This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles

for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

Designation: D8225 − 19

Standard Test Method for

Determination of Cracking Tolerance Index of Asphalt
Mixture Using the Indirect Tensile Cracking Test at
Intermediate Temperature1
This standard is issued under the fixed designation D8225; 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 (´) indicates an editorial change since the last revision or reapproval.

1. Scope 2. Referenced Documents

1.1 This test method covers the procedures for preparing, 2.1 ASTM Standards:2
testing, and measuring asphalt mixture cracking resistance D8 Terminology Relating to Materials for Roads and Pave-
using cylindrical laboratory-prepared asphalt mix samples or ments
pavement cores. Testing temperatures are selected from the D3203/D3203M Test Method for Percent Air Voids in Com-
long-term pavement performance (LTPP) database intermedi- pacted Asphalt Mixtures
ate temperatures. The test method describes the determination D3666 Specification for Minimum Requirements for Agen-
of the cracking tolerance index, CTIndex, and other parameters cies Testing and Inspecting Road and Paving Materials
determined from the load-displacement curve. These param- D6373 Specification for Performance Graded Asphalt
eters can be used to evaluate the resistance of asphalt mixtures Binder
to cracking. D6925 Test Method for Preparation and Determination of
the Relative Density of Asphalt Mix Specimens by Means
1.2 The values stated in SI units are to be regarded as of the Superpave Gyratory Compactor
standard. No other units of measurement are included in this 2.2 AASHTO Standards:3
standard. R 30 Practice for Mixture Conditioning of Hot Mix Asphalt
(HMA)
1.3 The text of this standard references notes and footnotes
M 320 Specification for Performance-Graded Asphalt
which provide explanatory material. These notes and footnotes
Binder
(excluding those in tables and figures) shall not be considered
M 332 Specification for Performance-Graded Asphalt
as requirements of the standard. Binder Using Multiple Stress Creep Recovery (MSCR)
1.4 This standard does not purport to address all of the Test
safety concerns, if any, associated with its use. It is the
3. Terminology
responsibility of the user of this standard to establish appro-
priate safety, health, and environmental practices and deter- 3.1 Definitions:
mine the applicability of regulatory limitations prior to use. 3.1.1 For definitions of terms used in this standard, refer to
1.5 This international standard was developed in accor- Terminology D8.
dance with internationally recognized principles on standard- 3.2 Definitions of Terms Specific to This Standard:
ization established in the Decision on Principles for the 3.2.1 CTIndex, n—cracking tolerance index, value used to
evaluate mixture resistance to cracking.
Development of International Standards, Guides and Recom-
mendations issued by the World Trade Organization Technical 3.2.2 Gf, n—failure energy (Joules/m2) required to induce a
Barriers to Trade (TBT) Committee. unit surface area of a crack and calculated as the work of

2
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
1
This test method is under the jurisdiction of ASTM Committee D04 on Road Standards volume information, refer to the standard’s Document Summary page on
and Paving Materials and is the direct responsibility of Subcommittee D04.26 on the ASTM website.
3
Fundamental/Mechanistic Tests. Available from American Association of State Highway and Transportation
Current edition approved April 1, 2019. Published April 2019. Originally Officials (AASHTO), 444 N. Capitol St., NW, Suite 249, Washington, DC 20001,
approved in 2019. DOI: 10.1520/D8225-19. http://www.transportation.org.

Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
Copyright by ASTM Int'l (all rights reserved); Mon Dec 28 19:48:40 EST 2020
1
Downloaded/printed by
University Of Nevada Reno (University Of Nevada Reno) pursuant to License Agreement. No further reproductions authorized.
 D8225 − 19
failure divided by specimen diameter (150 6 2 mm) and 4.2 This test procedure considers both crack initiation and
normalized thickness of 62 mm. propagation in asphalt mixtures, and is developed based on
3.2.3 l65, n—displacement (mm) corresponding to 65 % of fracture mechanics.4
the peak load at the post-peak stage. See Fig. 1. 5. Significance and Use
3.2.4 l75, n—displacement (mm) corresponding to 75 % of 5.1 The indirect tensile cracking test is used to determine
the peak load at the post-peak stage. See Fig. 1. asphalt mixture cracking resistance at an intermediate tempera-
3.2.5 l85, n—displacement (mm) corresponding to 85 % of ture which could range from 5 °C to 35 °C, depending on local
the peak load at the post-peak stage. See Fig. 1. climate. The specimens are readily obtained from Superpave
P 2P gyratory compactor compacted cylinders with a diameter of
3.2.6 |m75|, n—slope (N/m) calculated as | l 852l 65 | using a 150 6 2 mm, with no cutting, gluing, notching, drilling, or
85 65
linear regression with all data points between P85 (l85) and P65 instrumentation required. Similarly, field cores can be tested to
(l65). See Fig. 1. measure remaining cracking resistance of in-place asphalt
mixtures.
3.2.7 P65, n—65 % of the peak load (kN) at the post-peak
stage. See Fig. 1. 5.2 The CTIndex of an asphalt mixture is calculated from the
failure energy, the post-peak slope of the load-displacement
3.2.8 P75, n—75 % of the peak load (kN) at the post-peak
curve, and deformation tolerance at 75 % of the peak load. The
stage. See Fig. 1.
CTIndex is a performance indicator of the cracking resistance of
3.2.9 P85, n—85 % of the peak load (kN) at the post-peak asphalt mixtures containing various asphalt binders, asphalt
stage. See Fig. 1. binder modifiers, aggregate blends, fibers, and recycled mate-
3.2.10 Wf, n—work of failure (Joules) calculated as the area rials. Generally, the higher the CTIndex value, the better the
under the load-displacement curve. cracking resistance and, consequently, the less the cracking
amount in the field. The range for an acceptable CTIndex will
4. Summary of Test Method vary with mix types and associated specific applications.4
Users can employ the CTIndex and associated criteria to identify
4.1 A cylindrical specimen is centered in the fixture. The crack-prone mixtures during mix design and production quality
load is applied such that a constant load-line displacement control/assurance.
(LLD) rate of 50.0 6 2.0 mm/min is obtained and maintained
for the duration of the test. Both the load and LLD are 4
Zhou, F., Im, S., Sun, L., and Scullion, T., “Development of an IDEAL
measured during the entire duration of the test and are used to Cracking Test for Asphalt Mix Design and QC/QA,” Road Materials and Pavement
calculate the CTIndex. Design, Vol 18, Supplement 4, 2017, pp. 405–427.

