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6th ICPT, Sapporo, Japan, July 2008

11

ANALYSIS OF SHEAR STRESS IN ASPHALT PAVEMENTS UNDER

ACTUAL MEASURED TIRE-PAVEMENT CONTACT PRESSURE

Kai SU

Special Researcher, Airport Research Center, Port and Airport Research Institute1-1, Nagase 3, Yokosuka 239-0826, [email protected]

Lijun SUN

Professor, Department of Transportation Engineering,

Tongji University

1239, Siping road, Shanghai, 200092, China

[email protected]

Yoshitaka HACHIYA

Service Center of Port Engineering (SCOPE), 3-3-1, Kasumigaseki, Chiyoda-ku, Tokyo,100-0013, Japan

[email protected]

Ryota MAEKAWA

Senior Researcher, Airport Research Center, Port and Airport Research Institute1-1, Nagase 3, Yokosuka 239-0826, Japan

[email protected]

ABSTRACTRutting is one of the most important load-induced distresses found in asphalt pavements. The

primary mechanism of rutting is associated with shear deformation rather than densification.

Recently, top-down cracking probably attributed to shear failure has also been a frequent

occurrence in asphalt pavements. Clearly, shear stress is one of the critical factors affecting

pavements performance, and there is a great need to fully comprehend shear stress in asphalt pavements. However, most conventional pavement design methodologies assume tire-

pavement contact stress is equivalent to tire inflation pressure and uniformly distributed over

a circular contact area. In fact, tire-pavement contact is not circular and contact pressure is

neither uniform nor the same as tire inflation pressure. To obtain an accurate account of the

influence of actual tire-pavement contact pressure on pavement response, this study evaluates

the shear stress in asphalt mixture layers produced by non-uniform stresses applied to the pavement surface, in a simulation of field conditions. Then a solid analysis is carried out for a

semi-rigid asphalt pavement. The calculated results indicate the maximum shear stress occurs

at a point approximately 60 mm under the tire edge, and that both tire inflation pressure and

load distinctly affect shear stress. Bonding at the interface between the asphalt mixture layerand the base course obviously affects shear stress as well.

KEY WORDS

shear stress, rutting, top-down cracking, measured tire-pavement contact pressure, 3D finite

element method

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Analysis of Shear Stress in Asphalt Pavements under Actual Measured Tire-Pavement Contact Pressure

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INTRODUCTION

Rutting in asphalt pavement includes densification and shear flow of hot-mix asphalt, but the

majority of severe instable rutting results from shear flow within the asphalt mixtures1,2). In

recent years, another type of surface distress called Top-Down Cracking (TDC), which is

usually found in longitudinal path, has become more common in asphalt pavements 2,3,4); this

is also considered as a shear-related failure5)

. As a result, shear stress is believed to be one ofthe critical factors affecting pavements performance, and it is necessary to well understand

shear stress in asphalt pavements.

However, traditional methods of pavement analysis assumed that contact pressure is the same

to tire inflation pressure and that it is uniformly distributed over a circular contact area and

acts in the vertical direction6). In fact, it has been recognized that the tire-pavement contact

area is not circular and that contact stress is neither uniform nor equal to tire inflation

pressure1,2,3,4,7,8). To gain an accurate understanding of the effect of shear stress on pavement

performance, a laboratory method of applying tire-pavement contact pressure is employed in

this paper. The results are compared for differing loading conditions. The effects of tire

pressure and stress components in terms of vertical and horizontal stress on shear stress arecomprehensively investigated by three-dimensional finite element method (3-D FEM). In

addition, the effects of asphalt layer thickness and interface conditions are also discussed.

MEASUREMENT OF TIRE-PAVEMENT CONTACT PRESSURE

In order to measure the tire-pavement contact pressure distribution under realistic condition, a

static laboratory test device was developed, as shown in Figure 19,10). In this device, a series

of instrumented pins are embedded in the asphalt concrete specimen to measure the stress

induced by the tire. The tire load is applied using servo-hydraulic actuators. All measurements

are automatically recorded using a data logger. Though this method may seem simple andmay result in slight inaccuracy as compared with an actual pavement, the results are

undoubtedly much more reliable than when uniform contact pressure is assumed.

