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Section 6: Approved Pavement Design Methods

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6.1 Introduction

Use one of the following analytical methods for designing pavements:

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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.

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  • 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.
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  • 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. 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.
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  • 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.”
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  • 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.
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  • 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.”
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  • The “Modified Texas Triaxial Design Method” 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.”
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  • 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.
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  • 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, Non-Destructive Evaluation of Pavement Structural Properties). Note that back-calculated modulus used in FPS 21 is not the same as the resilient modulus used in the AASHTO design procedure.
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  • 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.

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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. Evaluating a soil for its Texas Triaxial Classification is covered in “ Tex-117-E, Triaxial Compression for Disturbed Soils and Base Materials.”

When the FPS design system was first developed in the 1970s, solutions produced for some lightly trafficked highways that had an occasional heavy load were found to be under-designed. The Modified Texas Triaxial Design Method was developed to overcome shortcomings of the FPS design procedure by determining the required pavement thickness to ensure protection against shear failure in unbound layers due to heavy wheel loads.

The modified triaxial method requires the use of the subgrade or base Texas Triaxial Class as derived from laboratory test results. Since the testing procedure requires the soil sample or base be moisture-conditioned to establish its triaxial classification (capillary absorption time based on material plasticity), the evaluation represents the soil’s strength at a weakened state.

The engineer may determine that this saturation level is not likely to occur in situ for a particular environment (like west Texas) and, therefore, not the overriding design consideration. Additional credit is given for bound materials within the structure that will allow a reduction in the calculated coverage above the evaluated unbound layer. This method has been automated and is included as a post-design check module in FPS 21. The method can also be used as a standalone tool for designs where traffic loading cannot be easily evaluated in terms of 18-kip ESALs, such as parking lots, temporary detours, etc. Results of this check may be waived based on local experience. When soil testing cannot be performed to establish the triaxial classification, soil maps may be used to identify the general soil type, and approximation of the soil triaxial classification can be made using historical test results ( Soil_Series.xls). Also, within the FPS 21 software, the designer can opt to estimate the soil triaxial class if the in situ soil PI is known or if the in situ soil type for the project is known.

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6.4 TxCRCP-ME (for Continuously Reinforced Concrete Pavements)

The TxCRCP-ME program is the only approved design method for CRCP projects at TxDOT. This design method was developed under TxDOT research project 0-5832, “Develop Mechanistic/Empirical Design for CRCP.” The program performs an analysis of the pavement system for given inputs in estimating the frequency of punchouts, the primary structural distress of CRCP.

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6.5 AASHTO 93 Design Procedure (for CPCD rigid pavement designs)

The AASHTO (originally AASHO) pavement design guide was first published as an interim guide in 1972. Updates to the guide were subsequently published in 1986 and 1993. The AASHTO design procedure is based on the results of the AASHO Road Test conducted from 1958-1960 in Ottawa, Illinois.

Approximately 1.2 million axle load repetitions were applied to specially designed test tracks in the most comprehensive pavement test experiment design conducted to that point. The original AASHO design process was strictly empirical in nature; subsequent updates have included some mechanistic provisions, such as, classifying the subgrade stiffness in terms of resilient modulus and accounting for seasonal variation in material stiffness.

AASHO design originated the concept of pavement failure based on the deterioration of ride quality as perceived by the user. Thus, performance is related to the deterioration of ride quality or serviceability over time or applications of traffic loading.

Also developed at the AASHO Road Test was the rendering of cumulative traffic loading in terms of a single statistic known as the 18-kip equivalent single axle load (ESAL).

The 1993 AASHTO Guide for Design of Pavement Structures is the only approved design method for CPCD projects at TxDOT. This design produces a rigid slab thickness in inches required to support the estimated traffic under a selected serviceability interval and estimated support and environmental conditions. The design procedure is available in automated form in the AASHTO DARWin® 3.1 program and Web Application at http://www.pavementinteractive.org/1993-aashto-rigid-pavement-structural-design-application/.

For more information on using the AASHTO CPCD design procedure, refer to the 1993 AASHTO Guide for Design of Pavement Structures.

Reinforcing steel design is reflected in the department’s recommended CRCP and CPCD standards, found under the Pavements section on the Roadway Standards webpage.

The TxCRCP-ME and AASHTO DARWin® 3.1 programs are available to TxDOT personnel through the district pavement engineer. Consultants may obtain the TxCRCP-ME program from the district pavement engineer or the Pavement Asset Management Section of the Maintenance Division.


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