08 - AASHTO Thickness Design

Thickness Design

1972 AASHTO Method

AASHTO Method

Pavement engineers recognized early that pavementswere being worn out by high axle loads. Lackingbasic design principles, states were forced to imposeaxle load limits to keep their pavements intact. Atfirst, each state had its own limits. During WWII,AASHO recommended an 18-kip load limit for dual-tire, single-axle trucks. After WWII, this limit wasadopted as FHWA policy along with a 32-kip limitfor tandem-axle trucks.

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AASHTO Method

In an attempt to develop rational pavement designmethods, engineers conducted experiments using teststrips with different pavement layer thicknesses. Atfirst these were built on existing U.S. highways andused existing traffic. Eventually, these gave way totest roads built specifically for experimentation thatused trucks with specific axle loads that continuouslytraversed the test sections.

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AASHTO Method

The first, built in Idaho in 1951, consisted of twoidentical test loops. On each loop, the northboundstraightaways used 2" of hot mix asphalt over a 4"gravel base and the southbound straightaways used4" of hot mix asphalt over a 2" gravel base. Eachstraightaway was separated into 300' sections. Thedifferent test sections were built with 0", 4", 8", 12",and 16" of subbase.

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AASHTO Method

One loop was traversed exclusively by 18-kip singleaxle loads in the inner lanes and 22.4-kip single axleloads in the outer lanes. The other used 32-kip and40-kip tandem axle loads.

The test ran for 18 months, accumulating 238,000vehicle passages between the end of 1952 and thebeginning of 1954.

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AASHTO Method

In 1956, the American Association of State HighwayOfficials (AASHO) followed up with an even morecomprehensive road test in Ottawa, Illinois. The testroad was constructed on the right-of-way of whatwas to become I-80. The setup consisted of fourlarge loops and two small loops of 4-lane highwaybroken into 836 100-foot test segments. All of thenorthbound lanes were hot-mix asphalt and all of thesouthbound lanes were portland-cement concrete.

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AASHO Road Test

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AASHO Road Test

Source: WSDOT Pavement Guide Interactive CD-ROM

AASHTO Method

The flexible pavement sections were constructedwith 1", 2", 3", 4", 5", or 6" of HMA surface course;0", 3", 6", or 9" of base course; and 0", 4", 8", or 12"of subbase. Four types of base were used: gravel,crushed stone, cement-treated, and asphalt-treated.

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AASHO Road Test


AASHTO Method

The rigid pavements were constructed with slabsfrom 3½" thick to 12½" thick, in 1½" increments.The slabs were poured on 0", 3", 6", or 9" of subbaseconsisting of a sand and gravel mix with a CBR of35%. The joints between the slabs were kept alignedwith dowel bars of various lengths and diameters.These keep the slabs from moving independentlyunder wheel loads so the individual slabs act as onecontinuous concrete road surface.

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AASHO Road Test


AASHTO Method

One of the small loops was left to serve as a control.The other was loaded by trucks with single axleloads of 2000-lb and 6000-lb. The larger loops wereloaded by tractor-trailers with single axle loads of12, 18, 22.4, and 20 kips and tandem axle loads of24, 32, 40, and 48 kips. A fleet of 60 trucks operated18½ hours a day, 6 days a week, 6 vehicles per laneat a speed of 35 mph for nearly 2 years. By the end ofthe test, 1.1 million axle loads had been applied!

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AASHO Road Test


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AASHO Road Test


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AASHO Road Test


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AASHO Road Test

Source: http://www.fhwa.dot.gov

AASHTO Method

This test generated an enormous amount of data. Tomake sense of it all, they analyzed rigid and flexiblepavements separately.

For the flexible pavements, they reduced everythingto just three index variables encapsulating (a) thestructure of the pavement, (b) the condition of thepavement, and (c) the traffic loads applied to thepavement at any point in time.

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AASHTO Method

The structure of the pavement is quantified by thestructural number (SN) which captures the thicknessand stiffness of the various pavement layers.

The condition of the pavement at any point in time isquantified by the present serviceability index (PSI)which captures the ride quality of the pavement.

The traffic loads are quantified by the number of 18-kip equivalent single axle loads (ESALs).

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Present Serviceability Rating

During the road test, the condition of the pavementwas periodically assessed by measuring things likethe amount of cracking, patching and rutting.

At the same time, the ride quality of the pavementwas assessed by teams riding in passenger cars. Eachteam member scored the ride quality on a scale from0 to 5 and indicated whether or not they found theride quality acceptable.

