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Aashto Flexible Pavement Design Manual LINK

Empirical equations are used to relate observed or measurable phenomena (pavement characteristics) with outcomes (pavement performance). This article presents the 1993 AASHTO Guide basic design equation for flexible pavements. This empirical equation is widely used and has the following form:

Aashto Flexible Pavement Design Manual


This equation is not the only empirical equation available but it does give a good sense of what an empirical equation looks like, what factors it considers and how empirical observations are incorporated into an empirical equation. The rest of this section will discuss the specific assumptions, inputs and outputs associated with the 1993 AASHTO Guide flexible pavement empirical design equation. The following subsections discuss:

The 1993 AASHTO Guide equation can be solved for any one of the variables as long as all the others are supplied. Typically, the output is either total ESALs or the required Structural Number (or the associated pavement layer depths). To be most accurate, the flexible pavement equation described in this chapter should be solved simultaneously with the flexible pavement ESAL equation. This solution method is an iterative process that solves for ESALs in both equations by varying the Structural Number. It is iterative because the Structural Number (SN) has two key influences:

In practice, the flexible pavement design equation is usually solved independently of the ESAL equation by using an ESAL value that is assumed independent of structural number. Although this assumption is not true, pavement structure depths calculated using it are reasonably accurate. This design process usually proceeds as follows:

This design utility solves the 1993 AASHTO Guide basic design equation for flexible pavements. It also supplies some basic information on variable descriptions, typical values and equation precautions.

The AASHTO Guide for Design of Pavement Structures (AASHTO, 1993) is the primary document used to design new and rehabilitated highway pavements. Approximately 80% of all states use the AASHTO pavement design procedures, with the majority using the 1993 version. All versions of the AASHTO Design Guide are empirical design methods based on field performance data measured at the AASHO Road Test in 1958-60.

Chapter 3 of this manual describes the evolution of the various versions of the AASHTO Design Guide. Geotechnical inputs to the 1993 AASHTO design procedure are detailed in Chapter 5. Chapter 6 provides some design examples using the 1993 AASHTO procedures.

The overall approach of the 1993 AASHTO procedure for both flexible and rigid pavements is to design for a specified serviceability loss at the end of the design life of the pavement. Serviceability is defined in terms of the Present Serviceability Index, PSI, which varies between the limits of 5 (best) and 0 (worst). Serviceability loss at end of design life, ΔPSI, is partitioned between traffic and environmental effects, as follows (see also Figure 3.8):

in which ΔPSITR, ΔPSISW and ΔPSIFH are the components of serviceability loss attributable to traffic, swelling, and frost heave, respectively. The structural design procedures for swelling and frost heave are the same for both flexible and rigid pavements; these are detailed in Appendix G of the 1993 AASHTO Guide. The structural design procedures for traffic are different for flexible and rigid pavement types. These procedures are summarized below in Sections C.2 and C.3, respectively. For simplicity, only the design procedures for new construction are summarized here. The design procedures for reconstruction are similar, except that characterization of recycled materials may be required. See the 1993 AASHTO Guide for details of additional procedures (e.g., determination of remaining structural life for overlay design) relevant to rehabilitation design.

Performance period refers to the time that a pavement design is intended to last before it needs rehabilitation. It is equivalent to the time elapsed as a new, reconstructed, or rehabilitated pavement structure deteriorates from its initial serviceability to its terminal serviceability. The term "analysis period" refers to the overall duration that the design strategy must cover. It may be identical to the performance period. However, realistic performance limitations may require planned rehabilitation within the desired analysis period, in which case, the analysis period may encompass multiple performance periods. Analysis period in this context is synonymous with design life in the 1993 AASHTO Guide. AASHTO recommendations for analysis periods for different types of roads are summarized in Table C-1.

Traffic is one of the most important factors in pavement design, and every effort should be made to collect accurate data specific to each project. Traffic analysis requires the evaluation of initial traffic volume, traffic growth, directional distribution, and traffic type.

