microstructural analysis of porous asphalt concrete mix subjected to rolling truck tire loads

7/25/2019 Microstructural Analysis of Porous Asphalt Concrete Mix Subjected to Rolling Truck Tire Loads

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Anupam, Srirangam, Varveri, Kasbergen and Scarpas 1

Microstructural Analysis of Porous Asphalt Concrete Mix1

Subjected to Rolling Truck Tire Loads2

3K. Anupam1, S.K. Srirangam2, A. Varveri3, C. Kasbergen4and A. Scarpas54

51Post Doctoral Researcher, 2,3Researcher, 4Senior Researcher, 5Professor6

7Section of Road Engineering8

Faculty of Civil Engineering & Geosciences9Delft University of Technology10Stevinweg 1, 2628 CN, Delft11

The Netherlands.1213

14

15161718

Total Number of Words1920

Number of words in text: = 4989 words21Number of figures: (7x 250) = 1750 words equivalent22

Number of tables: (3x 250) = 750 words equivalent2324

-------------------------------------- ------------------------------25

Total number of words = 7489 words equivalent2627282930313233

Corresponding author: K.Anupam34Section of Road Engineering35Faculty of Civil Engineering & Geosciences36Delft University of Technology37

Stevinweg 1, 2628 CN Delft38 The Netherlands.39E-mail:[email protected] Fax: +31 15 278 239440

4142434445

46474849

50July 201551
mailto:[email protected]:[email protected]:[email protected]:[email protected]


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Microstructural Analysis of Porous Asphalt Concrete Mix1

Subjected to Rolling Truck Tire Loads2

3K.Anupam, S.K.Srirangam, A.Varveri, C.Kasbergen and A.Scarpas4

56

ABSTRACT78

Porous asphalt concrete (PAC) course is best known for its noise reduction and improved wet skid9resistance characteristics. Nevertheless, the use of PAC is associated with reduced lifetimes and high10maintenance costs, mainly due to various distress mechanisms such as raveling. Therefore, it is necessary11to have a better understanding of the stress states associated at the micromechanical level, i.e. at the12masticaggregate interfacial zone and the mastic itself. For this purpose, it is necessary to develop13

micromechanical finite element (FE) models which comprise of realistic asphalt mix meshes with14

different phases that are subjected to rolling wheel loads. In this paper, a framework is presented to15develop a three dimensional FE model capable of simulating a rolling wide-base truck tire over an asphalt16pavement surface. From the results of FE simulations, the stress states at the mastic and aggregate-mastic17interfacial layer were studied.For the analyzed surface of the PAC mix, it was observed that the mastic18phase registered high stress states as compared to mastic-aggregate interfacial phase, which suggests that19the sample may experience a cohesive failure in the long run. The developed methodology also provides a20

tool to analyze the influence of tire operating conditions such as tire inflation pressures and loads on the21stress states of asphalt mixes. Finally, the micromechanical stress response of PAC mix was compared22

with other conventional asphalt mix designs and it was found that the magnitude of stresses developed in23the mastic of PAC are higher as compared to the conventional asphalt mix designs.24

25

Keywords: PAC mix; micromechanical analysis; FE simulations; rolling truck tire, mastic, mastic-26

aggregate interfacial zone2728293031323334353637

3839404142434445

46474849

50515253


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INTRODUCTION1Porous asphalt concrete (PAC) is an open-graded asphalt mixture that typically has an air void content2higher than 20% by volume. Its large amount of air voids results in high interconnectivity of the air void3network, which in some cases was found to reach 90% (1). The PAC surface course is very effective in4reducing noise caused by tire-pavement interaction. Also, due to its excellent water drainage5characteristics PAC is particularly used in countries like the Netherlands that experiences extreme6rainfall. Especially during wet seasons, the use of porous asphalt reduces the hydroplaning and splash7and spray effects and improves the quality of the runoff water increasing the wet skid resistance.8

The most common distress of porous asphalt is the loss of aggregates from the pavement surface,9namely raveling (2). Premature raveling results from a combination of traffic loading and environmental10conditions. The open structure of porous pavements facilitates the intrusion of oxygen and water in the11pavement, which leads to a degradation of the mechanical properties of the constituent materials, thus12decreasing the durability of pavements. However, over the years, it has been realized that PACs high13vulnerability against raveling results in reduced lifetimes and high maintenance costs. Its been observed14that the average lifetime of porous asphalt is about 11 years, which is much shorter than the lifetime of15

dense asphalt. The raveling of the PAC also decreases the noise reducing potential of the pavement layer16 and threatens the technical durability of the surface layer (3).17

