Pavement Design Principlesicivil-hu.com/Nedal/Pavement Design Principles for T3.… · ·...
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Pavement Design Principles Andrew Johnson, Ph.D., P.E. State Pavement Design Engineer Office of Materials and Research
Transcript of Pavement Design Principlesicivil-hu.com/Nedal/Pavement Design Principles for T3.… · ·...
Pavement Design Principles
Andrew Johnson, Ph.D., P.E.
State Pavement Design Engineer
Office of Materials and Research
Objectives
Historical perspective
Basic asphalt design inputsTraffic characterization
Subgrade characterization
Objectives
Design methodolgyAASHTO Design Equation
Conversion to actual pavement design
Sensitivity
Mechanistic-Empirical Design Guide
Objectives
Distress identificationFunctional versus structural
Nondestructive testingFalling Weight Deflectometer
What is the purpose ofpavement?Protect the subgrade from excessive
permanent deformationResist loss of structural capacity from
fatigue produced by repeated traffic loads
Provide adequate serviceability to users, without repair, for a given period of time
What is the goal of pavementdesign?
Provide the most cost-effective structure while optimizing the level of service provided to road users.
Roman Roads
Roman Empire built over 3000 miles of roads in Britain alone by 200 A.D.
These roads had ditches to aid in drainage and their thickness varied over weaker soils.
This indicates that the Romans had some understanding of basic soil mechanics
First Asphalt Pavement
Paris – 1854
The material was natural rock asphalt, i.e., limestone rock impregnated with asphalt.
The material provided a quiet, easily cleaned surfacing, but the skid resistance was very low in wet weather.
First Asphalt Pavement
The first asphalt concrete specifications appeared in the US in the 1890’s.
Concurrently, coal tar/aggregate mixtures were being used in Europe.
The first hot-mix asphalt plants were developed in the late 1920’s
Early Construction
First modern asphalt paver introduced mid-1950’s Prior to 1950’s, asphalt was placed by
form-riding finishers similar to PCC
Pavement Design
Two theoretical pavement typesRigid Pavement – Resists traffic loading by
resistance to bending. Concrete pavement is primarily rigid.
Pavement Design
Two theoretical pavement typesFlexible Pavement – Resists traffic loading
through “internal friction”. An unpaved road with a compacted crushed stone course is a pure flexible pavement.
Old-fashioned asphalt pavement (thin asphalt layer over a granular base) is mostly a flexible pavement.
Pavement Design
Modern asphalt pavements are difficult to analyze mathematically.Two key factors:
Vertical compressive strain in subgrade
Horizontal tensile strain at the bottom of the asphalt.
Pavement Design
Modern asphalt pavements are difficult to analyze mathematically.
Road Test Era, 1909-1961
Bates Road Test (1922-23) In 1920, Illinois passed a $200 million bond
issue to build 9000 miles of paved roads. To determine the best paving material,
they built sections of brick, asphalt, and concrete.
Developed first thickness design procedures and chose concrete for the Illinois pavements. Old WWI Army trucks with 18,000 pound wheel
loads were used to load the pavement.
Road Test Era, 1909-1961
AASHO Road TestDetermine relationship between traffic and
performanceDetermine effect of loads on bridgesPerform special studies (base types, paved
shoulders, tire pressures)Develop instrumentation, test procedures,
data, charts, graphs, and formulas for future highway design
AASHO Road Test Layout
OTTAWA
Loop 1Loop 2
Loop 3 Loop 4Loop 5Loop 6
23
178
71
N
AASHO Road Test
Started Nov. 1958, Ended Nov. 1960Loops 3-6:
6 vehicles/lane
10 vehicles/lane (Jan ‘60)
AASHO Road Test
Operation 18 hours, 40 minutes per day
6 days/week
1,114,000 Applications
Avg. ESAL - 6.2 million
Max ESAL - 10 million (Flex)
AASHTO Design Guides
AASHO Road Test, 1958-1960
AASHO Interim Guides, 1961 & 1962
Revised Interim Guide, 1972
Revised Chapter III (Rigid), 1981
AASHTO Guide for the Design of Pavement Structures, 1986
AASHTO Design Guides
AASHTO Guide for the Design of Pavement Structures (Overlays), 1993
Supplement to the AASHTO Guide for the Design of Pavement Structures (Rigid Pavement Design), 1998
Mechanistic-Empirical Guide for Design of Pavement Structures (work in progress), 2002
What does SCDOT use?
