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Conventional Flexible Pavement Structural
Design Methods
By:yCurtis F. Berthelot Ph.D., P.Eng.Department of Civil Engineering
Pavement Structural Design
Objective of road structural design is to optimize the structural composition of the road structure.
M b li d t May be applied to:• New construction.• Rehabilitation construction.
Conventional Flexible Pavement Structural Design2
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Pavement Structural Design
Structural design needs to consider:• Insitu subgrade material• Loadings:• Repeat axles over life• Critical state loadings
• Road structure material constitutive properties/climatic durability.
• Material layer interface compatibility.
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• Geometrics (grade width and height).• Constructability.• Future expansion requirements.
Pavement Structural Design
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Pavement Structural Design
Primed Surface
150 mm Granular Subbase150 mm Granular Drainage Sand
85 mm Hot Mix Asphalt Concrete
170 mm Granular Base
in situ Subgrade(Trimmed and Proof Rolled for Soft Spots)
350 mm Cement-Emulsion Strengthened in situ GranularPrimed Surface
85 mm Hot Mix Asphalt Concrete
in situ Subgrade(Trimmed and Proof Rolled for Soft Spots)
Primed Surface
85 mm Hot Mix Asphalt Concrete
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150 mm Asphalt Stabilized Base Coarse
150 mm Granular Base350 mm Full Depth Cement Strengthened Subgrade
Primed Surface
in situ Subgrade(Trimmed and Proof Rolled for Soft Spots)
Pavement Structural Design
Two primary classes of road users:• Private (cars and light trucks)• Commercial (heavy trucks)
Pavement structural design ensures adequate structural integrity to accommodate commercial vehicle loading.
Primary factors influencing road structural performance over the life of the road asset are:• Commercial truck loadings
Cli i ff
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• Climatic effects• Combined effects of both
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n I
nd
ices
InitialNew Road
Changing Field State Conditions (Saskatchewan)
icea
bil
ity
or C
ond
itio
n
Road Strengthening
Treatment
Poorly FundedStop-Gap
Ongoing Strategic Preservation
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Minimum Acceptable Serviceability
time
Ser
vi
Structural Deterioration Under Stop Gap Preservation
.....n-Year
Design Life
Identify Structural Distresses (Materials Structural Failure)
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Identify Structural Distresses (Materials Structural Failure)
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Changing Economy
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Changing Economy
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Provincial Highway System
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Standard Axles
2-Tire Steering Axle(5,500 kg)
4-Tire Single Axle (9,100 kg)
12-Tire Tridem Axle Group(23,000 kg)
8-Tire Tandem Axle Group(17,000 kg)
2 m
to
.5 m
2.3m
m to
7
m
0.20 m 0.45 m
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1.2 1
2.4 m to 2.6 m2.
4 m
3.7
Non-Standard Axles
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Based on the phenomenological rutting and fatigue cracking observations of asphalt pavements at the AASHO R d T h l f i bili
Traffic Load Equivalencies ESAL
AASHO Road Test, the loss of pavement serviceability resulting from the passage of any axle at a specified weight is equated to the loss in serviceability due to and Equivalent Single Axle Load (ESAL) of 80 kN.
The “fourth power law” is used to empirically equate axles at various loadings to the 80 KN single axle.
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The total number of “Design ESALs” for a given vehicle is the ratio of the number of 18-kip single axle l d li i i d li h d
Traffic Load Equivalencies ESAL
load applications required to replicate the damage inflicted to the pavement by the vehicle.
spacings) axle and loads axle (vehiclepavement fail to passes of #
axle) kip-(18pavement fail to passes of #ESAL
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4P
KΔPSI
Traffic Load Equivalencies ESAL
80KNP
KΔPSI
Where:PSI = Change in the Present Serviceability
Index due to one axle loadP = Applied axle load (KN)P S d d i l l l d (80 KN)
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P80KN = Standard single axle load (80 KN)K = Regression constant
Typical ESALs
15400 k
Gross Weight36200 kgs356 KN
80.000 LbsTruck Factor
USA
15400 kgs151 kN
34.000 Lbs1.11
15400 kgs151 kN
34.000 Lbs1.11
5440 kgs54 kN
12000 Lbs0.23
+2.45
5500 kgs54 kN
12125 Lbs0.24
17000 kgs167 kN
37500 Lbs1.67
17000kgs167 kN
37500 Lbs1.67
Gross Weight39500 kgs
428 kN87100 Lbs
Truck Factor3.58+
Canada
Gross Weight
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5500 kgs54 kN
12125 Lbs0.24
17000 kgs167 kN
37500 Lbs1.67
17000 kgs167 kN
37500 Lbs1.67
23000 kgs226 kN
50700 Lbs1.32
Gross Weight62500 kgs
614 kN137800 Lbs
Truck Factor4.90+ +
Note: Pt = 2.5; SN = 3
Canada Canada
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Empirically calibrated from the AASHO Road Test:
• Determined a universal load equivalency pavement
Traffic Load Equivalencies ESALs
damage relationship.