Two mechanisms are responsible for the loss of aggregates from the pavement surface, namely18cohesive and adhesive failure. Cohesive failure occurs within the mastic itself, while adhesive failure19involves debonding of the mastic film from the aggregate surface. The type of failure, which will20ultimately lead to raveling, depends on the arrangement of aggregates in the mix and the loading type and21the loading rate, as well as on the mechanical behavior of the mixture components (strength and stiffness)22

and their interfacial adhesive characteristics. At any given time, one of the two damage mechanisms will23be dominant.24

According to Lytton et al. (4), a critical parameter, which dictates the occurrence of one failure25type over the other, is the thickness of the mastic film. It was stated that asphalt mixtures with thin asphalt26films fail in tension primarily due to loss of their adhesive bond strength, while those with thicker mastic27

films fail because of damage within the mastic as opposed to interfacial adhesive failure. Nevertheless,28due to the large variation of mastic film thickness throughout a mixture, it is necessary to consider both29

mechanisms. Moreover, traffic loading in combination with the presence of a high percentage of coarse30aggregates in PAC, results in a wide range of stress and strain states in both the mastic and the interfacial31mastic-aggregate zone. Huurman et al. (5) (6) demonstrated that high stresses occur in the stone contact32regions which might lead to bonding failure under traffic loads. Therefore, information about the33mechanical behavior in the graingrain contact region is of great importance. Mo et al. (7) found out that34

tire shear stresses are critical for raveling resistance at high temperatures while the horizontal tensile35strains close to the pavement surface account for raveling at low temperatures. Therefore, the stress-strain36states that the components of the mixture experience under moving traffic loading are of great37importance, as they can provide valuable information on the critical damage locations in an asphalt38pavement.39

The viscoelastic behavior of asphalt mixtures containing unmodified and modified asphalt40binders was simulated using the DEM. The HMA microstructure was captured using images of vertically41

cut sections of specimens. The captured grayscale images were processed into black and white images42representing the mastic and the aggregate phases, respectively (8). Aggregates were modeled as rigid43objects and the viscoelastic interaction among the mix constituents was defined using a time-dependent44viscoelastic model.Dai et al. (9) used the finite element method (FEM) to study the effect of aggregates,45asphalt binder, and air voids. An equivalent lattice network system was applied to simulate interparticle46behavior. The model predicted a stiffer load deflection response with higher binder moduli.47

Although past studies considered various factors that influences the raveling of PAC mixes such48as loading, temperature, speed and mastic characteristics, their models are limited to idealized PAC mixes49and did not consider actual microstructural features of PAC mixes. Such models can predict the distress50mechanisms only on an average sense. However, the distresses of asphalt pavements fundamentally arise51at the micromechanical level, i.e. at the mastic and mastic-aggregate bond itself. Such analysis requires52large computational resources as millions of elements are required to carry it out.53


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The aim of this paper is to present a micromechanical analysis of PAC mixes by considering their1actual internal structure due to interaction with a heavy load carrying tire. In this study, FE simulations of2a wide-base truck tire rolling on FE meshes of different asphalt concrete mixes were performed. Such FE3simulations can capture the realistic contact stress distributions of a rolling truck tire at the tire-pavement4interface and their effect on the micromechanical pavement responses under moving vehicular loads.5These micromechanical critical responses include 3D stress distributions of the pavement at mastic and6aggregate-mastic interfacial zone levels.7

8

Objective and Scope9The objective of this paper is to develop a 3D FE model which is capable to predict the critical stresses10developed in the mastic and in the mastic-aggregate interfacial region of PAC mix due to rolling truck tire11loads.12

The scope of the proposed study include:13a) the determination of the influence of the loading conditions of the rolling truck tire on the14

structural response of pavement15

b)

the comparison of the performance of a PAC mix against other popular asphalt mix designs1617

Study parameters18A typical European articulated and super single trailer using a wide-base tire truck of configuration19425/65R22.5 was chosen to study the micromechanical response of asphalt concrete mixes. The tested20inflation pressures and tire loads for the wide-base tire are 520 kPa, 720 kPa, 950 kPa and 26 kN, 49 kN21and 53 kN respectively. These values give a basis to conduct a comparison of the relative aggression of22

each combination of tire inflation pressure and load on the 3D stress distributions within the given asphalt23mix. The speed of the truck tire is considered to be 8 km/h. A comparative study of a PAC mix with24conventional asphalt mixes was carried out by considering different asphalt concrete mixes: such as stone25mastic asphalt concrete (SMA), asphalt concrete-10 (AC-10) and thin surfacing asphalt (UTS), see Table261.27

