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  • Flexible Pavement Design Procedures

    Pavement Managers Workshop August 6-8, 1985

    Texas State Department of Highways and Public Transportation

    Highway Design Division Pavement Design Section


    Pavement Managers Workshop

    August 6-8, 1985



    Flexible Pavement Design Procedures

    Flexible Pavement Design System

    Texas Triaxial Design Procedures

    Swelling Clays In The FPS System

    The FPS Traffic Equation







    Appendix A Test Method Tex 117-E . . A-107 "Triaxial Compression Test for Disturbed Soils and Base Materials"

    Appendix B "Modified Triaxial Design Procedure For Use With The Flexible Pavement Design System (FPS)"




    Pavement Type Definitions

    A discussion of pavement design makes it necessary to define the type of pavement being discussed. The two very basic types are "flexible" and "rigid" pavements. Definitions are very general and due to this fact they are varied. A very basic definition of pavement types is as follows:

    Flexible - Asphalt Surface Rigid - Portland Cement Concrete Surface.

    These definitions are arbitrary. The flexible pavement may become 11 slab-like" and assume many of the rigid pavement characteristics. The rigid pavement may lose this rigid posture and behave somewhat like the flexible pavement.

    The flexible pavements may be subdivided into two very general areas:

    1. Thin Surfaced Flexible Pavements 2. Semi-Rigid and Stabilized Pavements

    The thin surfaced flexible pavements may be surfaced with either asphaltic concrete pavement or the surfacing may be single, double or triple surface treatment. The base course for the thin surfaced flexible pavements should be unbound or unstabilized. The key to the thin surfaced flexible pavement is its flexibility. The surfacing layer must be flexible enough to withstand

    the tensile strains without forming tensile cracks or fatigue cracking. The base material must be unstabilized so that tensile strains are not trans- mitted upward to the surfacing layer.

    The semi-rigid or stabilized pavements are characterized by their thick and rigid surfacing layers. The subbase(s) in this type of pavement are also frequently stabilized. The key thought in this type of pavement is the ability to withstand tensile stresses without premature fatigue failures. This ability to tolerate the tensile stresses is a function of the rigidness and thickness of the surfacing layer(s). With sufficient rigidness and thickness the pavement layer assumes a semi-rigid slab like behavior. This rigid or slab like behavior aids in disipating the wheel load stresses into

    the base. subbase and subgrade layers .. The semi-rigid layer may actually be composed of several layers. The layers must be bonded together so that

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  • shear stresses are transmitted between layers. The layers may be

    a series of asphaltic concrete layers or there may be a mixture such as asphaltic concrete on cement treated base.

    The surfacing layer thickness for the 11 thin surfaced flexible pave- ments" and the "semi -rigid and stabilized pavements .. and the reasoning for this concept will be discussed in another section of this report.

    Definition of Flexible Pavement Design

    Pavement design and especially flexible design divides itself into two tasks as follows:

    1. Mixture or Materials Design 2. Structure or Thickness Design

    Flexible pavement design divides itself into these two tasks but it is not intended to imply that these two tasks can be cleanly separated at the design stage and later joined into a happy reunion on the roadway. There must be interaction between the tasks; the thickness design assumptions must be achievable in the materials design stage and, likewise, the materials design stage must carry out the intent of the thickness design. Specifica- tions are the link between mixtures and thickness design. Specifications are also the means by which both design phases are linked to construction and to maintenance of the pavement.

    Figure 1, Pavement Relationships, point out that specifications are the link in the pavement design, construction and maintenance cycle. Also, the interaction or paths between functions is of interest. Obviously, all posssible pathways are not shown.

    The objective of this report will be to treat only the structural or thickness design phase of flexible pavement design.

    The Life Cycle of a Pavement

    The design of pavements is but one of a series of steps in the life cycle of a pavement. During the life cycle of a pavement some of the major

    events could be as follows:

    I. Planning 2. Preliminary Design

    3. Programming (Funding)

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  • 4. Detailed Design 5. Construction 6. Maintenance 7. Evaluation (Monitoring)

    When to Do a Pavement Design

    The question of when to do a pavement design is of extreme importance. Inevitably, the need for a pavement design occurs and before an adequate design can be made, other factors may have forced the use of an "estimate" of a pavement structure. Obviously the design must be well established be- fore the progra~ning or funding stage. In the previously noted life cycle of a pavement there are two design stages shown. If this first or prelimi- nary design is inadequate this deficiency will be reflected into all other phases of a pavement•s life cycle. The pavement as constructed will be inadequate,maintenance will be excessive, anticipated performance will not be achieved and the cycle returns prematurely to planning. It becomes necessary to anticipate needs and plan for these needs sufficiently in advance of a program call so that the allocated funding may correspond to the future needs of the project. The evaluation or pavement monitoring process should be used as a tool to anticipate both the current and the future needs of an existing pavement.

    Flexible Pavement Design Procedures

    There are a considerable number of well known flexible pavement design procedures. The key issue is "availability .. and "applicability" of these various procedures. The two procedures most readily available to Depart- mental pavement designers are:

    (1) Flexible Pavement Design System (FPS) (2) Texas Triaxial Design System

    The Flexible Pavement Design System (FPS) is the Highway Design Division•s official flexible pavement design procedure and as such, this procedure is presented in greater detail in this report.

    The Materials and Tests Division, sponsors Test Method Tex-117E, "Triaxial Compression Tests for Disturbed Soil and Base Materials... This

    test method is generally known as the Texas Triaxial Design System. The Texas Triaxial Design System will also be discussed later in this report. Also an abbreviated version of the Texas Triaxial Design System is

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  • recommended in the checking of designs generated by FPS and this will also be presented.

    There are of course many other flexible pavement design procedures

    both theoretical and emperical, that the designer may wish to consider as "second opinion .. type of procedures.




    FIGURE 1 Pavement Relationships

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    Introduction and History

    The Flexible Pavement Design System is known in the Department as FPS. The current version is FPS-11, which designates this as the eleventh major version of the system. FPS dates its beginning to the American Association of State Highway Officials (AASHO) Road Test, conducted in Ottawa, Illinois, in the late 1950 1 s and early 1960•s. The Test was extensive in its test features and produced design concepts for both flexible and rigid pavements.

    The major accomplishment, possibly, of the Road Test was the defining of the serviceability concept or the ability of a pavement to serve traffic for which it was designed. A second important feature defined at the Road Test was the 18-Kip Single Axle load equivalents. This feature allowed mixed traffic to be converted to a uniform common denominator in terms of pavement performance represented by the serviceability concept. By the use of factorial pavement designs the Road Test represented the most comprehensive relationships between performance, structural thickness and traffic loadings available at that time. To date, there have been no further developments that supersede the Road Test.

    There was one major flaw in the Road Test concept. The pavement designers in Texas and other states were faced with the problem of extrapolating the AASHO Road Test results to the individual states. The Test results were applicable to one environment, one subgrade, one set of construction materials and one set of construction procedures, all of which were unique to the Road Test site.

    In 1962, the Texas Highway Department took a significant step forward in flexible pavement design procedures. The Department contracted with Texas Transportation Institute (TTl) for a research task to adapt findings of the AASHO Road Test to Texas conditions. This task at TTl was under the direction and supervision of Mr. Frank H.