• Found that pavement deterioration in terms of damage occurring from an 80-KN axle load appeared to have a fourth power relationship to the actual axle load applied to the pavement.
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Conventional Flexible Pavement Structural Design19
4
80
(KN)LoadAxleDamage Pavement Relative
KN
Given that an 80 KN axle load is assumed to have a pavement damage equivalency of 1.0, by the fourth
Traffic Load Equivalencies ESAL
power pavement relationship, a 160 KN single axle load (twice that of an 80 KN axle load) inflicts 16 times the damage of that of an 80 KN axle load.
Likewise, a 40 KN axle load inflicts only 0.0625 times the damage of an 80 KN axle load.
The fourth pavement damage relationship does not
Conventional Flexible Pavement Structural Design20
p g papply to all pavement or field state conditions.
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Structural Design Methods
Across Canada, transportation agencies have developed standard pavement thickness equivalencies based on years of experienceyears of experience.
There are four primary methods to perform structural pavement design:• Empirical based layer equivalents• Nomograghs• Shell curves (SDHT)• Asphalt Institute curves
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• Asphalt Institute curves• Surface deflection methods• Multilayer mechanistic theory methods
Empirical Structure Layer Thickness Equivalencies
B.C. 1 mm Asphalt Conc.- 2 mm gravel base (mm)- 25 mm sandy gravel subbase
Alberta 1 mm Asphalt Conc.- 2.25 mm crushed gravelp g- 1.75 mm soil cement- 1.25 mm asphalt treated gravel
Saskatchewan Considered as a variable and therefore not used
Manitoba 1 mm Asphalt Conc.- 2 mm gravel base- 1.5 mm sand asphalt or soil cement- 2 mm lime treated clay
Ontario 1 mm Asphalt Conc.- 1 mm treated base( h l )
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(asphalt or cement)- 2 mm granular A base- 3 mm granular (B. C. D) subbase
1 mm Full Depth AC- 2.7 mm granular A base (tentative)
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Quebec 1 mm Asphalt Conc.- 2 mm crushed rock base- 2.5 mm gravel base or subbase- 5 mm sand subbase
Empirical Structure Layer Thickness Equivalencies
- 1.25 mm soil cement(150 mm thick or less)- 2 mm soil cement (more than 150 mm)- 33 mm lime stabilized clay- 18 mm asphalt stabilized base
Newfoundland 1 mm Asphalt Conc.- 25 mm graded crushed rock- 2.5 mm graded crushed gravel- 2 mm soil cement stabilized- 3 mm gravel subbase
4 mm sandy gravel
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- 4 mm sandy gravelNew Brunswick 1 mm Asphalt Conc.- 2 mm crushed rock
- 2 mm soil cement (150 mm thickness and over)- 2 mm (or less) asphalt stabilized base- 3 mm gravel subbase
AASHTO Flexible Pavement Design
Function of any road is to safely and smoothly carry vehicular traffic from one point to anotherp
• Serviceability: Ability of a pavement to serve the traffic for which it is designed
• Performance: Ability of the pavement to satisfactorily serve traffic over a period of time
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AASHTO Flexible Pavement Design
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Five basic steps to SMHI pavement structure thickness design of new roads:• D t i i d d i i t i f ti
SMHI Pavement Structural Design
• Determine required design input information:• traffic projections.• in situ subgrade performance properties.
• Determine pavement layer thicknesses;• Prepare construction plan (may be staged);• Perform economic evaluation (materials quantities
d h l di ) d
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and haul distances), and;• Iterate to final structural design solution.
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Theoretical SMHI structural thickness design procedure.
SMHI Pavement Structural Design
• Calculate design life ESALs.
• Determine in situ subgrade CBR.
• Balance fatigue and rutting performance in design.
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Developed in late 1920’s.