28TABLE 1 AC-10, SMA, UTS and PAC Asphalt Pavement Mix Compositions29

30

Composition (%)

Components 4/10 mm

aggregate

2/6.3 mm

aggregate

0/4 mm

aggregate

Limestone

filler

Binder

SMA 55.2 13.8 15.9 8.4 6.4

UTS 52.0 13.7 21.7 7.1 5.5

PAC 54.9 16.8 19.2 3.8 5.3

AC-10 17.9 33.9 37.7 4.7 5.8

31

32

Particle size distribution

Sieve size (mm) 14 10 6.3 4 2 1 0.063

Passing (%)

SMA 100 90 47 29 25 21 10.1

UTS 100 91 51 33 28 22 8.8

PAC 100 99 47 27 19 14 5.6

AC-10 100 97 80 51 41 16 7.8

33

Standard asphalt mix properties

Property

Max. density

(kg/m3)

Voids at 10

gyrations (%)

Voids at 50

gyrations (%)

Texture

depth(mm)

Bulk

density(kg/m3)

Voids

(%)

SMA 2400 11.9 6.3 1.5 2294 4.4


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UTS 2429 13.7 6.9 2.1 2104 13.4

PAC 2582 22.7 8.8 NA 1975 23.7

AC-10 2387 6.3 3.5 0.7 2348 3.7

In this study, the tire rubber and the mastic were represented as viscoelastic isotropic materials.1The rheological properties such as complex shear viscosity ( * ), elastic shear modulus ( G ' ) and viscous2shear modulus ( G" ) of the tire rubber and the mastic were measured by frequency sweep tests by using3

Dynamic Shear Rheometer (DSR) apparatus. The test data was then expressed in terms of Pronys series4by using the Generalized Maxwell model (10). Figure 1 shows a schematic of the generalized Maxwell5model which consists of a spring and Maxwell elements connected in parallel.6

7

FIGURE 1 Schematic of generalized Maxwell model8

9The stress in linear viscoelastic materials is given by the following constitutive equation, which is based10on the Boltzmans superposition principle:11

trel

0

d(t) G (t ) d

d

(1)12

where is the strain tensor and relG t is the linear relaxation (shear) modulus also known as the13

relaxation kernel (11).14

The relaxation modulus of this model is given by15

m

rel i i

i 1

2 2m m' ''i i i i

rel 2 2 2 2i 1 i 1i i

G G G exp t /

G GG G i G iG

1 1

(2)16

where is the shear modulus of the single spring and iG and i are respectively the moduli and17

relaxation times associated with the springs in the m viscoelastic components, is the frequency.18Table 2presents the Pronyscoefficients obtained for the tire rubber and mastic materials using19

the above mentioned approach. The modulus of elasticity and Poissons ratio of aggregate was considered20to be 45 GPa and 0.2 respectively.21

22

m

G

G

G1

1

G2

2

Gm

m


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1

TABLE 2 Typical Pronys constants for mastic and tire rubber2

Mastic Rubber

Gi i Gi i

2.2373E-04 5.37E+07 4742623 1.74E-06

2.1491E-03 10648722 2814458 0.005098

4.4863E-04 2.48E+06 1365902 17.55506

3.0252E-01 0.001578 105709 279605.4

7.2054E-02 0.764437

G

: 5.17 MPa

6.2538E-05 300106.7

1.7631E-01 0.024376

G

: 7.78 GPa

3DESCRIPTION OF 3D FINITE ELEMENT MODEL4

5FEM mesh generation of asphalt concrete specimens6The authors had discussed in detail the generation of FEM mesh of a given Asphalt Concrete specimen in7

their earlier studies (12) (13). However, in order to ensure a better readability, a brief description is8provided about the procedure to obtain the Finite Element mesh of a given Asphalt Concrete sample.9

The asphalt concrete slab of a given mixture i.e. PAC was prepared in the laboratory using a10sector roller compactor. A specimen of 60 mm in height and 150 mm in diameter was cored out of the11slab. This asphalt specimen was used for the X-ray scanning to capture their surface morphologies by12

using an X-ray tomographer. The internal structure of the asphalt mixture was determined by means of an13X-ray computed tomographer (CT). A fan-beam CT scanner with a source X-ray intensity of 225 kV was14

used to scan the asphalt specimen with a distance in the vertical direction of 0.4 mm; grey scale images of151430 pixels 1430 pixels in resolution. Figure 2 also shows the image analysis technique that was used16to distinguish the different phases, such as, air voids, aggregates and asphalt binder. During the17segmentation process, the noise of the X-ray CT scan image was removed by applying a linear spatial18filter.19

