AASHTO Method for Highway Flexible Pavement Design
The AASHTO method for highway flexible pavement design is based on the AASHO Road Test results and has been revised over the years to provide guidelines for pavement structural design. It emphasizes performance period, time constraints, and design variables to ensure long-lasting and cost-effective pavement solutions. The selection of the performance period is crucial and influenced by factors like pavement classification and traffic volume. This method is a valuable resource for engineers and professionals involved in highway and transportation engineering.
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Structural Design of Highway Flexible Pavement Design , AASHTO Method, Design Variables and Equations Highway and Transportation Engineering Al-Mustansiriyah University 2019-2020 Dr. Rana Amir Yousif & Dr. Abeer K. Jameel Yoder; E. J. and M. W. Witczak, Principles of Pavement Design , A Wiley- Interscience Publication, John Wiley & Sons Inc., U.S.A., 1975.
References Nicholas J. Garber and Lester A. Hoel. Traffic and Highway Engineering , Fourth Edition. Yoder; E. J. and M. W. Witczak, Principles of Pavement Design , A Wiley- Interscience Publication, John Wiley & Sons Inc., U.S.A., 1975. Yaug H. Huang, Pavement Analysis and Design , Prentic Hall Inc., U.S.A., 1993. AASHTO Guide for Design of Pavement Structures 1993 , AASHTO, American Association of State Highway and Transportation Officials, U.S.A., 1993. Oglesby Clarkson H., Highway Engineering , John Wiley & Sons Inc., U.S.A., 1975. Yoder; E. J. and M. W. Witczak, Principles of Pavement Design , A Wiley- Interscience Publication, John Wiley & Sons Inc., U.S.A., 1975.
AASHTO METHOD The design procedure recommended by the American Association of State Highway and Transportation Officials (AASHTO) is based on the results of the extensive AASHO Road Test conducted in Ottawa, Illinois, in the late 1950s and early 1960s . The AASHO Committee on Design first published an interim design guide in 1961 . It was revised in 1972 and 1981 . In 1984-85, the Subcommittee on Pavement Design and a team of consultants revised and expanded the guide under NCHRP Project 20-7/24 ; they issued the guide in 1986 . The guide was revised in 1993 with practically no change in the design method presented in this section . Yoder; E. J. and M. W. Witczak, Principles of Pavement Design , A Wiley- Interscience Publication, John Wiley & Sons Inc., U.S.A., 1975.
11.3 AASHTO METHOD 11 .3.1 Design Variables Several general design variables related to both flexible and rigid pavements are presented in this section . Time Constraints To achieve the best use of available funds, the AASHTO design guide encourages the use of a longer analysis period for high-volume facilities, including at least one rehabilitation period . Thus, the analysis period should be equal to or greater than the performance period, as described below. Performance Period The performance period refers to the time that an initial pavement structure will last before it needs rehabilitation or the performance time between rehabilitation operations . It is equivalent to the time elapsed as a new, reconstructed, or rehabilitated structure deteriorates from its initial serviceability to its terminal serviceability. Yoder; E. J. and M. W. Witczak, Principles of Pavement Design , A Wiley- Interscience Publication, John Wiley & Sons Inc., U.S.A., 1975.
11.3 AASHTO METHOD 11 .3.1 Design Variables The designer must select the performance period within the minimum and maximum allowable bounds that are established by agency experience and policy. Note The selection of performance period can be affected by such factors: The functional classification of the pavement. The type and level of maintenance applied. The funds available for initial construction. Life cycle costs. and other engineering considerations. Yoder; E. J. and M. W. Witczak, Principles of Pavement Design , A Wiley- Interscience Publication, John Wiley & Sons Inc., U.S.A., 1975.
11.3 AASHTO METHOD 11 .3.1 Design Variables Analysis Period The analysis period is the period of time that any design strategy must cover . It may be identical to the selected performance period performance limitations may necessitate the consideration of staged construction or planned rehabilitation for the desired analysis period . Table 11.13 contains general guidelines for the length of the analysis period . .However, realistic Yoder; E. J. and M. W. Witczak, Principles of Pavement Design , A Wiley- Interscience Publication, John Wiley & Sons Inc., U.S.A., 1975.
11.3 AASHTO METHOD 11 .3.1 Design Variables Traffic:The design procedures are based on cumulative expected 18-kip (80-kN) equivalent single-axle load (ESAL) . If a pavement is designed for the analysis period without any rehabilitation or resurfacing, all that is required is the total ESAL over the analysis period . However, if stage construction is considered and rehabilitation or resurfacing is anticipated, a graph or equation of cumulative ESAL versus time is needed so that the ESAL traffic during any given stages can be obtained . Note Yoder; E. J. and M. W. Witczak, Principles of Pavement Design , A Wiley- Interscience Publication, John Wiley & Sons Inc., U.S.A., 1975.
