Micromechanical investigation of open-graded asphalt friction courses’ rutting mechanisms

Construction and Building Materials 44 (2013) 25–34

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Construction and Building Materials

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Micromechanical investigation of open-graded asphalt friction courses’rutting mechanisms

0950-0618/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.conbuildmat.2013.03.027

⇑ Corresponding author. Tel.: +1 510 305 6246.E-mail addresses: [email protected] (E. Coleri), [email protected]

(J.T. Harvey), [email protected] (K. Yang), [email protected](J.M. Boone).

1 Tel.: +1 530 754 6409.2 Tel.: +1 916 734 0314.3 Tel.: +1 916 734 3158.

Erdem Coleri a,⇑, John T. Harvey a,1, Kai Yang b,2, John M. Boone b,3

a University of California Pavement Research Center, Department of Civil and Environmental Engineering, University of California, Davis, One Shields Avenue, Davis, CA 95616, USAb Department of Radiology, University of California, Davis Medical Center, 4860 Y Street, Suite 3100 Ellison Building, Sacramento, CA 95817, USA

h i g h l i g h t s

� Rutting mechanisms of open-graded asphalt friction courses (OGFC) under full-scale loading is investigated.� We measured the full-scale load related changes in OGFC microstructure using X-ray CT imaging.� Thick DGAC layers under OGFC might also induce early rutting failures.� Densification is the major factor controlling OGFC and DGAC rutting.� OGFC layer thickness is the major factor affecting the OGFC rutting mechanisms and performance.

a r t i c l e i n f o

Article history:Received 28 November 2012Received in revised form 12 March 2013Accepted 13 March 2013

Keywords:MicrostructureRuttingX-ray computed tomographyAsphalt concreteOpen-graded friction coursePermeable friction courseParticle trackingDensificationShear

a b s t r a c t

This study aims to identify the permanent deformation accumulation mechanisms of open graded frictioncourse (OGFC) mixes under full-scale loading. Changes in OGFC and underlying dense graded asphalt con-crete (DGAC) layers’ microstructures under full scale trafficking were determined using X-ray computedtomography (CT) images taken before and after accelerated pavements tests (APT). Image processing andparticle tracking methods were used to identify changes in air-void and aggregate distributions to eval-uate the rutting mechanisms of multi-layered (OGFC on DGAC) asphalt concrete sections. Densificationwas observed to be the major factor controlling the rutting on test sections while shear related deforma-tion was just a minor contributor. Based on APT and laboratory test results, OGFC layer thickness wasdetermined to be the major factor affecting the OGFC rutting mechanisms and performance. Displace-ment vector distributions for the OGFC layers appeared to be larger than the distributions for the DGAClayers due to the high initial air void content for the OGFC layers. However, significant deformation of theDGAC layers suggested that thick underlying DGAC layers might also induce early rutting failures. Resultsof this study provide experimental data for continued development of multi-scale continuum mechanicsand discrete element method analysis approaches for multi-layered flexible pavement structures.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction performance [5]. According to the survey carried out by the Na-

Open-graded friction courses (OGFC) have been used in the Uni-ted States since 1950 [1]. The major advantages of OGFCs are fasterdrainage, less splash from vehicles, improved driver visibility, re-duced traffic noise, improved surface reflectivity and increasedsurface friction [2]. Cleaner runoff is also another advantage ofOGFC mixes [3,4]. Although most of the states have reported goodperformance, some states stopped using OGFC layers due to poor

tional Center for Asphalt Technology (NCAT) on design and con-struction practices for OGFCs [1], only half of the surveyed stateagencies had good experience with OGFC.

Although there are many proposed mix design proceduresavailable for OGFC mixes, a particular mix design procedure withunique test methods is not available to date [6]. Suresha et al. [7]conducted laboratory studies to characterize the OGFC mixes fordifferent compaction efforts using Marshall compaction method.Alvarez et al. [8] analyzed the internal structure of field and labo-ratory compacted OGFC mixes using X-ray CT imaging and maderecommendations to improve current laboratory specimen fabrica-tion procedures to better reproduce the air void characteristics offield compacted mixes.