After filtering, a threshold segmentation algorithm was used to transform the grey scale image20into binary ones, corresponding to each constituent of the asphaltic mix microstructure. Once the21segmentation is completed, FE meshes for the asphalt pavements were obtained by means of the22SimpleWare software (14). The FE meshes of asphalt mix specimens used in the rolling tire- pavement23interactions are shown in 2b. 6.8 million Elements of three-dimensional four-node linear tetrahedral24elements, C3D4, were used to develop the pavement model. Sets containing elements of different phases25such as Mastic phase, Aggregate phase, Voids phase and Mastic-aggregate interfacial layer phase were26

created during mesh generation. Thin layers of 0.1 mm around aggregate was considered as the interfacial27layer. The interfacial layer comprises of wedge elements with average material properties between the28mastic and the aggregate phase.29


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1

2(a) Different phases of asphalt mix obtained by image analysis technique3

45

(b) Micromechanical FE mesh of asphalt pavement surfaces6

7FIGURE 2 Generation of asphalt surface FE mesh8

9Truck Tire10A wide-base truck tire with a configuration of 425/65R22.5 is considered in the present study. The tread,11sidewalls and inner liner of truck tire is made up of rubber. The generalized Maxwell viscoelastic (VE)12material model was used to define the tire rubber components in the form of Pronys coefficients as13

discussed in previous section. Three-noded and four-noded linear, hybrid with constant pressure, axi-14symmetric elements with twist as an additional degree of freedom were used to mesh the rubber15compounds. The rebar technique was used to model the reinforcement materials (belts and ply) in the16rubber continuum with an elastic material model. The rebar elements used are generalized 2-node linear,17with twist, axi-symmetric surface elements. The rim was modeled with two rigid bodies associated with18

two reference nodes located on a prescribed axis. It was assumed that there is always a full contact19between the rim and the tire.20

Asphalt mix specimenAir voids

Aggregate Asphalt binder

PA surface mesh AC-10 surface mesh


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The FE mesh for the 3D truck tire model is obtained by revolving the 2D cross-section about the1axis of revolution, Figure 3. A full description of 3D tire modeling was described in the authors previous2publications (12) (13).3

4

56

FIGURE 3 FE mesh of 425/65R22.5 truck tire78

Validation of truck tire9

Validation of the 3D truck tire model was made against the wheel load testing results obtained by the10TireView software (15). The comparison of the FEM results of the contact area and contact stresses of11425/65R22.5 tire at different inflation pressures (520, 720, 950 kPa) and their respective load classes (2612kN and 49 kN) were made against the experimental tests results of the same tire integrated in the13

TireView program, Table 3.14Except for 5 out of 25 simulations, the percentage of difference between the TireView and FEM15

results are under 10%, which shows that the developed FE model simulates correctly the footprints of the16tire and corresponding 3D stresses under different loading conditions. Even for these 5 simulations, the17difference between the measured and predicted results of the truck tire is under 14%. This is a very18important benchmark in the simulations for the next stage of assessing tire-pavement interactions of19rolling truck tires traversing over the micromechanical asphalt pavement meshes.20

TABLE 3 Comparison of experimentally measured and numerically predicted results of truck tires21

425/65R22.5tire

Inflationpressure (kPa)

Tire load(kN)

Exp. FE

Contact area(mm2)

520

26 41297 36796

49 59606 60202

72026 35839 38706

49 54148 48733

95026 29445 32478

49 47755 44126

Max. verticalcontact stress

(MPa)

52026 0.98 1.01

49 0.88 1.11

72026 1.2 1.31

49 1.22 1.35

95026 1.31 1.21

49 1.4 1.49


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Max.longitudinal

contact stress(MPa)

52026 0.19 0.17

49 0.16 0.155

720

26 0.09 0.084

49 0.09 0.1

95026 0.06 0.055

49 0.15 0.17

Max. transversecontact stress

(MPa)

