Cooling cycle effects on low temperature cracking ...

ORIGINAL ARTICLE

Cooling cycle effects on low temperature crackingcharacteristics of asphalt concrete mixture

Behzad Behnia • William G. Buttlar •

Henrique Reis

Received: 7 August 2013 / Accepted: 8 April 2014 / Published online: 18 April 2014

� RILEM 2014

Abstract The effect of cooling cycles on the low

temperature behavior of asphalt concrete mixtures is

investigated using a recently developed acoustic

emission (AE) test device. In an attempt to link the

local AE response of the asphalt mixtures to the

pavement global response obtained through mechan-

ical tests, the AE-based results were validated using

traditional mechanical pavement performance testing

methods namely, the disk-shaped compact tension

[DC(T)] test and the indirect tensile test (IDT) method.

Field-aged pavement cores, which were expected to

have a gradient in binder aging properties (more aging

near the surface of the pavement), were collected and

tested. Test results revealed that significant damage

resulted from cyclic cooling, effecting the fracture

energy and stress relaxation ability of the asphalt

mixture. The AE results collected were consistent with

the results obtained using the DC(T) and the IDT test

methods. The so-called Felicity effect was observed

by evaluating AE activity occurring in a sample

subjected to multiple cooling cycles and an AE based

healing index was introduced to evaluate the amount

of healing that resulted from warming cycles. Low

temperature induced microdamage was also investi-

gated using X-ray computer micro-tomography, in an

effort to better understand the physical nature of

microcracking in asphalt mixtures at low temperatures

and the source of AE emissions detected.

Keywords Low temperature cracking �Cooling cycle � Mechanical performance tests �DC(T) � IDT � Non-destructive tests � Acoustic

emission � Felicity effect �Kaiser effect �Healing

index � X-ray computed tomography

1 Introduction

Unless rehabilitated regularly, all asphalt pavements,

will eventually fail. Low temperature cracking, also

referred to as thermal cracking, is considered as a

major form of distress in asphalt pavements built in

regions with cold climates. It results from the

contraction of the pavement during cooling tempera-

tures under restrained conditions. As temperature

drops, thermally induced contraction strains generate

tensile stresses inside the restrained pavement layer.

B. Behnia (&) � W. G. Buttlar

Department of Civil and Environmental Engineering,

University of Illinois at Urbana-Champaign, 205 N.

Mathews Ave., Urbana, IL 61801, USA

e-mail: [email protected]

W. G. Buttlar


H. Reis

Department of Industrial and Enterprise Systems

Engineering, University of Illinois at Urbana-Champaign,

104 S. Mathews Ave., Urbana, IL 61801, USA


Materials and Structures (2014) 47:1359–1371

DOI 10.1617/s11527-014-0310-y

Author's personal copy

Once these thermally induced tensile stresses reach the

tensile strength of asphalt mixture, thermal cracks

begin to form in the pavement. Under severe cooling

cycles, very low temperatures and/or very fast cooling

rates, thermal cracks may develop within the pave-

ment under one or very few cooling cycles [11, 14].

Low temperature cracks manifest themselves as

transversely-oriented surface-initiated cracks of vari-

ous lengths and widths as shown in Fig. 1. This form

of distress drastically affects asphalt pavement life

span, rideability, and leads to millions of dollars in

repair and maintenance annually. Detrimental effects

of low-temperature cracking have motivated a number

of studies in an effort to experimentally design and

control asphalt properties related to the low-temper-

ature performance of asphalt pavements. However,

accurate predictions of thermal cracking and associ-

ated failure mechanisms still remain a challenge. The

current study for evaluation of low-temperature

cracking performance of asphalt pavements is driven

by the need to address the aforementioned challenges

asphalt pavements are facing in cold climates. The

intent of this paper is to investigate effects of cooling

cycles on low-temperature cracking characteristics of

asphalt concrete material through non-destructive

(NDT) evaluation techniques along with performing

advanced pavement performance mechanical tests.

During the course of this study, lab compacted

samples as well as field cores specimens taken from

an outdoor testing section were evaluated. Two

advanced pavement performance tests namely, disk-

shaped compact tension (DC[T]) test and indirect

tensile test (IDT) were employed to evaluate/charac-

terize asphalt concrete’s macro-structure. The micro-

structure of asphalt concrete material was assessed

using two NDT tests namely: acoustic emission (AE)

and X-ray computed micro-tomography imaging.

