implementation of nondestructive testing and mechanical performance approaches to assess low temperature fracture properties of asphalt binders

Accepted ManuscriptImplementation of Nondestructive Testing and Mechanical Performance Ap-proaches to Assess Low Temperature Fracture Properties of Asphalt Binders Salman Hakimzadeh, Be

Accepted Manuscript

Implementation of Nondestructive Testing and Mechanical Performance

Ap-proaches to Assess Low Temperature Fracture Properties of Asphalt Binders

Salman Hakimzadeh, Behzad Behnia, William G Buttlar, Henrique Reis

To appear in: International Journal of Pavement Research and

Technology

Received Date: 29 June 2016

Revised Date: 19 January 2017

Accepted Date: 20 January 2017

Please cite this article as: S Hakimzadeh, B Behnia, W.G Buttlar, H Reis, Implementation of Nondestructive Testing and Mechanical Performance Approaches to Assess Low Temperature Fracture Properties of Asphalt

Binders, International Journal of Pavement Research and Technology (2017), doi: http://dx.doi.org/10.1016/j.ijprt 2017.01.005

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Implementation of Nondestructive Testing and Mechanical Performance Approaches to Assess Low Temperature Fracture Properties of Asphalt Binders

ABSTRACT

In the present work, three different asphalt binders were studied to assess their fracture behavior

at low temperatures Fracture properties of asphalt materials were obtained through conducting the Compact Tension [C(T)] and Indirect Tensile [ID(T)] strength tests Mechanical fracture tests were followed by performing Acoustic Emissions test to determine the “embrittlement temperature” of binders which was used in evaluation of thermally induced microdamages in binders Results showed that both nondestructive and mechanical testing approaches could successfully capture low-temperature cracking behavior of asphalt materials It was also observed that using GTR as the binder modifier significantly improved thermal cracking resistance of PG64-22 binder The overall trends of AE test results were consistent with those of mechanical tests

Keywords: Thermal Cracking; Indirect Tensile Strength Test; Compact Tension Test;

Nondestructive Approach; Acoustic Emission Test; Embrittlement Temperature

1 INTRODUCTION

Low temperature cracking, a.k.a thermal cracking, is one of the most dominant distresses in asphalt pavements in areas with cold climates This type of cracking manifests itself with series

of top-down evenly spaced cracks which are perpendicular to the flow of traffic as shown in Figure 1 The mechanism of low-temperature cracking is related to the tensile stresses induced within the pavement layer due to significant drop in temperature As a continuous layered system without any periodic joints, asphalt pavements when subjected to cold temperatures are restrained from contraction As a result thermally induced tensile stresses will build up within the pavement and progressively increase as the surrounding temperature decreases Eventually, when the induced tensile stresses exceeds the tensile strength of the pavement material, thermal cracks initiate from the surface of the pavement and propagate downwards leading to more types of distresses in the pavement system (especially after infiltration of water) resulting in further reduction of performance, service life, and structural integrity of the pavement structure

Figure 1: Typical thermal cracking in asphalt pavements

Significant number of research studies have been conducted to tackle the thermal cracking problem in pavements [1-11] Based on recent studies, low-temperature fracture characteristics

of asphalt binder is one of the most important factors controlling thermal cracking of asphalt pavements Current specification utilizes the Bending Beam Rheometer (BBR) test along with the Direct Tension Test (DTT) to determine binder stiffness, the m-value, which reflects the ability of binder to relax the induced stresses, and the failure strain of asphalt materials at low-temperatures [12, 13] Although the parameters obtained from the Strategic Highway Research Program (SHRP) tests such as stiffness and failure strain are necessary to characterize the behavior of asphalt binders at low-temperatures, they alone are not adequate to evaluate the resistance of asphalt binders to premature cracking Recent studies have shown that accurate evaluation of the low-temperature cracking performance of asphalt binders, especially polymer-modified asphalt materials still remain a challenge [14,15] As a result, there is still a need for new binder testing methods to accurately capture low-temperature properties of asphalt binders

