AASHTO Flexible Pavement Design 1

8/3/2019 AASHTO Flexible Pavement Design 1

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AASHTO Flexible Design Procedure

Dr. Christos Drakos

University of Florida

Topic 7 AASHTO Flexible Pavement Design

1. Development

1.1 AASHO Road Test

AMERICANASSOCIATION OF STATE HIGHWAY AND TRANSPORTATION OFFICIALS

http://www.aashto.org/

1.2 Performance MeasurementsEstablishment of performance criteria is critical

Late 50s road test in Illinois Objective was to determine the

Provided data for the design criteria

AIAASHTO VsFunctional Structural


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AASHO Road Test performance based on user assessment:

Difficult to quantify ( Highly variable

1.2 Performance Measurements (cont)

0-1 V. Poor1-2 Poor2-3 Fair3-4 Good4-5 V. Good

A panel of experts drove around in standardvehicles and gave a rating for the pavement

Measurable characteristics (performance indicators): Visible Surface friction Roughness (


Establish correlation between user assessment (ride experience)and performance indicators (measurable characteristics)

1.3 AASHTO Performance Relations

0-1 V. Poor1-2 Poor2-3 Fair3-4 Good4-5 V. Good

USER ASSESSMENT PERFORMANCE INDICATORS

MeasureMeasureMeasure

Present Serviceability Index (PSI)PSI = A0 + A1F1 + A2F2 + A3F3A0 A3 = Regression CoefficientsF1 = Measure of roughnessF2 = Measure of ruttingF3 = Measure of cracking

How does the true (user) performancecorrelate to the measured performance?

calculated the regressioncoefficients for the PSI equation


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1.3 AASHTO Design Equations

1.3.1 Performance Requirements & Design Life

PSI

Time (age)

Terminal PSI (known) Pvt isno longer functional

AASHTO performance requirement =

PSI scale: 1 (V. Poor) 5 (V. Good)


1.3.2 Performance Relation

PERFORMANCE(PSI)

What are the three factors affecting performance (PSI)?


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1.3.3 Definition of Structural Number

BASE

AC

SUB-BASE

D1

D2

D3

Structural Coefficient (a):a = fnc (

a1

a2

a3

Basic Procedure: Determine the traffic (ESAL) Calculate the

Select the performance level ( Solve for the required


1.3.4 Design Notes

i. Different combination of materials & thicknessesmay result in the same SN

ii. Your job as a designer is to select the mosteconomical combination, using available materialsand considering the following:

iii.AASHTO assumes that pavement structural layerswill not be overstressed: Must check that


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2. Design Inputs

2.1 General Design Variables

Design Life Material Properties Traffic Reliability

Degree of certainty that the pavement will lastthe design period

Uncertainty in:


2.2 AASHTO Reliability Factor (FR)

Adjust traffic for reliability:

R1818 FwW =

Where:W18 =w18 =

FR= fnc (

Reliability levelchosen

Overall Standard Deviation: Traffic Variation Performance prediction

variation Materials (subgrade)Steps:

1. Define functional class (Interstate/Local)2. Select reliability level (R) Table 11.14

3. Select a standard deviation (S0) Flexible:

No traffic variation: S0=0.35 With traffic variation: S0=0.45

Rigid: No traffic variation: S0=0.25 With traffic variation: S0=0.35


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2.3 Performance Criteria

Design for serviceability change:

PSI = PSI0 PSIt PSI0 =

PSIt =

2.4 Material Properties

2.4.1 Effective Subgrade Resilient Modulus

Obtain MRvalues

Separate year into time intervals Compute the2.32

R8

f M101.18u

=


Compute average uffor entire year

Determine effective MRusing average uf

2.4.1 Effective Subgrade Resilient Modulus (cont)

2.32

R8

f M101.18u

=


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2.4.2 Pavement Structural Layers

Layer coefficient ai; relative quality as a structural unit:

2 of material with a=0.2 provides

Initially layer coefficients were derived from AASHO road testresults; have subsequently been related to resilient modulus

Hot-Mix Asphalt

AASHTO does not

require test to determine

HMA modulus; usually

assume aHMA=0.44


2.4.2 Pavement Structural Layers (cont)

Can estimate the base layer coefficient from Figure 7.15 for: Untreated base Bituminous-treated base Cement-treated base

For untreated base can also use the following (instead ofinterpolating from the figure):

Untreated and Stabilized Bases

Granular Sub-bases

Can estimate the sub-base layer coefficient from Figure 7.16 Can also use the following (instead of interpolating from the

figure):


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2.5 Drainage

AASHTO guide provides means to adjust layer coefficients

depending Define quality of drainage of each layer based upon:

Determine drainage modifying factor (m) from Table 11.20 iiii mDaSN =


2.6 Computation of Required Pavement Thickness

Determine the required SN for design traffic Identify trial designs that meet required SN

2.6.1 Basic Approach

2.6.2 Nomograph to Solve for SN


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2.6 Computation of Required Pavement Thickness (cont)