2 Experimental methods and results

The asphalt mixture samples used in this study were

obtained from field cores as well as laboratory

compacted gyratory samples. The first set of samples

consisted of field cores taken from a 50 meter long

outdoor testing section located at Advanced Trans-

portation Research Engineering Laboratory (ATREL)

in Rantoul, IL. This test section was constructed in

September 2010 mainly to conduct NDT tests on

pavements. Field cores were taken from the test

section in March 2012. From the time of construction

till the time when the cores were extracted, testing

section experienced two winters and one summer. The

highest and lowest air temperatures at the test section

in 2011 and 2012 were 100 �F (38 �C) and -8 �F (-

22.2 �C), and 102 �F (39 �C) and 6 �F (-14.4 �C),

respectively. During that period of time the air relative

humidity changed between 34 and 95 %. What makes

this test section unique is the absence of mechanical

(traffic) loads throughout the pavement’s life. This

means that there is no load-related distress in the

pavement and all existing damage is caused solely by

the environment. Presence of traffic loads make the

problem very complicated. On one hand traffic load

may improve and accelerate the healing process of

thermally induced cracks in asphalt concrete by

causing slight increase in the pavement temperature;

On the other hand, the traffic load itself can further

damage the pavement by inducing load related cracks

in the pavement. Investigating the low temperature

cracking characteristics of asphalt pavements sub-

jected to both mechanical and thermal loads is out of

the scope of this study.

Asphalt pavement of the test section has been

exposed to the environment (snow, rain, hot and cold

weather) reacting with the oxygen in the air even at the

moderately low temperatures, causing oxidative

aging. A total of six field cores of 200 mm high and

150 mm diameter were extracted from the test section.

Figure 2 schematically illustrates the test section from

which asphalt mixture field cores were extracted. In

addition, three 180 mm high, 150 mm diameter lab

compacted samples were prepared using loose PG64-

22 asphalt mixture from the test section. Loose asphalt

Fig. 1 Thermal cracking formation and non-uniform thermal

stresses distribution through the pavement thickness [14]

1360 Materials and Structures (2014) 47:1359–1371


mixture was sampled at the time of construction of test

section, properly sealed and stored to avoid aging. Lab

compacted samples can be considered to have the

same aging level as the test section asphalt mixture

right after the pavement construction. In order to

fabricate testing samples for mechanical tests, field

cores and laboratory samples were sliced into two

50 mm thick cylindrical specimens: one from the top

and another one from the bottom of the sample. To

avoid possible end-effects, around 10 mm of both

sides was trimmed off before slicing the samples as

shown in Fig. 2. AE, and X-ray tomography tests as

well as DC(T) and IDT tests were performed on both

field cores and laboratory samples.

2.1 Disk-shaped compact tension [DC(T)] test

To evaluate the low-temperature fracture characteris-

tics of asphalt mixture samples, DC(T) test was

performed for all samples in accordance with ASTM

D7313 at -12, ?10 �C higher than the PG low

temperature grade of the test section asphalt binder

(PG64-22) [3]. The DC(T) geometry is a 50 mm thick

circular specimen with a single edge notch loaded in

tension. Due to its simple geometry, DC(T) specimens

can be obtained easily from field cores or lab compacted

samples. In this study DC(T) test was selected as the

fracture test, in order to ensure compatibility with

cylindrical specimens and to allow a 19 mm nominal

maximum aggregate size mixture to be tested with

sufficient fracture ligament area [7].

The loading rate for the DC(T) test is controlled

through opening displacement at the crack mouth. A

constant crack mouth opening displacement (CMOD)

rate of 1 mm/min was used in this study in accordance

with ASTM D7313-07. The CMOD gage mounted to

the DC(T) is shown in Fig. 3. Fracture energy of the

specimens is determined by calculating the normal-

ized area under the Load-CMOD curve. Normaliza-

tion is done to obtain the fracture energy required to

produce a unit fracture area. The DC(T) testing setup

and DC(T) specimen geometry are shown in Fig. 3.

To investigate effects of cooling cycles on cracking

performance of asphalt mixtures, half of laboratory

compacted samples were subjected to intense one

cooling-cycle by placing them inside a cooling

chamber at -50 �C for 2 h to induce thermal damage.