FIG. 1 Recorded Load (P) versus Load-Line Displacement (l) Curve

Copyright by ASTM Int'l (all rights reserved); Mon Dec 28 19:48:40 EST 2020
2
Downloaded/printed by
University Of Nevada Reno (University Of Nevada Reno) pursuant to License Agreement. No further reproductions authorized.
 D8225 − 19
NOTE 1—The quality of the results produced by this standard are the specimen as in Fig. 2. The outer edges of the loading strips
dependent on the competence of the personnel performing the procedure shall be beveled slightly to remove sharp edges.
and the capability, calibration, and maintenance of the equipment used.
Agencies that meet the criteria of Specification D3666 are generally 6.1.3.1 Option A—The loading strips can be part of a test
considered capable of competent and objective testing, sampling, fixture similar to that shown in Fig. 2, in which the lower
inspection, etc. Users of this standard are cautioned that compliance with loading strip is mounted on a base having two perpendicular
Specification D3666 alone does not completely ensure reliable results. guide rods or posts extending upward. The upper loading strip
Reliable results depend on many factors; following the suggestions of shall be clean and freely sliding on the posts. Guide sleeves in
Specification D3666 or some similar acceptable guideline provides a
means of evaluating and controlling some of those factors. the upper segment of the test fixture shall direct the two loading
strips together without appreciable binding or loose motion in
6. Apparatus the guide rods.
6.1 Test Apparatus—An indirect tensile cracking test appa- 6.1.3.2 Option B—The upper and lower loading strips, as
ratus consists of an axial loading device, a load cell, loading shown in Fig. 3, are parts of an axial loading device. They are
strips, specimen deformation measurement devices, and a data permanently attached to the top loading actuator and the base
acquisition system. Alternatively, the load cell, loading strips, plate, respectively.
specimen deformation measurement devices, data acquisition 6.1.3.3 Option C—The upper and lower loading strips, as
system, or combinations thereof can be integrated into a test shown in Fig. 4, are part of a test fixture integrated with a load
fixture. cell, loading strips, specimen deformation measurement
6.1.1 Axial Loading Device—The loading apparatus shall be devices, and a data acquisition system.
capable of delivering loading in compression with a capacity of 6.1.4 Internal Displacement Measuring Device—The dis-
at least 25 kN. It shall be capable of maintaining a constant placement shall be measured to a resolution of 60.01 mm. The
deformation rate of 50 6 2.0 mm/min, which may require a machine stroke linear variable differential transformer (LVDT)
closed-loop, feedback-controlled servo-hydraulic load frame. or other type of displacement transducer can be used if its
An electromechanical, screw-driven frame may be used if it resolution is sufficient to meet the requirement. The displace-
can maintain the constant deformation rate. ment data measured during the test may need to be corrected
6.1.2 Load Cell—The load cell shall have a resolution of for system compliance through standardizing the test system.
10 N and a capacity of at least 25 kN. 6.1.5 External Displacement Measuring Device—If an in-
6.1.3 Loading Strips—Steel loading strips with a concave ternal displacement measuring device does not exist or has
surface having a radius of curvature equal to the nominal radius insufficient precision, one or more external displacement mea-
of the test specimen. For specimens with a nominal diameter of suring devices such as LVDTs can be used (Fig. 3).
150 6 2 mm, the loading strips shall be 19.05 6 0.3 mm wide. 6.1.6 Data Acquisition System—Time, load, and LLD (using
The length of the loading strips shall exceed the thickness of either internal or external displacement measuring devices)

FIG. 2 Traditional Indirect Tension Test Fixture

Copyright by ASTM Int'l (all rights reserved); Mon Dec 28 19:48:40 EST 2020
3
Downloaded/printed by
University Of Nevada Reno (University Of Nevada Reno) pursuant to License Agreement. No further reproductions authorized.
 D8225 − 19

FIG. 3 Loading Strips Embedded in an Axial Loading Device

data are collected at a minimum of 40 sampling data points per in thickness; for the mixtures with a NMAS of 25 mm or larger,
second to obtain a smooth load-LLD curve. specimens shall be 150 6 2 mm in diameter and 95 6 1 mm
6.2 Conditioning Chamber—An environmental chamber or in thickness. All specimens are prepared without cutting or
water bath capable of maintaining the target intermediate test trimming.
temperature 61.0 °C for conditioning specimens before test- 8.2.2 Aging—Laboratory-compacted test specimens shall be
ing. properly conditioned before the compaction.
6.3 Gyratory Compactor—A gyratory compactor and asso- NOTE 2—For laboratory-mixed and laboratory-compacted (LMLC)
ciated equipment for preparing laboratory specimens in accor- mixes, specimens should be conditioned properly before the compaction.
dance with Test Method D6925 are needed. One option is to condition the loose mix for 4 h according to AASHTO R
30 for mixture mechanical property testing. For plant-mixed and
6.4 Saw—A laboratory saw capable of trimming field cores, laboratory-compacted mixes (PMLC), specimens may be compacted after
if needed. reheating the mix to its compaction temperature. The acceptable CTIndex
criteria are dependent on the aging method used. It may be necessary to
6.5 Sample Measurement Device—A caliper accurate to adjust CTIndex criteria or establish LMLC and PMLC to account for the
60.1 mm shall be used to measure specimen thickness and effect of aging. A Superpave gyratory compactor, according to Test
diameter. Method D6925, is preferred for compacting test specimens, but other
types of compactors (such as Marshall hammer) are allowed as long as the
7. Hazards specimens meet dimensional requirements.

