Part2_Chapter2_Materials

Property Deterioration

Materials that are subjected to load-related fatigue distress may experience a severe degradation of properties with time and load repetitions. Micro-cracks may develop, leading to a reduced stiffness or modulus. This reduction in modulus will, in turn, lead to an increase in stress states within the pavement and a greater possibility of permanent deformation. The Design Guide considers the impact of load-related damage on property degradation.

Time-Temperature Effects

All asphaltic materials are highly sensitive to temperature and the rate of loading. Because asphalt is a viscoelastic-plastic material, the modulus of an asphalt mix may approach that of an unbound granular material at high temperatures and long loading rates (i.e., slow speed of passing vehicles). In contrast, at cold temperatures and very short load rates, the material will tend to behave in a pure elastic mode and have modulus values that approach that of PCC material. In the Design Guide, the methodology for asphaltic mixtures will take into account the range of temperatures expected in the design period. In addition, the use of an asphalt master curve, based on time-temperature superposition principles, will allow the engineer to account for the approximate speed of the vehicle under consideration.

Asphalt is not the only material that is influenced by vehicular speed. For example, rigid pavements show a 30 percent reduction in corner deflection and PCC edge strain as vehicle speeds increase from 3 to 60 mph. The dynamic modulus of PCC under typical Interstate vehicle speeds is approximately 20 percent higher than the static modulus historically used in rigid pavement analysis. Soft, wet clay subgrades that are viscoelastic-plastic may also exhibit a time-dependent modulus response; however, the ability to adjust subgrade moduli for a particular design speed is not intrinsically a part of the Guide. If a particular design situation requires that this factor be considered, direct modulus testing can be altered to characterize the material behavior under the specified loading rate.

The rate of load effect upon material response is a function not only of the vehicular speed, but also of the location of the material within the pavement structure. In general, as one proceeds deeper into the pavement, the length of the stress pulse acting on a given material will increase, suggesting that the time of the load pulse will also increase. The Guide does not automatically consider this aspect of material behavior, except in the analysis of asphaltic mixtures. Asphalt materials are characterized by a master curve incorporating time and temperature effects directly into the solution methodology.

Non-Linear Behavior

A material is considered non-linear if the value of the elastic modulus depends on the state of stress in the material. While many materials start to exhibit this behavior at very high stress states, the only materials that are considered to be non-linear in this Guide are unbound base/subbase and subgrade materials. Because the Guide inputs have been developed in a hierarchical structure, the most advanced level of M-E analysis can incorporate this factor into the design analysis methodology. If a non-linear analysis is desired due to use of non-linear

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ACKNOWLEDGMENT OF SPONSORSHIP

This work was sponsored by the American Association of State Highway and Transportation Officials, in cooperation with the Federal Highway Administration, and was conducted in the National Cooperative Highway Research Program, which is administered by the Transportation Research Board of the National Research Council.

DISCLAIMER

This is the final draft as submitted by the research agency. The opinions and conclusions expressed or implied in the report are those of the research agency. They are not necessarily those of the Transportation Research Board, the National Research Council, the Federal Highway Administration, the American Association of State Highway and Transportation Officials, or the individual states participating in the National Cooperative Highway Research Program.

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PART 2—DESIGN INPUTS

CHAPTER 2 MATERIAL CHARACTERIZATION

2.2.1 INTRODUCTION

This chapter describes the material characterization required for the mechanistic-empirical (M-E) design approach described in this Guide. The interaction of the materials inputs with other components of the mechanistic-empirical design procedure outlined in the Guide is shown in figure 2.2.1. The interaction between the materials, climatic, traffic, structural response, and performance prediction components is apparent from the figure. To provide a common basis for understanding the material requirements, the following M-E-based subcategories have been developed:

• Material properties required for computing pavement responses.
• Additional materials inputs to the distress/transfer functions.
• Additional materials inputs required for climatic modeling.

Figure 2.2.1. Interaction between materials module with other components of the M-E framework in the Design Guide.

In the first category are material properties required to predict the states of stress, strain, and displacement within the pavement structure when subjected to an external wheel load. In the M-E approach selected for the Design Guide, these properties include elastic modulus (E) and Poisson's ratio (μ) of the material. These properties are mandatory inputs for each pavement layer in the system.

In the second category are all the materials-related inputs that enter the distress or smoothness models directly. As stated in PART 1, Chapter 1, the key distress types or performance measures considered

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in flexible pavement design include load-related fatigue fracture (top-down and bottom-up), permanent deformation, and transverse fracture. The primary performance measures considered in rigid pavement design include load-related fatigue fracture of PCC (top-down and bottom-up) in JPCP, faulting of transverse joints in JPCP, and punchouts in CRCP. For each of these distresses, the critical structural response under a given wheel and climatic loading condition is affected by pavement material properties such as modulus and Poisson's ratio. In addition, parameters such as strength, expansion-contraction characteristics, friction between slab and base, erodibility of underlying layers, layer drainage characteristics, plasticity and gradation, and other material attributes directly influence how a material contributes to a given distress mechanism. These additional materials inputs are specific to the pavement type and distress model under consideration.