Mixture density and air void content are the major parametersused for OGFC mix design. Most of the studies focused on the mac-roscopic evaluations of mixture density and air void content while

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26 E. Coleri et al. / Construction and Building Materials 44 (2013) 25–34

only a few research studies investigated the effects of OGFC mix-ture microstructure on pavement performance [9,10]. Additionalresearch is required to further analyze the internal structure ofOGFC mixes and determine the changes in OGFC microstructureunder full-scale loading. In-situ rutting mechanisms of OGFC mixesshould be investigated to be able to develop effective mix designprocedures for OGFC mixes.

While most state agencies focused on development of effectivemix design methods for OGFC mixes, there are no structural designmethods specifically developed for OGFC thickness design. Cooleyet al. [11] indicated that practitioners have been using current struc-tural design methods for designing OGFC layer thicknesses byassigning a structural value ranging from equating OGFC to DGACto giving OGFC no structural value. Van Der Zwan et al. [12] devel-oped a mechanistic-empirical design approach by using the mea-sured dynamic modulus for OGFC and DGAC mixes. Timm andVargas-Nordcbeck [13] developed a structural coefficient for OGFClayers based on the results of full-scale testing to be used in mech-anistic-empirical design. Although these studies focused on modify-ing the current structural design and performance predictionmethods developed for DGAC mixes to account for OGFC mixes,none of the studies in the literature have attempted to identify thedifferences between the rutting failure mechanisms of OGFC andDGAC mixes.

In this study, changes in OGFC and underlying DGAC layers’microstructures under full scale trafficking were determined usingX-ray CT images taken before and after accelerated pavementstesting. Asphalt concrete blocks with OGFC and DGAC layers fromevery test section were sawn and scanned before full-scale acceler-ated pavement testing (APT) to determine the initial air void,aggregate and mastic distributions. Scanned blocks were installedback into their original positions with a fast-setting epoxy andaccelerated pavement rutting tests were conducted on sectionswith scanned asphalt blocks. At the end of the tests, deformed as-phalt blocks were re-sawn to perform after-testing X-ray CT imag-ing in order to determine the changes in air void distributions andparticle movement by comparing before and after testing images.Changes in air-void and aggregate distributions were used to eval-uate the rutting mechanisms of multi-layered (OGFC on DGAC) as-phalt concrete sections. Rutting performance of OGFC mixes werealso evaluated based on Hamburg wheel tracking device (HWTD)laboratory test results.

Results of this study will provide important information for de-sign of OGFC layers. Results will also provide experimental data forcontinued development of multi-scale continuum mechanics anddiscrete element method analysis approaches.

2. Objectives

The main objectives of this study were:

(1) Use the X-ray CT imaging method to identify changes inOGFC and DGAC layers’ microstructures caused by full-scaletrafficking.

(2) Use the experimental measurements to perform a first-levelassessment of the micromechanical phenomena to provideinsight into rutting mechanisms for mix and structuraldesign of OGFC layers.

Table 1Measured air-void contents for OGFC layers of HVS and HWTD test sam

Section Mix type NMAS (mm) Air-v

653HB PG76-22PM 9.50 26.7655HB PG76-22PM 4.75 27.8657HB PG64-10 4.75 29.5

(3) Compare the measured response of the multi-layered struc-ture to the response of flexible pavement structures withone layer.

(4) Evaluate the OGFC mix performance using Hamburg wheeltracking device (HWTD) laboratory test results.

(5) Provide data to support development and understanding oflaboratory tests for OGFC design.

(6) Provide experimental data for continued development ofmulti-scale continuum mechanics and discrete elementmethod analysis approaches for multi-layered flexible pave-ment structures.

3. Heavy Vehicle Simulator testing

The Heavy Vehicle Simulator (HVS) is a mobile load frame thatuses a full-scale wheel (dual or single) to traffic the pavement testsection. The trafficked test section is 8 m long, of which 1 m oneach end are used for turnaround of the wheel and are discarded.The failure criterion for rutting tests was defined as an averagemaximum downward rut of 12 mm over the full monitored sec-tion. The pavement temperature at 50 mm depth was maintainedat 50 �C ± 2 �C to assess rutting potential under typical pavementconditions.