7.1 Standard laboratory caution should be exercised when 8.2.3 Air Void Content—Prepare a minimum of three speci-
handling, compacting, and fabricating test specimens and mens at the target air void content 60.5 %.
asphalt mixtures. NOTE 3—The specimen air voids can be calculated using Test Method
D3203/D3203M. The typical air void target for highway pavements is
8. Sampling, Test Specimens, and Test Units 7.0 %. Other target air voids can be used, but specimens with significantly
8.1 The indirect tensile cracking test may be conducted on different air voids (larger than 60.5 %) are not comparable.
laboratory-prepared test specimens or field cores. 8.3 Samples Cored From Asphalt Pavements:
8.2 Laboratory-Compacted Asphalt Mixture Samples: 8.3.1 Roadway cores can be used if pavement layer thick-
8.2.1 Specimen Size—For the mixtures with a nominal ness is greater than 38 mm. Roadway core specimens shall be
maximum aggregate size (NMAS) of 19 mm or smaller, the 150 6 2 mm in diameter, with all surface of the perimeter
specimens shall be 150 6 2 mm in diameter and 62 6 1 mm perpendicular to the surface of the core within 6 mm. Trim top

Copyright by ASTM Int'l (all rights reserved); Mon Dec 28 19:48:40 EST 2020
4
Downloaded/printed by
University Of Nevada Reno (University Of Nevada Reno) pursuant to License Agreement. No further reproductions authorized.
 D8225 − 19

FIG. 4 Loading Strips Embedded in a Test Fixture With Load Cell, Displacement Measurement Device, and Data Acquisition System

and bottom surface of all cores to the same thickness with these where:
guidelines. Roadway core test specimen shall be prepared as PG IT = intermediate performance grade temperature (°C),
thick as possible, but in no case be less than 38 mm. While a PG HT = climatic high-performance grade temperature (°C), and
PG LT = climatic low-performance grade temperature (°C).
thickness correction is applied in the calculation of CTIndex,
testing specimens at a uniform thickness will reduce test error.
NOTE 4—Care shall be taken to avoid damage to the cores during 9.2 Inspect the fixture to ensure all contact surfaces are
handling and transportation prior to testing. A core bit of 156 mm in
clean and free of debris.
diameter may be needed in order to obtain cores 150 6 2 mm in diameter.
The air voids of the core specimens should be determined. Additionally, 9.3 Insert the specimen in the fixture, ensuring the specimen
the CTIndex values of core specimens are relatively comparable but may is centered and making uniform contact on the support.
not be equal to those of laboratory-compacted specimens due to different
aging conditions.
Generally, it is sufficient to center the specimen by eye.
8.4 A minimum of three specimens shall be tested for 9.4 Apply load to the specimen in LLD control at a rate of
LMLC or PMLC specimens. A minimum of three roadway 50 6 2.0 mm/min. Stop the test when the load drops below
core specimens shall be tested. 100 N. During the testing, record the time, load, and displace-
ment at a minimum sampling rate of 40 data points per second.
9. Procedure 9.5 Testing shall be completed in 4 min or less after removal
9.1 Precondition test specimens in an environmental cham- from the environmental chamber to maintain a uniform speci-
ber or water bath at a target intermediate test temperature men temperature.
61.0 °C for 2 h 6 10 min.
10. Calculation or Interpretation of Results
NOTE 5—The typical target intermediate test temperature is 25 °C.
Other target intermediate test temperatures can be used. One choice for the
10.1 The work of failure (Wf) is calculated as the area under
target intermediate test temperature is PG IT defined in Specification the load versus LLD curve (see Fig. 1) through the quadrangle
D6373, AASHTO M 320, or AASHTO M 332 and provided in Eq 1: rule provided in Eq 2:

( S ~l D
n21
PG HT1PG LT 1
PG IT 5 14 (1) Wf 5 i11 2 l i ! 3 P i 1 3 ~ l i11 2 l i ! 3 ~ P i11 2 P i ! (2)
2 i51 2

