Diseño de pavimeno Flexible AASHTO 93

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AASHTO 1993Flexible Pavement Design Equation

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Outline

1. AASHO Road Test

2. Present Serviceability Index (PSI)

3. Equation and terms4. Example



AASHO Road Test (1)

1958 - 1961

AASHO Road Test

Picture from: Highway Research Board Special Report 61A-G



AASHO Road Test (2)

• Construction: August 1956 - September 1958

• Test Traffic: October 1958 - November 1960

• Special Studies: Spring and early summer

1961

AASHO Road Test



Test Loops (1)


AASHO Road Test



Test Loops (2)

AASHO Road Test




Environment

• Mean Temperature (July) 76°F

• Mean Temperature (January)27°F

• Annual Average Rainfall 34

inches

• Average Frost Depth 28inches

(for fine-grained soil)

AASHO Road Test



Flexible Materials

• HMA– Dense-graded

– 85-100 pen asphalt

• Base Course

– Crushed limestone– 10% passing No. 200

– Average CBR = 107.7

• Subbase Course

– Sand/gravel mixture

– 6.5% passing No. 200

– CBR = 28 – 51

• Subgrade

– A-6 soil (silt/clay)

– 82% passing No.

200– Average CBR = 2.9

– Optimum wc =13%

AASHO Road Test



Flexible Sections

• HMA

– 1 to 6 inches thick

• Base Course– 0 to 9 inches thick

• Subbase Course– 0 to 16 inchesthick

• Thickest section– 6 inches HMA

– 9 inches base

– 16 inches subbase– Used for heavy loads

– 2.6 to 3.6 PSI at test end

• Thinnest section

– 1 inch HMA– Used for light loads

– 8 to 25 ESALs to failure

AASHO Road Test



Flexible Performance

• Majority failed

• Even thickest sections sustainedappreciable damage

• Most failed during spring thaw

– Frost action was a major contributor

– Thicker base & subbase helped tomitigate frost action

AASHO Road Test



Rigid Materials

• Cement– Type I

– 564 lb/yd3

• Portland Cement Concrete– Maximum w/c = 0.47

– 14-day compressive strength = 3500 psi

– 14-day flexural strength = 550 psi (1/3 point)

– Slump = 1.5 to 2.5 inches

– Maximum aggregate size = 1.5 and 2.5 inches

• Subbase and subgrade were the same asflexible sections

AASHO Road Test



Rigid Sections

• Slabs

– 2.5 to 12.5 inchesthick

• Subbase Course

– 0 to 9 inches thick

• Dowel Bars

– All had dowel bars

– Sizes varied

• Thickest section

– 12.5 inch slab

– 9 inches subbase

– Used for heavy loads

– 4.2 to 4.5 PSI at test end

• Thinnest section

– 2.5 inch slab

– Used for light loads– 4.2 to 4.4 PSI at end

AASHO Road Test



Rigid Performance

• Majority did not fail

• Most sections PSI at the test endwas around 3.8 to 4.4

AASHO Road Test

AASHO R d T



Trucks

AASHO Road Test

Picture from: Highway Research Board

Special Report 61A-G

AASHO R d T t



Subgrade Support Variation

AASHO Road Test

Picturefrom

:HighwayRese

arch

BoardSpe

cialRep

ort61A-G



Test Tracks Today

NCAT Test Track



AASHO Road Test



Present Serviceability Rating (PSR)

AASHO Road Test


AASHO Road Test



Present Serviceability Rating (PSR)

AASHO Road Test




Present Serviceability Index (PSI)

• Calculated value to match PSR

( ) P C SV PSI +−+−= 9.01log80.141.5

SV = mean of the slope variance in the two wheelpaths

(measured with the CHLOE profilometer or BPR Roughometer)

C, P = measures of cracking and patching in the pavement surface

C = total linear feet of Class 3 and Class 4 cracks per 1000 ft2

of pavement area.A Class 3 crack is defined as opened or spalled (at the surface) to a width of

0.25 in. or more over a distance equal to at least one-half the crack length.

A Class 4 is defined as any crack which has been sealed.

P = expressed in terms of ft2 per 1000 ft2 of pavement surfacing.

Basic Equations



Basic Idea

Time

Ser

vic

eability

(P

SI) p0

pt

p0 - pt

Basic Equations

Basic Equations



Basic Relationship

∀ β and ρ depend on pavement structure (thickness andstiffness) and loading

∀ β determines the shape of the graph

∀ρ is the number of loads at which p = 1.5

( )β

ρ

−=−

W p p p p t o 0

Basic Equations

Basic Equations



Basic Equation

( )

( ) 07.8log32.2

1

109440.0

5.12.4log

20.0)1log(36.9log

19.5

018 −+

++

−∆

+−++×= R R M

SN

PSI

SN S Z W

Basic Equations

• Choose these values– Reliability (Z

Rand S

0)

– p0, p

tΔPSI

• Measure MR



Explanation of Terms

( )

( ) 07.8log32.2

1

109440.0

5.12.4log

20.0)1log(36.9log

19.5

018−+

++

−∆

+−++×= R R M

SN

PSI

SN S Z W

W18

Base 10 logarithm of the predicted number of ESALs overthe lifetime of the pavement. The logarithm is taken basedon the original empirical equation form from the AASHO

Road Test.