3.1. Material and structure properties

Three open graded asphalt mixture types were used in this studyas surface friction courses, a polymer modified (PM) mixture withPG76-22PM binder and 4.75 mm nominal maximum aggregate size(NMAS), a PM mixture with PG76-22PM binder and 9.5 mm NMASand a conventional mixture with PG64-10 binder and 4.75 mmNMAS. Fibers are not used in any of the mixes. All open-graded layerswere constructed on top of a 70 mm thick dense graded asphaltlayer. The thickness of the aggregate base layer was 150 mm. The de-sign open graded layer thicknesses for the PG76-22PM – 9.5 mmNMAS and PG76-22PM – 4.75 mm NMAS mixes were 25 mm whilethe design layer thickness for the PG64-10 – 4.75 mm mix was12 mm. It should be noted that as-built open-graded layer thicknessfor the 12 mm thick section was around 16 mm (about 33% higherthan the design thickness) due to the limited control during con-struction. However, this level of difference between as-built and de-sign thicknesses for thin OGFC layers is expected in highwayconstruction. Average OGFC layer air-void contents of the HVS testsections were determined using the core samples taken from thesections (Table 1). Air-void contents of the HWTD test samples werealso determined before testing (Table 1).

3.2. Experiment program

The HVS loading program for each section is summarized in Ta-ble 2. All trafficking was carried out at 8.7 km/h (5.4 mph) with adual-wheel configuration with the centerlines of the two tiresspaced 360 mm apart, using radial truck tires inflated to 690 kPa(100 psi), in a channelized (no wander), unidirectional loadingmode in which the wheel travels one direction loaded and is liftedoff the pavement when returning [14]. Channelized trafficking isused to simulate the tracking of radial tires in the wheelpath once

ples.

oid content – HVS (%) Air-void content – HWTD (%)

28.425.726.8

Table 2Summary of HVS loading program.

Section Mix type NMAS (mm) Design thickness(mm)

As-built thickness(mm)

Wheel load(kN)

Temperatures at 50 mm Total repetitions

Average (�C) SDa (�C)

653HB PG76-22PM 9.50 25 23 40 49.2 2.1 10,000655HB PG76-22PM 4.75 25 22 40 49.8 1.1 5000657HB PG64-10 4.75 12 16 40 49.5 1.5 50,000

a SD: standard deviation.

E. Coleri et al. / Construction and Building Materials 44 (2013) 25–34 27

a small rut forms, and is more aggressive than field conditions inthe initial stages of rutting.

4. Hamburg wheel tracking device laboratory testing

HWTD test is used to measure the rutting resistance of asphaltmixes by rolling a steel wheel with constant load across the surfaceof asphalt concrete samples under controlled temperature. HWTDtests were conducted for all three OGFC mixes of this study. Fourreplicate HWTD tests were conducted for each OGFC mix type.All tests were conducted at 50 �C, which was also the target tem-perature for HVS testing. All HWTD samples were prepared to havean equal thickness of 60 mm in order to evaluate the sole effect ofmix type on rutting resistance by isolating the layer thicknesseffect.

5. Procedure for X-ray CT image data collection

General procedure developed by Coleri et al. [14] is used for X-ray CT image data collection. The procedure is also illustrated withphotographs in Fig.1. After the construction of the open-gradedlayers on top of the dense graded layers (Fig. 1a), square 17 cm as-phalt concrete blocks were sawn from the HVS test sections(Fig. 1b). One block was sawn from every test section. Asphalt con-crete blocks were located so that one half of the block was in thewheelpath while the other half was next to the wheelpath in orderto monitor the lateral shear flow. X-ray CT images for the removed