52026 0.08 0.09

49 0.11 0.12

72026 0.07 0.08

49 0.13 0.14

95026 0.09 0.1

49 0.15 0.16

Exp.: Experimental results1FE: Finite element model results2

3

Rolling tire-asphalt pavement interaction4The contact between the truck tire and the asphalt surface was simulated using the General Contact5algorithm available in ABAQUS ( (16). The contact boundaries between the tire and the asphalt surface6were represented by two deformable surfaces. The tangential contact constraint was enforced using a7penalty algorithm. A master and slave surface contact pair was considered where the elements of the8rubber wheel acted as the slave surface while those of the base surface acted as the master surface. In this9model, the friction coefficient between the tire and the asphalt surface is not input but an output quantity10which is calculated on the basis of hysteretic component of total friction.11

Hysteretic friction reflects the amount of energy dissipated within the rubber material when it is12alternately compressed and expanded as it slides or rolls over the pavement surface texture. A brief13procedure to obtain the hysteretic coefficient of friction is given below:14

By using Figure 1, the total shear stress ( ) expressed in Equation (1) can be re-written as15

m

i vi

i 1

G G

(3)16

where is the total shear strain and v inelastic shear strain.17

The shear stress in the dashpot of the Maxwell element ( M ) is defined proportional to the inelastic shear18

strain rate v 19

M i vi (4)20Also, the stress in the spring of the Maxwell element can be computed as21

M i viG (5)22On the basis of Equations (4) and (5),23

i vi i viG (6)24Also, the response of each one of the viscous components can be expressed as25

i ivi vi

i i

G G

(7)26

Finally, Equation (3) can be re-written as27

i

i

Etm t s

i

i 1 0

G G s e ds

(8)28

Total energy dissipation can be expressed as29


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Tm

i vi

i 1 0

W dt

(9)1

i

i

Gt t s

i i ei i

0

G G e s ds

(10)2

i

1t t s

vi ei

i i 0

1 1e s ds

(11)3T Tm

T T 0i vi

i 1 0

Tm mT T T T T T T/2

i vi i vi

i 1 i 1T T

T T TT T T T T/2 i i

i

W dt , W 0

W W dt W

= W W2

(12)4

where i indicates the Maxwell component index, vi inelastic shear strain rate at the ith component, i is5

relaxation time in ith component, i total shear stress in the ith component, iG is shear stiffness modulus6

in the ith component7

From which, the hysteretic coefficient ( hys ) of friction can be expressed as8

9T

hys

N

W

F l (13)10

where NF is the normal load, l is the contact length of the tire with the pavement surface, WTis the energy11

dissipation calculated as the sum over all integration points of the product of W and the associated12volume.13

The rolling analysis of the truck tire on a given asphalt concrete pavement is carried out in two steps:141) Steady state rolling;152) Transient rolling.16

Because of computational limitations, it is not possible to develop a micromechanical FE mesh long17enough for the tire to achieve the desired speed and other operating conditions. Instead, as a first step, a18

steady state rolling (SSR) at the desired operating conditions is achieved by means of a steady state19solution of the tire rolling over a smooth pavement surface. The SSR uses an Eulerian analysis based on20the steady state transport (SST) feature of Abaqus in which only the material moves at the specified21

velocity through the fixed FE mesh.22Results from the steady state rolling phase are imported to the transient rolling step and are23

brought to equilibrium against the stationary asphalt micromechanical pavement mesh. Once equilibrium24is established, the tire is forced to travel over the surface of the pavement mesh at prescribed operating25

conditions (velocity, slip ratio, inflation pressure and normal load). The slip ratio is defined as the ratio of26the slip speed to the vehicle speed. This two-stage procedure drastically improves computational time and27is thus adopted in this study. Even then, the entire procedure required a LINUX cluster with at least 828computing node, which consists of 16 CPUs running at 4 GHz which shares 512 GB of memory. Figure294 shows the developed FE model of a wide-base truck tire traversing over a micromechanical PAC30surface and the corresponding stress profile of PAC mix due to the truck tire loading.31

32


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12FIGURE 4 FEM simulation of truck tire-PAC pavement interaction3

4

RESULTS AND DISCUSSION5In this section, the results of FE simulations of a rolling truck tire traversing over PAC mix are presented.6

