A Computational Model for Predicting the Effect of Tire

A Computational Model for Predicting the Effect of Tire Configuration on Asphaltic Pavement Life Roberto F. Soares* — David H. Allen* — Yong-Rak Kim* — Curtis Berthelot** — Jorge B. Soares*** — Mark E. Rentschler**** * College of Engineering, 114 Othmer Hall University of Nebraska-Lincoln, Lincoln, NE 68588-0642 [email protected] [email protected] [email protected] **Department of Civil Engineering University of Saskatchewan, 57 Campus Drive, Saskatoon SK, Canada S7N5A9 [email protected] ***Department of Transportation Engineering Universidade Federal do Ceará, Campus do Pici, Bloco 703, Pici 60455760 - Fortaleza, CE – Brasil [email protected] ****Department of Surgery 985045 Nebraska Medical Center, Omaha, NE 68198-5045 [email protected]

Road Materials and Pavements Design. Volume X – No X/2007, pages 1 to n

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This paper proposes a model for predicting the mechanical behavior and performance life of asphalt pavements subjected to various tire configurations, layer thickness, and material properties. A viscoelastic two-dimensional finite element model was developed and utilized in order to predict pavement life depending on different combinations of these design variables. The effects of truck loads on the pavement performance were studied by simulating three tire configurations: two types of the new generation wide-base single tire and one conventional dual tire configuration. Also, two different types of hot mix asphalt (HMA) and three variations of the HMA layer thickness were evaluated. Results showed that the use of conventional dual tires produces approximately 50 per cent longer asphalt service life when compared to the use of wide tires. The service life is shown to increase by increasing the HMA layer thickness. In fact, simulation results suggest that a 200 mm thick HMA layer provides a 15 per cent longer life than a 100 mm thick layer. Asphalt material results also suggest that the quality of materials can significantly affect pavement performance and service life. Furthermore, service life is significantly reduced when a poor quality material is combined with a thin asphalt layer. The simulation model presented here will be useful for future pavement design and material selection. ABSTRACT.

KEYWORDS:

Asphalt Pavement, Hot Mix Asphalt, Model, Tire Configuration, Viscoelasticity.

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1. Introduction Accurate truck loading estimates and analysis of the impact of heavy traffic on pavement performance are important issues for pavement designers. Recently, a wide-base single tire (shown in Figure 1) has been proposed as a replacement for the conventional dual tires used by most semi-trailer traffic in the USA. These widebase tires are currently more commonly used in Europe and Canada, and are slowly appearing in the USA market as a dual tire configuration replacement. The motivation for this trend is that it has been shown that a single wide-base tire offers advantages for fuel efficiency, productivity and vehicle stability (John and Glauz, 1995; Sebaaly et al., 1998). However, the effects of the single wide-base tire on pavement wear characteristics are still being examined.

Figure 1. Dual tires vs. wide-base single tire

A new computational model that can be used to perform a pavement service life prediction based on a mechanistic analysis using a finite element method has been developed. It includes viscoelastic properties of materials to help account for energy dissipation of the hot mix asphalt (HMA) and base layers. Viscoelastic properties have been used in several studies (Park and Kim, 1998; Blab and Harvey, 2002; AlQadi et al., 2002, 2005; Collop et al., 2003; Mun et al., 2004; Elseifi et al., 2006) to generally predict short-term stresses and strains in pavement systems. The newlydeveloped pavement design guide, so-called Mechanistic-Empirical Pavement Design Guide (NCHRP 1-37A, 2004) also uses the loading time (or frequency)dependent viscoelastic dynamic modulus as the primary material property for asphalt

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concrete layer, but performance analysis is carried out based on layered elastic theories, and design life of pavements is determined by incorporating mechanistic pavement responses with empirical transfer functions. More recently, a seminal study by Desai (2007) presented a purely mechanistic unified constitutive modeling approach called the Disturbed State Concept (DSC) to characterize and to predict the behavior of materials in both rigid and flexible pavements. However, to the authors’ best knowledge, a purely mechanistic model to predict long-term pavement life has rarely been attempted due to the significant analytical/computational complexities. This study is purely computational, and does not include all environmental conditions at this time. The current goal is a realistic model, with the least number of empirical variables and assumptions, which intends to predict long-term pavement service life.