Fig. 1. The general procedure followed

asphalt concrete blocks were acquired at UCDMC (Fig. 1c). X-ray CTimages were processed to obtain the aggregate, air-void (Fig. 1d)and mastic distributions in the asphalt blocks. Extended cuts thatwere created during sawing of the blocks due to the circular shapeof the concrete saw were filled with an emulsion-sand mixture toavoid localized failures (Fig. 1e). Scanned blocks were installedback into their original position with fast-setting epoxy (Fig. 1fand g). After the HVS tests were performed on the sections with as-phalt concrete blocks (Fig. 1h), asphalt blocks with accumulatedpermanent deformation were re-sawn to acquire the after-testingimages (Fig. 1i). Acquired images were processed to determinethe after-testing distributions of the aggregate, mastic and air-voidphases. By comparing the before and after testing images, changesin air-void distributions and permanent particle movement withHVS trafficking were determined. These results were used to eval-uate the rutting mechanisms of multi-layered asphalt pavementsystems of this study. Profilometer measurements taken alongthe test sections showed that localized failures around the blocksare not evident. Deformations on the blocks appeared to be closeto the deformations observed along the test sections.

6. X-ray CT image acquisition and processing

X-ray CT images of the asphalt concrete blocks were acquired atUniversity of California Davis Medical Center (UCDMC). Threedimensional images of the asphalt blocks were created by combin-ing a series of 2D cross-sectional images generated by the scanner.

for X-ray CT image data collection.

Fig. 2. Distributions of air voids in one of the asphalt concrete blocks (655HB-Thick-PM-4.75mm) from two perspectives. Note: width of the block is 167 mm.

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The quality of the 3D image depends on the X-ray CT image reso-lution. In this study, horizontal planar images were acquired at1 mm intervals while the resolution cell for the other two dimen-sions was determined to be 0.24 mm. At each sampling point with-in the sample volume, each pixel from a CT image measures a valuethat is related to the density and atomic number of the material atthat particular point. These measured values are then converted tograyscale values [15]. The gray scale intensities for the aggregates,air voids and mastic range from �1000 to 3095, but these valueswere rescaled to the range from 0 to 255. Because aggregates aredenser than mastic and air voids, they occupy the higher portionof the intensity scale. Low density air voids are at the lowest inten-sity portion while mastic intensity is between aggregate and airvoid intensities.

Simpleware software [15] was used to create masks for the airvoid domain based on measured intensity groups by followingthe procedure developed by Coleri et al. [14]. Air void percentagesfor each asphalt mixture sample were determined by using thestandard CoreLok method [16]. The total volume of each specimenwas calculated for the complete intensity range, 0–255. The upperlimit for the air void intensity range was determined by trial and er-ror to match the measured air void contents. The air-void intensityranges for the asphalt concrete blocks of 653HB-Thick-PM-9.5mm,655HB-Thick-PM-4.75mm and 657HB-Thin-Conv.-4.75mm HVStest sections were determined to be 0–116, 0–113 and 0–117,respectively. After the thresholds for the air void domain weredetermined, masks with specific colors were assigned to clearlyvisualize the distribution of air void domain in the total block vol-ume. Fig. 2 shows the distribution of air voids for the 655-HB-Thick-PM-4.75mm test section. Developed 3D images were furtherprocessed to obtain the distribution of air voids with depth.

In this study, tracking algorithm of Imaris Version 7.2 softwarewas used to determine the permanent particle movement withHVS trafficking. The procedure developed by Coleri et al. [14]was used to create aggregate domain in asphalt concrete blocks.The ‘‘spot object’’ feature of Imaris, which was proven to be the

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Fig. 3. Comparison of average downward rut from HVS tests.

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(c) Fig. 4. Comparison of average rut from HWTD tests. (a) 653HB-Thick-PM-9.5 mm(b) 655HBThick-PM-4.75 mm (c) 657HB-Thin-Conv.-4.75 mm.

Fig. 5. Tested HVS test sections and HWTD test samples (a) HVS – 653HB-Thick-PM-9.5mm (b) HVS – 655HB-Thick-PM-4.75mm (c) HVS – 657HB-Thin-Conv.-4.75mm(d) HWTD – PG76-22PM with NMAS = 9.50mm (e) HWTD – PG76-22PM with NMAS = 4.75 mm (f) HWTD – PG64-10 with NMAS = 4.75 mm.