1972 Interim Guide USC performed a study in the late 1980’s to
evaluate the 1986 Guide.
SCDOT decided that resilient modulus was not practical for routine analysis.
Statistical reliability concepts led to designs that contradicted previous practice.
SCDOT does use a modified form of the 1993 Guide procedures for NDT of existing pavement.
Design Inputs
TrafficHow do we handle vehicles of different size
and weight? Fourth power relationship
Double the weight on an axle, and the damage increases by 2x2x2x2 = 16-fold.
Design Inputs
TrafficHow do we handle vehicles of different size
and weight? Equivalent Single Axle Loads
Damage from various axle loads and configurations are equated to the same damage from a single 18,000 pound axle.
1 ESAL = 1 axle load of 18,000 pounds
ESALs are different for concrete and asphalt because each pavement type responds differently to increasing loads
Design Inputs
TrafficHow do we handle vehicles of different size
and weight? SCHD Load Study (1966-1970)
Looked at average ESALs per truck since not all trucks are loaded to the maximum.
Load study has been updated using weigh-in-motion data over the years, but the changes have not been major.
Design Inputs
TrafficHow do we handle vehicles of different size
and weight? Road Groups
Represent typical truck load combinations for different conditions.
See page 5 of Pavement Design Guidelines
Road Group F = 0.3774 ESALs per truck
Road Group O = 0.9027 ESALs per truck
Fully loaded five-axle tractor-trailer = 2.37 ESALs
Design Inputs
Subgrade CharacterizationAASHO Road Test - Ottawa, IL
Subgrade: silty-clay (A-6), CBR = 2-4
Soil Support Value Triaxial
R-value
CBR
Resilient Modulus
Classification
Design Inputs
Subgrade CharacterizationPractical considerations
Soil may vary considerably within a project
Borrow from outside the project limits can vary
Project size increases variability
Undercut remains subjective
Design Inputs
Subgrade Characterization SCDOT method
Take borings at 500 foot intervals Visually classify each layer Perform AASHTO Soil Classification on each layer in
each boring Determine the predominant soil classifications Assign a SSV to each layer based on previous tests Perform several CBR tests (typically one per centerline
mile) to verify the previous work Determine SSV based on value exceeded by 85% of soil Make adjustments based on potential variability Assign SSV for entire project
Design Inputs
Subgrade Characterization Problems for non-SCDOT designers
Historical soil information not available.
SCDOT hesitant to release information for liability reasons.
Do not use SSV relationships from other states with SCDOT inputs.
Strongly recommend using AASHTO soil classification.
May want to increase CBR testing to two per mile.
Can call Mike Lockman at 737-6692 to check the reasonableness of SSV figures.
Bottom line – engineering judgment is critical.
Design Inputs
Serviceability What constitutes failure?
AASHTO used road users to define Present Serviceability Index (PSI) 0 = impassable
5 = total perfection
4.2 = typical new road
Found PSI correlates to roughness
Design Inputs
Serviceability What does SCDOT use for design PSI?
2.5 for interstate and limited access
2.0 for all others
AASHTO 1972 DesignEquation – Page 6
)0.3(372.01
log
1
10944.0
5.12.42.4
log2.0)1log(36.9)log(
19.5
SSVR
SN
P
SNESAL
t
ESAL = Equivalent Single Axle LoadSN = Structural NumberPt = Terminal ServiceabilityR = Regional FactorSSV = Soil Support Value
Structural Number (SN)
How to equate pavements made of different materials?Coefficient of Relative Strength (a)
Have no direct physical meaning Derived from statistical correlation
Structural Number = a x thicknessTwo pavements with equivalent SN
theoretically have the same load-carrying capability
Coefficients of Relative Strength – Page 10
Hot Laid AC Surface – 0.44 Hot Laid AC Binder – 0.44
First 400 psy only, a=0.34 beyond 400 psy
Sand-Clay Base – 0.12 to 0.20 Coquina Base – 0.12 Graded Aggregate Base – 0.18 Hot Laid Asphalt Aggregate Base – 0.34 Cement Stabilized Aggregate Base – 0.34 Old PCC Pavement – 0.40 Earth-Type Subbase – 0.08 Cement Modified Subbase – 0.15
Derivation of Coefficients Dr. Busching at Clemson University did
research project to determine these coefficients in the late 1960’s.