Used by highway departments for evaluation of road
SMHI Pavement Structural Design
soils.
Estimate of bearing ratio of soils determines amount of load that soil can carry.
Saskatchewan Highways uses correlation of California bearing ratio to AASHTO Group Index.
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SMHI Pavement Structural Design
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Conventional SMHI pavement structure thickness design attempts to satisfy two traffic load related
i i
SMHI Pavement Structural Design
criteria:
• Vertical compressive strain at the top of the subgrade (εvc= rutting).
• Horizontal tensile strain at the bottom of the asphalt concrete layer (εt= fatigue cracking).
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Correlating fatigue cracking to the limiting tensile elastic stress and strain at the bottom of the asphalt
l i li i d i h i h ll
SMHI Pavement Structural Design
concrete layer is limited in the assumption that all factors that influence fatigue cracking must be linearly related to the stresses and strains at the bottom of the asphalt layer.
Similarly, rutting to the limiting vertical compressive elastic stress and strain at the top of the subgrade is li i d i h i h ll f h i fl
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limited in the assumption that all factors that influence rutting must be linearly related to the stresses and strains at the top of the subgrade.
AASHTO relationships used to correlate fatigue cracking of asphalt concrete to limiting elastic tensile
i d/ h b f h h l
SMHI Pavement Structural Design
strain and/or stress at the bottom of the asphalt concrete layer may be expressed in powerlaw form as:
2
1f
tf fN
2
1f
tf fN
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Phenomenological-EmpiricalSubgrade Strain Rutting Model
Mechanistic Rutting Model
SMHI Pavement Structural Design
vc
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Statistical Correlations of Vertical Compressive Strain at Top of Subgrade
Nd=f4(vc)-f5
Viscoelastic Continuum Behavior of All Road Structure Layers
Ref: Berthelot, C.F. PhD. Dissertation
The asphalt Institute incorporated asphalt concrete stiffness into the traditional AASHTO power law fatigue
l i
SMHI Pavement Structural Design
correlations as:32 )()(1ff
tf EfN 32 )()(1ff
tf EfN
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Mechanistic Fatigue Cracking Model
Phenomenological - EmpiricalTensile Flexure Strain Model
SMHI Pavement Structural Design
MicrocrackZones
TT
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Micromechanical Microvoid Model of Mode I, II, and III
Fatigue Crack Growth
Statistical Correlation of Fatigueto Failure Tensile Stress/Strain at
Bottom of Asphalt Concrete Layer32 -f
t-f
1f )((E)fN Ref: Berthelot, C.F. PhD. Dissertation
SMHI Pavement Structural Design
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Traffic = ConstantS bgrade S pport Constant
Traffic = ConstantSubgrade Support Constant
SMHI Pavement Structural Design
Subgrade Support = Constant Subgrade Support = Constant
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Granular Layer Thickness Granular Layer Thickness
Thickness Curve for Constant VerticalCompressive Strain on Subgrade
Thickness Curve for Constant TensileStrain at Bottom of Asphalt Layer
Traffic = Constant
SMHI Pavement Structural Design
hal
t C
oncr
ete
Th
ickn
ess
Subgrade Support = Constant
Subgrade Strain Criterion
Asphalt Strain Criterion
Design Target
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Asp
h
Granular Layer Thickness
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SMHI uses a series of thickness design curves that relate asphalt concrete and granular thickness to traffic l di
SMHI Pavement Structural Design
loading.
The performance related properties of the subgrade significantly affect thinner pavement thickness design because the subgrade is the foundation to all other layers of the structure.
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SMHI Pavement Structural Design
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Each design chart:
• Based on a subgrade design California Bearing Ratio
SMHI Pavement Structural Design
(CBR).
• Displays the asphalt thickness on the vertical axis and total granular thickness on the horizontal axis.
• Contains a series of curves representing various levels of design traffic loadings in terms of the number of 80kN axle passes expected in the design
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p p glane during the life of the structure.
Each curve:
• Represents an infinite number of combinations of
SMHI Pavement Structural Design
the layer thicknesses which will meet the criteria of limiting the horizontal tensile and vertical compressive strains in the asphalt .
• It is divided into zones (usually three) of CBR 20, CBR 40, and CBR 80 granular material. Each of these zones represents the minimum CBR value for
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the material to be placed in that particular level of the structure.
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Given that stresses resulting from an applied load disseminates downward from the pavement surface, the
l i l i h h hi h CBR l h ld b
SMHI Pavement Structural Design
granular material with the highest CBR value should be placed closest to the surface of the road.