Fig. 6. Segmented air voids in one of the asphalt concrete blocks (655HB-Thick-PM-4.75mm) from two perspectives (colored volumes are air-voids). Note: Width of the blockis 167 mm.


most effective method for particle tracking in asphalt concreteblocks [14], was used to determine the particle movement. Dis-placement vectors developed by particle tracking were quantifiedlater in this paper to evaluate the shear and densification responseof the three structures.

7. Results and discussion

7.1. HVS test results

The rutting test results of the three test sections are shown inFig. 3 (average downward rut, defined as the downwarddeformation averaged across 13 measurements, transverse profilesat 0.5 m intervals along the 6 m wheelpath). The applied wheel loadfor all the tests was 40 kN. All sections had almost the same level ofdownward rut after the completion of the HVS tests. The sectionwith thin OGFC layer and conventional PG64-10 binder (657HB)showed the best rutting performance.

7.2. HWTD test results

HWTD test results of the three OGFC mix types are given inFig. 4. All OGFC samples had the equal thickness of 60 mm. It can

be observed that rutting resistance of the Conv.-4.75mm mix is sig-nificantly lower than the PM-4.75mm and PM-9.5mm mixes. Thisresult indicated that binder type is controlling the rutting resis-tance of the equal height OGFC mix samples during the HWTDtests. However, by comparing HVS and HWTD test results (Fig. 3and Fig. 4), it can be concluded that rutting resistance of theConv.-4.75mm mix can be improved by reducing the layer thick-ness. Because the as-built OGFC layer thickness of the HVS test sec-tion with the Conv.-4.75mm mix (657HB) was about 35–45%smaller than the thickness of the sections with PM mixes (653HBand 655HB), section 657HB showed the best rutting performance.This result suggested that layer thickness is the major factor con-trolling the in situ rutting failure of OGFC mixes. Tested HVS testsections and HWTD test samples are shown in Fig. 5.

7.3. Changes in 3D air-void distributions with HVS trafficking

Rutting in the asphalt layers accumulates with increasing num-ber of load applications and is highly sensitive to variations in thetemperature and traffic levels [17]. Volume change (densification)and shape distortion (shear related deformation) are the two defor-mation modes that control the rutting accumulation. Volumechange is the deformation of a material with equal principal strains

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(a) (b)Fig. 7. Reduction in air void content with HVS trafficking (a) wheelpath and (b) hump right next to the HVS wheel).

Table 3Densification related rutting calculated from X-ray ct images.

Section 1HOGFC (mm) Mix type 2Den. rut: OGFChump(mm)

Den. rut: OGFC wheelpath(mm)

Den. rut: DGAC hump(mm)

Den. rut: DGAC wheelpath(mm)

653HB 23 PG76–22PM 0.28 1.57 0.15 1.07655HB 22 PG76–22PM 0.85 1.72 0.34 0.96657HB 16 PG64–10 1.05 2.02 0.34 1.46

Note: 1HOGFC: As-built OGFC thickness; 2Den.: densification; 3Total asphalt rutting on blocks for the sections 653HB, 655HB and 657HB were 3.76 mm, 3.56 mm and 3.95 mm,respectively.

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(c)(b)(a)Fig. 8. Distributions of rutting caused by densification (a) 653HB-Thick-PM-9.5mm (b) 655HB-Thick-PM-4.75mm and (c) 657HB-Thin-Conv.-4.75mm.


(a)

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Fig. 9. Distribution of displacement vectors for one of the 20 mm segments of theasphalt concrete blocks for all test sections (front view): (a) section 653HB-Thick-PM-9.5mm, (b) section 655HB-Thick-PM-4.75mm and (c) section 657HB-Thin-Conv.-4.75mm. Note: Direction of HVS traffic is out of the page.


in all dimensions. Bulk modulus (K) is the resistance of a materialto volume change. Shear distortion can be defined as the deforma-tion without any change in volume. Resistance of a material toshear distortion is represented with the shear modulus (G) [18].Typically, rutting accumulation rate is higher under initial loadingdue to the combined effect of densification and shear distortion.The deformation accumulation rate starts to decrease as the layersbecome stiffer with the reduction in air-voids and reorientation ofaggregates.