DC(T) test were performed on both intact, i.e., virgin,

and thermally-damaged samples. Obtained fracture

energy results are presented in Fig. 4. Figure 4a shows

the DC(T) fracture energy results for both thermally-

damaged and intact laboratory samples. It is observed

that the average CMOD fracture energy of thermally

damaged samples is about 12 % less than fracture

energy of intact samples. This clearly shows the

detrimental effect of a severe applied cooling cycle on

fracture resistance of asphalt concrete. Reduction in

fracture resistance of the asphalt concrete after one

intense cooling cycle indicates the presence of ther-

mally induced micro-damages inside the conditioned

samples. Figure 4b compares the fracture energies of

top and bottom specimens obtained from field cores.

Fig. 2 Schematic

representation of test section

and field cores [6]

Materials and Structures (2014) 47:1359–1371 1361


Figure 4b shows that the fracture energy of top

specimens are lower than the fracture energy of

specimens extracted from the bottom of the pavement

layer. The average fracture energy of the top speci-

mens is around 306 J/m2, whereas for the bottom

specimens the fracture energy is 362 J/m2. The

(a) (b)82.5

Fig. 3 a DC(T) test setup

b geometry and

recommended dimensions

for the DC(T) sample

Fig. 4 DC(T) CMOD fracture energy results for field cores and

lab compacted samples: a fracture energy for thermally

damaged and intact lab specimens, b fracture energies for top

and bottom specimens extracted from field cores and c,

d compare fracture energies of field core samples with

laboratory compacted samples



observed difference in fracture energies can be

associated with the presence of environmentally

induced damages in the specimens extracted from

the top of the field cores, which negatively affect the

fracture cracking resistance of the asphalt mixture.

Figure 4c, d compares CMOD fracture energies of

field core samples versus laboratory compacted sam-

ples. The average CMOD fracture energy of the

bottom field core samples 362 J/m2, is very close to

that of the intact laboratory samples, 354 J/m2. This is

because field core bottom specimens were not directly

exposed to the environment; i.e., they were somehow

protected from oxidative aging as well as damaging

intense thermal cooling gradients. That’s why field

cores bottom specimens and intact laboratory samples

have the same performance as undamaged test section

mixture, i.e., they exhibit similar cracking resistance.

Surprisingly, the fracture energies of the field core top

samples are close to the samples that were submitted to

1-cooling cycle, which implies that the 2 h of intense

cooling of the laboratory samples caused more or less

the same level of thermally induced micro-damages

that exist in the top field core samples.

2.2 Indirect tensile test (IDT)

IDT creep tests were also performed to further

investigate the cooling cycle effects on low-temper-

ature cracking performance of the asphalt mixtures.

The IDT tests were performed on both laboratory

compacted and field core samples. Testing was

conducted in accordance with AASHTO T322 testing

procedure utilizing three test replicates [1]. Creep

testing was conducted at 0, -10 and -20 �C through

application of static loading for the duration of

1,000 s. Creep compliance was determined using the

horizontal and vertical deformations measured in the

center of the specimen. Applied loads should be

carefully selected so that the load-induced horizontal

and vertical strains remain in the linear viscoelastic

range during the creep test. The induced strains were

measured 30 s after starting the creep test and

compared with the allowable lower and upper limits

to ensure that measurements are conducted within the

linear range of material response. Figure 5 schemat-

ically illustrates the specimen for IDT test [1, 9].

Creep compliance master-curves were generated

through time–temperature superposition. A power-law

model was fitted to the master-curves using Eq. (1)

that shows the fitting function. The parameters D0, D1

and m are determined using a least-square fitting

procedure. The m value of the power law is associated

with the rate of creep deformation and stress relaxation

in viscoelastic materials.

DðtÞ ¼ D0 þ D1tm ð1ÞThe creep compliance results from IDT creep tests

are presented in Fig. 6, which shows the correspond-

ing fitted power-law curves along with the exponential

terms from the power-law model (m value). The

m value represents the ability of the material to relax

induced stresses during the relaxation period. The

higher the m value, the better the cracking perfor-

mance of the material. As expected, the aged-hardened

field cores (i.e., top samples) had the lowest m value

and creep compliance values, which indicate they are

prone to thermal cracking. On the other hand, the other

field cores (i.e., from the pavement bottom) samples

ID(T) Sample

ID(T) Epsilon Gages

Fig. 5 IDT geometry and dimensions

Fig. 6 Creep compliance master-curves from IDT test results



exhibited the best performance by exhibiting the

highest m values and creep compliance. In the case of

laboratory compacted samples, test results followed

the same trend; intact, i.e., virgin, samples showed

higher m values and creep compliances as compared to

thermally-damaged samples. Interestingly, even

though field cores bottom specimens and laboratory

compacted intact samples were almost at the same

aging level, field cores bottom samples performed

better than lab intact samples. This can be linked to

better compaction of asphalt concrete in the field

which resulted in an asphalt mixture with lower

potential for thermal cracking.