11.3 AASHTO METHOD 11 .3.1 Design Variables Reliability:The design procedures are based on cumulative expected 18-kip (80-kN) equivalent single-axle load (ESAL) . Yoder; E. J. and M. W. Witczak, Principles of Pavement Design , A Wiley- Interscience Publication, John Wiley & Sons Inc., U.S.A., 1975.
11.3 AASHTO METHOD 11 .3.1 Design Variables Application of the reliability concept requires the selection of a standard deviation that is representative of local conditions . It is suggested that standard deviations of 0 .49 be used for flexible pavements and 0 .39 for rigid pavements. These correspond to variances of 0 .2401 and 0 .1521, which are nearly the same as those shown in Table 10.12 .. When stage construction is considered, the reliability of each stage must be compounded to achieve the overall reliability ; that is, (11.28) in which n is the number of stages being considered . For example, if two stages are contemplated and the desired level of overall reliability is 95%, the reliability of each stage must be (0 .95) ^1/2 , or 97.5% . Yoder; E. J. and M. W. Witczak, Principles of Pavement Design , A Wiley- Interscience Publication, John Wiley & Sons Inc., U.S.A., 1975.
11.3 AASHTO METHOD 11 .3.1 Design Variables Environmental Effects :The AASHO design equations were based on the results of traffic tests over a two-year period . The long-term effects of temperature and moisture on the reduction of serviceability were not included . If problems of swell clay and frost heave are significant in a given region and have not been properly corrected, the loss of serviceability over the analysis period should be estimated and added to that due to cumulative traffic loads . Figure 11.23 shows the serviceability loss versus time curves for a specific location . Yoder; E. J. and M. W. Witczak, Principles of Pavement Design , A Wiley- Interscience Publication, John Wiley & Sons Inc., U.S.A., 1975.
11.3 AASHTO METHOD 11 .3.1 Design Variables Yoder; E. J. and M. W. Witczak, Principles of Pavement Design , A Wiley- Interscience Publication, John Wiley & Sons Inc., U.S.A., 1975.
11.3 AASHTO METHOD 11 .3.1 Design Variables The environmental loss is a summation of losses from both swelling and frost heave . The chart may be used to estimate the serviceability loss at any intermediate period, for example, a loss of 0 .73 at the end of 13 years . Of course, if only swelling or frost heave is considered, there will be only one curve on the graph . The shape of these curves indicates that the serviceability loss due to environment increases at a decreasing rate. This may favor the use of stage construction because most of the loss will occur during the first stage and can be corrected with little additional loss in later stages. Yoder; E. J. and M. W. Witczak, Principles of Pavement Design , A Wiley- Interscience Publication, John Wiley & Sons Inc., U.S.A., 1975.
11.3 AASHTO METHOD 11 .3.1 Design Variables ServiceabilityInitial and terminal serviceability indexes must be established to compute the change in serviceability, APSI, to be used in the design equations . The initial serviceability index is a function of pavement type and construction quality . Typical values from the AASHO Road Test were 4.2 for flexible pavements and 4.5 for rigid pavements. is the lowest index that will be tolerated before rehabilitation, resurfacing, and reconstruction become necessary. The terminal serviceability index Yoder; E. J. and M. W. Witczak, Principles of Pavement Design , A Wiley- Interscience Publication, John Wiley & Sons Inc., U.S.A., 1975.
11.3 AASHTO METHOD 11 .3.1 Design Variables An index of 2.5 or higher is suggested for design of major highways and 2.0 for highways with lower traffic. For relatively minor highways where economics dictate a minimum initial capital outlay, it is suggested that this be accomplished by reducing the design period or total traffic volume, rather than by designing a terminal serviceability index less than 2.0 . Yoder; E. J. and M. W. Witczak, Principles of Pavement Design , A Wiley- Interscience Publication, John Wiley & Sons Inc., U.S.A., 1975.
11.3 AASHTO METHOD 11 .3.2 Design Equations The original equations were based purely on the results of the AASHO Road Test but were modified later by theory and experience to take care of sub-grade and climatic conditions other than those encountered in the Road Test. Original Equations The following are the basic equations developed from the AASHO Road Test for flexible pavements (HRB, 1962) : + 9.36 log (SN + 1) 4.79 log ( Yoder; E. J. and M. W. Witczak, Principles of Pavement Design , A Wiley- Interscience Publication, John Wiley & Sons Inc., U.S.A., 1975.