In this study, densification (air void reduction) of OGFC andDGAC layers were determined by using the air void distributionsfrom X-ray CT images taken before and after testing. Measureddensifications for OGFC and DGAC layers were used to determinethe contributions of each layer’s densification (air void reduction)to total downward surface rut. First, asphalt concrete blocks weredivided into 1.76 mm thick volumes in the vertical direction. Vol-umetric air void content for each volume was calculated with Sim-pleware software [15] to determine the distribution of air voids

within the depth of the asphalt blocks. Fig. 6 shows the segmentedair void content distribution before HVS trafficking for the asphaltconcrete block of section 655HB-Thick-PM-4.75 mm. Segmentedimages were used to calculate the change in air void content withdepth. In order to determine the change in air void content withHVS trafficking, air void content distributions, similar to Fig. 6,for both before and after trafficking images were developed forhump (right next to HVS wheel) and wheelpath locations. After-trafficking air void content distributions were subtracted fromthe before-trafficking distributions to calculate the reduction inair void content distributions with HVS trafficking.

Fig. 7 shows the reduction in air void content with HVS traffick-ing. The areas under change in air void content curves for the unitvolume thickness (1.76 mm) were also calculated to determine thedistribution of densification related rutting along the thickness ofthe asphalt concrete layers. Fig. 8 shows the calculated distribu-tions of rutting caused by densification for all HVS test sections.Layer interface between OGFC and DGAC layers are shown by hor-izontal lines in Figs. 7 and 8. It can be observed from Figs. 7a and 8that air void reduction is concentrated at the bottom of OGFC lay-ers for all test sections while the section 657HB-Thin-Conv.-4.75mm has a significantly higher air void reduction at the top partof the OGFC layers as well. Due to the low initial DGAC air voidcontent at the start of the tests, reductions in air void contents ofthe DGAC layers for the 1.76 mm thick unit volumes appeared tobe significantly smaller than the OGFC layers. However, sinceDGAC layers were about 3–4 times thicker than the OGFC layers,cumulative densification related rutting for the DGAC layers wereclose to the cumulative densification related rutting for the OGFClayers. This result suggested that thick underlying DGAC layersmight also induce early rutting failures.

It can also be observed from Fig. 8 that shear flow from underHVS wheel to the hump part also created densification at the humppart of the OGFC layers. Negligible change in air-voids at the humppart of the DGAC layers suggested insignificant shear related defor-mation for these layers. However, in order to further investigatethe actual shear flow at the DGAC and OGFC layers, particle move-ment with HVS trafficking was also investigated using a trackingalgorithm later in this paper.

Total densification related rutting for each layer calculated byusing the data from Fig. 8 is also given in Table 3. It can be observedthat most of the densification accumulated at the OGFC layerswhile densification of DGAC layers under the wheelpath was alsosignificant. Total asphalt concrete rutting on the wheelpath ofthe blocks for the sections 653HB, 655HB and 657HB were deter-mined to be 3.76 mm, 3.56 mm and 3.95 mm, respectively. Bydividing the total densification related rutting under the wheel-path by the total asphalt concrete rutting on the blocks, contribu-tion of densification to total asphalt rutting was determined to be70%, 75% and 88% for the sections 653HB, 655HB and 657HB,respectively. This result suggested that densification is the majorfactor controlling the rutting on tests sections while shear relateddeformation was just a minor contributor.

Although at the bottom of the OGFC layer, the reduction in airvoid content distributions are close for all three sections, higherair void reduction at the top part of the 657HB-Thin-Conv.-4.75 mm layer might have caused higher loss in permeability. Thisresult suggested that although 657HB-Thin-Conv.-4.75 mm sectionshowed better rutting performance, it may have lost its drainagefunctionality (permeability) due to rutting. To be able to explainthe direct effects of air-void reduction on permeability, a numericalmodel of flow in the pore network of before and after testing blocksshould be developed [19]. The changes in structural pore networkanisotropy with permanent deformation accumulation and its ef-fects on permeability should be determined by using the X-rayCT images collected in this study.