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Present Serviceability Rating

22

Acceptable Ride Quality

PSR Acceptable?

3.0 88%

2.5 45%

2.0 15%

23

Present Serviceability Index

The highly subjective ride quality ratings were latercorrelated with the measurable quantities of distressto form the present serviceability index (PSI). Thatway, you didn’t need teams of riders any more; youcould predict what the average rider would assess theride quality to be from the measurements of rutting,cracking, etc.

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Having reduced the very complex road test results tojust three variables (PSI, SN, and ESALs), AASHOengineers developed a regression model relating thechange in PSI over time to the number of ESALs asa function of the structural number. Pavements witha high SN can withstand more ESALs before theyfail than can pavements with a low SN.

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Ride Quality Over Time

5

4

3

2

1

0

PSI

ESALS (log scale)

Low SN High SN


The average ride quality of the pavements when theywere first constructed was PSI = 4.2. The engineersdeemed PSI = 1.5 to represent failure. At that point,none of the ride quality evaluators found the ride tobe of acceptable quality.

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Very good

Good

Fair

5

4

3

2

1

0

PSI

ESALS (log scale)

Failure

pi = 4.2

pf = 1.5 Poor

Very poor


You really don’t want the roads to reach the point offailure because (a) the motoring public would deemthe roads unfit and (b) they would be unsafe becauseit would be difficult to keep your car in the lane.

The goal, then, is to design the road to accommodatethe requisite number of ESALs over its design lifewithout the ride quality falling below some terminalserviceability level. The road would then be repavedor rebuilt and the clock would start over.CIVL 3137 29

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5

4

3

2

1

0

PSI

ESALS (log scale)

Failure

pi = 4.2

pf = 1.5

Terminal Serviceability Level (pt)


For low volume roads (like residential streets) youmight allow the ride quality drop as low as PSI = 2before remediating the pavement.

For high volume, lower speed roads (like Poplar Ave.or Germantown Rd.) you would typically design fora terminal serviceability level of 2.5.

For high-speed roads (like highways) it would beunsafe to let the PSI drop much below 3.

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Very good

Good

Fair

5

4

3

2

1

0

PSI

ESALS (log scale)

Limit for Low-Volume Roads

Limit for High-Volume Roads

pi = 4.2

pf = 1.5 Poor

Very poor

Limit for High-Speed Roads

AASHTO Design Equation

The final regression model developed by the AASHOengineers relates the log of the number of ESALs tothe structural number of the pavement system (SN)and the terminal serviceability level (pt) you want toachieve before rehabilitating the pavement.

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LifetimeESALs

SubgradeSupport

Ride QualityThreshold

Structural Number(Flexural Rigidity)

t

10

10 18 10 i

5.19

4.2 plog

14.2 1.5log W 9.36log SN 1 0.20 log 0.372 S 3.0

1094 R0.4SN 1

RegionalFactor

1966


One drawback to this equation is that it completelyneglects the subgrade support. The pavements for theroad test were all built on a 3-foot embankment of thelocal clayey subgrade soil (CBR = 2) so there was nodata to account for the effects of subgrade quality northe effects of seasonal changes in subgrade support.

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After publishing the 1966 design equation, engineersset out to incorporate subgrade support in the model.

Their initial attempt (published in 1972) incorporateda regional factor (R) to account for seasonal changesin subgrade support and a soil support value (S) thatcould be related to measures such as the CBR, thegroup index, and the Hveem resistance value.

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LifetimeESALs

SubgradeSupport



t

10

10 18 10 i

5.19

4.2 plog

14.2 1.5log W 9.36log SN 1 0.20 log 0.372 S 3.0

1094 R0.4SN 1

RegionalFactor

1972

Regional Factor (R)

The regional factor was based on an index value thatranged from a high of 5 for wet, sloppy subgradessuch as occur during the spring thaw to a low of 1/5for frozen subgrades such as occur in the middle ofthe winter in northern parts of the country.

These regional factors could be averaged over theentire year to arrive at a single value that could beused in the design equation.

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Regional Factor (R)

Condition R value

Roadbed materials frozen to a depth of 5 in. or more (winter)

Roadbed materials dry (summer and fall)

Roadbed materials wet (spring thaw)

0.2 – 1.0

0.5 – 1.5

4.0 – 5.0

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Regional Factor (R)

Soil Support Value (S)

The regional factor was carefully chosen so it wouldhave a value of 1 for Ottawa, Illinois. That way, theR term in the design equation dropped out, leavingthe original model alone.