11.3 AASHTO METHOD 11 .3.2 Design Equations Here, logarithm of the ratio of loss in serviceability at time t to the potential loss taken at a point where or , noting that 4.2 is the initial serviceability for flexible pavement. A function of design and load variables, as shown by Eq. 11.30 , that influences the shape of versuscurve; A function of design and load variables, as shown by Eq. 11.31 , that denotes the expected number of load applications to a of 1.5 , as can be seen from Eq. 11.29, where = when = 1.5; Axle load application at the end of time t ; = Yoder; E. J. and M. W. Witczak, Principles of Pavement Design , A Wiley- Interscience Publication, John Wiley & Sons Inc., U.S.A., 1975.
11.3 AASHTO METHOD 11 .3.2 Design Equations serviceability at end of time t ; load on one single axle or a set of tandem axles, in kip ; axle code 1 for single axle, 2 for tandem axle ; structural number of pavement, which was computed as in which : a l , a2, and a3 : are layer coefficients for the surface, base, and subbase, respectively, and D1 , D2 , and D3 : are the thicknesses of the surface , base, and subbase, respectively. Yoder; E. J. and M. W. Witczak, Principles of Pavement Design , A Wiley- Interscience Publication, John Wiley & Sons Inc., U.S.A., 1975.
11.3 AASHTO METHOD 11 .3.2 Design Equations The procedure is greatly simplified if an equivalent 18-kip (80-kN) single axle load is used . By combining Eqs. 11.29, 11.30, and 11.31 and setting L1 = 18 and L2 = 1, we obtain the equation Equation 11 .33 is applicable only to the flexible pavements in the AASHO Road Test with an effective sub-grade resilient Modulus of 3000 psi (20 .7 MPa). in which : : is the number of 18-kip (80-kN) single-axle load applications to time t. : is the terminal serviceability index. Yoder; E. J. and M. W. Witczak, Principles of Pavement Design , A Wiley- Interscience Publication, John Wiley & Sons Inc., U.S.A., 1975.
11.3 AASHTO METHOD 11 .3.2 Design Equations Modified Equations For other sub-grade and environmental conditions, Eq.11.33 is modified to . in which : : is the effective roadbed soil resilient modulus. When MR = 3000 psi (20 .7 MPa), Eq . 11 .34 is identical to Eq . 11 .33 . Note that Yoder; E. J. and M. W. Witczak, Principles of Pavement Design , A Wiley- Interscience Publication, John Wiley & Sons Inc., U.S.A., 1975.
11.3 AASHTO METHOD 11 .3.2 Design Equations To take local precipitation and drainage conditions into account, Eq.11.32 was modified to. in which : : is the drainage coefficient of base course and. :is the drainage coefficient of sub base course . Equation 11.34 is the performance equation that gives the allowable number of 18-kip (80- kN) single-axle load applications Wt18 to cause the reduction of PSI to pt. Note that Yoder; E. J. and M. W. Witczak, Principles of Pavement Design , A Wiley- Interscience Publication, John Wiley & Sons Inc., U.S.A., 1975.
11.3 AASHTO METHOD 11 .3.2 Design Equations If the predicted number of applications = the reliability of the design is only 50%, because all variables in Eq.11.34 are based on mean values. To achieve a higher level of reliability, must be smaller than by a normal deviate , Here: : is the normal deviate for a given reliability R, and : is the standard deviation. Yoder; E. J. and M. W. Witczak, Principles of Pavement Design , A Wiley- Interscience Publication, John Wiley & Sons Inc., U.S.A., 1975.
11.3 AASHTO METHOD 11 .3.2 Design Equations can be determined from Table 10.1. or, more conveniently, from Table 11.15 . Yoder; E. J. and M. W. Witczak, Principles of Pavement Design , A Wiley- Interscience Publication, John Wiley & Sons Inc., U.S.A., 1975.
11.3 AASHTO METHOD 11 .3.2 Design Equations Combining Eqs . 11.34 and 1 .36 and replacing (4.2 ) by yields. Equation 11.37 is the final design equation for flexible pavements . Figure 11.25 is a monograph for solving Eq . 11.37 . Yoder; E. J. and M. W. Witczak, Principles of Pavement Design , A Wiley- Interscience Publication, John Wiley & Sons Inc., U.S.A., 1975.
11.3 AASHTO METHOD 11 .3.2 Design Equations Steps to use 11.25 is a monograph for solving Eq . 11.37 . Steps 1 Steps 2 Steps 3 Reliability T(11.14) The number of 18-kip Standard Deviation T(11.15) Steps 4 Steps 6 Steps 5 Design Structure Number, SN Design Serviceability Loss Effective Roadbed Soil (MR) Yoder; E. J. and M. W. Witczak, Principles of Pavement Design , A Wiley- Interscience Publication, John Wiley & Sons Inc., U.S.A., 1975.
11.3 AASHTO METHOD 11 .3.2 Design Equations Step 1 Yoder; E. J. and M. W. Witczak, Principles of Pavement Design , A Wiley- Interscience Publication, John Wiley & Sons Inc., U.S.A., 1975.