2.3 Acoustic emission-based (AE) test

AE phenomenon refers to the generation of transient

elastic mechanical waves caused by the sudden release

of local stresses (strain energy) in a material. This

technique is applicable wherever the stresses in a

material stimulate the release of strain energy in the

form of a transient stress wave, whose detection can be

used as a possible indication of failure. AE waves are

produced in a material due to the dynamic local

rearrangement of its internal structure. When a

material is mechanically or thermally stressed to the

point where inelastic deformation occurs, it emits AE

waves. The energy of AE during crack growth is

drawn from the stress field around the crack, which

relaxes momentarily as the crack propagates. Emis-

sion of acoustic energy results from a momentary

instability and a sudden movement of the crack tip to

restore equilibrium. Emitted AE elastic waves prop-

agate outward from the source and can be detected

using sensitive piezoelectric sensors mounted on the

surface of the test sample [4].

The use of AE for evaluating asphalt materials

dates back several decades. AE has been used

extensively to assess the microscopic fracture pro-

cesses and to evaluate damage growth in asphalt

materials. Khosla and Goetz [13] used AE techniques

to detect crack initiation and propagation in IDT

specimens at low temperature. Hesp et al. [12] used

AE measurements to detect crack initiation and

propagation in restrained specimens at low tempera-

tures (-20 to -32 �C). Li and Marasteanu [15–17]

implemented AE technique to analyze the microstruc-

tural phenomena of damage and the corresponding

macroscopic behavior in asphalt mixtures tested at low

temperatures. Apeagyei et al. [2] and Behnia et al.

[6–8] utilized embrittlement temperature from AE

measurements to evaluate the suitability of the AE

technique for low-temperature characterization of

asphalt binders and mixtures.

This study employs the AE technique to detect and

assess occurrence of micro-damages in asphalt mix-

tures due to cooling cycles. Unlike mechanical tests

this technique evaluates the material from the micro-

scale standpoint. Essentially, asphalt mixtures are

heterogeneous materials composed of the asphalt

binder and aggregates. Upon rapid cooling of the

asphalt mixture, due to differential thermal contrac-

tion between the asphalt mastic and aggregates,

thermal stresses build up inside the specimen prior to

the onset of material fracture. As the temperature

drops, material behavior gradually changes from

quasi-brittle to brittle state where resistance to fracture

is generally very low, allowing microcracks to develop

readily. Microcracks in asphalt mixtures result pri-

marily from a combination of asphalt mastic brittle-

ness and the action of thermally-induced tensile

stresses within the material [17].

Semicircular-shaped asphalt mixture samples hav-

ing a thickness of 50 mm, and a diameter of 150 mm

were prepared for AE testing. AE samples were

positioned on a steel block acting as a heat sink in the

cooling chamber as shown in Fig. 7. AE tests were

conducted in a polystyrene box containing dry ice as

the coolant. Wideband AE sensors (Digital Wave,

Model B1025) with a nominal frequency range of

20 kHz to 1.5 MHz were utilized to monitor and

record acoustic activities of the sample during the test.

High-vacuum grease was used to couple the AE

sensors to the test sample. AE Signals were pre-

amplified 20 dB using broad-band pre-amplifiers to

reduce extraneous noise. The signals were then further

amplified 21 dB (for a total of 41 dB) and filtered

using a 20 kHz high-pass double-pole filter using the

Fracture Wave Detector (FWD) signal condition unit.

The signals were then digitized using a 16-bit analog-

to-digital converter (ICS 645B-8) using a sampling

frequency of 2 MHz and a length of 2,048 points per

channel per acquisition trigger. The outputs were

stored for later processing using Digital Wave soft-

ware (Wave-Explorer TM V7.2.6). Sample tempera-

ture was continuously recorded through the test using

a K-type thermocouple placed on the specimen

surface. Typical temperature versus time cooling plot



is shown in Fig. 8. The average cooling rate was

around 0.7 �C/min. The AE testing setup and speci-

men dimensions are schematically shown in Fig. 7a, b,

respectively [2].