In 1986, Little and Mahboub investigated the use of the J-integral method for fracture properties

of plasticized sulfur binders and suggested that the JIC value could be used for binder

performance testing at low-temperatures [16] In 1994, Lee et al developed a three-point bending configuration test which was a more practical method for measuring fracture toughness and fracture energy of asphalt binders in the linear-elastic regime [17] Ponniah et al (1996) utilized three point notched bending beam test to determine fracture toughness and fracture energy of asphalt binders [18] In 2001, using the notched BBR test, Anderson et al showed that fracture toughness (KIC) provides a more definitive ranking of resistance to thermal cracking as compared to Superpave criteria [19] In 2004, Andriescu et al used double-edge-notched tension specimen to determine the essential work and plastic work of asphalt binder fracture [20] Hoare

et al (2006) used results obtained from the three-point notched bending beam test and showed that fracture toughness and fracture energy are sensitive to factors such as stiffness, binder’s morphology, and polymer content [15] In another investigation in 2006, Edwards et al developed a new compact tension test for the grading of asphalt binders [21] Behnia et al (2010) developed an acoustic emission-based testing method to evaluate the behavior and embrittlement temperature of asphalt binder at low-temperature [22] In 2011, Rosales et al proposed a new Single-Edge-notched beam test configuration to determine the stiffness and fracture energy of modified binders [23] Recently, Roque et al (2012) developed a new binder direct tension test to determine fracture energy of asphalt binder at intermediate temperatures [24]

With recent advances in the field of fracture mechanics, development of valid fracture tests for asphalt binder seems to be an important step and a plausible endeavor in the evolution of asphalt binder selection to control thermal cracking The present study focuses on characterization of low-temperature fracture properties of asphalt materials (i.e fracture toughness, fracture energy, tensile strength, and embrittlement temperature) using both mechanical fracture performance tests, the Indirect Tensile (IDT) test and the Compact Tension (CT) test, as well as a nondestructive testing method, i.e the acoustic emission-based test Different types of asphalt binders (modified and unmodified) at different temperatures are evaluated and the results are presented and discussed

2 MATERIALS AND METHODS

In the present work, three different asphalt binders including: PG64-22, PG64-22 plus 10% Ground Tire Rubber (GTR) by weight, and PG 70-22 (Styrene-Butadiene-Styrene (SBS)-modified) were utilized The objective was to evaluate the effect of GTR on PG64-22 asphalt binder and also comparing the GTR-modified PG64-22 asphalt binder against one grade higher asphalt binder, i.e PG70-22 Two mechanical performance tests, i.e., the Indirect Tension test [ID(T)] and the Compact Tension test [C(T)], which are commonly used for testing asphalt mixtures, were implemented to assess low-temperature fracture performance of asphalt binders

In addition to conducting fracture tests, a nondestructive acoustic emission-based testing approach was performed to determine the embrittlement temperature of asphalt materials and to provide a better perspective of fracture behavior of asphalt binders in the micro-scale level

2.1 Indirect Tensile Test [ID(T)]

The Superpave Indirect Tension test [ID(T)], developed under the Strategic Highway Research Program (SHRP), commonly used to determine the creep compliance and indirect tensile strength of asphalt mixtures, was utilized to measure the tensile strength of asphalt binders The ID(T) strength test was performed in accordance with AASHTO TP9-96 [25], which involves application of compressive load through the diametrical axis of a cylindrical binder sample and

measuring the displacements and the compressive peak load at failure The tensile strength of the binder sample can be calculated using the following equation:

 =2

where:

 : ID(T) tensile strength (kPa)

 : Compressive load at failure (kN)

 : Cylinder thickness (mm)

 : Diameter of cylinder (mm)

To conduct the ID(T) strength test, 150 mm diameter, 50 mm thick cylindrical binder samples were fabricated and utilized The geometry of the ID(T) specimen and the test setup are shown in Figure 2 A silicon rubber mold was used to prepare specimens ID(T) binder samples were prepared in the laboratory by pouring 135°C asphalt binder into the silicon rubber mold Prepared samples were cooled down at room temperature for 2 hours, then conditioned for another 2 hours at -10 oC until they become solidified and easy to demold, Figure 3 The ID(T)

binder specimens were then placed in the cooling chamber for 2 hours before testing to reach to the target testing temperature ID(T) test was conducted on conditioned samples at the following three different temperatures -12, -22, and -30oC with the loading rate of 12.7 mm/min All testing

was performed using an Instron 8500 servo-hydraulic load frame with an environmental chamber capable of controlling the temperature ranging from 40°C to -40°C within ±0.1°C Figure 4 shows the experimental setup and specimen after failure

