test
within the range of linear viscoelastic response.
load,andthereaftershallbewithin 610mNoftheaveragetest
5.3 The low-temperature thermal cracking performance of
load. Details of the loading pattern are shown in Fig. 2.
asphalt pavements is related to the creep stiffness and the
7.3.2 Loading Shaft—Aloadingshaftcontinuousandinline
m-value of the asphalt binder contained in the mix.
with the load cell and deflection measuring transducer with a
5.4 The creep stiffness and the m-value are used as
spherically shaped end 6.3 6 0.3 mm in radius.
performance-based specification criteria for asphalt binders in
7.3.3 Load Cell—A load cell to measure the contact load
accordance with AASHTO Method of Practice MP1.
and the test load. It shall have a minimum capacity of no less
6. Interferences
than 2.00 N and a resolution of at least 2.5 mN. It shall be
6.1 Measurements for which the mid-point deflection of the mounted in line with the loading shaft and above the fluid level
in the controlled temperature bath.
test specimen is greater than 4.0 mm are suspect. Strains in
excess of this value may exceed the linear response of asphalt 7.3.4 Linear Variable Differential Transducer (LVDT)—A
linearvariabledifferentialtransducerorothersuitabledeviceto
binders.
FIG. 1 Schematic of Test Device
D 6648
FIG. 2 Definition of Loading Pattern
section 11.5. A platinum resistance thermometer meeting DIN Standard
measure the deflection of the test specimen. It shall have a
43760 (Class A) is recommended for this purpose.
linearrangeofatleast6mm,andbecapableofresolvinglinear
movement of 2.5 µm. It shall be mounted axially with and
7.5 Controlled-Temperature Fluid Bath—A controlled-
above the loading shaft.
temperature liquid bath capable of maintaining the temperature
7.3.5 Sample Supports—Two stainless steel or other non-
at all points in the bath to within 6 0.1°C of the test
corrosive metal supports with a 3.0 6 0.3 mm contact radius
temperature in the range of -36°C to 0°C. Placing a test
and spaced 102 6 1.0 mm apart. The spacing of the supports
specimen in the bath may cause the bath temperature to
shall be measured to 6 0.3 mm and the measured value shall
fluctuate 6 0.2°C from the target test temperature. Conse-
be used in the calculations in Section 14. The supports shall be
quently bath fluctuations of 6 0.2°C during iso-thermal con-
dimensionedtoensurethatthetestspecimenremainsincontact
ditioning shall be allowed.
with the radiused portion of the support during the entire test.
7.5.1 Bath Agitator—A bath agitator for maintaining the
See Fig. 3.
required temperature homogeneity with agitation intensity
7.3.5.1 The width of the test specimen support in contact
such that the fluid currents do not disturb the testing process
with the test specimen shall be 9.50 6 0.25 mm. See Fig. 3.
and mechanical noise caused by vibrations is less than the
7.3.5.2 Avertical alignment pin 2 to 4 mm in diameter shall
resolution specified in 7.3.3 and 7.3.4.
be provided at the back of each support to align the test
7.5.2 Circulating Bath (Optional)—A circulating bath unit
specimen on the supports. The front face of the pins shall be
separate from the test frame, which pumps the bath fluid
6.75 6 0.25 mm from the middle of the support. See Fig. 3.
through the test bath. If used, vibrations from the circulating
7.4 Temperature Transducer—A calibrated temperature
system shall be isolated from the bath test chamber so that
transducer capable of measuring the temperature to 0.1°C over
mechanical noise is less than the resolution specified in 7.3.3
the range from -36°C to 0°C and mounted within 50 mm of the
and 7.3.4.
geometric center of the test specimen.
7.6 Data Acquisition and Control Components—A data
acquisition system that resolves loads to the nearest 2.5 mN,
NOTE 1—The required temperature measurement can be accomplished
test specimen deflection to the nearest 2.5 µm, and bath fluid
withanappropriatelycalibratedplatinumresistancethermometer(PRT)or
a thermistor. Calibration of the PRT or thermistor can be verified as per temperature to the nearest 0.1°C. The data acquisition system
FIG. 3 Schematic of Specimen Supports
D 6648
shall sense the point in time when the signal to switch from the shall be used in the calculation of the thickness of the test
contactloadtothetestloadisactivated.Thistimeshallbeused specimens when using the equations in section 13.2.3.1.
as the zero loading time for the test load and deflection signals.
7.8.2 Stainless Steel (Thin) Beam for Overall System
Using this time as the reference for zero time, the data
Check—One stainless steel beam 1.0 to 1.6 mm thick by 12.7
acquisition system shall provide a record of subsequent load
6 0.1 mm wide by 127 6 5 mm long with an elastic modulus
and deflection measurements at 8.0, 15.0, 30.0, 60.0, 120.0,
reported to three significant figures by the manufacturer of the
and 240.0 s.
BBR. The manufacturer of the BBR shall measure and report
7.6.1 Filtering of Acquired Load and Deflection Signals—
thethicknessofthisbeamtothenearest0.01mmandthewidth
The load and deflection signals shall be filtered with a low pass
to the nearest 0.05 mm. The dimensions of the beam shall be
analog and/or digital filter that removes components with
used to calculate the modulus of the beam during the overall
frequencies greater than 4 Hz from the load and deflection
system check (see section 11.3).
signals. Filtering may be accomplished by averaging five or
7.8.3 Standard Masses—Standard masses for verification
more digital signals equally spaced in time about the time at
and calibration as follows:
whichthesignalisreported.Theaveragingshallbeoveratime
7.8.3.1 Verification of Load Cell Calibration—One or more
period less than or equal to 6 0.2 s of the reporting time. For
masses totaling 100.0 6 0.2 g and two masses of 2.0 6 0.2 g
example, the load and deflection signals at 8.0 s may be the
each for verifying the calibration of the load cell (see section
average of signals at 7.8, 7.9, 8.0, 8.1, 8.2 s.
11.3).
7.7 Test Specimen Molds—Test specimen molds with inte-
7.8.3.2 Calibration of Load Cell—Four masses each of
rior dimensions of 6.35 6 0.05 mm wide by 12.70 6 0.05 mm
known mass 6 0.2 g, and equally spaced in mass over the
deep by 127 6 5 mm long fabricated from aluminum or
range of the load cell (see A1.2).
stainless steel as shown in Fig. 4, or from silicone rubber as
shown in Fig. 5.
7.8.3.3 Daily Overall System Check—Two or more masses,
7.7.1 The thickness of the two spacers used for each mold
each of known mass to 60.2 g for conducting overall system
(small end pieces used in the metal molds) shall be measured
check as specified by the manufacturer (see section 11.4).
with a micrometer and shall not vary from each other in
7.8.3.4 Accuracy of Masses—Accuracy of the masses in
thickness by
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