Different scenarios are discussed to analyze the critical stress situation of the mastic and mastic-aggregate7interfacial layer in the PAC mix design. The critical stress state of a PAC mix is due to the combination of8normal (compressive/tensile) and shear stresses under given tire and pavement characteristics. The state9of these stresses can vary within/near vicinity of the tire footprint in lateral, longitudinal and normal10directions forcing the pavement engineers to consider multi-axial stress states rather than one11

dimensional. However, it is difficult to envisage such multi-axial stresses and to predict the failure of a12

given pavement due to a particular component of stress.13In this study, the stress response was expressed in terms of stress invariants in order to get an14

insight into the performance of the mastic of a given PAC mix. Past researchers (15) showed that the15stress invariants are the realistic representative of failure of a given asphalt pavement mix under different16

material and loading conditions. It is noted here that the model assumes that the material properties will17follow linearity. The critical stresses were considered to be the regions of asphalt mixture loaded by the18

truck tire directly above and near vicinity of tire contact patch. The structural response of a given asphalt19mix under given loading conditions can be expressed in terms of 6 stress components, 3 normal (20

11 22 33, , ) and 3 shear ( 12 23 13, , ). For a given asphalt material and loading conditions, the stress21

invariants ( 1I and 2J ) can be computed as:22

1 11 22 33I trace (14)23

2 2 2 2 2 22 11 22 33 12 13 231

J s s s 22

(15)24

Asphalt Concrete mesh(mastic+ Aggregate

+ interfacial zone)

425/65R22.5

wide-base truck tire

Stress development within the

PAC mix due to rolling tire load


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11 11

22 22

33 33

s 1 3*trace

s 1 3*trace

s 1 3*trace

(16)1

The 1I invariant is used as an indicator of the normal stresses and the 2J acts as an indicator of the2

deviatoric or shear stresses.3

For every Gauss point of interest, two pairs of ( 1I , 2J ) were filtered, one with the highest 1I value4

and one with the highest 2J value. The ( 1I , 2J ) data pairs of several simulations were combined in a5

stress invariant plots for various asphalt mix surfaces. In this 1I - 2J system, the following cases were6

analysed:71) the stress invariants of the mastic and the aggregate-mastic interfacial layer of a PAC mix82) the influence of inflation pressure and tire loads on the stress invariants of PAC mix9

3)

the comparison of the performance of PAC mix with conventional asphalt mix designs in terms of10stress invariants11

12Stress invariants of the mastic and the aggregate-mastic interfacial layer of a PAC mix13Figure 5 shows the critical local stress response of the mastic and the aggregate-mastic interfacial layer of14PAC mix under the loading of rolling wide-base truck tire. In this figure, the negative X-axis represents15the compressive stress state and the positive X-axis represents the tensile stress state. The points of higher16

2J represent the maximum shear and the points of higher 1I represent values on the compressive or17

tensile stresses that the PAC mix would experience under given loading conditions.18As mentioned before, the model contains millions of elements, which makes it impossible to19

compute stress variants at all the points in the model. Thus in the load influenced deformation zone a total20

of 740 points are selected. These 740 points are scattered in all three direction within deformation zone.21Figure 5a and 5b shows the stress invariants of the mastic of a PAC mix. The points of higher normal22

stress invariants ( 1I

p3

) on the compressive side of the envelope were located inside of the ribs of the23

truck tire. The stress invariants on the tension side of the envelope were obtained away from the loading24area of the truck tire. Figure 5a shows that the mastic of PAC is subjected to high local normal and local25shear stresses. Although, the mastic can sustain high local compressive stresses, the local failure of mastic26might occur due to the high local tension and local shear stresses. During the course of time these stresses27results in the raveling of the PAC mix. This type of trend is understandable as the mastic is being28punished by the rolling truck tire which can penetrate through the voids of the PAC mix and exerting high29contact pressure on the mastic component.30

Comparing Figures 5a and 5b, the mastic zone of PAC mix experiences higher stress invariants31compared to the aggregate-mastic interfacial zone for the given loading type and material characteristics.32This type of trend represents the combination of normal and shear stresses that might lead to the cohesive33failure of raveling.34


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1

(a) Stress invariants of mastic of PAC mix2

3

(b) Stress invariants of aggregate-mastic interfacial zone of PAC mix4

FIGURE 5 Schematic of stress invariants of a PAC mix56

Influence of inflation pressure and tire loads on the stress invariants of PAC mix7Figure 6 shows the critical local stress response of the mastic of PAC mix for different loading conditions8

of rolling wide-base truck tire. Figure 6a compares the predicted stress response of mastic for 26 kN, 499kN and 53 kN for a constant inflation pressure of 720 kPa.10

0

2

4

6

8

10

12

14

-15 -12 -9 -6 -3 0

0.0

0.2

0.4

0.6

0.8

1.0

1.2

-0.8 -0.6 -0.4 -0.2 0 0.2

Wide-base tire 425/65R22.5

Speed: 8 km/h

Inflation pressure: 720 kPa

Load: 49 kN

Wide-base tire 425/65R22.5Speed: 8 km/h


Load: 49 kN

1Ip3

(MPa)