Figure 2: Schematic representation of ID(T) sample dimensions and testing setup details (a) ID(T) binder

specimen dimensions; (b) ID(T) strength testing setup

150

50 mm

Figure 3: ID(T) binder sample preparation procedure using silicon rubber mold

(a) (b)

Figure 4: (a) Experimental setup for ID(T) strength test along with the binder specimen (b) IDT binder

sample after failure at the end of the test

2.2 Compact Tension Test [C(T)]

The Compact Tension test, C(T), is one of the most commonly used tests in the field of fracture mechanics to assess fracture toughness as well as fracture energy of materials Fracture toughness or critical stress intensity factor is an important property of engineering materials which describes the ability of the material to resist fracture In this study, the C(T) test was conducted in accordance with ASTM E399-05, the standard test method for plane-strain fracture toughness for metallic materials [26] Figure 5 schematically illustrates the geometry and dimensions of the C(T) binder specimen The C(T) test involves applying tensile load through metal pins inserted into the loading holes and measuring Crack Mouth Opening Displacement (CMOD) with a clip-on gauge as shown in Figure 6 Similar to ID(T) samples, C(T) samples were fabricated by pouring 135oC asphalt binder in to the prepared C(T) silicon rubber mold The mold was prefabricated with the 65 mm notch and 12.5 mm diameter loading holes in place After demolding and conditioning the specimens at target temperature, the compact tension test was performed at -22 oC applying CMOD loading rate of 0.2 mm/min using a sensitive 1 kN

load cell The test setup and specimens before and after failure is shown in Figure 7

Figure 5: Geometry and dimensions of the Compact Tension (CT) specimen

Figure 6: Typical Load-CMOD curve obtained from C(T) binder fracture test

(a) (b)

Figure 7: Compact Tension Test: (a) C(T) testing setup, (b) C(T) binder specimen before and after failure

Fracture energy (Gf), the energy required to generate a unit cracked surface area, can then be calculated by measuring the area under the load-CMOD curve normalized by the fractured surface area Fracture toughness can be obtained using the following equation [27]:

=   16.7    

− 104.7  

+ 369.9  $ 

− 573.8  ' 

+ 360.5  ( 

) Where,

= Critical stress intensity factor (Fracture Toughness)

 = Tensile load at failure

 = Thickness of the specimen

 = Crack length

 =Width of the specimen

2.3 Acoustic Emission-based Test

Acoustic Emission (AE) approach is a powerful nondestructive testing method commonly used

to detect and locate microdamages in materials under the stress The AE technique is classified

as a nondestructive testing method due to the fact that it is a passive testing approach that only listens to the acoustic response of the material and unlike some mechanical tests it doesn’t directly cause any sort of damage in the material which is being tested The AE method is used

to monitor, record, and quantify the microdamages occurring within the material

Analysis

CMOD Gauge

Acoustic emission phenomenon is defined as the spontaneous release of localized strain energy

in the form of transient mechanical elastic waves within a stressed material Behnia et al

developed an acoustic emission-based testing technique to evaluate low-temperature cracking

characteristics of asphalt binders [6-9] The AE binder sample is a thin layer of asphalt binder

bonded to a granite substrate AE binder samples are placed in the ULT-25 portable freezer and

cooled down from 20oC to -50oC at the average cooling rate of 2oC/min Different thermal

contraction of asphalt binder sample and granite slab causes progressively higher thermal stress

within the binder sample resulting in thermal crack formation Acoustic emissions result from the

strain energy release in the form of transient elastic mechanical waves during the formation of

these thermal cracks in the binder sample

Analysis of AE activity of asphalt materials was performed on recorded AE signals and

associated testing temperatures Here, an AE event is defined as a rapid physical change such as

micro-fracture in the material which releases energy in the form of transient stress waves and can

be detected as an AE signal with the voltage and energy equal to or greater than 0.1 V and 4 V2