7.4. Aggregate and mastic movement with HVS loading – particletracking

The spot tracking algorithm of Imaris was used to generate dis-placement vectors. Adjacent voxels, which is a volume elementrepresenting a value on a regular grid in 3D space, with closeintensity values were combined to form spots in before and aftertrafficking X-ray CT images. Coordinates for each spot in beforeand after trafficking asphalt concrete blocks were determinedand used for displacement vector development. The resolution ofthe X-ray CT images is not expected to affect the particle trackingbecause the spot intensity does not change during testing. How-ever, since the particle tracking algorithm excludes before-testingspots that cannot be matched in the after testing image, higherX-ray CT image resolution will result in more displacement vec-tors. It should be noted that higher X-ray CT image resolutioncan only be achieved by reducing the block dimensions.

Spots that were furthest from the HVS load (at the bottom of theblock) were assumed to be stationary after HVS trafficking [14].These spots were used as reference points to match before andafter trafficking images. It should be noted that no significant dif-ferences between the displacement fields were observed when dif-ferent spots (furthest from the HVS load) were used as reference.This result suggested that movements of the spots that are furthestfrom the HVS load were negligible and they can be used as refer-ence points for particle tracking.

Fig. 9 shows the distributions of displacement vectors for theasphalt concrete blocks. In order to improve the visualization, dis-placement vectors for one of the 20 mm segments of the asphaltconcrete blocks were shown. It can be observed that most of thedisplacement was accumulated at the open graded layers for allthe test sections while the displacements at the dense graded lay-

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(c) Fig. 10. Cumulative distributions of the displacement vector lengths for the four separawheelpath.

ers were also significant. Significantly higher air-void contents forthe OGFC layers induced the densification related deformation andresulted in larger deformations. No significant shear flow was ob-served at the OGFC layers. In addition to densification in the verti-cal direction, HVS wheel appeared to push the under-tire OGFCasphalt mix to the sides creating densification at the humps aswell. This result explains the high densification observed at theOGFC humps measured by X-ray CT imaging (Fig. 8). On the otherhand, two shear flow patterns were observed for the DGAC layers,one flow to the hump and the other to the opposite direction underthe tire. Most of the shear flow at the DGAC layers appeared to bein the downward direction while upward shear flow to the humps,previously observed for thick rubberized gap graded mixes byColeri et al. [14], was not observed. This result suggested that shearflow at the DGAC layers did not significantly increase the height ofthe humps and affect maximum total rutting.

In order to quantitatively evaluate the particle movementdistributions for the complete asphalt blocks, first displacementvectors in the OGFC and DGAC layers were separated. Then, dis-placement vectors in the hump of the blocks were isolated fromthe wheelpath as shown with dashed lines in Fig. 9. Cumulativedistributions of the displacement vector lengths for the four sepa-rated volumes are given in Fig. 10 for all test sections. It should benoted that displacement vector length distributions were calcu-lated for the total particle movement in all three dimensions anddo not directly reflect the downward rutting. It can be observedfrom Fig. 10a that movement at the hump of the asphalt block oftest section 653HB is significantly smaller than the other two testsections, which might be a result of the large NMAS for the 653HBmix. However, it can be observed from Figs. 10c and d that dis-placement vector distributions under the HVS wheel are very closefor all test sections. Displacement vector distributions for the

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(c) (d) Fig. 11. Cumulative distributions of the vertical components of the displacement vector lengths for the four separated volumes: (a) OGFC-hump, (b) DGAC-hump, (c) OGFC-wheelpath and (d) DGAC-wheelpath. Note: Negative values indicate upward movement.


OGFC-wheelpath appeared to be larger then the distributions forthe DGAC layers due to the high initial air void content for theOGFC layers. However, it should be noted that deformation of theDGAC layers is also a significant factor affecting the total surfacerutting. This result suggested that thick underlying DGAC layersmight also induce the early rutting failures. This result is alsocoherent with the densification related rutting values given in Ta-ble 3.