2. Background

2.1. Pavement Mechanics Pavement mechanics and the effect of different tire configurations have been studied for many years (Burmister, 1945; Yoder and Witczak, 1975). Early work included studying of the response of various flexible pavement structures subjected to different tire configurations (Sebaaly and Tabatabaee, 1989; Bonaquist, 1992). Results showed that tensile strain under the wide-base tires was 50 per cent greater than those under the dual tires and compressive stresses at the interface were lower for the dual tires. A more recent study demonstrated that wide-base tires can cause significantly more damage to the pavement compared to conventional dual tires (US DOT, 2000). Results showed that the wide-base tires appear to cause 1.5 times more rutting than dual tires on flexible pavements. These results are supported by many other studies which conclude that wide-base tires are generally more damaging to the pavement than conventional dual tires (Huhtala et al., 1989; Akram et al., 1992; Perdomo and Nokes, 1993; Myers et al., 1999). Recently, advances in materials and tire design have led to the development of a new generation of wide-base tires designed to cause no additional infrastructure wear as compared to dual tires. This new design includes a wider flatter transverse profile, providing a uniform pressure distribution. The effect of this new technology on pavements was analyzed by using the methods of equivalent single axle loads (ESAL) and showed that the current axle load limit of 6,000 kg in Canada for single tires can be increased up to 8,000 kg for wide-base tires with no more damage than dual tires at 10,000 kg (Ponniah, 2003). To help understand the effects of the new wide-base tires better, numerous field experiments including measurements of stress-strain behavior in a HMA layer were previously performed under different loading configurations, environmental conditions, and at different traffic speeds (AlQadi et al., 2002, 2005). Results from these performance analyses showed that the

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newly developed tires and the conventional dual tire produce approximately the same tensile strains at the bottom of asphalt concrete layer. However, the vertical compressive stresses induced by the wide-base tire are greater on the upper HMA layers.

2.2. Finite Element Methods The finite element technique has been receiving increased attention from the pavement mechanics community due to its extremely versatile implementation of mechanical characteristics in addressing issues such as inelastic constitutive behavior, irregular pavement geometry, (Helwany et al., 1998; Wang, 2001; Blab and Harvey, 2002; Erkens et al., 2002; Al-Qadi et al., 2005; Saad et al, 2006) and growing damage (Collop et al., 2003; Mun et al., 2004; Kim et al., 2005, 2006). Recently, researchers performed a three-dimensional finite element analysis using a viscoelastic constitutive model to simulate the pavement responses to different vehicular loadings (Al-Qadi et al., 2005). They also measured pavement responses to two new-generation wide-base tires, however no cyclic load and long-term performance was considered in their study. They found greater fatigue damage from the wide-base tires than the conventional dual-tire assembly. Furthermore, they found that the new wide tires demonstrated slightly greater subgrade rutting damage as well. However, another study experimentally evaluated dual tires and wide tires and showed no statistical difference in pavement response between the wide-base and dual tire configurations (Timm and Priest, 2006). In addition to experimental work, Timm and Priest also modeled the pavement using layered elastic system and since no source for energy dissipation was considered, the model overpredicted the response under the wide-base tires when compared to the field data. While some of these studies have incorporated the viscoelastic effects of asphaltic pavement, they have not been conducted over long periods of time. A more accurate prediction of tire configuration effect on asphaltic pavement performance should include long-term studies, meaning the analysis should consider the cyclic loading of trucks over the period of the pavement life.

3. Roadway Model A simulation model of a standard two-lane asphalt roadway was used to investigate mechanical behavior of pavement subjected to different loading configurations, layer thicknesses, and asphalt material properties. The model presented here employs advanced algorithms to produce a time-marching computational simulation capable of predicting the spatial and temporal variations in stresses, strains, and displacements in viscoelastic roadways subjected to these various configurations. Energy dissipation due to several effects, such as

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viscoelasticity, crack-associated damage, aging of asphalt (Krishnan and Rajagopal, 2005) and environmental effects should be included to accurately predict the longterm behavior of asphalt pavements. As a first-step the current analysis included material viscoelasticity while future models will further incorporate the other dissipative methods. Therefore, pavement life herein was defined by examining when critical permanent deformation (12.5 mm) due to viscoelastic energy dissipation occurs on the surface of the pavement structure as a representation of rutting. The multiple layers of a typical two-lane asphalt roadway were modeled. These layers include the surface layer, base layer, subbase layer, and subgrade layer (insitu soil). The top layer is the HMA followed by the base typically made of either bituminous millings or crushed aggregates. The subbase is composed of soils stabilized with lime or fly ash. From symmetry, only one lane was modeled as shown in Figure 2. In this model the roadway is 12 m wide. It is comprised of two 3.50 m wide traffic lanes and two 2.50 m shoulders. The traffic lane is sloped to the shoulder at 2 per cent grade and the shoulders have a grade of 4 per cent.

