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8/11/2019 2005 Wagnoner Buttlar Paulino Disk-shaped Compact Tension Test
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8/11/2019 2005 Wagnoner Buttlar Paulino Disk-shaped Compact Tension Test
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Fig. 1--Different fracture specimen geometries for asphalt
concrete:
a) Single-edge notched beam; 3,6-9 b) Semi-
circular bend; ]~ ]] c) Disk-sh aped comp act tension
Fig. 2--Example of a core obtained at a reflective crack
location in a pavemen t structure
current work is motivated by the need to characterize the
fracture behavior of asphalt concrete fro m standard cyl indri-
cal f ield and laboratory specim ens.
One prospect ive fracture test that can be performed us-
ing cyl indrical samples is the semi-circular bend (SCB) test
(Fig. l (b)) . T he SCB test has been used to obtain the fracture
toughness of asphalt concrete for use in est imating fat igue
cracking l~ and for invest igat ing the frac ture energy of as-
phal t concrete at low temperatures . l l
One concern wi th the SCB tes t a r rangement fo r asphal t
coi 'icrete is that the 150 mm diame ter of a s tandard laboratory-
compacted asphalt specimen leads to a relat ively short frac-
ture l igame nt i n the SC B specimen . Without conclusive e v-
idence of the s ize of the fracture process zone '4 of asphalt
concrete (the relat ively large grain s ize of the aggregates is
apparent in Fig. 2), the ini t ial l igament length should be as
large as possible to produce a rel iable fracture property and
fracture test . Also, the crack propagation with the SCB ge-
ome try creates an arching effect with high com pressive s tress
as the crack approaches the top edge. The high compressive
stresses could arrest the crack, which further reduces the ef-
fect ive l igament, creat ing potent ial ly inval id test ing results .
Other geometries , such as notched direct tension, 12 have
been used for obtaining fracture propert ies of asphalt con-
crete, but these geo metries have the sa me l imitat ion of being
restricted to non-standard labo ratory compactio n procedures.
Therefo re , the deve lopment o f a d i sk -shaped compact t ens ion
test (Fig. l(c)) was pursued in this investigation.
Disk shaped Com pact Tension Geome try for
Asphalt Concrete
The A ST M E399 Standard Test Metho d 13 describes the
D
W
a
d
C
R e c o m m e n d e d
D i m e n s i o n s m m )
150
l l 0
25
27.5
25
35
L _
ff
L
I +el -
i s
i.~-:-~
dd
Fig. 3~Re comm ende d DC T) geometry for asphalt
concrete
DC(T) specimen for obtaining the plane-strain fracture
toughness o f metal l ic specimens. A compa ct tension test has
previously been used to obtain the fracture toughness of as-
phal t concrete; 14 however, the spec imen geom etry w as o f the
standard rectangular-shaped form , which could not be easi ly
fabricated from field cores or s tandard laboratory com paction
methods.
The AS TM E399 specificat ion was used as a s tart ing point
in this s tudy for the developmen t of a DC(T) specime n geom -
etry for fracture test ing of asphalt concrete. A fter several i ter-
at ions in the laboratory, a specime n geome try that max imize s
l igamen t length but prevents specim en rupture at the loading
holes was developed. The resul t ing spec imen dimensions are
given in Table 1 along with the AST M E399 dim ensions. The
proposed specimen geometry is schematical ly displayed in
Fig. 3.
The d imens ions o f the p roposed DC(T) spec imen fo r as -
phal t concrete are within the tolerances given in the ASTM
E399 specificat ion, with the exception of the locat ion of load-
ing holes. During the develop mental s tages of the DC(T ) ge-
ometry, the loading holes were placed closer to the edges of
the specime n to create a longer l igament length. However, th is
geomet ry had a t endency to fa i l (45 0% of the spec imens) a t
the loading holes (see Fig. 4(a)) due to the geometrical con -
figurat ion with insufficient material betwe en holes and outer
boundaries . The loading holes were m oved towards the center
of the specimen to ensure sufficient material was present and
to reduce the chance of the specimen fai l ing at the loading
9 Society for Experimental Mechanics Experimental Mechanics 9
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Fig. 4--Crack growth in the initial and final specimen geom-
etry (a) init ial specimen geom etry with fai lure at the loading
points; (b) final specimen geometry with desirable fracture
plane
T BLE
1--COMPARISON OF DC(T) SPECIMEN DIMEN-
SIONS GIVEN IN ASTM E399 AND THOSE RECOM-
MENDED FOR ASPHALT CONCRETE
Dimensions in mm
Recommended for
ASTM E399 Asphalt Concrete*
D 150 150
W 111 110
max 56 50
Bmin 28
C 28 35
qb 28 25
d
31 25
amax 61 27.5
ami n
50
* Based upon experiments on asphalt mixtures with 9.5 mm
nominal maximum aggregate size and sma ller
holes. After modifying the loading hole locations (Fig. 4(b)),
over 80 specimens have been tested without a single failure
at a loading hole.
