aashto flexible design method

8/10/2019 AASHTO Flexible Design Method

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AASHTO FLEXIBLEDESIGNMETHOD

One of the major objectives of the AASHTO road test was to provide

information that could be used to develop design criteria and design procedures.The current issue of AASHTO was issued on 1986 AASHTO Guide for Design

Pavement Structures.

This current version incorporates the various design inputs:

traffic

reliability

subgrade soil property

environmental effects

performance criteria into the design equation and the design chart

as shown in Figure 16-11 to determine: structural number

combined structural capacity of the pavement

required for the pavement

combination of the layer thickness

property of the materials


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TRAFFIC

The total load applications due to all mixed traffic within

the design period are converted to the 18-kip ESAL, W18, usingthe axle load equivalency factors for each axle group providedin Guide.

= + +

= directional distribution factoralthough it is generally 0.5, it has shown that it mayvary from 0.3 to 0.7 depending on which direction isloaded and unloaded

= lane distribution factor

=the cumulative 2 directional 18-kip ESAL

No. of lanes inEach direction

Percent 18-kip ESALIn design lane

1

2

3

4

100

80-100

60-80

50-75


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RELIABILITY

Reliability design factor provides a predetermined

level of assurance (R) that pavement section will survivethe period for which they are designed. For a given

reliability level, reliability factor is a function of the overall

standard deviation (So). The standard deviations of 0.45

and 0.35 respectively are suggested by the Guide forflexible and rigid pavements.

oRR SZF 10log


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ENVIRONMENTALEFFECTS

The long term effects of temperature, moisture, and

material aging on pavement performance could not

be directly accounted for the road test data.

Also, if the effects of swell clay and frost heave

of a subgrade soil on the performance of the

pavement in a specific region are significant, the

loss of serviceability over the design period should

be estimated and added to that due to traffic loads.


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SERVICEABILITY

Initial serviceability ()and terminal serviceability ()indexes must be established to compute the change in

serviceability (PSI) in Figure 16-11. Typical values

from AASHTO road test were:

=4.2 (flexible) and 4.5 (rigid)

= 2.5 for major highway

= 2.0 for other pavements

PSI=

The change in serviceability (PSI) should alsoinclude the loss of serviceability during the design period

due to potential subgrade swelling and frost heave.


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EFFECTIVEROADBEDSOILRESILIENT

MODULUS

An effective resilient modulus is then established

that is equivalent to the combined effect of the

subgrade resilient modulus of all seasonal resilient

moduli. Figure 16-12 is a work sheet given in the

Guide for estimating effective roadbed resilientmodulus. A year is divided into 24 periods, and the

resilient modulus of the roadbed soil in each period

is determined and entered in second column in the

figure.


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16-12

= 1.81 10

.


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DETERMINATIONOFREQUIREDSTRUCTURAL

NUMBER

The nomograph is constructed from equation in Fig.

16-11:

the inputs are:W18estimated future traffic

Rreliability

Sooverall standard deviation

Mreffective resilient modulusPSI design serviceability loss


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SELECTIONOFPAVEMENTTHICKNESS

DESIGNS

It is necessary to determine the thickness of

the various layers in a flexible pavement that will

provide the required load carrying capacity that

corresponds to design number.

SN = a1D1+ a2D2m2+ a3D3m3

ai = layer coefficient of layer i

Di= thickness of layer i (in)mi = drainage modifying factor for layer i


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Layer coefficients (ai)is a measure of relative

effectiveness of a given material to function as a

structural component of the pavement. The layercoefficients of materials are below

Asphalt concrete surface course. Fig. 16-13

presents a chart that can be used to determine the

layer coefficient of a dense graded asphalt concretesurface course based on its elastic modulus at

68oF.

Bituminous treated bases. Fig.16-14 presents a

chart that can be used to estimate the layercoefficient of a bituminous treated based on its

elastic modulus or its Marshall stability value.


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ASPHALTCONCRETESURFACE()

16-13


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BITUMINOUS

TREATED

BASE()

16-14


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Granular base and subbase layers.Fig. 16-15 and

Fig.16-16 can be used to estimate the layer coefficient

of a granular base material and granular subbasematerial based on different laboratory test results of the

material.

Cement treated bases.Fig.16-17 presents a chart that

can be used to estimate the layer coefficient of a cement

treated base from its unconfined compressive strength

or elastic modulus.

Drainage modifying factor (mi) Table 16-15 presents

the drainage coefficients for untreated base and

subbase materiaals. The coefficient depend on thequality of drainage and percentage of time the pavement

structure is saturated. The quality of drainage is

measured the length of time it takes for water to be

removed from base or subbase.


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GRANULAR

BASE()

16-15


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CEMENT

TREATEDBASE

()

16-17


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If Fair and

30% exposure,

then miis 0.80.


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16-18

G P


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GENERALPROCEDUREFORSELECTIONOF

LAYERTHICKNESS.

1. Using E2as Mr and Fig. 16-11, determine the structural

SN1required to protect the base and compute thethickness of layer 1 (D1)

D1 SN1/a1

D1 5/0.44 = 11.3636 in

2. Using E2 as Mr and Fig.16-11, determine thestructural number SN2required to protect the subbase andcompute the thickness of layer 2 (D2)

D2 ()

3. Using the roadbed resilient modulus and Fig 16-11determine the structural number SN3 required to protectthe roadbed soil and compute the thickness of layer 3 (D3)

D3 ()


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aashto flexible design method

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