Figure 2. A typical two-lane asphalt roadway with boundary conditions imposed

Mixed boundary conditions are needed to accurately model the complex layered pavement structure. The HMA and base layers are modeled as viscoelastic materials similarly to other studies (Al-Qadi et al., 2005). The subbase and subgrade materials usually show anisotropic and nonlinear elastic behavior. However, as a first approximation, they are assumed to behave as isotropic linear elastic materials (AlQadi et al., 2004; Elseifi and Al-Qadi, 2006). The boundary conditions include no horizontal displacement on the left vertical boundary and no vertical displacement on the bottom boundary. Also, traction is applied on the surface of the pavement. Accurate modeling requires these complexities, which lead to the need for numerical computation to reach a solution.

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In this study, a computational model based on finite element techniques was used so that inelastic material behavior and irregular pavement geometry can be realistically represented for predicting mechanistic pavement performance.

3.1. HMA and Base Layers The anisotropic nature of the HMA and base layers was approximated in this early model as isotropic viscoelastic medium. This approximation will have an affect on the accuracy of the results, however, such an approximation has been shown (Masad et al., 2006) to only marginally affect pavement fatigue lives. Masad et al. (2006) also state that the permanent deformation of HMA and base layers were always greater for the anisotropic model. This approximation will consistently affect all of the results, so that our comparison of pavement lives will only be scaled. As separate layers, each can realistically dissipate energy due to the viscoelastic nature at a different loading rate. Constitutive behavior of the HMA and base layers can be represented by the following linear viscoelastic convolution integral: t

VE σ ij ( xk , t ) = ∫ Cijkl (t − τ ) 0

where

∂ε kl ( xk ,τ ) dτ ∂τ

[1]

σ ij ( xk , t ) = stress as a function of time and space,

ε ij ( xk , t ) = strain as a function of time and space, VE Cijkl = stress relaxation modulus which is time-dependent,

xk = spatial coordinates, t = time of interest, and τ = time-history integration variable.

With all the equations and state variables posed, the constitutive equations are transformed into an incremental form in order to be used with a finite element technique. Briefly this technique involves the use of numerical approximations that leads a simple set of algebraic equations to be solved in order to extract the finite element solution. For a complete solution of the algorithm regarding the incrementalization technique, refer to Zocher et al. (1997). Isotropic viscoelastic materials can be modeled by a Prony series based on the generalized Maxwell model shown in Figure 3. This representation has been proved to be accurate that is indistinguishable from the experimental data (Zocher et al., 1997). The Prony series representation of the generalized Maxwell model can be written as:

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Road Materials and Pavements Design. Volume X – No X/2007 n  E EVE (t ) = E∞ + ∑ E p exp  − p  η p =1 p 

where

 t  

[2]

E∞ and E p = spring constants in the generalized Maxwell model,

η p = dashpot constants in the generalized Maxwell model, n = the number of dashpots, and t = time of interest.

σ

E1

E2

E3

En −1

En

E∞

η1

η2

η3

η n −1

ε

ηn

σ Figure 3. Generalized Maxwell model

The reduced master curves (stress relaxation moduli) for the two different HMA materials and the two base materials used in this study are shown in Figure 4. The Prony series constants were obtained from the experimental study of Chehab et al. (2002) in which Superpave mixes were tested in uniaxial tension. Note that HMA I and Base I constitute material set No.1 and HMA II and Base II represent material set No. 2. The position of the relaxation curves demonstrates that material set No. 2 is stiffer than No. 1, as shown in Figure 4.