AE tests were conducted on both lab samples and

field cores specimens. Analyses of thermally induced

AE activities of asphalt concrete samples were

performed on accumulated AE events extracted from

AE tests. Each AE event represents a rapid physical

change such as microdamages in the material test

sample that releases energy in the form of a transient

stress wave, and appears as an AE signal. Tempera-

tures corresponding to the first major high energy

event, the embrittlement temperature, as well as total

number of AE events counts are presented in the

Table 1. AE results of laboratory prepared samples

show that the first cooling cycle specimens

consistently have much warmer embrittlement tem-

peratures, TA, and significantly higher number of AE

events counts as compared to those of the second

cooling cycle samples. The main source of AE events

of the asphalt concrete samples subjected to cold

temperatures is generation of thermally-induced mi-

crocracks within samples. During the first cooling

cycle, shortly after starting the test, as soon as

thermally induced local tensile stresses within the

sample exceed the strength of the material, asphalt

concrete samples encounter large number of micro-

cracks. Formation of microcracks in the sample

generates a large number of AE events, see Table 1.

AE asphalt concrete samples sit for 48 h at room

temperature, 20 �C, after manufacturing. Due to

viscoelastic properties of asphalt concrete (i.e. stress

relaxation), stresses induced within asphalt concrete

during manufacturing (compaction and cutting) of

samples are completely relieved after 48 h. It appears

reasonable to assume that AE samples were in stress-

free condition before applying the first cooling cycle.

Here, the room temperature is termed as the ‘‘stress-

free temperature’’, TC, and it is used as the reference

temperature. Before starting the second cooling cycle,

the samples’ temperature is raised to stress-free

temperature (i.e., thermal unloading) by letting them

sit at room temperature, 20 �C, for 2 h. During this

period of thermal unloading, a portion of microcracks

(perhaps the smaller microcracks) are healed, while

the reminder of the microcracks (i.e., the larger

microcracks) may need additional time and higher

temperatures to heal. During the second cooling cycle,

mainly due to the presence of thermally-induced

Fig. 7 a AE testing set-up, b AE testing specimen dimensions

Fig. 8 Typical temperature–time cooling plot with the average

cooling rate of 0.7 �C/min



microcracks created during the first cooling cycle, the

internal structure of asphalt concrete samples has

become softer; yet, because of the indeterminate

nature of the microstructure, it is still capable of

sustaining an increase in thermal stresses with lower-

ing the temperature. As a result, it takes lower

temperatures (higher in magnitude) to generate new

microcracks within the sample. This can be clearly

observed in the relative large differences between

embrittlement temperatures of sample obtained during

the first cooling cycle, TA, and during the second

cooling cycle, TD. Clearly, the embrittlement temper-

ature obtained during the second cooling cycle is

lower than the one obtained during the first cooling

cycle. The difference between embrittlement temper-

atures obtained during the first and second cooling

cycles is schematically depicted in Fig. 9, and

reported in Table 1.

The observed phenomenon is known as the Felicity

Effect. This phenomenon is witnessed during cyclic

loading of materials and is defined as the absence of

detectable AE activities during reloading of the

material until a load level below the previous maxi-

mum applied load, in the current case a thermal load.

In other words, during reloading process, the material

test sample generates AE activity before the previous

maximum loading level is reached. The Felicity effect

is also schematically illustrated in Fig. 9, which shows

the cumulative AE events counts versus applied

thermal load. The loading path from B to C (unload-

ing) and C to D to E (reloading) clearly indicates the

absence of AE activity up to a loading level (TD),

which is below the level of the previous loading cycle

(TB). It should be noted that another AE cyclic loading

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Kaiser effects occurs when no AE activity happens

until the previous maximum load level is reached,

which makes the Kaiser effect a particular case of the

Felicity effect (i.e., points B and D coincide). When

the Kaiser effect is valid indicates that no microcrack

healing occurs in the sample. Considering that the

adhesive nature of asphalt concrete material depends

on the chemical composition and the oxidative aging

level of the asphalt mixture, some level of microcracks

healing takes place within asphalt concrete during

thermal unloading. This means that in thermal cyclic

loading of adhesive materials such as asphalt mixtures

with high potential of microcrack healing, the Felicity

effect is the one that would occur rather than the

Kaiser effect. This is illustrated in Fig. 9 and reported

in Table 1.