Pressure sensor

Fixed frame

Testing tire

Hydraulic system

Specimen

Sensor wire

Test controller

Figure 1 Static test system for tire-pavement contact pressure

The tire used for testing was new longitudinally pattern truck tire (11.00-20) as illustrated in

Figure 2 a). In the subsequent analysis, the tire loading area had to be simplified to build a

more efficient three-dimensional finite element model. This simplified pattern is presented in

Figure 2 b). In the tests, tire inflation pressure ranged from 0.46 MPa to 1.05 MPa, while tire

loading varied from 19 kN to 50 kN. In all, six combinations of tire pressure and tire loading

are studied in this paper, that is, 0.46/25, 0.60/25, 0.81/19, 0.81/25, 0.81/50, 1.05/25, in which

the first item meant tire pressure (MPa) and the later item symbolized the tire load (kN). For

example, 0.60MPa/25kN meant the combination of tire inflation pressure of 0.60 MPa and

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Su, Sun, Hachiya and Maekawa

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tire load of 25 kN. It should be noted that the case of 0.60MPa/25kN, with an average vertical

contact pressure of 0.70 MPa (load/average area), is generally considered as the standard

loading condition11).

a) 11.00-20 Tire b) Simplified contact area

Figure 2 Test tire tread and contact area simplified for 3-D FEM

Table 1 Pavement structures in FEM model

LayerThickness

(mm)Material

Modulus(MPa)

Poisson’sratio

Wear course 50Asphalt concrete 3,000 0.35

Binder course 100

Base course 300 Cement-stabilized aggregate 6,000 0.25

Subgrade / soil 85 0.40

PAVEMENT STRUCTURE AND 3-D FEM MODEL

In some countries, asphalt pavements consisting of a cement stabilized base and asphalt layerare widely used to take advantage of the superior strength of semi-rigid materials10,11). A

fairly typical semi-rigid asphalt pavement structure was selected for modeling in this finiteelement analysis11,12), as tabulated in Table 1. It consists of two asphalt mixture layers, a base

course and a semi-infinite subgrade. Each material is assumed to be homogenous, isotropic

and linearly elastic. The modulus of the cement-stabilized base course with a cement content

of 5% was determined by back-calculation based on the measured deflection10).

Figure 3 shows the entire 3-D FEM model by ANSYS, in which the load is a single axle

fitted with dual tires at each side. This configuration is considered based on the assumption

that the effect of interactions between front and rear axles on shear stress is negligible10)

. The

dimensions of the model are 6m long, 4m wide and 6m deep; this size gives a goodconvergence of results. The y axis represents the tire travel direction and the x axis thetransverse direction. The origin of the coordinate system is located in the mid-point between

the two left tires. The loading area varies with loading conditions. However, the dual tirespacing (center to center) is kept constant with 300mm in every case10). The boundary

conditions are that the side faces (front, back, left and right) are fixed in the normal direction,

and the other two directions are free; the bottom is completely fixed in all directions. The

interfaces between different layers are all assumed to be perfectly bonded except the section

of effect of bonding condition.

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Figure 3 3-D FEM model

EFFECT OF TIRE INFLATION PRESSURE AND TIRE LOAD

In this section, vertical load is only considered. The maximum shear stress occurs directly

under the tire edge irrespective of loading conditions. Figure 4 shows that, in all cases, shearstress under the tire edge increases initially with depth, reaching the maximum value at a

depth approximately 60 mm after which it decreases from the peak value.

It seems that loading conditions have little effect on the location of the shear stress maximum.

However, both tire load and tire pressure make a significant contributions to the magnitude ofshear stress. Relatively, overloading is more dangerous than overpressure. This is clear from

the shear stress in the case of loading at 0.81MPa/50kN, which is far greater than that at

0.81MPa/25kN, whereas the shear stress at 1.05MPa/25kN is only slightly larger than that at

0.81MPa/25kN.