2J (MPa)

2J (MPa)

1Ip3

(MPa)


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12

3FIGURE 6 Schematic of stress invariants of a PAC mix for different loading conditions4

It can be seen that as the load increases, the change in vertical contact stress distribution of a5rolling tire has small effect on the microstructural response of mastic. The reason can be attributed to the6

proportionate increase in the in the tire contact area with load which ultimately results in the uniform7stress distributions in the mastic. Although a slight increase in the values of stress invariants are observed,8the maximum percentage change observed is less than 10%.9

0

2

4

6

8

10

12

14

-15 -13 -11 -9 -7 -5 -3 -1 1

720kPa_26 kN

720 kPa_49 kN

720 kPa_53 kN

0

2

4

6

8

10

12

14

-15 -13 -11 -9 -7 -5 -3 -1 1

49 kN_520 kPa

49 kN_720 kPa

49 kN_950 kPa

PAC mix surface


Speed: 8 km/hInflation pressure: 720 kPa

Load: 26 kN;49 kN; 53 kN

2J (MPa)


Speed: 8 km/h

Load: 49 kN

Inflation pressure: 520 kPa; 720 kPa; 950 kPa

a)

b)

1Ip3

(MPa)

1Ip3

(MPa)

2J (MPa)


15/17


In contrast, the vertical contact stress tends to increase as the tire inflation pressure increases for a1constant tire load. Increase in the inflation pressure renders the truck to be rigid in the central contact area2which ultimately increases the local normal and shear stress states of mastic of PAC mix. Because of an3increase in the values of stress invariants can be observed, Figure 6b. An increase of around 37% in the4values of stress invariants is observed when inflation pressure of truck tire increases 520 kPa to 950 kPa.5

6Comparison of the performance of the PAC mix with other asphalt mix designs7Next, the FE results of the PAC mix was compared against different asphalt mix designs for the given8truck tire loading. By comparing the Figure 5a and Figure 7, it is apparent that the PAC mix is subjected9to stress invariants higher than those on any other asphalt mix design surfaces.10

11

12FIGURE 7 Schematic of stress invariants of different asphalt mixes (continued)13

0.0

0.5

1.0

1.5

2.0

2.5

3.0

-4 -3 -2 -1 0 1

0.0

0.5

1.0

1.5

2.0

2.5

3.0

-3 -2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5

AC-10 mixWide-base tire 425/65R22.5

Speed: 8 km/h


Load: 49 kN

SMA mix


Speed: 8 km/h

Inflation pressure: 720 kPaLoad: 49 kN

a)

b)

1Ip 3 (MPa)

2J (MPa)

1Ip3

(MPa)

2J (MPa)


16/17


1

23

FIGURE 7 Schematic of stress invariants of different asphalt mixes45

The trend suggest that lower mastic content and higher void ratio allows the tire rubber to6

penetrate through the voids of the PAC mix and exert high contact stresses on it. For the7

remaining mix designs, a thin surfacing mix design develops the next higher stress invariants on8both normal stress axis and shear stress axis followed by AC-10 mix and SMA.9

CONCLUSIONS10PAC is known to be associated with pavement distresses such as raveling during its design life mainly11because of its high voids content in the mixture. The vehicular loading combined with the presence of12high percentage of stone fraction in PAC results in a wide range of stress states in both the mastic and the13interfacial zone between mastic and aggregate. These stresses are the potential causes of raveling which14occurs in the form of either cohesive or adhesive damage. Therefore, information about the mechanical15behavior at the graingrain contact region is of great importance for understanding the reason of such16distresses.17

In contrast with the past research studies, the present study utilizes micromechanical pavement18meshes which consist of mastic, aggregate and an interfacial zone between them. A 3D FE19micromechanical model was developed to simulate the rolling truck tire-asphalt mix surface interaction.20The resulting stresses due to microstructural response of asphalt pavement mixes was expressed in terms21of stress invariants.22

High values of stress invariants were observed in the asphalt pavement mixes when compared23with past research studies that used homogeneous pavement surfaces. This type of trend is due to the24punching mechanism induced by rolling truck tire on the mastic of asphalt mixes and exerting high25

contact stresses on the mastic component. For the tested PAC mix, the mastic zone experienced higher26stress invariants compared to the aggregate-mastic interfacial zone which points towards the cohesive27

failure of PAC mix.28In the present study, the effect of loading conditions on the microstructural response of asphalt29

mixes were also studied. It was observed that the increase in load has a marginal influence of the stress30 response of a given PAC mix. However, higher inflation pressures induces higher localized stresses on31the mastic of PAC mix resulting higher stress invariants.32

