As Sh to Pavement Design

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Design of Flexiable Pavementto ASSHTO

Guide for Design of Pavement Structures

Accompanying Documentto Design Spreadsheet

Officine Maccaferri SpaVia Agresti, 640123 Bolognatel. +39-051-6436000 fax. +39-051-236507www.maccaferri.com March 2007



IntroductionThis document describes the design of flexible pavements, in accordance withthe American Association of State Highway and Transportation Officials(AASHTO) Guide for Design of Pavement Structures. This design method isan empirical method.

Flexible pavements consist of a prepared subgrade layer which is the roadbedsoil compacted to a specified density. A subbase course is constructed on topof the prepared roadbed, and may be omitted if the subgrade soil is of a highquality. The base course is constructed on the subbase course, or if nosubbase is used, directly on the roadbed soil. It usually consists of aggregatessuch as crushed stone, or crushed gravel and sand. On top of the basecourse is the surface course that typically consists of a mixture of mineralaggregates and bituminous materials.

Design Approach

The approach to flexible pavement design is similar for reinforced andunreinforced pavements and can be divided into two steps:

1. The structural number of the pavement is determined. This isindependent of the reinforcement,

2. The depth of the pavement materials can be determined.

Determining the Structural NumberThe AASHTO design approach uses the empirically based Structural Number(SN) to quantify the structural strength of a pavement required for a givencombination of soil support, total traffic, reliability, and serviceability level. Therequired SN is converted to actual thickness of surfacing, base and subbase,by means of appropriate layer coefficients representing the relative strength ofthe construction materials.

The design equation is as follows:

33322211 m Dam Da DaSN ++= (1)

Where,

ia is the ith layer coefficient,

i D is the thickness of the ith layer,

im is the drainage coefficient for the ith layer.

The number of the layers starts at the upper most and continuous downward.The 1st layer is the bituminous layer, while the 2nd layer the base course. Thedrainage of the bituminous layer is not considered in design.

The basic empirical design equation in the ASSHTO design guide is asfollows:



( )

( )

07.8)(log32.2

1

109440.0

5.12.4log

20.0)1(log36.9log 10

19.5

10

101810 −+

++

⎥⎦

⎤⎢⎣

⎡

−

∆

+−−+= Ro RM

SN

PSI

SN S Z W

(2)

Where,

18W is the number of 18 kip (80 kN) equivalent single axle load (ESAL)

applications,

R Z is the standard normal deviate,

oS is the combined standard error of the traffic prediction and performance

prediction,PSI ∆ is the difference between the initial and terminal serviceability indexes.

r M is the resilient modulus of the formation soil in psi.

By solving equation (2) for the structural number SN, the required depths ofthe pavement layers can be determined from equation (1).

The calculation of the structural number requires the following parameters tobe determined:

1. The reliability parameter,2. The serviceability criteria,3. The combined standard error,4. The resilient modulus of the formation soil,5. The layer coefficients,6. The drainage coefficients,7. The number of 18 kpi (80 kN) equivalent axle loads (ESAL)

applications.

The reliability parameter The reliability parameter is a means of incorporating a degree of certainty intothe design process to ensure that the various design alternatives will performover the analysis period (deign life of the pavement). The level of reliability isa function of the volume of traffic, the importance of the roadway and the riskof not performing to expectation.

The level of reliability suggested in ASSHTO guide are presented in Table 1.

Table 1. Level of reliability for different road types.Functional classification Recommended level of reliability (%)Interstate and other freeways 85-99.9 80-99.9Principal arterials 80-99 75-95Collectors 80-95 75-95Local 50-80 50-80

The standard normal deviate R Z is directly related to the level of reliability.

Note: R Z is a negative value.



The serviceability criteria The serviceability of a pavement is defined as its ability to serve the type oftraffic which uses the pavement, the measure of serviceability is the PrimeServiceability Index (PSI) which ranges from 0 (impossible road) to 5 (perfectraod. The ASSHTO guide uses the total change in serviceability index ( PSI ∆ )

as the serviceability design criteria, which is defined as:

t o p pPSI −=∆

Where,

o p is the initial serviceability index.

t p is the terminal serviceability index, which is based on the lowest index

that will be tolerated before rehabilitation.

Washington State Department of Transportation suggest that:

5.4=o p

0.3=t p giving,

5.1=∆PSI

The combined standard error The combined standard error variable defines how widely the two basicdesign inputs, traffic and performance, can vary. Its value should be selectedto represent the local conditions. Typical values of So used are 0.40 to 0.50for flexible pavements and 0.35 to 0.40 for rigid pavements.

The resilient modulus of the formation soil The empirical ASSHTO design equation (2) is based on the resilient moduluswhich is correlated with the CBR value of the foundation soil as follows:

(%)1500)( CBR psi M r =

Note : That equation (2) is empirical and therefore r M must be inputted in

units of psi.

In the design spreadsheet r M is inputted in kPa and converted by the

spreadsheet to psi. This was to insure consistency of units.

The modulus in kPa can be calculated directly from CBR as follows:

(%)10335)( CBRkPa M r =

The layer coefficients Layer coefficients are empirical relationships between structural number (SN)and layer thicknesses which expresses the relative ability of a material tofunction as a structural component of the pavement. The magnitude of thelayer coefficient is a function of the modulus of the material layer.



ASSHTO provides guidance in the form of charts relating modulus to layercoefficient, Figures 1,2,3,& 4. These charts are material type specific.

