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AUSTROADS Pavement Design
Dr Bryan Pidwerbesky
Group Technical ManagerFulton Hogan Ltd
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Pavement
Subgrade
Load, W
P0
P0
P1
P1
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Pavement
Subgrade
Compression Tension
Load, W
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Pavement Life Cycle StrategiesPavement Strategies
Life Cycle Analysis Period (years or esa)
P e
r f o r m a n c e M e a s u r e
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AUSTROADS Pavement Design Guide 2003
Two design processes for Flexible Pavements
Empirical Design Chart
• flexible pavements consisting of unbound
granular materials, sprayed seal surfaceMechanistic
• flexible that contain one or more bound
layers
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8
AUSTROADS Guide Figure 8.4
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Fundamentals of Mechanistic Design
• Pavement performance related to elasticstrain
• Advantages
– rational / scientific
– flexible
– portable• Really semi-empirical
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Definitions of Stress & Strain
Strain (ε)unit movement / unit length
dimensionless (µ m/m)
Elastic strain
100% rebound after load is removed
Recoverable (resilient) strain
strain that rebounds after load is
removedResidual (permanent) strain
Stress (σ)unit load / unit area
kN/m2 = kPa
Modulus
• stress / strain
• Elastic modulus
• Flexural modulus
• Resilient modulus
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Mechanistic Design Process
Design Traffic (ESA)
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Single
ε2
Single tyre – Single Axle
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ε1
ε2
Direction of Travel
ε1 ε2= Permanent strain = Resilient strain
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Dual Tyre
ε4
Individual tyre’s effect
Combined effect
Dual tyre – Single AxleEquivalent Standard Axle (esa)
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ε3
ε4
Direction of Travel
ε3 ε4= Permanent strain = Resilient strain
Tandem
Combined effect
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Traffic Loading
• For seal design: use equivalent light vehicles -convert heavy vehicles to elv
• For highway pavement design: ignore light
vehicles, use only HCVs, converted to EquivalentStandard Axles (esa)
• For ports & log handling yards, DON’T use esa
or 4th power rule for converting loads!• For airports, special loading formula
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Mechanistic Design Process
Design Traffic (ESA)
Proposed Pavement Model
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Pavement Model
• ESA loading• layers defined by E, υ, h
• anisotropic (value depends on
direction)
– subgrade
– unbound
• isotropic (same in any
direction)– asphalt
– cemented
E1,υ1, h1
E2,υ2, h2
E3,υ3, h3
En,υn, hn
ESA Load
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Typical, Reasonable Input Values
Material Modulus (MPa) Poisson’s Ratio
Asphalt (> 75 mm thick) 1500 – 4500depends on mix
properties & vehiclespeed
0.40
Unbound Base 200 – 450(!) 0.35
Unbound Subbase 150 – 300 0.35
Subgrade 10 – 250 0.45
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Sub-layering
Asphalt Modulus = 3200 MPa
Subgrade Modulus = 100 MPa
Granular Layer Modulus = 400 MPa
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Selected subgrade materials• 1992 Guide: Ev = 10 * Design Subgrade CBR forentire layer thickness
• concern that this resulted in higher moduli thangranular materials
• new procedure developed to sublayer selectmaterial,
• modulus is limited by modulus of underlyinginsitu subgrade
• modulus doubles every 150 mm thickness ofselect material, up to max 10 x CBR
150) /thicknesslmat'subgradeselected(
insitu 2 xE =select E
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New granular sublayering rules
divide total granular thickness into 5 equally thick layers
5
4
3
2
1
Sublayer
T/5
T/5
T/5
T/5
T/5
Modulus
(MPa)
Thickness
(mm) First step: calculatemodulus of top
sublayer
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Modulus of top granular sublayer
and that determined using the following formula:
doubling of modulus every 125mm
125) /thickness granular(SubgradesublayergranularTop 2 xE E =
ETop granular sublayer
Modulus of Cover1Material (MPa)Thickness of
OverlyingMaterial
1000 2000 3000 4000 5000
40 mm 350 350 350 350 350
75 mm 350 330 310 290 280100 mm 320 280 260 240 220125 mm 280 240 210 190 170150 mm 250 200 160 150 150175 mm 220 160 150 150 150200 mm 180 150 150 150 150>=250 150 150 150 150 150