A Model for Predicting Pavement Life

Relaxation Modulus (MPa)

1.00E+05

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Base II HMA II Base I HMA I

1.00E+04

1.00E+03

1.00E+02

1.00E+01

1.00E+00 1.0E-03 1.0E-02 1.0E-01 1.0E+00 1.0E+01 1.0E+02 1.0E+03 1.0E+04 1.0E+05 1.0E+06 Reduced Time (s)

Figure 4. Reduced master curves of two HMA materials and base materials

The combined thickness of the HMA and the base layers was maintained at 500 mm. For each simulation the top layer of HMA was varied between 100 mm, 150 mm, or 200 mm. Depending on the selected thickness of the HMA layer, base layer thickness was adjusted to maintain the same total thickness of 500 mm.

3.2. Subbase and Subgrade Layers

The subbase and subgrade layers were treated as linear elastic, similar to many other studies (Rowe et al., 1995; Papagiannakis et al., 1996; Siddharthan et al., 1998, 2002; Elseifi and Al-Qadi, 2006). The linear elastic constitutive relationship can be expressed as: E σ ij ( xk , t ) = Cijkl ε kl ( xk , t )

where

[3]

E Cijkl = elastic modulus which is not time-dependent.

Elastic properties for the subbase and subgrade layers are presented in Table 1. These are typical values similar to other studies (Blab and Harvey, 2002; Huang, 2003). Thicknesses for the subbase (300 mm) and the subgrade (1,100 mm) were held constant for all simulations.

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Layer

Subbase Subgrade

Properties Young’s modulus (MPa) Poisson’s ratio 800 0.35 200 0.35

Table 1. Elastic properties of subbase and subgrade materials used in this study

4. Truck Model

4.1. Tire Configurations

Three different tire configurations were used in the simulations: two types of the new generation wide-base single tire (44.5 cm and 49.5 cm) and a conventional dual tire system (two 29.5 cm tires in dual mode). Replacing both sides of dual tires by one wide tire results in a smaller contact area. Since wide tires carry the same load as the dual tires, the pressure is larger in the wide tire case. The axle length, the distance defined as length from the left wheel center to right wheel center, was not treated as a variable and it was fixed at 1.80 m for all simulations.

4.2. Loading

Most states in the USA have limitations for the maximum weight per axle and overall weight. Nebraska Department of Roads (NDOR, 2005) sets limits of 15,400 kg for tandem axles and a maximum gross-vehicle weight of 36,300 kg. The tandem axle configuration shown in Figure 5, applies a load of 151 kN to the roadway, assuming the axle is symmetrically loaded. The front, steer axle has 5,500 kg and applies a wheel load of 27 kN. The load distribution and wheel configurations analyzed in this study are shown in Figure 5.

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Figure 5. Truck with dual tires or wide-base tires configuration and weight distribution

A 20 m truck traveling at 120 km/h takes 0.6 second to pass over a fixed point in the pavement. Using this approximation, each of the five axles applies a cyclic load to the fixed pavement point during the 0.6 second of travel time. For this study it was estimated that three trucks pass through the fixed point each minute. This estimation is based on the average daily traffic from one of the highest volume interstate highways in the state of Nebraska and it was kept the same throughout the simulation. Ramp functions with a peak load of 75 kN representing the trailer axles and 54 kN representing the steering axle were implemented to the problem, as shown in Figure 6.

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0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.0 -10.0 Load (kN)

-20.0 -30.0 -40.0 -50.0 -60.0 -70.0 -80.0 Time (s)

Figure 6. Loading sequence to simulate truck trafficking

5. Finite Element Analysis

5.1. Mesh Convergence

One of distinct characteristics of finite element structural analysis is that the solution is dependent on the size of elements selected (mesh density). Fine meshes have the disadvantage of increasing the number of equations that must be subsequently solved, while choosing a relatively coarse mesh will result in an inaccurate numerical solution. Therefore, to reach an appropriate mesh density producing accurate results, an analysis of mesh convergence of each case was conducted first. By recreating the mesh with a denser element distribution, results from different meshes were compared until the results converge satisfactorily. The mesh convergence for the dual tire configuration is shown in Figure 7. Stress on the pavement surface was monitored as the mesh density increased. Meshes with more than 1,423 elements showed little improved accuracy.