Had AASHO Road Test material shipped to Clemson.
Built many test pavements with both AASHO and SC material.
Compared deflections of AASHO pavements with SC pavements
Assumed equal deflections = equal SN
Example SN calculation200 psy Surface
200 psy Binder
8 inches GAB
Assume 105 psy/inch
200/105 = 1.90”
200/105 = 1.90”
8.00”
* 0.44 = 0.84
* 0.44 = 0.84
* 0.18 = 1.44_____
SN= 3.12
Pavement Design Example
Traffic = 15,000 ADT for 10 years
Future travel lanes = 4
Percent trucks = 5
Industrial area, Road Group M
Soil Support Value = 2.0
Not limited access, PT = 2.0
What is the recommended pavement?
Pavement Design Example
Step 1 – Convert traffic to ESALs 15,000 ADT 2-way = 7500 ADT 1-way
For 4-lanes, critical lane factor = 0.8
Critical Lane ADT = 0.8 * 7500 = 6000
Critical Lane Trucks = 6000 * 5% = 300
Road Group M = .7713 ESALs/truck
Daily ESALs = 300 * .7713 = 231.4
Design ESALs = 231.4 * 365 * 10 = 844,610
Design Pavement of SN=4.13
From our previous example:
200 psy Surface
200 psy Binder
8 inches GAB
Assume 105 psy/inch
200/105 = 1.90”
200/105 = 1.90”
8.00”
* 0.44 = 0.84
* 0.44 = 0.84
* 0.18 = 1.44_____
SN= 3.12
3.12 – 4.13 = -1.01 = SN deficit
Design Pavement of SN=4.13 To complete design with AAB,
1.01 / 0.34 = 2.97 inches * 105 psy/in = 312 psy Use 350 psy
200 psy Surface
200 psy Binder
350 psy AAB
8 inches GAB
200/105 = 1.90”
200/105 = 1.90”
350/105 = 3.33”
8.00”
* 0.44 = 0.84
* 0.44 = 0.84
* 0.34 = 1.13
* 0.18 = 1.44_____
SN= 4.25
Design can be abused…
Bituminous Surfacing (Triple)
5 Feet Earth Type Subbase
0.50”
60.00”
* 0.35 = 0.18
* 0.08 = 4.80_____
SN= 4.98
Design Sensitivity What happens if the thickness is below the
requirement? Suppose that a typo causes 300 psy AAB to be
specified instead of 350 psy.
200 psy Surface
200 psy Binder
300 psy AAB
8 inches GAB
200/105 = 1.90”
200/105 = 1.90”
300/105 = 2.86”
8.00”
* 0.44 = 0.84
* 0.44 = 0.84
* 0.34 = 0.97
* 0.18 = 1.44_____
SN= 4.09
Use AASHTO nomograph to recalculate SN
4.09 = 799,000 ESALs
4.13 = 856,000 ESALs
4.25 = 1,048,000 ESALs
Design Sensitivity What happens if the thickness is below the
requirement? Suppose that a typo causes 350 psy AAB to be
specified instead of 450 psy.
799,000/856,000 = 93% of design ESALs799,000/1,048,000 = 76% of specified ESALs
Design Reliability
When does a 10-year pavement design fail?Target is 90% reliability (only 1 in 10
designs last 10 years or less)Average life is closer to 13-15 yearsVariables
Traffic Material Construction Practice Subgrade Climate
Practical Considerations
Current specifications require granular base courses to placed in no more than 8 inch lifts. Will be increased to 10 inches soon.
Total granular base thickness should not exceed 12 inches.
Cement Modified Subbase should be either 6” or 8” thick.
Practical Considerations
For surface courses, thickness range is 150 to 200 psy. Should only use one course of surface.
For intermediate (binder) courses, thickness range is 200 to 300 psy.
For Asphalt Aggregate Base courses, thickness range is 300 to 450 psy.
Practical Considerations
For construction traffic, the perpendicular drop-off between lanes must not exceed 1”. With a slope, the drop-off must not exceed 2”.
Costs can vary depending on the project setting.Designs with the minimum number of
operations may be cheaper in urban settings.