An important parameter is the felicity ratio (FR),

which is defined as the ratio of the load at which

emissions occur to the previous maximum load. In the

present study, it is defined as the ratio of lengths of pre-

cracking regions of the first and the second cooling cycles

with respect to TC as the reference temperature, Eq. (2).

The average Felicity ratio for tested asphalt mixture is

around 0.702. To better quantify crack healing of asphalt

mixtures, a new parameter called the ‘‘Healing Index’’ is

introduced and defined by Eq. (3). The Healing Index is

an indication of the amount of healing that occurred in

asphalt mixture between the loading cycles over the

period of thermal unloading. Depend on the sample

temperature (i.e., thermal cycle), type of binder, and

presence of moisture, the Healing Index of asphalt

mixture ranges from 0 % (no healing) to 100 % (fully

healed). The average Healing Index of the tested samples

is around 29.75 %, which indicates partial healing of the

asphalt concrete mixtures during the rest period between

the first and second cooling cycles.

FR ¼ TD � TC

TB � TC

ð2Þ

Healing index ð%Þ ¼ 100 ð1 � FRÞ ð3ÞThe AE results of the field cores specimens show that

the bottom field core specimens have higher number of

AE events counts as compared to the top samples.

Similarly to the laboratory compacted samples, this can

be linked to presence of thermally induced damages in

the already age-hardened top specimens due to exposure

to the environment and severe thermal gradients.

Another important observation regarding field cores is

that the embrittlement temperatures (TA) of the top

samples are warmer than those of bottom samples.

Oxidative aging of top samples lowers their thermal

cracking resistance as it is also observed in DC(T) test

results, see Fig. 4. When subjected to cold temperature,

the aged-hardened top samples develop microcracks at

warmer temperatures than the bottom samples.

Comparison of AE events counts of the second

cooling cycle of laboratory samples and the field cores

top specimens is also interesting. Even though both

have encountered thermally-induced damages, sur-

prisingly the field cores top samples have higher

number of AE event counts as compared to the second

cooling cycle samples. The average cumulative AE

event counts of field core top specimens is around

24,000 while for the second cooling cycle samples the

cumulative AE events counts is only 356. This can be

attributed to the healing mechanisms (adhesive healing

at the asphalt-aggregate interface and cohesive healing

within asphalt mastic), which occurs in the field core

top samples. The second cooling cycle was conducted

shortly after the first cooling cycle; as a result,

laboratory samples didn’t have enough time for the

healing mechanism to occur to the same extent as the

field core tope samples. After being severely ther-

mally-damaged during the winter, the field core top

samples had enough time for the healing mechanism to

occur and to reduce the micro-damages in the pave-

ment especially during the summer months.

2.4 X-ray computed micro-tomography imaging

Asphalt mixtures have a complex heterogeneous micro-

structure. The macro properties of asphalt mixtures

strongly depend on their microstructure. X-ray computed

micro-tomography (micro-CT) technique is a powerful

tool for acquiring the internal microstructure of hetero-

geneous materials. It is a nondestructive evaluation

technique for microscopic visualization of interior fea-

tures within materials and for obtaining digital informa-

tion on their 3-D geometries and properties. Tomography

refers to the cross-sectional imaging of an object from

either transmission or reflection data collected by

illuminating the object from many different directions

[10]. In X-ray micro-CT device, X-rays were sent at

different angles through a rotating specimen, collected

through a flat-panel detector, and a computer was used to

reconstruct a 3D image of the sample’s interior [5].



Using X-ray micro-CT, numerous images of the

cross-section of asphalt concrete specimens were

collected. Each image has several levels of gray

intensity corresponding to different densities of the

materials and X-ray attenuation in the material.

Attenuation is mainly a function of X-ray energy and

the density and composition of the material being

imaged.

In the current study, X-ray micro-CT was employed

to quantitatively visualize the cooling cycle effects on

asphalt mixture. Cubic samples of 1 inch on-the-side

were prepared from intact, i.e., virgin, compacted lab

samples utilized as testing specimens. To investigate

the cooling cycle effects on asphalt mixture micro-

structure, X-ray micro-CT conducted on samples

before and after conditioning samples at -50 �C for

2 h. Figure 10 depicts sectional images of both

undamaged and thermally-damaged mixtures. Ther-

mally-induced damages, circled in red, are easily

detectable by comparing high resolution images

before and after conditioning the sample. There are

two main types of micro-damages observed: thermal

microcracking inside the asphalt mastic and ther-

mally-induced debonding between aggregate and

asphalt mastic. Debonding at the interface happens

due to differential thermal contraction between aggre-

gates and asphalt mastic. Thermal microcracking of

the asphalt mastic results primarily from a combina-

tion of asphalt mastic brittleness and the action of the

thermally-induced tensile stresses within the material.