1. Cover material is either asphalt or cemented material or a combination of these materials.
vertical modulus of top sub-layer, minimum of
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New granular sublayering rules
5
43
2
1
Sublayer
4160
57607960
10960
15060
Modulus
(MPa)
Thickness
(mm)
5
1
=subgrade
sublayergranulartop
E
E R
Modulus of each Sublayer calculated from ratio of
modulus of top granular sublayer and subgrade:
R = (150/30)1/5
= 1.38
Subgrade modulus = 30
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Definition of Project ReliabilityBecause of this lack of certainty in performance of
the constructed pavement, an appropriate measure ofanticipated performance of the proposed pavement is
its Project Reliability:
“The Project Reliability is the probability that the
pavement when constructed to the chosen design will
outlast its design traffic before major rehabilitationis required”
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Typical Desired ProjectReliabilitiesTable 2.1
Typical Project Reliability Levels
Road Class Project Reliability (%)
Freeway 95-97.5Highway: lane AADT>2,000 90-97.5
Highway: lane AADT<2,000 85-95
Main Road: lane AADT>500 85-95Other Roads: lane AADT<500 80-90
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Desired reliability achieved by using
reliability factors in the performancerelationships
mk
RFN
=
µε
Reliability Factor varies with selecteddesired project reliability
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Cemented materials
Characterisation for Pavement Design• isotropic (Ev=Eh)
• elastic modulus
• Poisson’s ratio 0.2
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Improved guidance on presumptivemoduli for cemented materials
• Subbase gravels, 4-5% cement E= 2,000 MPa• Crushed rock, 2-3% cement E= 3,500 MPa
• Base with 4-5% cement E= 5,000 MPa
• Lean mix concrete (rolled) E= 7,000 MPa
• Lean mix concrete (screeded) E=10,000 MPa
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Cemented materials fatigue relationship
• Modified to allow to design to desired project
reliability, includes Reliability Factor (RF)
( )12
8040 191000113
+=
µε
.E / ,RFN
Desired Project Reliability
80% 85% 90% 95% 97.5%
4.7 3.3 2.0 1.0 0.5
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Asphalt
Characterisation for Pavement Design• isotropic (Ev=Eh)
• elastic modulus
• Poisson’s ratio 0.4
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asphalt fatigue relationship• modified to allow to design to desired project
reliability, includes Reliability Factor (RF)
5
360
+=
eS
1.08) V6918(0.856 RF N
.mix
B
µ
D e s i r e d P r o j e c t R e l i a b i l i t y
I n t ’ l 8 0 % 8 5 % 9 0 % 9 5 % 9 7 . 5 %
5 t o 1 0 2 . 5 2 . 0 1 . 5 1 . 0 0 . 6 7
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Mix Stiffness
• Resilient modulus is a measure of ‘stiffness’• As stiffness increases, load is spread over a
wider area
• Smix (t, T) ∝ σ / ε
• Smix (t, T) = mix stiffness at a particular rate
of loading (t) and temperature (T)
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GyratoryCompaction
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Advantages of GyratoryCompaction• Better replication of
mixes laid in the field
• Opportunity to varycompaction conditions
• Gives information onmix compactibility
• Rational
• Affordable• Safe
• Allows fullercharacterisation ofmixes at design stage:♦ air voids at several
compaction levels
♦ compaction to refusaldensity (250 cycles)
♦ assessment of mixcompactability
• Allows optimisation ofaggregate blend
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Resilient Modulus Test
• Apply vertical load, P, ata specified rate (Pulserepetition period, 3.0 ±0.05 seconds) and for a
specified time (Risetime, 0.04 ± 0.005seconds)
• Different rates of loading
and different load pulsetimes simulate differentloading conditions.
Extensometersmeasurehorizontaldisplacement
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MATTA• IPC Universal Materials
Testing Apparatus• Indirect tensile test setup,
for measuring stiffnessmodulus
• Normally measured at 25ºC, but differenttemperatures can be
specified depending onfield conditions.
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FH MATTA Results
Modulus
Bitumen
Location
Modulus
BitumenLocation
Mix
Bitumen
3390 MPa
@ 4.8%Dunedin
2880 MPa
@ 5.1%Nelson
2970 MPa
@ 5.5%
Nelson
3350 MPa
@ 5.4%
Waikato
2930 MPa
@ 4.5%Southland
1570 MPa
@ 5.6%Nth Hrbr
1570 MPa
@ 5.8%Southland
2300 MPa
@ 5.8%Auckland
40 mm
B80
20 mm
B80
14-16 mm
B80
14-16 mm
B60
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Increase Resilient Modulus by:
• Reducing bitumen content
– Relative to Marshall optimum binder content
• Increasing bitumen viscosity
– decreasing penetration grade
• Coarser gradation
• Increasing compactive effort (to a critical limit forsome mixes)
• Reducing temperature
• Aging (generally increases modulus)
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Effect of Temperature on Modulus
0
2000
4000
6000
8000
0 10 20 30 40 50 60
Temperature (C)
M o d u l u s ( M P a )
Mix10
Mix16
Mix20
• 120 Gyratory
compaction cycles
• 80/100 pen bitumen
• Auckland basalt• Air voids:
– Mix 10 = 4.8%
– Mix 16 = 4.8%
– Mix 20 = 4.6%
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Repeat Load Triaxial Apparatus