Rehabilitation
“You can observe a lot by watching”
-Yogi Berra
Distress Identification
To describe pavement distress, three pieces of information are requiredDistress type
Severity level
Extent
Distress Identification
Pavement distress falls into two broad categories Functional distress – Distress that causes a loss of
serviceability but does not affect the structural capacity of the pavement
Examples: Rutting
Block cracking
Faulting
Skid resistance
Distress Identification
Pavement distress falls into two broad categoriesStructural distress – Distress that is caused
by the loss of the structural capacity of the pavement structure
Examples: Fatigue/Alligator Cracking
Distress Identification
Manual of Distress Identification was developed for SHRP LTPP. Is available online at:
www.tfhrc.gov/pavement/ltpp/reports/03031/
Fatigue Cracking
Generally a sign of insufficient pavement strength for the traffic and subgrade conditions.
Often associated with localized poor drainage.
In the terminal condition, will start to rut and deform the subgrade.
Block cracking
Generally an age-related functional distress.
Is a “top-down” process.Does not indicate deeper pavement
problems.Prefer to mill the existing pavement
prior to overlay to prevent the cracks from reflecting.
Block cracking
Can be distinguished from fatigue cracking by the shape of the crack pattern.
Often occurs along with fatigue cracking.
Longitudinal cracking
Can be a function of using a stiff asphalt mixture.
Hard to distinguish from low-severity fatigue cracking.
In non-fatigue situations, is typically a top-down type of crack. These cracks appear on either side of the wheelpath.
Reflection cracking at PCCJoints
Most pre-1950’s primary routes were paved with PCC.
Many urban streets were also PCC.
Some degree of reflection cracking is difficult to inhibit.
If the pavement is not pumping, may not be a serious concern.
Reflection cracking at PCCJoints
High severity reflection cracking
Rutting
Two types of ruttingDeep rutting – Results from permanent
deformation of the subgrade due to excessive stress. This type of rutting is a structural distress.
Shallow rutting – Due to lack of stability in the asphalt layers. This type of rutting is a functional distress.
Raveling
Loss of aggregate from the surface.
Can be caused by a “dry” mix.
Overlay Analysis
Assume that the pavement loses structural capacity as it is exposed to traffic and weather.
At a given point in time, the pavement has a SN that is less than when it was new.
Two methods for determining in situ SN.
Overlay Analysis
Nondestructive testingFalling Weight Deflectometer
Overlay Analysis
Nondestructive testingFalling Weight Deflectometer
Overlay Analysis
Coefficient DepreciationAssume that the original structural
coefficients are reduced. Surface and Binder – 60% (0.26 vs. 0.44)
Base – 70%
Subbase – 80%
Depth of depreciation dependent on pavement condition.
Overlay Design
200 psy Surface
200 psy Binder
450 psy AAB
8 inches GAB
200/105 = 1.90”
200/105 = 1.90”
350/105 = 3.33”
8.00”
* 0.44 = 0.84
* 0.44 = 0.84
* 0.34 = 1.13
* 0.18 = 1.44_____
SN= 4.25
Coefficient Depreciation ExampleUse the previous example pavement:
Overlay Design
Coefficient Depreciation ExampleVisual inspection indicates the pavement
has: Widespread low severity raveling
Widespread low severity fatigue cracking
Isolated high severity fatigue cracking
No rutting
Overlay Design
Coefficient Depreciation ExamplePavement design indicates a 5.0 SN is
necessary for the next 10-year period.
Overlay Design
Coefficient Depreciation ExampleCracking probably extends fully through
the asphalt. Assume the surface/binder/AAB are
depreciated.
No rutting, so assume GAB is intact. No depreciation on base.
Overlay Design
200 psy Surface
200 psy Binder
350 psy AAB
8 inches GAB
200/105 = 1.90”
200/105 = 1.90”
350/105 = 3.33”
8.00”
* 0.26 = 0.49
* 0.26 = 0.49
* 0.21 = 0.70
* 0.18 = 1.44_____
SN= 3.12
Coefficient Depreciation Example
Overlay Design
Coefficient Depreciation Example 3.12 - 5.0 = -1.88
Mill 2” to remove raveled material and replace 1.9 inches of 0.26 material with 0.34 material (0.34 - 0.26) * 1.9” = 0.15
SN = 3.12 + 0.15 – 5.0 = -1.73
Overlay with 200 psy surface and 200 psy binder = (400 psy/105 * 0.44) = 1.68
