Illinois DOT From the AASHO Road Test To 20 Years of Mechanistic Pavement Experience ……and Counting David L. Lippert, PE Bureau of Materials and Physical Research Illi i D t t fT t ti Illinois Department of Transportation Transportation Research Board 87 th Annual Meeting 87 Annual Meeting January 13, 2008 Outline • AASHO Road Test History • Illinois Adoption of Mechanistic Illinois Adoption of Mechanistic • Current Updating Efforts 60 70 80 90 100 0 10 20 30 40 50 60 0 10 20 30 40 50 Probability of Failure, % 9-in CRCP without DC 9-in CRCP with DC 0 10 20 30 40 50 Cumulative ESALs, million “AASHO Road Test”
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Illinois DOTFrom the AASHO Road Test
To 20 Years of Mechanistic Pavement Experience
……and Counting
David L. Lippert, PEBureau of Materials and Physical Research
Illi i D t t f T t tiIllinois Department of Transportation
Transportation Research Board87th Annual Meeting87 Annual Meeting
January 13, 2008
Outline• AASHO Road Test History• Illinois Adoption of MechanisticIllinois Adoption of Mechanistic • Current Updating Efforts
• Collect static weight data from enforcement scalesscales.
• Load spectrum by axle/vehicle type. – Single.g– Tandem. – Triple.S f &• ESAL factor by FHWA vehicle class & road
type.– Class I – Interstate and multi lane– Class I – Interstate and multi lane.– Class II – Two lane over 2000 ADT.– Class III - 750 to 2000 ADT.– Class IV – Under 750 ADT.
• Summarize into PV, SU and MU groups.
PV SU and MUPV, SU and MUPassenger Vehicles (PV)
CarsSingle Unit (SU)
ClassCars
Light Trucks
Buses 4Class 2
Class 3
2 Axle 5
3 A l 6
Multiple Unit (MU)
3 Axle 6
4 Axle 74 Axle 7
Class 8 to 13
Traffic MapsTraffic MapsTotal ADT MU ADTTruck ADT
PV = Total ADT –Truck ADT SU = Truck ADT – MU ADT
Current ESAL Factors (Flex)Current ESAL Factors (Flex)Road Class
PV SU MUClass
Class I 0.0004 0.394 1.908
Class II 0.0004 0.372 1.554
Class III 0.0004 0.355 1.541C ass 0 000 0 355 5
Class IV 0.0004 0.350 1.523
Design Minimums: Interstate – 1,500 MU, 500 SU
N I 900 MU 300 SUNon Interstate – 900 MU, 300 SU
Rigid Nomographg g p
C SS
SOIL
AFF
IC
KN
E
TRA
THIC
T
Flexible Nomographg p
C
.N.
AFF
IC SOIL
S
TRA
4.75
AASHO AdvancesAASHO Advances
•Equivalent 18K Single Axle Loads (ESALs)
•Thickness Designs for both Flex & PCC
•“Equivalent” Pavements
•Cost Allocation•Cost Allocation
AASHO LimitationsAASHO Limitations
•One Set of Materials.
•Two Years of Weathering.
1 1 Milli A l•1.1 Million Axles.
•Totally Empirical – need to extrapolateTotally Empirical need to extrapolate to 100’s of millions of axles.
Structural Number ConceptStructural Number Concept444” = 1 76.444 = 1.76
thi k t f hi h l f ilitithick pavements for high volume facilities.• New materials very difficult to relate back to
d t t f l ffi i troad test for layer coefficient.• Modern facility traffic well beyond road test
t ffitraffic. • Valid procedure??
M h i ti D iMechanistic -
Mechanistic DesignMechanistic
“Concerning the Relationships Between Applied Forces and Material Responses.”Material Responses.
i iBasic Premise -
Low Deflections = Long LifeLow Deflections = Long Life
Mechanistic EngineeringMechanistic Engineering
IL-AAHSTO vs MechanisticIL AAHSTO vs. Mechanistic30
es
20
25
s, I
nche
10
15
Thic
knes
s
AASHTOMech
Perpetual HMA Design
5
10
HM
A T
00 20 40 60 80
Traffic ESAL'sTraffic, ESAL s
Illinois Mechanistic-Empirical D iDesign
• Research completed in 1987Research completed in 1987.• Load spectrum discussed – dismissed.
D i b d 18K ESAL’• Designs based upon 18K ESAL’s.• Results very complex. • Many designer inputs.• Policy decisions needed to simplifyPolicy decisions needed to simplify.
Fatigue Theory
High Strain = Short Lifeg
Low Strain = Long Life18K
Fatigue Life
Fatigue Cracking
Repeated Leads topBending
e ds oFatigue Cracking
Fatigue Cracking
Repeated Leads topBending
e ds oFatigue Cracking
Illinois Mechanistic Loop pPavement Model
18,000 Pounds
80 PSI
100Performance Calibration
405060708090
100
Probabilityof Failure, %
i i h
Load Model0
102030
0 10 20 30 40 50Cumulative ESALs, million
9-in CRCP without DC9-in CRCP with DC
Why Load Spectrum Not Used
• Data reliabilityData reliability.– Calibration.
Maintenance of equipment– Maintenance of equipment.• Limited data collection ability.
– Expense.– People – Head count limits.
• Data fit into performance calibration??• Department understanding of ESAL’s.p g
Inputs – Full-Depth AsphaltInputs Full Depth Asphalt• TrafficTraffic.• Soil Support (Eri).
L ti (t t / d l l ti )• Location (temperature/modulus relations).• Asphalt grade.• Mix air voids and gradation.• Crack initiation at bottom of HMACrack initiation at bottom of HMA.• Reliability.