Declare the known variables W18, ZR, S0, PSI & MR Give an initial estimate for the SN Allow the equation solver (Matlab, Maple, Mathcad, Excel,

etc.) to iterate for the solution

2.6.3 Solving the Equation

log W 18( ) Z R S 0( ) 9.36 log SN 1+( )+ 0.2log

PSI

4.2 1.5

0.41094

SN 1+( )5.19

+

+ 2.32 log M R( )+ 8.07


2.6.4 Pavement Structural Layers

SN = a1D1 + a2D2m2 + No Unique Solution! Many

Optimize the design; consider the following: Design constraints drainage, minimum thickness, available materials Construction constraints minimum layer thickness Economics

2.6.5 Layered Design Analysis

Nomograph determines the SN required to protect the

subgrade However, each structural layer must be protected against

overstressing Procedure developed using the AASHTO design nomograph

Determine the SN required to protect each layer


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SNtotal

MReff

First we need to protect the subgrade; use the nomographto get SN needed to provide adequate protection

BUT,

Only top (AC) layer

E1, a1

E2, a2, m2

SN1

E3, a3, m3

SN2


2.6.6 General Procedure

1. Using E2 as the MRvalue, determine from Figure 11.25 the structuralnumber SN1 required to protect the base and compute the thickness oflayer 1 by

2. Using E3 as the MRvalue, determine from Figure 11.25 the structuralnumber SN2 required to protect the subbase and compute the thickness oflayer 2 by

3. Based on the roadbed soil resilient modulus MReff, determine from Figure11.25 the total structural number SN3 required and compute the thicknessof layer 3 by

1a

1SN

1D =

2m2a

*1

D1

a2

SN

2D

3m

3a

2m*

2D

2a*

1D

1a

3SN

3D


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2.7 Other Thickness Considerations

64> 7,000,000

63.52,000,000 7,000,000

63500,000 2,000,000

42.5150,000 500,000

4250,000 150,000

41< 50,000

Aggregate BaseAsphalt ConcreteESAL

2.7.1 AASHTO Suggested Minimums

2.7.1 Construction / Stability

Layer must be thick enough to act as a unit:

Thickness > 2* (Maximum Aggregate Size)

Maximize crushed stone thickness minimize AC thickness Can also stabilize base to use less HMA

Use gravel only for fill or frost


2.8 Cost Considerations

Consider: Different combination of materials Cost of materials Cost of excavation (cut areas)

Express cost as a unit contribution to SN

Asphalt Concrete

Pit-Run Gravel

Crushed Stone

$/unit SNmiai$/sq.yd.-inMaterial


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WORK EXAMPLE ON THE BOARD

2.9 AASHTO Design Example 1

--5,000Roadbed Soil

0.700.1014,000Granular Subbase

0.800.1430,000Crushed Stone

-0.42400,000AC

miaiMRMaterialGiven:

Reliability = 90% Overall Std. Dev. = 0.35 W18 = 10 million Design Serviceability Loss = 2.0


2.10 AASHTO Design Example 2

0.3212,000Pit-Run Gravel

0.80500,000Cement-Stabilized Base

0.25-Excavation

0.4025,000Crushed Stone1.20350,000Bituminous-Treated Base

1.70300,000Asphalt Concrete

Cost ($/sq.yd.-in)Modulus (psi)Material

Given: Reliability = 90% Performance period = 20 years Overall Std. Dev. = 0.45 W18 = 5.26 million Design Serviceability Loss = 2.0


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2.10 AASHTO Design Example 2 (cont)

0.32SubbasePit-Run Gravel

0.40SubbaseCrushed Stone

0.40BaseCrushed Stone

1.20BaseBituminous-Treated Base

0.80BaseCement-Stabilized Base

1.70BaseAsphalt Concrete

1.70SurfaceAsphalt Concrete

$/Unit SNmiai$/sq.yd-inLayerMaterial

Construct a material information table:

Next step is to fill in the information



Asphalt Concrete structural coefficient (a) Figure 7.13:

0.37


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Bituminous-treated base structural coefficient (a) Figure 7.15:



Cement-stabilized base structural coefficient (a) Figure 7.15:

0.118


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Crushed stone base structural coefficient (a) Figure 7.15:



Crushed stone subbase structural coefficient (a) Figure 7.16:

0.16


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4.440.80.0900.32SubbasePit-Run Gravel

2.081.20.1600.40SubbaseCrushed Stone

2.781.20.1200.40BaseCrushed Stone

4.001.00.3001.20BaseBituminous-Treated Base

6.781.00.1180.80BaseCement-Stabilized Base

6.181.00.2751.70BaseAsphalt Concrete

4.591.00.3701.70SurfaceAsphalt Concrete

$/Unit SNmiai$/sq.yd-inLayerMaterial

Are there any obvious conclusions?

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AASHTO Flexible Pavement Design 1