conventional flexible pavement structural design methods design course... · 1 conventional...

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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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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.


• Geometrics (grade width and height).• Constructability.• Future expansion requirements.



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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



Primed Surface



150 mm Asphalt Stabilized Base Coarse

150 mm Granular Base350 mm Full Depth Cement Strengthened Subgrade

Primed Surface



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


• 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


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)


Changing Economy


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Changing Economy


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


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.


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


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


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)


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


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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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


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


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


• 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 )


(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


- 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


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


and haul distances), and;• Iterate to final structural design solution.

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Theoretical SMHI structural thickness design procedure.


• Calculate design life ESALs.

• Determine in situ subgrade CBR.

• Balance fatigue and rutting performance in design.


Developed in late 1920’s.

Used by highway departments for evaluation of road


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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Conventional SMHI pavement structure thickness design attempts to satisfy two traffic load related

i i


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


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


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


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


vc


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


correlations as:32 )()(1ff

tf EfN 32 )()(1ff

tf EfN


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Mechanistic Fatigue Cracking Model

Phenomenological - EmpiricalTensile Flexure Strain Model


MicrocrackZones

TT


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



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Traffic = ConstantS bgrade S pport Constant

Traffic = ConstantSubgrade Support Constant


Subgrade Support = Constant Subgrade Support = Constant


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


hal

t C

oncr

ete

Th

ickn

ess

Subgrade Support = Constant

Subgrade Strain Criterion

Asphalt Strain Criterion

Design Target


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


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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Each design chart:

• Based on a subgrade design California Bearing Ratio


(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


p p glane during the life of the structure.

Each curve:

• Represents an infinite number of combinations of


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


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


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.


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


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


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


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



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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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


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


660 mm Total Granular450 mm

AC 80 mm


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.


• 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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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


p g p jN15 can be determined on the actual in situ CBR chart.

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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


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.



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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.



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


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


meet our goals as an organization.

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