In cases where high quality aggregate is scarce, full depth strengthened or full depth asphaltic concrete systems may be used.
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Example: Given the design chart CBR 8.0, determine three equivalent pavement structures that can be
l d f h hi k d i h if h d i
SMHI Pavement Structural Design
selected from the thickness design chart if the design traffic is 7.3 x 104 ESALs.
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SMHI Pavement Structural Design Example
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Solution: Table below summarizes the results of the alternate thickness design as per the SDHT method.
SMHI Pavement Structural Design Example
Material Alternate 1 Alternate 2 Alternate 3Asphalt Concrete 150 50 0Top Base Coarse (CBR 80) 0 50 100Bottom Base Coarse (CBR 40) 0 100 100
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Subbase (CBR 20) 0 100 100Total Thickness 150 300 300
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SMHI Overlay Thickness Design Method
All transportation agencies have developed standard overlay thickness design procedures based on two
i icriterion:
• Empirical performance experience
• Construction experience
• Deflection measurements
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SMHI Overlay Thickness Design Method
The SDHT pavement structure design charts can also be used to determine overlay thickness design.
Most overlays in Saskatchewan are designed to accommodate future traffic expected over the next 15 years (N15).
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SMHI Overlay Thickness Design Method
The overlay design procedure is based on four primary steps:• Determine in situ CBR.• Determine the maximum required HMAC overlay
thickness for worst case scenario (i.e. existing HMAC pavement no longer acceptable);
• Determine minimum required HMAC overlay thickness for best case scenario (i.e. HMAC pavement in good condition and structurally sound
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pavement in good condition and structurally sound.• Determine the actual HMAC overlay thickness
required based on survey of existing pavement condition.
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Existing AC = 210 mmExisting granular = 450 mm
Minimum AC = 0 mmMaximum AC = 80 mm Rutting
= 150 mm Fatigue
1.5 x 107
AC = 200 mm
AC = 150 mm
AC = 80 mm
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660 mm Total Granular450 mm
AC 80 mm
SMHI Overlay Thickness Design Method
Backcalculate in situ CBR:
• Determine the in situ CBR at the point in time historic traffic NH applied to the road up until failure is the actual design to failure traffic volume.
• Since we know the existing granular thickness and the existing asphalt thickness, the exact intersection of these three criterion may be determined from the CBR design charts.
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• The CBR design chart that best suits the specified intercept of the above criterion is the in situ CBR of the subgrade.
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SMHI Overlay Thickness Design Method
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SMHI Overlay Thickness Design Method
To determine the maximum HMAC overlay thickness to accommodate future traffic:
• Use the actual in situ CBR chart to determine the overlay thickness.
• Assume the existing HMAC pavement is granular material (i.e. failed).
• The maximum required HMAC overlay thickness corresponding to the projected future traffic volume
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p g p jN15 can be determined on the actual in situ CBR chart.
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SMHI Overlay Thickness Design Method
Minimum overlay thickness is designed with respect to the total traffic, historic and future:
NT = NH + N15
• Where: NT = Total trafficNH = Historic trafficN15 = 15 year design traffic
Gi n th ri in l CBR d i n h rt th t t l d i n
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Given the original CBR design chart, the total design traffic and existing granular thickness, the minimum required AC thickness may be determined from the original CBR design chart.
SMHI Overlay Thickness Design Method
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Mechanistic Structural Thickness Design
The magnitude of pavement deflection is an indicator of the pavement’s ability to withstand traffic loading. R b d d fl i d d d di dRebound deflections measured under a standardized loading may be used to evaluate structural adequacy of pavements and determine the overlay thickness required.
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Mechanistic Structural Thickness Design
Pavement reflections/deflections:
• Should be taken in areas of non-failed pavement area representative of pavement structure.
• Are measured with the Benkleman Beam using a reflection test.
• Deflection bowls measured with falling weight deflectometer.
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Benkelman Beam Overlay Plot
Mechanistic Structural Thickness Design
More to come in mechanistic design section.
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DiscussionBe competent. If you have heard of the Peter Principle, understand that we do not practice it here. We depend on you knowing your job. p y g y jBravado won’t make up for a lack of competence. If you need additional training, ask. A willingness to learn and improve your skill level is a sure sign of growth potential. The more competent you become, the more we can rely on you to help us meet our goals as an organization
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meet our goals as an organization.