Section 6: Approved Pavement Design Methods

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Use one of the following analytical methods for designing pavements:

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  • FPS-19W for flexible pavements (FPS-11 may be used under certain circumstances)
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  • AASHTO design procedure (1993) for flexible and rigid pavement designs
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  • Modified Texas Triaxial Design Method for flexible pavements.
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Flexible Pavement System – Windows version (FPS-19W)

For most flexible pavement design work, especially higher-volume highways (>10,000 ADT, 5 M ESALs), the Flexible Pavement Design System (FPS-19W) is the preferred method for designing flexible pavements. FPS-19W should be used as a check for all flexible designs as described in Chapter 2, Pavement Design Process. This design procedure training is available to department personnel through CST-M&P.

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  • FPS-19W 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-19W 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-19W program assumes that a smaller deflection means smaller stresses or strains and, therefore, longer life.
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  • The program uses a “District Temperature Constant” that assigns a cold region multiplier to those areas of the state more susceptible to thermal cracking as the only environmental input. However, the current recommendation is to nullify this parameter by assigning a value of “31” corresponding to climate in Central Texas. See Chapter 5, FPS-19W Design Parameters, for additional discussion.
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  • All other environmental influences including seasonal changes in material stiffness, frost heave, or moisture susceptibility of materials are not directly considered by the program. Input fields are provided to assess the impact on performance if the project location has swelling foundation soils. Adding thickness to overcome swelling effects is not encouraged, except in very limited cases. For more information, go to Chapter 3, 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 growth. 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-19W (or the older FPS-11) 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, FPS-19W Design Parameters.
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  • The “Modified Texas Triaxial Design Method” is included in FPS-19W 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 HMAC layers and full-depth rut life of the structure with options to use several strain-based performance models.
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  • FPS-19W uses back-calculated modulus to characterize the pavement layer strength (stiffness) based on Falling Weight Deflectometer (FWD) deflection measurements (see “Pavement Evaluation.”). Note that back-calculated modulus used in FPS-19 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 back-calculation.

Other Acceptable Design Procedures

Although FPS-19W is the preferred design method for analytical design of flexible pavements, FPS-11 and the 1993 AASHTO design procedure are also acceptable, provided the designer is versed in their respective methodologies. Used extensively by the department until the mid-1990s, FPS-11 is a DOS-based design routine with design inputs that are substantially the same as those in FPS-19.

The two biggest limitations with the FPS-11 system are the use of material stiffness coefficients (versus modulus, also, not the same as AASHTO layer coefficients) to characterize the strength of the material layers and the required supporting deflection testing device (Dynaflect), which is no longer maintained at the department level. Other than the subgrade stiffness coefficient, layer stiffness coefficients can not be directly derived through the complementary STCOEF2 back-calculation routine, nor is there a robust correlation with back-calculated moduli.

Training for FPS-11 is no longer supported at the department level and related documentation is scarce. Familiarity with the AASHTO procedure must be gained through participation in an NHI class or equivalent or through self-study. Pavement Design staff from the Construction Division can consult with individuals interested in using the AASHTO design procedures.

Basic Design Types

FPS-19W uses a menu of five basic design types:

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HMA or surface treatment




HMA Overlay

flexible base

asphalt stabilized base

asphalt stabilized base

flexible base

existing HMA



flexible base

stabilized subbase/subgrade

existing base






In addition, a previously generated design input file can be recalled. The number of distinct layers that can be evaluated is limited to that shown in the table. The designer may opt to consolidate two or more layers or ignore a minimal contributing layer (such as select fill) in a design as a work-around if the number of layers in the proposed structure exceeds that available in any design option. In the case of consolidating multiple layers, a combined modulus must be assumed; this procedure would require some discretion to ensure adequacy of the overall design. This philosophy applies to the design of full-depth or "perpetual" type HMA pavements.