0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.400.18

0.16

0.14

0.12

0.10

0.08

0.06

0.04

0.02

0.00

D e p t h ( m )

Shear stress (MPa)

0.46MPa/25kN

0.60MPa/25kN

0.81MPa/19kN

0.81MPa/25kN

0.81MPa/50kN

1.05MPa/25kN

Figure 4 Shear stress distribution as a function of depth

The shear stress distributions over the horizontal plane through the point with the maximum

shear stress are illustrated in Figure 5. Only three load conditions are compared here. The

distribution of shear stress over the horizontal plane differs under different load conditions.The larger shear stresses are focused at the tire edge in the case of overloading and standard

load conditions, while in the case of overpressure, shear stress is similar at the edge and at the

center of the tire.

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15

0.10

0.15

0.20

0.25

0.10

0.15

0.20

0.25

-0.10

-0.05

0.00

0.05

0.10

T r a v

e l l i n

g d i r e c t i o

n ( m

)

T r a n s v e r s e d i r e c t i o n ( m )

0.10

0.15

0.20

0.25

0.30

0.10

0.15

0.20

0.25

0.30

.35

.40

-0.15

-0.10

-0.05

0.00

0.050.10

0.150.20

T r a v

e l l i n

g ( m

)


0.10

0.15

0.20

0.25

0.30

0.10

0.15

0.20

0.25

0.30

-0.10

-0.05

0.00

0.05

0.10

T r a v

e l l i n

g d i r e c t i o

n ( m )


a) 0.60MPa/25.0kN b) 0.81MPa/50.0kN c) 1.05MPa/25.0kN

Figure 5 Shear stress distribution over the transverse plane (one tire)

From these calculated results, it can be concluded that high shear stress action occurs at the

edge of the loading area. This implies that a shear plane may develop near the tire edge if the

shear stress is great enough, forcing the asphalt concrete away from the tire. This might be

the mechanism responsible for shear deformation. On the other hand, shear stress may also

play a role in the formation of TDC, which occurs along the driving path; it is believed thatwhen the maximum shear stress is great enough, it can easily cause TDC at the tire edge

under repeated application of loading. Even if shear stress is not primarily responsible, it is at

least one of the major factors which develop TDC.

EFFECT OF HORIZONTAL STRESS

The load combination of 0.60MPa/25kN was used to study the effect of horizontal pressure

on shear stress. The horizontal stress coefficient, defined as the ratio of horizontal stress to

vertical stress, varied from 0.0 to 0.711)

. Figure 6 depicts the relationship between shear stressand horizontal stress. As can be seen from this figure, shear stress gradually increases when

the horizontal stress is very low, and then rapidly increases once the horizontal stresscoefficient exceeds 0.2. This means that higher shear stress generally occurs in areas where

braking or turning wheels cause significant horizontal stress. This agrees with the observedsevere rutting on highway gradients and at bus stops. Asphalt mixtures in such areas should

be carefully designed to give greater shear strength, enabling them to resist shear

deformation.

-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0

0.2

0.3

0.4

0.5

0.6

0.7

0.8

M a x i m u m s h e a r s t r e s s ( M P a )

Horizontal stress coefficient

Figure 6 Maximum shear stress versushorizontal stress coefficient

10 15 20 25 30

0.10

0.15

0.20

0.25

0.30

0.35

M a x i m u m

s h e a r

s t r e s s ( M P a )

Asphalt mixture layer thickness (cm) Figure 7 Shear stress at different asphalt

mixture layer thickness

EFFECT OF ASPHALT MIXTURE LAYER THICKNESS

For asphalt mixture layers of 100 mm to 300 mm in thickness, the maximum shear stress

remains almost constant. This is illustrated in Figure 7, which shows that a thinner asphalt

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mixture layer does not lead to significantly greater shear stress than a thicker one. In other

words, for a particular asphalt mixture, a thinner layer leads to barely greater risk of rutting

and TDC compared with a thicker one.

