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
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AASHO Road Test (1)
1958 - 1961
AASHO Road Test
Picture from: Highway Research Board Special Report 61A-G
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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
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Test Loops (1)
Picture from: Highway Research Board Special Report 61A-G
AASHO Road Test
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Test Loops (2)
AASHO Road Test
Picture from: Highway Research Board Special Report 61A-G
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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
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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
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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
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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
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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
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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
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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
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Trucks
AASHO Road Test
Picture from: Highway Research Board
Special Report 61A-G
AASHO R d T t
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Subgrade Support Variation
AASHO Road Test
Picturefrom
:HighwayRese
arch
BoardSpe
cialRep
ort61A-G
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Test Tracks Today
NCAT Test Track
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AASHO Road Test
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Present Serviceability Rating (PSR)
AASHO Road Test
Picture from: Highway Research Board Special Report 61A-G
AASHO Road Test
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Present Serviceability Rating (PSR)
AASHO Road Test
Picture from: Highway Research Board Special Report 61A-G
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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
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Basic Idea
Time
Ser
vic
eability
(P
SI) p0
pt
p0 - pt
Basic Equations
Basic Equations
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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
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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
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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.
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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).
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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
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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).
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Structural Number
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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
=> ∫ ∫
∞∞
∞−
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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.
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Solving the Equation
• Iterative process
– Both ESAL and structural equationhave SN
• Often solved assuming ESAL values
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1 AA HT tructura
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1 AA HT tructuraDesign
Step-by-Step
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Step 1: Traffic Calculation
• Total ESALs
– Buses + Trucks
– 2.13 million + 1.33 million = 3.46
million
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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
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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%
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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
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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
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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
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Step 5: Calculate Design
• 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
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Step 5: Calculate Design
• 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
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Step 5: Calculate Design
• 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