Figure 1. Relationship between elastic modulus and layer coefficient, 1a , for

dense graded asphalt concrete.

Figure 2. Variation of layer coefficient, 2a , with different strength parameters.



Figure 3. Variation of layer coefficient, 2a , with different strength parameters

for bituminous treated base course.

Figure 4. Variation of layer coefficient, 2a , with different strength parametersfor cement treated base course.



The drainage coefficients The drainage characteristics of the pavement is accounted for through the useof modified layer coefficients. Table 2 presents the definitions of drainagelevels suggested in the ASSHTO guide. The drainage of the bituminous layer

(Layer 1) is not considered in design.

Table 2. ASSHTO drainage conditionsQuality of Drainage Water Removed Within

Excellent 2 hoursGood 1 dayFair 1 weekPoor 1 month

Very poor Water will not drain

Table 3 presents the ASSHTO recommendedi

m values as a function of the

drainage quality and the percentage of time during the year the pavementstructure would normally be exposed to moisture levels approachingsaturation.

Table 3. Recommended values of the drainage coefficients, im

Percentage of time pavement structure is exposed tomoisture levels approaching saturation

Quality ofdrainage

< 1 % 1 – 5 % 2 – 25 % > 25 %Excellent 1.40 – 1.35 1.35 – 1.30 1.30 – 1.20 1.20Good 1.35 – 1.25 1.25 – 1.15 1.15 – 1.00 1.00Fair 1.25 – 1.15 1.15 – 1.05 1.00 – 0.80 0.80Poor 1.15 – 1.05 1.05 – 0.80 0.80 – 0.60 0.60Very poor 1.05 – 0.95 0.95 – 0.75 0.75 – 0.40 0.40

The number of 18 kpi (80 kN) equivalent axle loads (ESAL) applications The ESAL is a function of the current traffic estimate, the anticipated trafficgrowth factor, the number of lanes on the carriageway, the reliability of thepavement and environmental effects. The calculation of the 18 kpi (80 kN)equivalent axle load is considered beyond the scope of this document and thedesign spreadsheet.

Modification of the ASSHTO method to incorporate reinforcementThe structural contribution of the reinforcement placed in the base course ona flexible pavement can be quantified by the increase in the layer coefficientof the aggregate base course. Equation (1) now becomes:

33322211 m Dam LCRDa DaSN ++= (3)

Where, LCR is the layer coefficient ratio, which has a value greater than unity.

The LCR is generally determined based on the results of laboratory testing onflexible pavements system incorporating reinforcement. A value of 1.4 for



extruded polypropylene and polyethylene geogrids and 1.25 for wovenpolypropylene geotextiles are suggested.

The Structural Number for the pavement is calculated using equation (2) inthe same manner as an unreinforced pavement.

The reduction in aggregate base course thickness can be calculated usingequation (3) as follows:

22

333112

LCRma

m Da DaSN D

−−=

Alternatively the asphalt thickness can be reduced. The thickness of theasphalt thickness can be calculated as follows:

1

3332221

am Dam LCRDaSN D −−=

Operation of the design spreadsheetThe spreadsheet is divided into two steps. The first is the calculation of thestructural number which is independent of any reinforcement type present.The second step is to determine the thickness of each layer based on thecalculated structural number. The spreadsheet allows this calculation for bothunreinforced and reinforced pavements for a direct comparison.

The design spreadsheet is interactive allowing the user to select values fromtables were necessary. The following are the required inputs:

1. The reliability parameter, which is selected from a table,2. The resilient modulus of the formation soil, inputted in units of kPa and

converted by the program to units of psi for use in calculations,3. The combined standard error,4. The layer coefficients,5. The drainage coefficients, which are selected from a table6. The number of 18 kpi (80 kN) equivalent axle loads (ESAL)

applications,7. The Layer Coefficient Ratio for the reinforcement is the pavement,

8. The depth of the asphalt, base course and subbase course for depthcalculations.

The design spreadsheet has an input/output user interface and a calculationssheets. Values are only entered in the yellow boxes. On no account shouldvalues be entered in the orange boxes. The user interface and calculationsheet are presented in Figures 5 & 6.

In calculating the structural number the “Calculate” button must be clicked.The drainage coefficient can be selected by clicking the “Select m based on drainage conditions in base and subbase layers” . This opens a new window

where the im coefficients can be selected for each layer, Figure 7.



The reliability parameter is also selected by clicking the “Select Level of reliability” button, which opens a new window where the value is selected. Thestandard normal deviate value is automatically determined by the designspreadsheet for the level of reliability.

With the structural number determined, the thickness of the constructionlayers can be varied to arrive at a satisfactory design. The spreadsheet allowsthe user to vary the depth of the asphalt, base course and subbase course forboth the reinforced and unreinforced sections. In each case two depths arerequired, while the third will be calculated. It should be noted that once thestructural number has been calculated the depth of the layers can be variedindependent.

Where the pavement does not include either a base course or subbase, thelayer coefficient or depth of the layer can be set to zero to remove the layerfrom the calculation.

The input sheet also allows minimum depth to be set for each layer. If on thecalculation of a layer thickness this minimum value is pass a warningmessage will appear on the sheet. The use can either reduce the minimumdepth of the layer or vary the depths of the other pavement layers.

Figure 5. The input/output user interface.



Figure 6. The calculation sheet.

Figure 7. Selection of drainage coefficient.



Figure 8. Selection of the reliability parameter.

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As Sh to Pavement Design