Explanation of Terms

( )

( ) 07.8log32.2

1

109440.0

5.12.4log

20.0)1log(36.9log

19.5

018−+

++

−∆

+−++×= R R M

SN

PSI

SN S Z W

SN

Structural number. An abstract number expressing thestructural strength of a pavement required for givencombinations of soil support (MR), total traffic (ESALs) and

allowable change in serviceability over the pavement life( ΔPSI).



Structural Number

• Converted to a layer depth usingcoefficients.

– SN = a1D

1+ a

2D

2m

2+ a

3D

3m

3+ …

a= layer structural coefficient

D= layer depth (inches)

m= layer drainage coefficient



Structural Number

Material a-valueSurface course

HMA (asphalt concrete) 0.44

Base course

Crushed stone 0.14

Stabilized base material 0.30 – 0.40

Subbase course

Crushed stone 0.11

Drainage Coefficient (m)Generally, quick draining layers that almost never saturatecan have drainage coefficients as high as 1.4, while slow-draining layers that often saturate can have drainagecoefficients as low as 0.40. Most often, the drainage

coefficient is neglected (i.e. set as m = 1.0).



Structural Number



Reliability (ZR, S0)

X = Probability distribution of stress

(e.g., from loading, environment, etc.)

Y = Probability distribution of strength

(variations in construction, material, etc.)

Probab

ility

Stress/Strength

Reliability = P [Y > X] [ ] ( ) ( ) dxdy y f x f X Y P x

y x

=> ∫ ∫

∞∞

∞−



Reliability (ZR, S0)

Reliability ZR

99.9 -3.090

99 -2.327

95 -1.64590 -1.282

80 -0.841

75 -0.674

70 -0.52450 0S0

Typical values for flexible pavement are 0.40 to 0.50. S0

cannot be calculated from actual traffic or constructionnumbers so it is almost always assumed to be 0.50.



Solving the Equation

• Iterative process

– Both ESAL and structural equationhave SN

• Often solved assuming ESAL values



1 AA HT tructura



Date goes here

1 AA HT tructuraDesign

Step-by-Step



Step 1: Traffic Calculation

• Total ESALs

– Buses + Trucks

– 2.13 million + 1.33 million = 3.46

million



Step 2: Get MR Value

• CBR tests along Kailua Road show:

– CBR ≈ 8

• MRconversion

( ) ( ) psiCBRM R 000,12815001500 ===

( ) ( ) psiCBRM R 669,982555255564.064.0===

AASHTO Conversion

NCHRP 1-37A Conversion



Step 3: Choose Reliability

• Arterial Road

– AASHTO Recommendations

FunctionalClassification

Recommended Reliability

Urban Rural

Interstate/freeways 85 – 99.9 85 – 99.9

Principal arterials 80 – 99 75 – 95

Collectors 80 – 95 75 – 95

Local 50 – 80 50 – 80

WSDOT

95

85

75

75

Choose 85%



Step 3: Choose Reliability

Reliability ZR

99.9 -3.090

99 -2.327

95 -1.645

90 -1.282

85 -1.037

80 -0.841

75 -0.674

Choose S0 = 0.50



Step 4: Choose ΔPSI

• Somewhat arbitrary

– Typical p0= 4.5

– Typical pt= 1.5 to 3.0

– Typical ΔPSI = 3.0 down to 1.5



Step 5: Calculate Design

• Decide on basic structure

• Note: AASHTO doesn’t differentiate betweentypes of HMA and base but many agencies do– Differentiation may not based on any testing

Resilient Modulus (psi)

Layer a Typical Chosen

HMA 0.44 500,000 at 70°F 500,000

ACB 0.44 500,000 at 70°F 500,000

UTB 0.13 20,000 to 30,000 25,000

Aggregate 0.13 20,000 to 30,000 25,000




• Solve equation for 2 layers– HMA and ACB is one layer

– UTB and aggregate is the other

• Solve for each layer using the MR of

the layer directly underneath

• Divide up HMA and ACB

• Divide up UTB and aggregate




• Preliminary Results

– Total Required SN = 3.995

– HMA/ACB

• Required SN = 2.74

• Required depth = 6.5 inches

– UTB and aggregate

• Required SN = 1.13• Required depth = 9 inches




• Apply HDOT rules and common sense

– HMA/ACB

•Required depth = 6.5 inches

•2.5 inches Mix IV (½ inch Superpave)•4 inches ACB (¾ inch Superpave)

– UTB and aggregate

•Required depth = 9 inches

•Minimum depths = 6 inches each– 6 inches UTB

– 6 inches aggregate subbase



Comparison

Layer California AASHTO

HMA Surface 2.5 inches 2.5 inches

ACB 7.0 inches 4.0 inches

UTB 6.0 inches 6.0 inches

Aggregatesubbase

6.0 inches 6.0 inches

Diseño de pavimeno Flexible AASHTO 93

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