Section 5: Approved Pavement Design Methods
Anchor: #i1009322Introduction
Use one of the following approved methods for designing pavements:
- FPS-19W for flexible pavements (FPS-11 may be used under certain circumstances)
- AASHTO design procedure (1993)
- Modified Texas Triaxial Design Method for flexible pavements. A check using this procedure must be made on all FPS-derived designs. Can be used as a stand alone procedure for locations where estimating traffic loading in equivalent single axle loads (ESALs) is impractical, such as, parking areas/lots or temporary detours. Results of this check may be waived based on local experience.
- The AASHTO design procedure for rigid pavements.
- Experience, based on demonstrated/documented performance under similar traffic loading, materials, and environment. Also, 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).
Flexible Pavement System – Windows version (FPS-19W)
The Flexible Pavement Design System (FPS-19W) is the preferred method for designing flexible pavements. Training is available for department personnel for this design procedure through CST-M&P.
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 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.
Although FPS-19W is the preferred design method for flexible pavements, FPS-11 and the 1993 AASHTO design procedure are also acceptable provided the designer is versed in their respective methodologies. FPS-11 is a DOS-based design routine that was used extensively by the department until the mid-1990s, with design inputs that are substantially the same as those used 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.
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.
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. 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.
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.
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, pages 2-18 .
The design check procedure is included in FPS-19W in a post-design check module. In addition, 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.
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.
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.
Basic Design Types
FPS-19W uses a menu of five basic design types:
|
1 |
2 |
3 |
4 |
5 |
|---|---|---|---|---|
|
HMA or surface treatment |
HMA |
HMA |
HMA |
HMA Overlay |
|
flexible base |
asphalt stabilized base |
asphalt stabilized base |
flexible base |
existing HMA |
|
subgrade |
subgrade |
flexible base |
stabilized subbase/subgrade |
existing base |
|
|
|
subgrade |
subgrade |
subgrade |
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 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.
Anchor: #BGBFHHGIAASHTO Design Procedure
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 in 1959-1960 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.
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.
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.
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.
Anchor: #BGBIFIGEModified 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 contained 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.
This method has been automated and is included as a post-design check module in FPS-19-W. The method can also be used as a stand-alone tool for designs where traffic loading can not be easily evaluated in terms of 18-kip ESALs, such as, parking lots, temporary detours, etc.
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).