Design reliability is defined as the probability that a pavement section will perform satisfactorily over the design period. It must account for uncertainties in traffic loading, environmental conditions, and construction materials. The AASHTO design method accounts for these uncertainties by incorporating a reliability level R to provide a factor of safety into the pavement design and thereby increase the probability that the pavement will perform as intended over its design life. The levels of reliability recommended by AASHTO for various classes of roads are summarized in Table C-2.

The AASHTO design equations also require specification of the overall standard deviation S0. For flexible pavements, values for S0 typically range between 0.35 and 0.50, with a value of 0.45 commonly used for design.

The initial serviceability index po corresponds to road conditions immediately after construction. A typical value of po for flexible pavements is 4.2. The terminal serviceability index pt is defined as the lowest serviceability that will be tolerated before rehabilitation or reconstruction becomes necessary. A terminal serviceability index of 2.5 or higher is recommended for design of major highways. Thus, a typical allowable serviceability loss due to traffic for flexible pavements can be expressed as:

Same as for flexible pavements; see Section 0. Note that the truck factor Tf will not in general be the same for rigid and flexible pavements. Refer to the 1993 AASHTO Design Guide or standard pavement engineering textbooks like Huang (2004) for determination of the truck factor.

Similar to flexible pavements; see Section 0. A typical value of po for rigid pavements is 4.4. As for flexible pavements, a terminal serviceability index of 2.5 or higher is recommended for design of major highways. Thus, a typical allowable serviceability loss due to traffic for rigid pavements can be expressed as:

Other layer properties include the modulus of rupture Sc and elastic modulus Ec for the Portland cement concrete slabs, an empirical joint load transfer coefficient J, and the subbase drainage coefficient Cd. The PCC parameters Sc and Ec are standard material properties; mean values should be used for the pavement design inputs. The joint load transfer coefficient J is a function of the shoulder type and the load transfer condition between the pavement slab and shoulders; recommended values are summarized in Table C-4. See Section 5.5.1 for determination of the drainage coefficient Cd.

The empirical design equations for flexible and rigid pavements in Eqs. (C.2) and (C.6) are implicit relationships for the required structural number SN and slab thickness D, respectively. Consequently, an iterative solution algorithm is required. The 1993 AASHTO Design Guide provides nomographs for the graphical evaluation of these equations. They can also be evaluated easily using a spreadsheet, e.g., via the Solver tool in Microsoft Excel. DARWin, a comprehensive software program tied to the 1993 AASHTO Design Guide procedures, is also available through AASHTO. Additional information on DARWin can be found at

Purpose: The primary functions of the New Hampshire Department of Transportation (NHDOT) Highway Design Manual (HDM) are to provide requirements and guidance on current highway design methods and policies, and to assure uniformity of design practice throughout the NHDOT in conjunction with the American Association of State Highway and Transportation Officials (AASHTO) and the Federal Highway Administration (FHWA). Our mission is to plan, construct, and maintain the best possible transportation system and State facilities in the most efficient and economical manner utilizing quality management techniques consistent with available resources and mandated controls. This manual should not supersede the application of sound engineering principles by experienced design professionals.

Variations from this manual will be necessary for special or unusual conditions, or between the issuances of new or revised source documents and any corresponding updates of the HDM. Consequently, instructions in this document are not intended to preclude the exercise of individual initiative and engineering judgment in reaction to site-specific conditions. It is important that there be consistency statewide in the application of this manual. The objective is uniformity of design for the same or similar conditions. Justifications for variations from this manual are to be appropriately documented.

Intended Audience: Contractors and Consultants interesting in performing pavement design work for TxDOT. Also anyone interested in learning about the latest pavement evaluation and design tools.

AASHTOWare Pavement ME Design is the next generation of AASHTOWare pavement design software, which builds upon the mechanistic-empirical pavement design guide, and expands and improves the features in the accompanying prototype computational software. ME Design supports AASHTO"s Mechanistic-Empirical Pavement Design Guide, Interim Edition: A Manual of Practice. ME Design is a production-ready software tool to support the day-to-day pavement design functions of public and private pavement engineers. 041b061a72


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