Cumulative distributions of the vertical components of the dis-placement vector lengths are shown in Fig. 11. Negative values inFig. 11 indicate upward movements. It can be observed fromFig. 11a and c that OGFC layers of the blocks for the sections653HB-Thick-PM-9.5 mm and 655HB-Thick-PM-4.75mm showsimilar responses while the section 657HB-Thin-Conv.-4.75mmhad significantly higher percentage of upward movement. This re-sult suggested that binder type, polymer modification and layerthickness are the major factors affecting the OGFC rutting mecha-nisms while NMAS is a minor factor. Fig. 11c and d suggest thatvertical displacement distribution for the OGFC-wheelpath is lar-ger than DGAC-wheelpath displacement distribution. However,deformation for the DGAC layers also had a major effect on totalsurface rutting.

Cumulative distributions of the displacement vector lengths(Figs. 10 and 11) given in this paper can be used for the calibrationor validation of multi-scale continuum mechanics and discrete ele-ment method models.

8. Conclusions

In this study, changes in OGFC and underlying DGAC layers’microstructures under full scale trafficking were determined usingX-ray CT images taken before and after HVS testing. Changes in air-

void and aggregate distributions were used to evaluate the ruttingfailure mechanisms of multi-layered (OGFC on DGAC) asphalt con-crete sections.

Results of micromechanical rutting failure investigations can besummarized as follows.

(1) According to the HWTD and HVS test results, OGFC layerthickness is the major factor controlling rutting.

(2) Densification was concentrated at the bottom of OGFC layersfor all test sections while due to the binder type, the section657HB-Thin-Conv.-4.75mm had a significantly higher airvoid reduction at the top part of the OGFC layers as well. Thisresult suggested that although 657HB-Thin-Conv.-4.75mmsection showed better rutting performance, it may have lostits drainage functionality (permeability) due to rutting.

(3) For the section 657HB-Thin-Conv.-4.75mm, about 88% of theasphalt concrete layer’s deformation appeared to be a resultof densification. Contributions of densification to totalasphalt rutting were 70% and 75% for the sections 653HB-Thick-PM-9.5mm and 655HB-Thick-PM-4.75mm, respec-tively. This result suggested that densification is the majorfactor controlling the rutting on test sections while shearrelated deformation was just a minor contributor.

(4) Due to the relatively lower initial DGAC air void content atthe start of the tests, reductions in air void contents of theDGAC layers for the 1.76 mm thick unit volumes appearedto be significantly smaller than the OGFC layers. However,since DGAC layers were about 3–4 times thicker than theOGFC layers, cumulative densification related rutting forthe DGAC layers were close to the cumulative densificationrelated rutting for the OGFC layers. This result suggestedthat thick underlying DGAC layers might also induce earlyrutting failures.


(5) In addition to densification in the vertical direction, HVSwheel appeared to push the under-tire OGFC asphalt mixto the sides creating densification at the humps as well. Neg-ligible change in air-voids at the hump part of the DGAC lay-ers suggested insignificant shear related deformation forthese layers.

(6) According to the particle tracking results, most of the shearflow at the DGAC layers appeared to be in the downwarddirection while upward shear flow to the humps, previouslyobserved for thick rubberized gap graded mixes by Coleriet al. [14], was not observed. This result suggested that shearflow at the DGAC layers did not significantly increase theheight of the humps.

(7) Displacement vector distributions for the OGFC-wheelpathappeared to be larger then the distributions for the DGAClayers due to the high initial air void content for the OGFClayers. However, deformation of the DGAC layers was alsoa significant factor affecting the total surface rutting.

(8) Cumulative distributions of the displacement vector lengthsdeveloped in this study can be used for the calibration orvalidation of multi-scale continuum mechanics and discreteelement method models.

Acknowledgments

This paper describes research activities that were requested andsponsored by the California Department of Transportation (Cal-trans), Division of Research and Innovation. Caltrans sponsorshipis gratefully acknowledged. The contents of this paper reflect theviews of the authors and do not reflect the official views or policiesof the State of California or the Federal Highway Administration.

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Micromechanical investigation of open-graded asphalt friction courses’ rutting mechanisms