Section 6: Approved Pavement Design Methods 6.1 Introduction Use one of the following analytical methods for designing pavements: • FPS 21 for flexible pavements • Modified Texas Triaxial Design Method for flexible pavements • TxCRCP-ME for continuously reinforced rigid pavements • AASHTO design procedure (1993) for CPCD rigid pavements. 6.2 Flexible Pavement Design System (FPS 21) For most flexible pavement design work, especially higher-volume highways (>10,000 ADT, 5 M ESALs), the Flexible Pavement Design System (FPS 21) is the required method for designing flexible pavements. FPS 21 should be used as a check for all flexible designs as described in “Pavement Design Process.” Design procedure training is available to department personnel through MNT – Pavement Asset Management.
CHAPTER 2 DESIGN PROCEDURES The 1993 AASHTO Guide and MEPDG were used in combination to help develop a revised WSDOT pavement catalog. The underlying design procedure for the revised design catalog remains the 1993 AASHTO Guide. The MEPDG was used to check the 1993 AASHTO Guide thicknesses at all ESAL levels. Topic 10 – AASHTO Rigid Pavement Design. Empirical design based on the AASHO road test: •. Over 200 test sections JPCP (15' spacing).
• FPS 21 provides a methodology for selecting a complete pavement design strategy. Such a strategy calls for action now (initial construction) and for future action (overlays or reconstruction). Depending upon the range of material layer thicknesses the designer is willing to consider, the output will consist of one or more recommended strategies.
For a given design analysis, initial construction costs as well as future costs are computed for each design strategy. The engineer selects a design strategy based on a multitude of considerations including past performance, cost, constructability, user delay, adjoining section, etc. • FPS 21 is a mechanistic-empirical design procedure that uses a performance model based on degradation of the serviceability index as defined in the AASHO Road Test research.
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Also borrowed from the AASHO Road Test is the standardization of cumulative traffic loading in terms of 18-kip equivalent single axle loads (ESALs). The FPS 21 program assumes that a smaller deflection means smaller stresses or strains and, therefore, longer pavement life. • Environmental influences including seasonal changes in material stiffness, frost heave, or moisture susceptibility of materials are not directly considered by the program. Impact of swelling foundation soils is no longer considered in FPS 21. Adding thickness to overcome swelling effects is not encouraged, except in very limited cases. For more information, go to Chapter 3, “Materials Investigation and Selection Information,” Section 2, “Geotechnical Investigation for Pavement Structures.” • The program uses a “confidence level” approach to account for variability in the in-place subgrade stiffness, construction variability, and traffic loading predictions.
A multiplier is assigned to the cumulative traffic loading as the desired level of confidence or reliability increases. • The system can generate designs that may fail under occasional heavy wheel loads. This circumstance is particularly acute for designs that have low cumulative loading in regions with poor subgrade. For this reason, designs obtained with the FPS 21 program must be checked with the “Modified Texas Triaxial Design Method.” Considerations for accepting this procedure as the governing method for determining design thickness are described in Chapter 5, Section 3, “FPS 21 Design Parameters.” • The is included in FPS 21 in a post-design check module. It can also be used as a standalone procedure using the graphs contained in the archived versions of “Tex-117-F, Triaxial Compression for Disturbed Soils and Base Materials.” • A mechanistic design check is provided to evaluate expected fatigue life of the HMA layers and full-depth rut life of the structure with options to use several strain-based performance models. It is highly recommended that the results of this check be considered for all pavement designs where the FPS-generated surface bituminous thickness is between 2 and 4 in.
• FPS 21 uses back-calculated modulus to characterize the pavement layer strength (stiffness) based on falling weight deflectometer (FWD) deflection measurements (see Chapter 4, “Pavement Evaluation,” Section 4, ). Note that back-calculated modulus used in FPS 21 is not the same as the resilient modulus used in the AASHTO design procedure. • It is incumbent upon the designer to have a recent set of deflection data for the project under consideration from which moduli can be generated, as well as institutional knowledge of material moduli when virgin or recycled materials are to be incorporated in the design. Each district should develop a database of typical moduli through a routine program of aggressive deflection testing and subsequent backcalculation. FPS-19W is the previous design program which has been replaced by FPS 21. Identical inputs used in FPS-19W will generate identical thickness designs in FPS 21, however FPS 21 is the required analytical method for designing flexible pavements. 6.3 Modified Texas Triaxial Design Method for Flexible Pavements The Texas Triaxial Classification of soils was developed in the late 1940s and early 1950s by the department as an indexed soil classification system related to soil shear strength.