Selecting the Input Level

For a given design project, inputs can be obtained using a mix of levels, such as concrete modulus of rupture from Level 1, traffic load spectra from Level 2, and subgrade resilient modulus from Level 3. No matter what input design levels are used, the computational algorithm for damage and distress is exactly the same. The same models and procedures are used to predict distress and smoothness no matter what input levels are used.

It is recommended that the designer use the highest level of inputs available at the time of design. The designer should recognize, however, that the standard error for each distress provided in Section 5 is used to determine the reliability of the trial design relative to the threshold value selected by the user. These standard errors were derived from the re-calibration effort completed under NCHRP Project 1-40D and were based on using the highest level of inputs for each pavement section (NCHRP, 2006). Table 6-1 provides a general listing of the predominant input levels used for the re-calibration effort to assist the user in judging the applicability of the standard error terms to the trial design.

Sections 9 through 11 provide guidance on determining the input level for each input group. If a user decides to routinely use all Level 3 inputs, the standard errors will probably be higher than included in the MEPDG and provided in Section 5. It is recommended that a user or agency decide on the pre- dominant input level to be used and if that decision deviates from the levels used in the re-calibration effort, the agency could definitely consider completing a local calibration to determine the appropriate standard errors for each distress prediction model. In the interim, designers may use the standard errors determined from the global calibration process.

Table 6-1. Predominant Input Levels Used in Recalibration Effort of the MEPDG

Input Group Input Parameter Recalibration Input_{Level Used}

Axle-Load Distributions (Single, Tandem, Tridem) Level 1

Truck Volume Distribution Level 1

Lane and Directional Truck Distributions Level 1

Tire Pressure Level 3

Axle Configuration, Tire Spacing Level 3

Truck Traffic

Truck Wander Level 3

Climate Temperature, Wind Speed, Cloud Cover,

Precipitation, Relative Humidity Level 1 WeatherStations

Resilient Modulus—All Unbound Layers Level 1;

Backcalculation

Classification and Volumetric Properties Level 1

Moisture-Density Relationships Level 1

Soil-Water Characteristic Relationships Level 3

Unbound Layers and Subgrade

Saturated Hydraulic Conductivity Level 3

HMA Dynamic Modulus Level 3

HMA Creep Compliance and Indirect Tensile Strength Levels 1, 2, and 3

Volumetric Properties Level 1

HMA

HMA Coefficient of Thermal Expansion Level 3

PCC Elastic Modulus Level 1

PCC Flexural Strength Level 1

PCC Indirect Tensile Strength (CRCP Only) Level 2

Material Properties

PCC

PCC Coefficient of Thermal Expansion Level 1

Unit Weight Level 1

Poisson’s Ratio Levels 1 and 3

All Materials

Other Thermal Properties; Conductivity, Heat Capacity,

Surface Absorptivity Level 3

Existing Pavement Condition of Existing Layers Levels 1 and 2

7.1 design/AnAlysis life

As noted under the definition of terms (Subsection 4.1), the design life of a new or reconstructed pave- ment is the time from initial construction until the pavement has structurally deteriorated to a specified pavement condition—the time when significant rehabilitation or reconstruction is needed. The design life of an overlay or CPR is the time from when the overlay is placed or CPR performed until signifi- cant rehabilitation or reconstruction is needed. The MEPDG can handle design lives from 1 year (e.g., detour) to over 50 years. The use of 50+ years as the design life is defined as a long-life pavement. The designer should remember that durability and material disintegration type surface distresses are not predicted with the MEPDG. These material disintegration distresses will limit the expected service life of all pavements. It is also important to note that few pavements were included in the global calibra- tion that exceeded 30 years of performance data. Thus, the designer should recognize the importance of adequate material and construction specifications (especially for the surface layer) for design periods exceeding 30 years.

7.2 construction And trAffic opening dAtes

Construction completion and traffic opening dates have an impact on the distress predictions. The designer may estimate the base/subgrade construction month, pavement construction month, and traffic open month. These can be estimated from the planned construction schedule. These dates were defined in Subsection 4.1 and are keyed to the monthly traffic loadings and monthly climatic inputs which affect all monthly layer and subgrade modulus values, including aging of HMA and PCC.

The designer may select the most likely month and year for construction completion of the unbound layer, placement of the bound layer, and opening the roadway to traffic. For large projects that extend into different paving seasons, each paving season could be evaluated separately. For example, there maybe portions of a project that are opened to traffic in the spring, summer, and fall. It is suggested that each be evaluated separately and judge the acceptability of the trial design based on the more conservative one. The MEPDG also has the capability to simulate an unbound aggregate base layer being left exposed for an extended period of time prior to placing the first HMA layer. When and if this condition is permit-

c H A p t e r 7

general project

information