Copyright by ASTM Int'l (all rights reserved); Mon Dec 28 19:48:40 EST 2020
5
Downloaded/printed by
University Of Nevada Reno (University Of Nevada Reno) pursuant to License Agreement. No further reproductions authorized.
 D8225 − 19
where: 11. Report
Wf = work of failure (Joules), 11.1 The report shall include the following parameters for
Pi = applied load (kN) at the i load step application, each test specimen:
Pi+1 = applied load (kN) at the i + 1 load step application,
li = LLD (mm) at the i step, and 11.1.1 Asphalt mixture type.
li+1 = LLD (mm) at the i + 1 step. 11.1.2 Test temperature, °C.
10.2 Failure energy (Gf) is calculated by dividing the work 11.1.3 Specimen preparation method and aging condition.
of failure (the area under the load versus the average LLD 11.1.4 Specimen air voids, %.
curve; see Fig. 1) by the cross-sectional area of the specimen 11.1.5 Specimen thickness, mm.
(the product of the diameter and thickness of the specimen): 11.1.6 Specimen diameter, mm.
Wf 11.1.7 Displacement, l75, mm.
Gf 5 3 106 (3)
D 3t 11.1.8 Post-peak slope, |m75|, N/m.
where: 11.1.9 Failure energy, Gf, Joules/m2.
Gf = failure energy (Joules/m2), 11.1.10 Work of failure, Wf, Joules.
Wf = work of failure (Joules), 11.1.11 Cracking tolerance index, CTIndex.
D = specimen diameter (mm), and
t = specimen thickness (mm). 12. Precision and Bias
10.3 Post-peak slope (|m75|) is the slope of tangential zone 12.1 The within-laboratory repeatability standard deviation
around the 75 % peak load point after the peak; see Fig. 1. of the CTIndex has been determined to be 13.5, based on one
10.4 Displacement (l75) is the displacement at 75 % the peak lab, 30 test replicates, and ten different mixtures (materials).
load after the peak. The between-laboratory reproducibility of this test method is
being determined and will be available on or before May 31,
10.5 Cracking tolerance index (CTIndex) is calculated from 2021. Therefore, this standard should not be used for accep-
the parameters obtained using the load-displacement curve, as tance or rejection of a material for purchasing purpose.
listed below:
t l 75 Gf NOTE 7—The CTIndex mean ranged from 31 to 255 for the ten different
CT Index 5 3 3 3 106 (4) materials used to develop this preliminary within-laboratory precision
62 D m 75? ? statement, and the specimens were molded with a Superpave gyratory
compactor and were tested with the fixture shown in Fig. 3.
where:
CTIndex = cracking tolerance index, 12.2 No information can be presented on the bias of the
Gf = failure energy (Joules/m2), procedure in this test method for measuring the cracking
|m75| = absolute value of the post-peak slope m75 (N/m), tolerance index because no material having an accepted refer-
l75 = displacement at 75 % the peak load after the peak ence value is available.
(mm),
D = specimen diameter (mm), and 13. Keywords
t = specimen thickness (mm).
t 13.1 asphalt mixture cracking resistance; cracking tolerance
NOTE 6— is a unitless correction factor for specimen thickness. 106 index; failure energy; indirect tensile cracking test; work of
62
is a scale factor in Eq 4. failure

ASTM International takes no position respecting the validity of any patent rights asserted in connection with any item mentioned
in this standard. Users of this standard are expressly advised that determination of the validity of any such patent rights, and the risk
of infringement of such rights, are entirely their own responsibility.

This standard is subject to revision at any time by the responsible technical committee and must be reviewed every five years and
if not revised, either reapproved or withdrawn. Your comments are invited either for revision of this standard or for additional standards
and should be addressed to ASTM International Headquarters. Your comments will receive careful consideration at a meeting of the
responsible technical committee, which you may attend. If you feel that your comments have not received a fair hearing you should
make your views known to the ASTM Committee on Standards, at the address shown below.

This standard is copyrighted by ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959,
United States. Individual reprints (single or multiple copies) of this standard may be obtained by contacting ASTM at the above
address or at 610-832-9585 (phone), 610-832-9555 (fax), or service@astm.org (e-mail); or through the ASTM website
(www.astm.org). Permission rights to photocopy the standard may also be secured from the Copyright Clearance Center, 222
Rosewood Drive, Danvers, MA 01923, Tel: (978) 646-2600; http://www.copyright.com/

Copyright by ASTM Int'l (all rights reserved); Mon Dec 28 19:48:40 EST 2020
6
Downloaded/printed by
University Of Nevada Reno (University Of Nevada Reno) pursuant to License Agreement. No further reproductions authorized.