The same was done with the soil support value. Itwas chosen to have a value of 3 for Ottawa, Illinoisso the S term in the design equation would evaluateto zero.

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LifetimeESALs

SubgradeSupport



t

10

10 18 10 i

5.19

4.2 plog

14.2 1.5log W 9.36log SN 1 0.20 log 0.372 S 3.0

1094 R0.4SN 1

RegionalFactor

1972


The way this equation is used, you determine valuesfor S and R based on your project location, choose asuitable terminal serviceability level (pt), then try tofind a value of SN that provides the required numberof ESALs (W18) over the design life of the pavement.

It is not possible to rearrange this equation to get SNon the left-hand side of the equal sign. It has to besolved by trial-and-error.

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To make matters worse, there were no such things aspersonal computers or pocket calculators in the late1960s and early 1970s; logarithms were found bylooking up values in tables and multiplication anddivision were done using slide rules!

To make the design equation usable, the engineersdeveloped nomographs, which are graphical equationsolvers.

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AASHTO Design Nomograph

Use for 20-yrdesign life

Use for ANYdesign life

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Example

A proposed highway near Vicksburg, MS will experience 1000 ESALs per day, on average, over the next 20 years. The subgrade soil is a silty clay with a CBR of 4. Find the required SN for a terminal serviceability level of 2.0.

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4.25

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4.25

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Regional Factor (R)

Vicksburg

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4.25

4.6

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Inc

rea

sin

g S

N

Inc

rea

sing

SN

BE CAREFUL!

Example

A proposed highway outside Pierre, SD will experience 2,000,000 ESALs over the next 25 years. The subgrade soil is a fat clay that has been amended with lime to produce a soaked CBR of 10. Find the required SN for a terminal serviceability level of 2.5.

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6.0

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6.0 2000

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Regional Factor (R)

Pierre

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AASHTO Design NomographUse for ANY

design life

6.02000

4.5

3.0

Structural Number (SN)

The structural number is an index value that tries tocapture the flexural rigidity of all the pavement layersabove the subgrade in a single value.

Each pavement layer is assigned a structural layercoefficient (ai) whose value depends on the qualityof the material used and its location in the pavementsystem. (For example, a sandy gravel has a lowervalue when used as a base course than as a subbasebecause the stresses are higher in the base course.)CIVL 3137 62

Structural Number (SN)

The structural number for the entire pavement isthen computed by multiplying each structural layercoefficient by the thickness of the layer (in inches).

The design goal, then, is to put together a pavementsystem that has the structural number determined bythe design nomograph. The choice of materials andlayer thicknesses depends on construction costs andthe local availability of materials.

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AASHTO Pavement Design

Subbase

Base

Asphalt

D2

D3

D1

Source: NCEES FE Supplied Reference Handbook

AASHTO Layer Coefficients

AASHTO published suggested layer coefficients touse for different materials in different layers. It alsopublished a series of charts and nomographs that thedesigner could use to determine appropriate layercoefficients based on measurements of strength andstiffness obtained from the actual materials beingused on the project.

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Asphalt Layer Coefficient (a1)

67

Base Layer Coefficient (a2)

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Subbase Layer Coefficient (a3)

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Pavement Design

Sandy GravelSubbase

Crushed StoneBase

Dense-GradedPlantmix Asphalt

10"

12"

5"

SN = ??


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Pavement Design

Sand Subbase(Esb = 12,500 psi)

Crushed StoneBase (CBR = 70)

Dense-GradedAsphalt (Eac = 350 ksi)

6"

8"

D1 = ?

SN = 2.7

Asphalt Layer Coefficient (a1)

73

0.39

Base Layer Coefficient (a2)

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0.13

Subbase Layer Coefficient (a3)

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0.09


In 1986, AASHTO revised their design equation oncemore, replacing the regional factor and soil supportterms with a single subgrade support term based on aseasonally adjusted resilient modulus (MR) value.

The seasonally adjusted value is a single year-roundMR that results in the same loss of ESALs as wouldoccur based on the seasonally varying MR values.

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t

10

10 18 10 10 R

5.19

4.2 plog

4.2 1.5log W 9.36log SN 1 0.20 2.32log M 8.07

10940.4

SN 1

LifetimeESALs

Seasonally AdjustedSubgrade Support



1986

08 - AASHTO Thickness Design

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