Figure 11 schematically shows the thermally induced

stresses inside the asphalt mastic, which results in

thermal microcracks in the specimen. The size of the

thermal microcracks was measured using Amira �

software for all samples. Measurements show that

thermally-induced microcracks have the average

width and length of *30 and 350 lm, respectively,

see Fig. 12. As mentioned earlier during the

1

2

Fig. 10 X-ray micro-ct

images of intact versus

thermally damaged samples



discussion of the DC(T) and IDT tests results, the

presence of these microcracks affects macro proper-

ties of asphalt concrete mixtures such as fracture

cracking characteristics and stress relaxation ability

(m value) of the material.

3 Summary and conclusions

The current study addressed the effects of cooling

cycles on low temperature cracking behavior of

asphalt mixtures. Two mechanical performance tests

namely, DC(T) and IDT, along with two NDT tests

namely, AE and X-ray micro-CT tests were conducted

on field cores and laboratory compacted samples. The

cooling cycle was simulated by conditioning samples

at -50 �C for 2 h. Based on laboratory testing results,

the following conclusions can be drawn regarding the

low-temperature cracking performance of asphalt

concrete mixtures:

• DC(T) test results showed that field cores top

specimens and thermally damaged laboratory

samples had lower fracture energies than field

cores bottom samples and intact, i.e., virgin

laboratory specimens, respectively. Results shows

that fracture energy measurements from

DC(T) test are sensitive to the presence of

thermally induced micro-damages inside the

asphalt concrete and capable of capturing the

effects of cooling cycles on the low-temperature

cracking performance of asphalt pavements.

• The trend observed in the DC(T) test was also

observed in the IDT test as thermally damaged

both laboratory and field cores specimens had

lower m value as compared to intact ones. This

indicates a lower rate of stress relaxation as well as

higher potential for thermal cracking. In addition,

field cores bottom samples exhibited the best

performance in terms of thermally induced stress

relaxation.

Thermally induced Tensile Stresses

Microcracks

Fig. 11 Thermally induced

stresses within the asphalt

mastic [10]

Fig. 12 Size of thermally

induced damages in the

mixture



• The AE testing technique appears to be a viable

approach for evaluating the extent of thermally

induced damages inside asphalt concrete mixtures.

Presence of microcracks in test samples signifi-

cantly reduced their AE activity. In other words,

the more damaged the material the lower its AE

activity. It should be noted that, AE-based tech-

nique has been developed and successfully

employed to assess low-temperature behavior of

asphalt binders and asphalt concretes and to

determine their ‘‘Embrittlement’’ temperature.

This temperature is thought to represent funda-

mental threshold in material behavior [15, 16].

• One important difference between the AE tech-

nique and mechanical bulk and fracture testing is

the response scale within the material. In the case

of mechanical tests, the mechanical test results

are a measure of the response of the whole

specimen (as a structure) and thus it is difficult to

extract local properties, while the AE procedure

yields results that are at the local scale of the

material response. Both mechanical tests and AE

techniques are recommended for complete eval-

uation of cracking resistance of pavement

mixtures.

• Microscopic visualization of microstructure of the

mixture was performed using X-ray micro-CT.

Thermally induced micro-damages were detected

and their dimensions were measured in high

resolution images. These micro-cracks and aggre-

gate-mastic debonding are thought to be the source

of the changes in the macro properties of the

mixture namely: fracture energy, m value.

• The results of this limited study suggest that

conducting mechanical performance tests in con-

junction with NDT tests could provide a thorough

assessment of thermal damage in asphalt mixtures.

Testing of a wider variety of asphalt mixtures with

different cooling cycles should be pursued.

Acknowledgments Authors are grateful for the support

provided by the National Cooperative Highway Research

Program—Ideas Deserving Exploratory Analysis (NCHRP-

IDEA) program. Furthermore, we would like to acknowledge

help from Dr. Sarfraz Ahmed from National University of

Science and Technology, Pakistan, in this study. Any opinions,

findings, and conclusions or recommendations expressed in this

publication are those of the authors and do not necessarily reflect

the views of the sponsoring agency.

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