There are also unique aspects to the performance algorithms with each type of design. For example, in design Type 1, the flexible base modulus is mathematically calculated based on the designer’s input modulus for the subgrade and the flexible base thickness. The designer may override this calculated value, but values that are considerably greater than the program-generated value should be carefully weighed; the intent of this design type is to scale the stiffness of the base modulus to the degree of support offered by overall base thickness and subgrade stiffness.

The Type 3 design also adjusts the flexible base modulus based on the subgrade modulus, but at a fixed ratio of 3:1. Using discretion, this value may be overwritten as described in Type 1 design.

The Type 4 design has no adjustments associated with it. When the pavement structure is not clearly a Type 1 or Type 2 or Type 3 design. Type 4 is the one recommended. All data in this pavement type may be overwritten to suit the structure being designed within the limitations already described.

The Type 5 design is intended for overlays of flexible pavements only and cannot be used reliably for overlay of concrete or HMA-surfaced structures on heavily stabilized bases.

For more information, refer to the Flexible Pavement Design System (FPS) 19: User’s Manual (TTI Research Report TX-02/1869-2). Contact the district pavement engineer (DPE) or CST-M&P to obtain a copy of the latest version of the FPS-19 computer program.


AASHTO Design Procedure (for flexible and 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 ever conducted. 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).

Flexible Design

Flexible design using the AASHTO procedure requires the designer to derive a structural number (SN) that is adequate for the anticipated traffic over the length of a desired performance period. The SN is equivalent to the sum of a layer coefficient (a), layer thickness (D), and drainage coefficient (m) for each layer.

The AASHTO procedure for flexible design is automated in the DARWin™ 3.10 software. One aspect that makes using this design procedure somewhat problematic is that layer coefficients are not directly correlated to any universal system of measurement. AASHTO does provide some guidelines for correlation to laboratory-derived resilient modulus and one design option using the DARWin software is to input layer resilient moduli instead of layer coefficients.

Rigid Design

The department uses the 1993 AASHTO procedure for rigid pavement design (in 1998, a rigid design supplement was published by AASHTO; the department does not use the 1998 supplement). The 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 AASHTO procedure for rigid design has been automated in a DOS routine referred to as TSLAB86 or the designer can use the DARWinTM software. Steel design is reflected in the department’s recommended CRCP standards, found under the Pavements section on the Roadway Standards webpage.

DARWinTM 3.10 is the latest automated version of the AASHTO design process. TxDOT has an unlimited license for this software for internal use only. The license cannot be shared with outside agencies. This software is scheduled to be retired by December 2011 and will no longer be supported by AASHTO. DARWinTM 3.10 is scheduled to be replaced by DARWin ME 2.0, AASHTO’s mechanistic-empirical design procedure.

Additional Information

For more information on using the AASHTO flexible or rigid pavement design procedures, refer to the 1993 AASHTO Guide for Design of Pavement Structures.


Modified Texas Triaxial Design Method

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 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. See also Chapter 2, Pavement Design Process for additional guidelines.

This method has been automated and is included as a post-design check module in FPS-19W. The method can also be used as a standalone tool for designs where traffic loading can not 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. See also Chapter 2, Pavement Design Process.

When soil testing can not 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).

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Experienced-Based Design

In 2009, the Pavement Design Task Force (PDTF) emphasized the need to scrutinze the performance of local highways. With a view of reverse engineering, compare the performance of good performing highways against predictions generated through the FPS/Modified Texas Triaxial procedures. A check of structural properties through deflection measurements and an evaluation of material properties (base classification, ACP mixture design, etc.) should be performed.

The potential to conserve limited construction funding can be realized where local experience has shown structural designs to outlast formal predictions. Quantification of their structural attributes can be used to improve prediction models. These proven strategies can supersede analytical design results provided documentation is added to the design report.

For special cases, see “Pavement Design Reports.” The pavement design for special cases will typically be based on engineering judgment, historical performance, district policy, and other guidelines (e.g., this guide, industry guidelines).

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