Because the DC(T) geometry is being developed as a prac-
tical engineering fracture test, the specimen thickness (B)
should reflect the typical pavement layer thicknesses present
in the field (25-100 mm). For test repeatability, it is often rec-
ommended that the smallest dimension of the specimen be at
least four times greater than the maximum aggregate size. 15
For this study, the maximum aggregate size in the mixture
was 9.5 mm. A specimen thickness of 50 mm was selected
for this study based upon these considerations. The influence
of specimen thickness on fracture energy has been observed
in Portland cement concrete 16 and deserves consideration for
asphalt concrete. However, the investigation of such an effect
was beyond the scope of this preliminary investigation.
The a~ ratio (see Fig. 3) for the present study was held
constant at 0.25, which is less than the ASTM range o f 0.45-
0.55. The ASTM specification has this recommendation to
ensure that the specimen meets the small-scale yielding cri-
teria. The 0.25 a~ ratio was selected to provide a larger
fractured face area, which should reduce the variability of
the test results caused by heterogeneity of asphalt concrete.
In order for the DC(T) geometry to be practical, the fab-
rication of the specimen should also be straightforward. The
specimen can be cut from a cylinder to the desired thickness
using a water-cooled masonry saw. Next, a template can be
used to mark the location of the loading holes, the flat edge at
the notch mouth, and the notch length. The specimen can then
be mounted on an X - Y table connected to a water-cooled
core drill with a 25 mm outside diameter core barrel to core
the loading holes. The use of a core drill was selected over
other methods because the core barrel produces a smooth
loading hole that is perpendicular to the specimen face. The
X - Y table was used to precisely locate the loading holes,
reducing the chance of eccentricity during loading. The flat
edge can be cut into the specimen using a water-cooled ma-
sonry saw. Careful consideration was taken in the fabrication
of the mechanical notch. The ASTM specification has the re-
quirement of a fatigue crack grown from the mechanical notch
tip to produce a sharp crack tip. The ability to produce a fa-
tigue crack of known length in asphalt concrete without dam-
aging the surrounding material is questionable and may not
be necessary if the width of the mechanical notch is narrow
compared to the aggregate structure. 9 To efficiently achieve
a narrow notch width, the mechanical notch was cut to ap-
proximately half of the intended length using a water-cooled
masonry saw with a notch width of 5 mm. Then, the mechan-
ical notch was finished using a handsaw with a carbide-grit
blade, producing a notch width of approximately 1 ram. The
two-step procedure was used to expedite the fabrication of
the notch and the entire notch could be fabricated using the
1 mm wide saw blade as well.
Experimental Procedure
An experimental design was developed to determine the
potential of using the DC(T) geometry for obtaining the frac-
ture energy o f asphalt concrete. A single mixture was used,
consisting of a 9.5 mm maximum aggregate size with a PG64-
22 asphalt binder. The PG64-22 asphalt binder, determined
by the American Association of State Highway and Trans-
portation Officials (AASHTO) MP1 Standard, 17 is a widely
used asphalt binder for the climate found in central Illinois.
The 9.5 mm maximum aggregate size mixture is typical of
a surface layer in an asphalt concrete pavement. Cylindrical
specimens were prepared in the laboratory following stan-
dard procedures using a gyratory compa cto r) 8 All specimens
were compacted to 934-1 of the theoretical maximum den-
sity of the mixture. From each gyratory-compacted cylinder
(115 mm high), two DC(T) specimens were fabricated with a
50 mm thickness following the procedures described above.
All testing was performed with an Instron 8500 servo-
hydraulic load frame with an environmental chamber capable
of controlling the temperature from 30~ to -30~ within
-t-0.1~ The loading fixture was fabricated following the
ASTM specification to ensure that the loading pins remained
frictionless throughout the loading. Load was measured with
a 10 kN load cell and the crack mouth opening displacement
(CMOD) with an Epsilon Model 3541-0020-250-ST clip-on
gage. Figure 5 shows the experimental setup with the loading
fixtures, specimen, clip-on gage, and crack detection wires:
one on the front (shown) and one on the back. The clip-on
gage was used as the feedback for the control loop to ensure
stable post-peak crack growth. The experimental design was
developed to detect the changes of fracture energy at various
temperatures (-20~ -10~ and 0~ and loading rates (10,
5, 1, and 0.1 mm mi n- 1). At each temperature and rate, three
replicates were tested to obtain the average fracture energy.