-µs thresholds, respectively It was observed that the majority of acoustic emission activity starts

at certain temperature which is termed the “embrittlement temperature (TEMB)” of the material

The embrittlement temperature is considered as the onset of thermally induced damages in

asphalt material The lower the embrittlement temperature, the better the low-temperature

cracking performance of that asphalt binder Figure 8 schematically depicts an AE binder sample

as well as testing set up A typical AE test result along with the embrittlement temperature is

illustrated in Figure 8(c) [8,22]

(a) (b)

(c)

Figure 8: Acoustic emission based test: (a) 6 mm thick AE asphalt binder sample; (b) AE testing setup; and

(c) Typical AE test plot to determine embrittlement temperature

The AE binder samples were prepared using aluminum molds identical to standard Bending Beam Rheometer (BBR) test Teflon tape was utilized as a debonding aid during molding A 10

mm thick square granite slab (150 mm by 150 mm) was used as the substrate To ensure proper bonding and restraint between the asphalt binder sample and the substrate, the granite substrate was preheated to approximately 135 °C Asphalt binder at a temperature of 135 °C was poured into the aluminum mold wrapped in Teflon tape placed on the heated slab Prepared samples were allowed to cooled down at room temperature for two hours and then positioned inside the freezer and cooled down to -50 oC Throughout conducting the AE test, the specimen temperature was continuously monitored and recorded using K-type thermocouple placed on the specimen surface Wideband AE piezoelectric sensors (Digital Wave, Model B1025) with a nominal frequency range of 50 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 specimen surface Since by nature the acoustic signals are of low energy, the sensor data is immediately fed into a preamplifier to minimize noise interference and prevent signal loss Signals from AE sensors were pre-amplified by 20 dB using broad-band pre-amplifiers Then, the signal was further amplified by 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 2048 points per channel per acquisition trigger The outputs were stored for later processing using Digital Wave software (WaveExplorerTM V7.2.6) [28, 29]

3 RESULTS AND DISCUSSION

The tensile strength of asphalt binders were determined through conducting ID(T) strength test Figure 9 illustrates obtained tensile strength values of asphalt binders at three different testing temperatures: -12, -22, and -30oC Each tensile strength presented is the average of at least three ID(T) test replicates The average Coefficient of Variation (CoV%) of ID(T) results was 12.5% Comparison of tensile strength of different binders shows that GTR rubber modified PG64-22 has the best performance, slightly better than PG70-22 Tensile strengths values of unmodified PG64-22 and GTR-modified PG64-22 at different temperatures are shown in Table 1 It is observed that adding GTR to PG64-22 binder significantly improves low-temperature tensile strength of asphalt Comparison of the tensile strength values of GTR-modified and regular PG64-22 binders at different temperatures shows that the improving effects of GTR was more pronounced at lower temperatures as the amount of increase in tensile strength due to the presence of GTR was 306% at -30 oC, which is twice as much as the amount of improvement occurred at -12oC, i.e., an increase of 154% in tensile strength As expected, it was also observed that asphalt binders exhibited lower tensile strength at lower temperatures This can be linked to the fact that as the temperature decreases, asphalt binders become more brittle and less resistant

to cracking

Figure 9: ID(T) Tensile Strength results for different asphalt binders at various temperatures: -12,-22,-30 o C

Table 1: Effects of GTR on Improving Tensile Strength of PG64-22 Asphalt Binder

Temperature (oC)

Binder Tensile Strength (kPa) Material

% Improvement PG64-22 PG64-22+GTR

Fracture toughness (critical stress intensity factor) and fracture energy of asphalt binders obtained from Compact Tension test are presented in Figures 10 and 11, respectively Each fracture test result represents the average of three C(T) tests with the CoV% values ranging from 7% for PG64-22 to 15% for PG 70-22 binders C(T) fracture tests were performed at the PG low-temperature of asphalt binders, i.e -22 oC Results show that GTR-modified PG64-22 had the highest fracture toughness and fracture energy, followed by PG70-22, and PG64-22 This is consistent with ID(T) tensile strength results as PG64-22+GTR exhibited the highest tensile strength compared to the other two binders Results clearly indicate that adding GTR has significantly improved low-temperature cracking resistance of PG64-22 asphalt binder