Finally, in the third category are materials-related inputs that enter the climatic module to help determine the temperature and moisture profiles through the pavement cross-section. These include engineering index properties (e.g., plasticity index), gradation parameters (porosity, effective grain sizes, etc), and thermal properties (absorptivity, heat capacity, coefficient of thermal expansion, and so on).

Table 2.2.1 is a tabular summary of the materials inputs required for M-E design arranged by the major material categories considered in the Design Guide including recycled asphalt and PCC layers used in rehabilitation design and construction. The material categories are explained later in this chapter.

2.2.1.1 Material Factors Considered

Time-Dependent Properties

Because some materials' properties undergo time-dependent changes, it is quite important to be able to consider these changes in any M-E analysis. Generally speaking, pavement material properties are constantly changing over time due to chemical and physical forces, influence of climate, as well as the onset of fracture or deformation. Consequently, the magnitude of the material properties either undergoes an enhancement or deterioration. These should be accounted for in design and are discussed below.

Property Enhancement

Long-term, time-dependent property enhancements occur due to one of the following conditions: chemical and physical hardening of asphalt binders due to short- and long-term aging of the asphalt binder, curing caused by the evaporation of moisture within asphalt emulsion systems, and pozzolanic reactions of cementitious materials. Normally, time-dependent property enhancements are viewed as increasing the relative modulus and strength of the material. Strength gain in PCC is an example of such a beneficial effect. Physical hardening of an asphalt layer is also beneficial in a flexible pavement system to the extent that it lowers the states of stress in the underlying layers and decreases the likelihood of permanent deformation occurring in the pavement system. However, in asphalt mixtures, the general increase in rigidity of the material layer also increases the possibility that fracture will occur due to load and environmental effects.

2.2.2

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2.2.3

Table 2.2.1. Major material input considerations by material group.

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Property Deterioration

Materials that are subjected to load-related fatigue distress may experience a severe degradation of properties with time and load repetitions. Micro-cracks may develop, leading to a reduced stiffness or modulus. This reduction in modulus will, in turn, lead to an increase in stress states within the pavement and a greater possibility of permanent deformation. The Design Guide considers the impact of load-related damage on property degradation.

Time-Temperature Effects

All asphaltic materials are highly sensitive to temperature and the rate of loading. Because asphalt is a viscoelastic-plastic material, the modulus of an asphalt mix may approach that of an unbound granular material at high temperatures and long loading rates (i.e., slow speed of passing vehicles). In contrast, at cold temperatures and very short load rates, the material will tend to behave in a pure elastic mode and have modulus values that approach that of PCC material. In the Design Guide, the methodology for asphaltic mixtures will take into account the range of temperatures expected in the design period. In addition, the use of an asphalt master curve, based on time-temperature superposition principles, will allow the engineer to account for the approximate speed of the vehicle under consideration.

Asphalt is not the only material that is influenced by vehicular speed. For example, rigid pavements show a 30 percent reduction in corner deflection and PCC edge strain as vehicle speeds increase from 3 to 60 mph. The dynamic modulus of PCC under typical Interstate vehicle speeds is approximately 20 percent higher than the static modulus historically used in rigid pavement analysis. Soft, wet clay subgrades that are viscoelastic-plastic may also exhibit a time-dependent modulus response; however, the ability to adjust subgrade moduli for a particular design speed is not intrinsically a part of the Guide. If a particular design situation requires that this factor be considered, direct modulus testing can be altered to characterize the material behavior under the specified loading rate.

The rate of load effect upon material response is a function not only of the vehicular speed, but also of the location of the material within the pavement structure. In general, as one proceeds deeper into the pavement, the length of the stress pulse acting on a given material will increase, suggesting that the time of the load pulse will also increase. The Guide does not automatically consider this aspect of material behavior, except in the analysis of asphaltic mixtures. Asphalt materials are characterized by a master curve incorporating time and temperature effects directly into the solution methodology.

Non-Linear Behavior

A material is considered non-linear if the value of the elastic modulus depends on the state of stress in the material. While many materials start to exhibit this behavior at very high stress states, the only materials that are considered to be non-linear in this Guide are unbound base/subbase and subgrade materials. Because the Guide inputs have been developed in a hierarchical structure, the most advanced level of M-E analysis can incorporate this factor into the design analysis methodology. If a non-linear analysis is desired due to use of non-linear

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