272 9 VoL 45, No. 3, June 200 5 9 200 5 Society for Experimental Mechanics
8/11/2019 2005 Wagnoner Buttlar Paulino Disk-shaped Compact Tension Test
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Fig. 5--Experimental setup for the DC(T) specimen with
crack detection wire
Several proposed techniques were found in the literature
for determining the fracture energy but, for this study, the
fracture energy was computed as the area under the load-
CMOD curve normalized by the area of the fractured sur-
face (initial ligament length times specimen thickness). In
the computation of fracture energy, unavoidable electronic
noise in the sensors creates unreliable estimates of area un-
der the load-CMOD curve. To circumvent this problem, the
CMOD data were smoothed using linear regression through
the CMOD-time curve. The use of linear regression is valid
since the test was in fact controlled at a constant CMOD rate.
Test esults
For the DC(T) geometry to provide a useful fracture prop-
erty, the variability of the fracture energy should be within an
acceptable range. The repeatability of the test and the mea-
sured quantities (load and CMOD) influence the fracture en-
ergy, and understanding these curves provides insight into
the fracture process. Figure 6 shows the load~CMOD curves
for three replicates tested at -1 0~ and 1 mm min -1 CMOD
rate. For the three replicates, the initial stiffness and peak
loads are very similar. The softening or post-peak responses
of the replicates vary in shape and length of the softening
tail. Crack detection wires have been placed at the notch tip
(Fig. 5) to indicate when the macrocrack initiates, which typ-
ically occurs after peak load (see Fig. 6). However, it must
be acknowledged that microcracking and localized damage
occur throughout the loading, especially at peak load. The
aggregate structure could have an effect on the crack path
and thus the softening response. The crack path can traverse
through aggregates, between aggregate-aspha lt binder inter-
faces, or through the asphalt binder. The fracture energy as-
sociated with each of these paths is expected to be quite dif-
ferent. Since the aggregate structure for each test replicate is
different, the softening response will likewise differ for the
aforementioned reasons. The Ioad-CMOD curves of Fig. 6
suggest that while the aggregate structure does not signif-
icantly affect the load required for crack initiation, it does
lead to some scatter between replicates in the load-CMOD
response after peak load is reached.
The coefficients of variation (COVs) of the fracture en-
ergy at each temperature and loading rate were obtained for
comparison (Table 2). In general, the COV decreases as the
Fig. 6---Load--CMOD curve for three experimental results
(a/I /V= 0.25) at- 10 ~
temperature increases. The reason for the increase in vari-
ability with decreasing temperature could be that the material
is more brittle at lower temperatures, where defects (voids,
weak aggregates, etc.) significantly influence the fracture pro-
cess. Based upon visual observation of the fracture surface,
the crack appeared to travel around aggregates at the high-
est temperature (0~ whereas at the lowest temperature,
the crack front often traveled through aggregates. Asphalt
binder properties have a strong dependence on temperature,
where the modulus and strength generally increase as the
temperature decreases with the exception of binder strength
at very low temperatures.19 The COV from the DC(T) geom-
etry was compared to the data found in the literature for the
SE(B)Tand SCB It geometries. The COV determined for the
DC(T) geometry was within the range determined with the
SE(B) (3-28 ) and SCB (15-34 ).