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Figure 7. Convergence of stress for 29.5 cm tires

(m)

The final roadway mesh for the dual tire configuration included 1,423 elements (Figure 8). The mesh density producing similar level of convergence for the widebase tire resulted in a mesh of 920 elements and 901 elements for the 44.5 cm widebase tire and the 49.5 cm wide-base tire, respectively. Thus, for each tire configuration used, a different mesh producing the same level of accuracy was used.

(m) Figure 8. Roadway mesh for the dual tire configuration (29.5 cm)

5.2. Simulation Parameters and Approximations

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Recently, three-dimensional (3D) finite element analysis has become a tool capable of capturing details of pavement response that traditional two-dimensional (2D) models could not (Shoukry et al., 2007). However, as illustrated in Figure 8, finite element analyses for this study have been conducted in 2D plane strain condition because the 2D analyses can significantly reduce computational effort while still providing a great deal of information and insight into the problem. The average computational time to run one case with the necessary number of truck passing cycles using a 2.2 GHz dual core multicomputer was about 40 hours, which seems untenable to run the same problem in 3D with the current computer power. By running a 2D problem, the loading is applied as a strip load in the third dimension and an overestimation of load is induced. Since a tire applies a nonuniform load to a pavement (Al-Qadi et al., 2005), implementing a uniform load for the 2D analysis is an approximation. A more accurate pressure distribution on a tire is one that is higher in the center and lower in the corners, making a sinusoidal curve in both directions for a reasonable 3D approximation. By doing a 2D analysis, only one dimension can be treated as a sinusoidal curve, since there is no variation on the third dimension. To provide a more accurate estimate using the 2D analysis, a factor showing the ratio between 3D and 2D was determined. The maximum displacements in the 2D analyses were then divided by this factor to correct and make a more realistic estimate. Two separate simulations were necessary to determine this factor. The first simulation was a 3D finite element analysis of a pavement structure using the commercial software ABAQUS version 6.6 (ABAQUS, 2006) with realistic contact areas provided by Al-Qadi et al. (2005) where the finite element models incorporated the exact contact pressure of each tire tread, based on the exact footprint shape and dimensions of the dual and wide-base tires provided by the manufacturer at a tire inflation pressure of 720 kPa. The second simulation was a 2D pavement structure with the same material properties, loads, and geometry as the 3D analysis. The 2D analysis was performed with the authors’ own code used for simulations presented in this study. A maximum displacement, 4.46E-01 cm was found for the 3D analysis, and 6.34E-01 cm was the maximum displacement for the 2D analysis. Therefore, the displacements are overestimated by a factor of 1.42. As a result, the displacements obtained from 2D simulations in the study presented here were corrected by being divided by this factor. It should be also noted that vertical stresses were primarily monitored to investigate pavement performance due to loading at this early stage of modeling. Horizontal stresses were not considered at this time, even though they may be significant (De Beer et al., 1997; De Beer, 2006). Failure criterion to define pavement life is necessary to compare mechanical behavior and corresponding service life of the pavement subjected to various tire configurations, layer thicknesses, and material properties. As previously mentioned, this study estimated pavement life by simply examining rutting (permanent deformation induced by materials viscoelasticity) in the near side wheel tracks at the surface (HMA) layer. Critical rut depth of 12.5 mm was selected based on many

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studies including results by Shook et al. (1982). At this stage of rutting, it has been noted that the surface is beginning to develop macrocracks, which leads to water ingress and pooling. Subsequent rapid deterioration follows due to synergistic effects by cracking, rutting, and moisture damage. Finite element computer simulation requires a significant amount of computer processing and would make the work practically unachievable to conduct over a longer period of time until the pavement completely fails. Since life predictions in this study are not associated with damage but simply based on viscoelastic energy dissipation, it might be possible to extrapolate the results after a certain number of cycles have been simulated. This process was conducted by running the problem up to 8.0E+05 seconds (i.e. 40,000 cycles of 20 seconds), instead of the full pavement life, and adding a trend line to the data for extrapolation. Two nodes on the surface with maximum and minimum permanent deformation around the load were selected to calculate the rut depth and consequently to take the extrapolation. The data presented in Figure 9 and Figure 10 help clarify this approach by illustrating permanent deformation on the surface versus time and followed by the same graph in logarithmic scale, making it easier to see the trend.

