AASHTO Flexible Pavement Design

AASHTO Flexible Design Procedure

Dr. Christos Drakos

University of Florida

Topic 7 AASHTO Flexible Pavement Design

1. Development

1.1 AASHO Road Test

AMERICAN ASSOCIATION OF STATE HIGHWAY AND TRANSPORTATION OFFICIALShttp://www.aashto.org/

1.2 Performance MeasurementsEstablishment of performance criteria is critical

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

number of load repetitions with the performance of various pavements

Provided data for the design criteria

AIAASHTO VsFunctional Structural


AASHO Road Test performance based on user assessment: Difficult to quantify (subjective) Highly variable Present Serviceability Rating (PSR)

1.2 Performance Measurements (cont)

0-1 V. Poor1-2 Poor 2-3 Fair 3-4 Good 4-5 V. Good

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

Measurable characteristics (performance indicators): Visible distress (cracking & rutting) Surface friction Roughness (slope variance)

Measure of how much slope varies from horizontal along the direction of traffic


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

1.3 AASHTO Performance Relations

0-1 V. Poor1-2 Poor 2-3 Fair 3-4 Good 4-5 V. Good

USER ASSESSMENT PERFORMANCE INDICATORS

Measure of RoughnessMeasure of RuttingMeasure of Cracking

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) performance correlate to the measured performance?

calculated the regression coefficients for the PSI equation


1.3 AASHTO Design Equations1.3.1 Performance Requirements & Design Life

PSI

Time (age)

PSI0

PSIt

PSI = (PSI0 - PSIt)

Terminal PSI (known) Pvt is no longer functional

AASHTO performance requirement = PSI PSI is such that PSIt is NOT reached before end of design life

Design Life

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


1.3.2 Performance Relation

PSI = fnc (MReff, SN, ESAL) MReff: Accounts for the environment SN: Index relating effectiveness of

PVT structure

PERFORMANCE(PSI)

What are the three factors affecting performance (PSI)?

Structural Efficiency of PVT

ESAL

Structural Number (SN)

MReff

=

known known known

Solve for SN


1.3.3 Definition of Structural Number

BASE

AC

SUB-BASE

D1

D2

D3

111 aDSN =

222 aDSN =

333 aDSN =

SN

Structural Coefficient (a):a = fnc (E, position in PVT)

a1

a2

a3SN = SN1 + SN2 + SN3

Basic Procedure: Determine the traffic (ESAL) Calculate the effective subgrade modulus (MReff) Select the performance level (PSI) Solve for the required SN needed to protect the subgrade


1.3.4 Design Notes

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

ii. Your job as a designer is to select the most economical combination, using available materials and considering the following: Geometry requirements (Cut/Fill) Drainage requirements Frost requirements

iii.AASHTO assumes that pavement structural layers will not be overstressed: Must check that individual layers meet structural

requirements


2. Design Inputs2.1 General Design Variables

Design Life Material Properties Traffic Reliability

Degree of certainty that the pavement will last the design period

Uncertainty in: Traffic prediction Performance prediction Materials & construction


2.2 AASHTO Reliability Factor (FR)Adjust traffic for reliability:

R1818 FwW =Where:W18 = Design ESALw18 = Predicted ESAL

FR = fnc (R, S0)

Reliability level chosen

Overall Standard Deviation: Traffic Variation Performance prediction

variation Materials (subgrade)Steps:

1. Define functional class (Interstate/Local)2. Select reliability level (R) Table 11.143. 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


2.3 Performance CriteriaDesign for serviceability change: PSI = PSI0 PSIt

PSI0 = Initial serviceability index Flexible: 4.2 Rigid: 4.5

PSIt = Terminal serviceability index Major highways: >2.5 Lower volume: 2.0

2.4 Material Properties2.4.1 Effective Subgrade Resilient Modulus Obtain MR values over entire year Separate year into time intervals Compute the relative damage value (uf) for each modulus

2.32R

8f M101.18u

=


Compute average uffor entire year

Determine effective MR using average uf

2.4.1 Effective Subgrade Resilient Modulus (cont)

2.32R

8f M101.18u

=


2.4.2 Pavement Structural Layers Layer coefficient ai; relative quality as a structural unit:

2 of material with a=0.2 provides the same protection as 1 material with a=0.4

Initially layer coefficients were derived from AASHO road test results; 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 of interpolating from the figure):

Untreated and Stabilized Bases

( ) 977.0log249.0 22 = EaGranular Sub-bases Can estimate the sub-base layer coefficient from Figure 7.16 Can also use the following (instead of interpolating from the

figure): ( ) 839.0log227.0 33 = Ea


2.5 Drainage AASHTO guide provides means to adjust layer coefficients

depending on the effectiveness of the drainage Define quality of drainage of each layer based upon:

Time required for drainage Percent time moisture levels approach saturation

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


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

PSI4.2 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 design configurations will meet

the required SN 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 by entering the nomograph using the MR of the layer in question


D1

1

11 a

SND =

SNtotal

MReff

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

BUT, have to protect each layer from overstressing; need to get required SN (level of protection) for each layer

Only top (AC) layer does not need protection

For example: Base needs SN1 protection. BUT, SN1= a1D1

So,

E1, a1

E2, a2, m2

SN1

E3, a3, m3

SN2


2.6.6 General Procedure1. Using E2 as the MR value, determine from Figure 11.25 the structural

number SN1 required to protect the base and compute the thickness of layer 1 by

2. Using E3 as the MR value, determine from Figure 11.25 the structural number SN2 required to protect the subbase and compute the thickness of layer 2 by

3. Based on the roadbed soil resilient modulus MReff, determine from Figure 11.25 the total structural number SN3 required and compute the thickness of layer 3 by

1a1SN

1D =

2m

2a

*1D1a2

SN2

D

3m

3a

2m*

2D

2a*1D1a3

SN3

D


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 / StabilityLayer 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

3.120.40 0.800.16

3.370.32 0.950.104.051.60 1.000.37

3.120.800.16

0.4 =


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 Stone

1.20350,000Bituminous-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


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


2.10 AASHTO Design Example 2 (cont)Asphalt Concrete structural coefficient (a) Figure 7.13:

0.37


2.10 AASHTO Design Example 2 (cont)Bituminous-treated base structural coefficient (a) Figure 7.15:


2.10 AASHTO Design Example 2 (cont)Cement-stabilized base structural coefficient (a) Figure 7.15:

0.118


2.10 AASHTO Design Example 2 (cont)Crushed stone base structural coefficient (a) Figure 7.15:


2.10 AASHTO Design Example 2 (cont)Crushed stone subbase structural coefficient (a) Figure 7.16:

0.16


2.10 AASHTO Design Example 2 (cont)

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