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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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