Figure 9. Permanent deformation versus time – this study

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Trend line

Figure 10. Permanent deformation versus time in log-scale – this study

5.3. Modeled Configurations

The life of a roadway is a function of several parameters such as layer thickness, lane width, contact area of the tire, pressure distribution, applied load, loading frequency, tire configurations, material properties, and many more. However, only three parameters were treated as variables in this study: tire width, viscoelastic properties of energy-dissipating layers (HMA layer and base layer for this study), and thickness of HMA layer. Only one parameter at a time was varied in each study, and all others were held fixed for comparison. The simulations performed here include three different tire types with different overall widths: 29.5 cm for the dual tire configuration and 44.5 cm and 49.5 cm for the new generation wide-base tire configurations. In addition to the tire configuration, the change in the HMA layer thickness, using values of 100, 150 and 200 mm with two different sets of HMAbase property was also evaluated. The goal was to compare the pavement service life for each case. Table 2 outlines which variables were evaluated in each case.

Case 1 Case 2 Case 3

Variable Tire width HMA layer thickness Material property

Description Dual tire (29.5 cm), Wide tire (44.5, 49.5 cm) 100, 150, and 200 mm Set No.1 and Set No.2

Table 2. Description of each tire and pavement case studied here

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6. Results

6.1. Case 1 and 3: Tire Width and Material Property

Table 3 presents simulation results with different tire configurations. Note that the pavement layer structure (150 mm and 350 mm for HMA layer and base layer, respectively) remained the same. The loading condition applied for all the cases is shown in Figure 6. Continuity conditions were satisfied at the layer interfaces where frictionless perfect bonding was assumed for this study.

Material

Set No.1

Set No.2

Tire width (cm) 29.5 44.5 49.5 29.5 44.5 49.5

Pavement life (years) 133.1 87.0 99.2 292.3 199.3 228.7

Table 3. Pavement life for different tire widths for HMA layer thickness of 150 mm

As shown in the table, the tire width 29.5 cm with dual format performed better than the other two wide-base single tires. The wide single tires with 44.5 cm always produced the shortest life. These results indicate that a truck with 29.5 cm dual tires gives a pavement life approximately 50 per cent longer than the wide tire 44.5 cm and around 30 per cent longer life than the wide tire 49.5 cm. The results also indicate that pavements composed of materials set No. 2 will last much longer (approximately 2.5 times) than pavements with materials set No. 1. This is expected since the set No. 2 is stiffer than set No. 1 resulting in more rut-resistance. With considerations of damage due to cracking, this may not be true any more. This study produced unrealistic pavement lives, as it can be seen from the table. However, the unrealistic pavement lives are not quite surprising because, as noted, only material viscoelasticity in the HMA and base layers was considered to produce permanent deformation due to energy dissipation. Other related energy-dissipating mechanisms (such as crack-associated damage and yielding) and environmental effects including aging of asphalt and the effects of moisture-temperature variation need to be included to accurately predict the pavement life. The pavement life herein was simply determined when the pavement structure reached total permanent deformation of 12.5 mm due to the viscoelastic energy dissipation. In spite of the unrealistic pavement life, the model is still able to predict a rank order of lives which significantly depend on structural design variables and layer thicknesses.

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6.2. Case 2 and 3: HMA Layer Thickness and Material Property

Next analysis was performed to compare life of pavement with different thicknesses of the HMA layer: 100, 150, and 200 mm. For this study, the tire width was fixed with 44.5 cm single configuration. As shown in Table 4, the results indicate that the thicker HMA layer is associated with longer performance life, even though total thickness of HMA plus base layers was fixed with 500 mm. This is due to a better rut-resistance from thicker HMA surface layer that is stiffer than base layer as illustrated in Figure 4.

Material

Set No.1

Set No.2

Asphalt layer thickness (mm) 100 150 200 100 150 200

Pavement life (years) 76.2 87.0 99.5 192.1 199.3 206.7

Table 4. Pavement life for different HMA layer thickness for tire width of 44.5 cm

Furthermore, it can also be noted that the effects of pavement layer thickness on performance life are incorporated with properties of layer materials. Pavements with materials set No. 1 presented a more significant increase in life than pavements with materials set No. 2, as the HMA layer thickness varies. This mechanical analysis of pavements can be a potential tool to aid pavement design and selection of pavement materials, since it gives a reasonable comparison of how the pavement response will be over the years and how to help to make a decision of which tire configuration and materials should be used to help lengthen the pavement service life.