With satisfactory repeatability o f fracture energy measure-
ments, the next step was to assess the ability of the DC(T) test
to detect the effects of temperature and loading rate on frac-
ture energy. Asphalt concrete shows a transition from brittle
to ductile fracture behavior as temperature increases. Fig-
ure 7 shows the Ioad- CMOD curves for a single loading rate
(1 mm mi n- I ) at the three test temperatures. The initial stiff-
ness of the material decreases with increasing temperature
(see Table 3) along with a change in the post-peak soften-
ing response. The curvature of the softening curve decreases
as the temperature increases, suggesting more load carrying
capacity, and thus more total energy is required to create the
fracture surface. In this case, more energy is spent on inelastic
deformation. The same phenomenon is seen for the loading
rates where the material appears more brittle as loading rate
increases. Table 2 shows the average fracture energy of three
replicates at all temperatures and loading rates. In general, the
fracture energy increases as the temperature increases, and
energy decreases with increasing loading rate. The current
analysis assumes the distribution of energy is dominated by
fracture energy, but creep strain dissipation, distributed ma-
terial damage, etc., will also consume energy. It is hoped that
current work to couple the experimental observations with
numerical analysis will lead to a truly fundamental material
9 Society for Experimental Mechanics Experimental Mechanics
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TABLE 2--AVERAGE FRACTURE ENERGY AND COV OF THE FRACTURE ENERGY AT VARIOUS TEMPERATURES AND
LOADING RATES fBASED UPON THRE E REPLICATES
Fracture Energy (J m -2) COV (%)
Rate Temperature (~ Temperature (~
(mm m n- 1 -20 -10 0 -20 -10 0
10 41 276 407 14.5 8.7 13.0
5 197 318 397 25.4 12.3 10.3
1 233 328 470 17.4 14.0 4.2
0.1 245 352 848 15.9 14.5 8.8
Fig. 8---Three types of crack paths observed from the tests
with the DC(T) geometry: (a) type I crack path; (b) type II
crack path; (c) type III crack path
Fig. 7--Load-CMOD curves for 1 mm/min loading rate at
three test temperatures
TABLE 3--INITIAL STIFFNESS FOR VAR-
IOUS TEMPERATURES AT 1 mm/min
LOADING RATE
Tempe rature Initial Stiffnes s
(~ (kN mm -1)
-20 60.4
-10 45.9
0 32.4
proper ty to be extracted from the DC T) tes t. Concurre nt tes t-
ing of s imilar materials with the SE B) tes t wil l be used to
val idate the analysis techniques, which are current ly under
development .
Discussion of Crack Trajectory
Although the tes t repeatabi l i ty and the t rends in the frac-
ture energy were sat isfactory, the observed deviat ions from
the mode I crack path were a cause for concern. Three dis-
t inct types of crack paths were observed during the tes t ing,
as shown in Fig . 8 . The mo de I or s traight crack path termed
type I ) was the second mos t comm on crack pa th and was a l so
the des ired c rack pa th . The m os t com mon crack pa th was the
type II path , where the crack ini t iates in a s traight path and
then deviates at a point along the l igament . The final crack
path , and also the least comm on, was the crack that in it iates
and propagates at an angle from the notch t ip . For el imina-
t ion of crack deviat ion, s ide grooves could be fabricated in
the specim en, but th is would require extra s teps in the fabri-
ca t ion p rocedure , wh ich wou ld be cumbe rsome and perhaps
impract ical . Table 4 shows the average crack deviat ion an-
gles for three repl icates at the temperatures and rates tes ted
during this s tudy. There are several possible causes fo r crack
turning and crack deviat ion, such as the presence of larger
and/or s tronger aggregates in the crack path . Other possible
factors include material heterog eneity and load eccentrici ty .
Statistical Analys is of Variance of Test Re sults
To support the observat ions that the fracture energy can de-
tect changes with tempe rature and loading rate, an analysis of
var iance ANOVA) was perfo rmed on the tes t da ta using SAS
Version 9.1 software. Two sep arate hypotheses w ere tes ted:
1) s tat is t ically different values of fracture energy can be ob-
tained with the propos ed DC T) tes t for the temperatures and
loading rates selected for th is s tudy; 2) fracture energy is not
correlated with c rack deviat ion angle. To tes t the hypotheses ,
the p-value, or the smallest s ignificance level at which the
observe d data w ould have caused reject ion, 2~ was used and
set at 0.05. For the fi rs t hypothesis , the p-va lue is muc h less
than the probabil i ty set 0 .05) as shown in Table 5 , indicat-
ing that s tat is t ical ly s ignificant d ifferences in fracture e nergy
can be de tec ted by the DC T) t es t s among the t es t t empera-
tures and loading rates s tudied. The second hypothesis that
the crack de viat ion does not affec t the fracture energ y is also
found to be t rue, as summarized in Table 6 . The correlat ion
betw een fracture energy and crack deviat ion is very low R =
-0.048) and the differences between the means do not d iffer
s ignificantly from zero p-va lue = 0 .7455).
Cohesive Zone Fracture Simulation of DC T)Test
To gain insight in to the fracture behavior of asphalt con-
crete, an in tegrated appr oach is being undertaken by cou pling
274 9 VoL 45, No. 3, June 200 5 9 200 5 Society for Experim ental Mecha nics
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TABLE 5--ANOV A TABLE FOR COMPARING FRACTURE ENERGY TO TEMPERATURE AND LOADING RATE
Source DF Type III SS Mean Square F-value P r > F
Tempem ture 2 751612.06 375806.03 216 .78
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