Current Approach to Pavement Design

8/13/2019 Current Approach to Pavement Design

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Current Approach to Pavement Design

CIV2242

Dr Jayantha Kodikara (Kodi)


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Pavement Design System(Austroads, 2001 draft)

Structural Design1. Flexible Pavements2. Rigid Pavements

3. Overlays

Design Traffic

PavementMaterials

SubgradeEvaluation

Environment

Construction

&Maintenance

Considerations

ComparisonOf Designs

Implementand monitor


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Flexible Pavement DesignFlexible Approach


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Distress Modes for Flexible Pavements(modified from Austroads, 2004)

DistressMode

Likely Causes Materials Affected

Rutting Deformation of underlying materials,specially subgrade

Unbound materials,(bound materials?)

Cracking Traffic associatedSingle or low repetitions of high loadMany repetitions of normal loads

Non-traffic associated

Thermal cyclingReflection of shrinkage cracksSwelling/shrinking of subgrade materials

Asphalt, cementedmaterials, granularmaterials

Roughness variability of density, swelling/shrinking ofsubgrade materials

All materials


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Design Chart for GranularMaterials with Thin Bituminous

Surfacing (Austroads, 2004)

)120/log()(log58)(log211219 2 DESACBRCBRt


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Example

Example: A unbound pavement is to be designed over a weak subgrade of CBR

4 to carry design ESAs of 2 x 10 6. Design a pavement with baseand one subbase layer over the subgrade. The base is to be madeof good quality crushed rock with CBR well over 80. The subbase

layer is to be made of marginal gravel material with a design CBR of30.From Figure 2, the thickness of the pavement needed above thesubgrade of CBR 4 is 480 mm.If we place, gravel above the subgrade, then the pavement

thickness needed above that layer is, by looking up the graph forCBR 30, is 130 mm.Therefore, the pavement layer thicknesses are: base (crushed rock)= 130 mm; subbase (gravel) = (480-130) = 350 mm.


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Mechanistic Pavement Design


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Mechanistic Pavement Design Model forFlexible Pavements (Austroads, 2001 draft)

Asphalt

300 300

1800 mm

165

2

3 3

Granular MaterialCementedMaterial

Subgrade

1 Tensile strain at bottom of asphalt2 Tensile strain at bottom of cemented material3 Compressive strain at top of subgrade

Critical locations

1

Uniform stress(equal to tyre

pressure)


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Material behaviour and inputparameters

Material Behaviour Properties

Asphalt Isotropic Eflex ,

Cement treated Isotropic Eflex ,

Unbound granular Anisotropic Ev, Eh, v, h, andf (shear modulus)

But,

Eh= 0.5 E v v = h =

f = E v/ (1+ )Ev is the resilientmodulus.


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Determination of pavement life


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100

1000

10000

100000

1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 1.E+07

Austroads (2004) Approach forSubgrade Modelling

Prevent the rutting failure by limiting the vertical strain onthe subgrade.Basically assume that soil behaves in a resilient mannerunder these strains.

79300

N

log N

log

How is is determined?


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Asphalt

Asphalt is a mixture of bituminious binder andaggregate which is spread and compacted whilehot to form a pavement layer.

The strength/stiffness of asphalt is derived fromfriction between the bitumen coated aggregateparticles and the cohesion resulted from bitumenbinder.

Main forms of distress are:Fatigue failureRutting and shoving due to inadequate strength andstiffness.


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Asphalt modulus (x1000, MPa)Typical Australian Dense-Graded Asphalt at 25 oC (Austroads, 2004)

Binder Mix Size (maximum particle size, mm)

10 14 20

Range Typical Range Typical Range Typical

Class170 2-6 3.5 2.5-3.5 3.7 2-4.5 4

Class320

3-6 4.5 2-7 5 3-7.5 5.5

Class

600

3-6 6 4-9 6.5 4-9.5 7

Multigrade

3.3-5 4.5 3-7 5 4-7 5.5

SBS 1.5-4 2.2 2-4.5 2.5 3-7 3

EVA 3-6.5 5.6


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Asphalt Fatigue Failure(Austraods, 2004 using Shell (1978) relationship)

500

1000

800

400

300

200

1001.0E+03

600

900

700

1100

1.0E+04 1.0E+07 1.0E+081.0E+05 1.0E+06

E=2000 MPa

Vb=10%E=2000 MPa

Vb=12%

E=4000 MPaVb=10%

E=4000 MPaVb=12%

M i c r o s t r a

i n

ESA Repetitions

V b=bitumen percentage by volume; E=asphalt resilient modulus

5

36.0)08.1856.0(6918

flex

b

E V RF N

RF=0.95


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Behaviour of Cemented Materials

Cemented materials may be described as amixture of a cementitious binder, granularmaterial and water, compacted and cured.The material has strength/stiffness more thanthe granular material (host material) but lessthan those of concrete.Typical binder contents used are 2 to 6% by

weight.In contrast to granular materials, these materialsdevelop some tensile strength.