EFFECT OF INTERFACE CONDITION

In a multi-layered pavement system, the condition of the interfaces between layers makes animportant contribution to pavement performance. Here, the effect of interface condition on

shear stress is evaluated, focusing on the cases of no bonding and full bonding between the

asphalt mixture layer and the base course. For these two cases, the shear stress contour on the

vertical plane at the edge of the tire is illustrated in Figure 8. Both the range and magnitude of

shear stress where there is no bonding are greater than where there is full bonding.

Correspondingly, poor bonding at the interface, which means a situation somewhere between

no bonding and full bonding, would result in higher shear stress than the full bonding case.

That is, inadequate bonding between the asphalt mixture layer and the base course is

detrimental not only in the sense that slippage failure may be induced, but also because it can

lead to rutting and TDC.

-0 .8 -0 .6 -0 .4 -0 .2 0 .0 0 .2 0 .4 0 .6 0 .8-0.50

-0.45

-0.40

-0.35

-0.30

-0.25

-0.20

-0.15

-0.10

-0.05

0.00

Tranverse d istance (m)

D e p t h

( m

)

0

3.000E4

6.000E4

9.000E4

1.200E5

1.500E5

1.800E5

2.100E5

2.450E5

a) Full bonding

-0 .8 -0 .6 -0 .4 -0 .2 0 .0 0 .2 0 .4 0 .6 0 .8-0.50

-0.45

-0.40

-0.35

-0.30

-0.25

-0.20

-0.15

-0.10

-0.05

0.00

Tranverse d istance (m)

D e p t h

( m

)

0

3.000E4

6.000E4

9.000E4

1.200E5

1.500E5

1.800E5

2.100E5

2.450E5

b) No bonding

Figure 8 Shear stress distribution through depth with different interface conditions

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CONCLUSIONS

This study has led to following conclusions:

1) In all cases, the maximum shear stress occurs at the tire edge, and this point of peak shear

stress is one of the major factors responsible for rutting and top-down cracking (TDC)

development.

2)

Both tire pressure and vertical load have a significant effect on shear stress, with verticalload having more pronounced influence.

3) Horizontal stress significantly affects shear stress, and particularly at higher magnitude of

horizontal stress.

4) The thickness of the asphalt mixture layer has little influence on shear stress.

5) Poor bonding between asphalt mixture layer and base course can lead to an increase in

shear stress, and this in turn increases the risk of rutting and TDC.

ACKNOWLEDGMENT

The authors express their hearty thanks to the National Science Found for Distinguished

Young Scholars of China for its assistance with this study.

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2) Myers, L. A., C. Drakos and R. Roque: The combined effects of tire contact stresses and

environment on surface rutting and cracking performance. Proceedings of the ninth

international conference on asphalt pavements, Copenhagen, Denmark, 2002.

3) Myers, L. A., R. Roque and B. E. Ruth: Mechanisms of surface-initiated longitudinalwheel path cracks in high-type bituminous pavements. Journal of the Association of

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Myers, L., R. Roque, B. Ruth and C. Drakos: Measurement of contact stresses for

different truck tire types to evaluate their influence on near-surface cracking and rutting.

Transportation Research Record , No. 1655, Transportation Research Board, pp.175-184,

1999.

5) Wang, L. B., L. A. Myers, L. N. Mohammad and Y. R. Fu: A micromechanics study on

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D.C., 2003.

6)

Huang, Y. H.: Pavement analysis and design. Prentice-Hall, Inc. Englewood Cliffs, New

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7)

De Beer, M., C. Fisher and F. Jooste: Determination of pneumatic tire/pavement interfacecontact stresses under moving loads and some effects on pavements with thin asphalt

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8) Himeno, K., T. Kamijima, T. Ikeda and T. Abe: Distribution of tire contact pressure of

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School of Transportation Engineering, Tongji University, China. Pp.20-27, 2007.11) Ministry of communication. Specification of asphalt pavement design. The peoples’

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