0

1

2

3

4

5

6

7

8

9

10

-10 -8 -6 -4 -2 0 2

Thin Surfacing mix


Speed: 8 km/hInflation pressure: 720 kPaLoad: 49 kN

c)

2J (MPa)

1Ip3

(MPa)


17/17


Finally, a comparative analysis of structural response of different asphalt mix designs subjected1to the loading of a rolling truck tire was carried out. The PAC mix experiences higher stresses due to2rolling truck tire than other asphalt mix surfaces. This trend can be attributed to the lower mastic content3and higher void ratio of PAC mix which might allow the tire rubber to penetrate through the voids of the4mix and exert high contact stresses on it.5

6

REFERENCES71.Varveri, A., S. Avgerinopoulos, C. Kasbergen, A. Scarpas, and T. Collop. Influence of air void content9on moisture damage susceptibility of asphalt mixtures. Computational Study. Transportation Research10Record: Journal of Transportation Research Board, Vol. 3, 2015, pp. 8-16.112.Klomp, A.J.G. Life period of porous asphalt. 1996.123.Hagos, E.T. The effect of aging on binder properties of porous asphalt concrete. 2008.134.Lytton, R.L., E. Masad, C. Zollinger, R. Bulut, and D.N. Little. Measurment of Surface Energy and its14Relationship to Moisture Damage. Texas, FHWA/TX-05/0-4524, 2005.155.Huurman , M., L.T. Mo, A.A.A. Molenaar, and S.P. Wu. Ravelling in porous asphalt concrete. , Ede,16

Netherlands, 2006.17 6.Huurman, M., A. Molenaar , F. van Reisen , R. Hofman, and J. Schreyer. Accelerated testing of double18

layer porous asphalt wearing courses. in APT'08 Program & Proceedings, 2008.197.Mo, L.T., M. Huurman, S.P. Wu, and A.A.A. Molenaar. Investigation into stress states in porous20asphalt concrete on the basis of FE modelling. Finite Elements in Analysis and Design , Vol. 43, 2007,21pp. 333343.228.Ala, A. A., M. Eyad, and P. Tom. Micromechanical Modeling of the Viscoelastic Behavior of Asphalt23

Mixtures Using the Discrete-Element Method. International Journal of Geomechanics, Vol. 7, no.24SPECIAL ISSUE: Constitutive Modeling and Computational Analysis of Asphalt Pavements, 2007, pp.25131-139.269.Dai, Q., M. H. Sadd, and Y. Zhanping. A micromechanical finite element model for linear and damage-27coupled viscoelastic behaviour of asphalt mixture. International Journal for Numerical and Analytical28

Methods, Vol. 30, no. 11, 2006, pp. 11351158.2910.Srirangam, S. K. Numerical Simulation of Tire-Pavement Ineraction (PhD Thesis). Delft University of30

Technology, The Netherlands, 2015.3111.Schapery, R. Nonlinear Viscoelastic and Viscoplastic Constitutive Equations with Growing Damage.32International Journal of Fracture, Vol. 97, 1999, pp. 33-66.3312.Srirangam, S.K., K. Anupam, A. Scarpas, and A. Kosters. Influence of temperature on tire-pavement34friction-I: Laboratory tests and finite element modeling. in TRB 92nd annual meeting, Washington D.C.,35

2013.3613.Srirangam, S.K., K. Anupam, A. Scarpas, C. Kasbergen, and M. Kane. Safety Aspects on Wet Asphalt37Pavement Surfaces through Field and Numerical Modeling Investigations. Transportation Research38record: Journal of Transportation Engineering, Vol. 3, 2014, pp. 37-51, Under publication.3914.SimpleWare. Simpleware, ScanIP 32-bit, Version 4.3. +CAD 32-bit, Version 1.3. 2011.40

15.Fernando, E.G., D. Musani, Park, and W. Liu. Evaluation of effects of tire size and inflation pressure41on tire contact stresses and pavement response. FHWA/TX-06/0-4361-1, 2006.42

16.ABAQUS. ABAQUS User's Manual. Version 6.13. 2013.43

microstructural analysis of porous asphalt concrete mix subjected to rolling truck tire loads

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