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Cemented Materials

Approximated to be linear elastic (under normaloperating conditions) and isotropic.Elastic modulus and Poissons ratio are needed

to characterise the material.Because the bending tensile fatigue failure isconsidered to be main distress mode undertraffic loading, flexural modulus is preferred.


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Parameters for Cemented Materials

Flexural modulus at 28 days curing in the road-bed is required.This may be obtained by:

Laboratory flexural beam tests, E flex Correlations of E flex with other laboratory tests (UCS)

E flex=1500 UCS ; UCS of lab specimens at 28 days curing.

Presumptive values.

Span/depth >3

P

P


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Presumptive Parameters forCemented Materials

MaterialCategory

Cemented Materials

Property(E in MPa)

Lean MixConcrete

Base4 to 5%Cement

Crushed Rock2 to 4%Cement

Subbasequality naturalgravel 4 to 5%

cementRange of E 5000 15000 3000-8000 2000 to 5000 1500 - 3000

Typical E 7000 (Rolled)10000 (Screed)

5000 3500 2000

Degree ofanisotrophy

1 1 1 1

Range ofPoissons ratio

0.1 - 0.3 0.1 - 0.3 0.1 - 0.3 0.1 - 0.3

Typical 0.2 0.2 0.2 0.2


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Failure of Cemented Materials Austroads (2004)

12

804.0 )191113000

(

E RF N

RF=0.95


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Granular materials

No failure criteria is used.It is assumed that the material property requirementshave been met (discussed under Pavement Materials).The material is considered as anisotropic elastic materialfor mechanistic analyses.The granular layer can be divided into sublayers toaccount for the variation of modulus due to differentstress levels.Need E v and Poissons ratio . For mechanisticdesigns, the soil is assumed to be anisotropic withEv/E H=2 and v= H. Another parameter (shear modulus)f=E/(1+ v).


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Presumptive Values for Granular UnboundMaterials (Austroads, 2004)

MaterialCategory

Unbound GranularHigh Quality CrushedRock

Base Quality Gravel Subbase Gravel

PropertyE in MPa

OverGranular

Material

Over StiffCemented

Material

OverGranular

Material

Over StiffCemented

Material

OverGranular

Material

Over StiffCemented

Material

Range E v 150-700 200-700 150-500 200-500 150-400 150-450

Typical E v 500 500 400 400 300 300

Degree of Anisotropy

2 2 2 2 2 2

Range of

v= H

0.25-0.4 0.25-0.4 0.25-0.4 0.25-0.4 0.25-0.4 0.25-0.4

Typical 0.35 0.35 0.35 0.35 0.35 0.35


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Project Reliability and ReliabilityFactor (RF)

Road classes Project reliability (%) Freeway 95-97.5

Highway: lane AADT >2000 95-97.5

Highway: lane AADT 2000 85-95

Main Road: lane AADT >500 85-95

Other Roads: lane AADT 500 80-90

Desired projectreliability

Reliability Factor (RF)

Asphalt Cementedmaterials

80% 4.7 2.585% 3.3 2.0

90% 2.0 1.5

95% 1.0 1.0

97.5% 0.5 0.67


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Summary of pavment design strategy

Construction &Maintenance

Influences

Environment

SubgradeEvaluation Traffic

Select TrialPavement

Accept?

& Collect Feedback

Comparsion of

Yes

CriteriaPerformance

Implement Design

Designs

MaterialsPavement

AnalysisPavement

No


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References

1. Austroads Pavement Design Guide (2004)2. Brown, S.F. (1996). Soil mechanics in pavement engineering, Geotechnique

46, No. 3, pp. 383-426.3. Lay, M.G. 1998). Handbook of road technology, Volume 1, Third Edition,

Transportation Studies volume 8, Gorden and Beach Science Publishers4. Shell (1978). Pavement Design Manual.


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END

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Current Approach to Pavement Design