7. Conclusions

A finite element method was used for the purpose of mechanically analyzing service life of asphalt pavements subjected to different tire configurations, layer geometries, and material properties. The primary motivations for this study were to model asphalt pavements in a purely mechanistic way and to predict which combination of design variables influences pavement life. The analysis gives a comparison of predicted pavement life for various roadway geometries and tire configurations.

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Simulation results indicate that dual tire configurations have the best performance compared to the other two new generation wide-base tire configurations. Additionally, for the wide-base tire case, as tire size increases, the damage potential decreases. In addition, results from analyzing pavement response to different layer thicknesses showed that the thicker HMA layer performs better than thinner layers, and the effects of layer thickness were more significant when it was associated with stiffness of layer materials. One of distinct characteristics of this type of modeling/analysis approach is that it can reduce the need of extensive laboratory and field work, since the predictions rely upon the computer simulation and the fundamental structural properties of each layer. However, because the current generation of the model merely takes into account energy dissipation due to materials viscoelasticity and does not provide any sources of energy dissipation in the form of damage and due to environmental effects, it has limitations that are left as future work.

8. References ABAQUS, “ABAQUS/CAE User's Manual and ABAQUS Theory Manual”, Version 6.6, 2006. Akram, T., Scullion, T., and Smith, R.E., “Estimating Damage Effects of Dual Versus Wide Base Tires With Multidepth Deflectometers”, Transportation Research Record, No. 1355, 1992, p. 59-66. Al-Qadi, I. L., Loulizi, A., Janajreh, I., and Freeman, T. E., “Pavement Response to Dual and New Wide Base Tires at the Same Tire Pressure”, Transportation Research Record, No. 1806, 2002, 38-47. Al-Qadi, I. L., Elseifi, M. A., and Yoo, P. J., “In-situ validation of mechanistic pavement finite element modeling”, Proc., 2nd Int. Conf. on Accelerated Pavement Testing, Minneapolis, 2004. Al-Qadi, I. L., Yoo, P. J., and Elseifi, M. A., “Characterization of Pavement Damage Due to Different Tire Configurations”, Journal of the Association of Asphalt Paving Technologists, No 74, 2005, p. 921-962. Blab, R., and Harvey, J. T., “Modeling Measured 3D Tire Contact Stresses in a Viscoelastic FE Pavement Model”, International Journal of Geomechanics, Vol. 2, No. 3, 2002, 271290. Bonaquist R., “An Assessment of the Increased Damage Potential of Wide Bases Single Tires”, Proc. of the 7th International Conference on Asphalt Pavements, Nottingham, UK, 1992, p. 1-16. Burmister D. M., “The General Theory of Stresses and Displacements in Layered Systems I”, J. Appl. Phys, Vol 16, No 89, 1945.

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A Model for Predicting Pavement Life

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Siddharthan, R. V., Yao, J., and Sebaaly, P. E., “Pavement Strain from Moving Dynamic 3D Load Distribution”, Journal of Transportation Engineering, Vol. 124, No. 6, 1998, p. 557–566. Siddharthan, R. V., Krishnamenon N., El-Mously, M. and Sebaaly, P. E., “Investigation of Tire Contact Stress Distributions on Pavement Response”, Journal of Transportation Engineering, Vol. 128, No. 2, 2002, p. 136-144. Timm, D. H., and Priest, A. L, “Mechanistic Comparison of Wide-Base Single versus Standard Dual Tire Configurations”, Transportation Research Record, Vol. 1949, 2006, p. 155-163. USDOT, “Comprehensive Truck Size and Weight Study”, FHWA-PL-00-029, Volume II, 2000. Wang, J., “Three-dimensional Finite Element Analysis of Flexible Pavements”, M.S. Thesis, University of Maine, 2001. Yoder, E.J., and Witczak, M.W., “Principles of Pavement Design. John Wiley and Sons, Inc.”, New York, New York, 1975. Zocher, M. A., Groves, S. E., and Allen, D. H., “Three Dimensional Finite Element Formulation for Thermoviscoelastic Orthotropic Media”, International Journal of Numerical Methods in Engineering, Vol. 40, No. 12, 1997, p. 2267-2288.