On-sample Axial Strain Measurement
LVDT core extensions
Minature LVDT
LVDT clamp screwed to target
glued on test specimen
LVDT extension bracket
screwed to brass target
glued on test specimen
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Non-linearity of Unbound Aggregates
For most bound materials,
modulus is independent of
confining pressure, but does
vary with load duration (speed)
For unbound aggregates, modulusdepends on confining pressure
(kPa), load magnitude (kN) &
duration (speed)
Modulus increases as confining /
applied pressure increases
0
50
100
150
200
250
300
Applied Pressure (kPa)
M o d u l u s
( M P a )
Non-linear Non-linear
Linear
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Mechanistic Design Process
Design Traffic (ESA)
Proposed Pavement Model
Calculate Critical Strains
CIRCLY
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3 Distress modes, 3 critical strains
Uniform stress(equal to tyrepressure)
1 Tensile strain at bottom of asphalt - asphalt fatigue
2 Tensile strain at bottom of cemented material - cement mat fatigue3 Compressive strain at top of subgrade - rutting & shape loss
Denotes likely locations of critical strains due to applied loading
3 Subgrade
1 Asphalt
2 Cemented Material
Spacing of Dual Wheels – Full Axle Configuration
330mm
Crushed Rock
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Critical Strains
• Critical strains– subgrade
– cemented layers
– asphalt
Asphalt
Cemented
Subgrade
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CIRCLY
• Accumulates ‘damage’ contributed by each axleload in the traffic spectrum at each analysis point
• Cumulative Damage Factor (CDF) is the sum ofdamage factors over all the loadings
– Pavement reaches its design life when CDF = 1.0
– If CDF < 1.0, then pavement has excess capacity, &CDF gives proportion of life consumed
– If CDF > 1.0, then pavement could fail early!
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Asphalt layer thickness
Compression Tension
STRAIN IN BOTTOM OF ASPHALT LAYERSTRAIN IN BOTTOM OF ASPHALT LAYER
Asphalt
LayerThickness
& Strains
50 mm
75 mm
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Thickness AC Tensile strain
25 50 75 100Asphalt
layer
thickness
Compressivestrain
Granular layers
Subgrade
Environmentissues
dominate
Structuralissues
dominate
Typical Multi-layer Flexible Pavement -
Design Parameters
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Typical Multi-layer Flexible Pavement -
Design Parameters
Thickness AC Tensile strain
25 50 75 100Asphalt
layer
thickness
Compressivestrain
Granular layers
Subgrade
Environmentissues
dominate
Structuralissues
dominate
EB 200 MPa
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Thickness AC Tensile strain
25 50 75 100Asphalt
layer
thickness
Compressivestrain
Granular layers
Subgrade
B
EB 400 MPa
Typical Multi-layer Flexible Pavement -
Design Parameters
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Mechanistic Design Process
Design Traffic (ESA)
Proposed Pavement Model
Calculate Critical Strains
Apply Performance Criteria
CIRCLY
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Performance Criteria
N
.
=
8511 7 14
µε Subgrade
N =
9300 7
µε
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100
1000
10000
10000 100000 1000000 10000000
Cumulative Load Repetitions
V e r t i c a l C o m p r e s
s i v e S u b g r a d e S t r
a i n ( µ m / m )
Shell, 1978 NZ Primary, 1983
NZ Secondary, 1983 AUSTROADS, 1992
AUSTROADS 2001
(1)
(2)
(3)
Dormon & Metcalf, 1965
(4)
(5)
Test Pavement Number
(2' )
P i dw e r b e s k y
(6)
(7)
Subgrade Strain Criterion
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Performance Criteria
Cemented N E
====++++
113000 19112
.804
µε µε µε µε
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Modified Materials
• Small proportion of binder ( < 2% )• Improved properties
– PI, workability, reduced water susceptibility
• Provides stable working platform
• Primarily compressive mode of failure
• Cracking not significant
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Cemented Materials
• Greater proportion of binder• Adds significant strength
• Relies on slab action
• Attracts stress
• Prone to fatigue cracking
• Prone to shrinkage cracking
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Definition of Modified Materials
Unbound Modified Cemented Lean-mix Concrete
Tensile Strength
< 80 kPa > 80 kPa
Dunlop (1978)
Increasing Binder Content
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Performance Criteria
N
.
=
8511
7 14
µε
Subgrade
Modified Subgrade Sub-layering
N =
93007
µε
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Performance Criteria
Asphalt N V
S
B
Mix
= +
6918 0 856 1080
5
( . . ).36
µε
N V
S
B
Mix=
+
6918 0 856 1080
5( . . )
.36 µε RF
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Tyre/Axle Load Configuration
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Mechanistic Design Process
Design Traffic (ESA)
Proposed Pavement Model
Calculate Critical Strains
Apply Performance Criteria
Determine Theoretical Service Life (esa)
CIRCLY
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Mechanistic Design Process
Design Traffic (ESA)
Proposed Pavement Model
Calculate Critical Strains
Apply Performance Criteria
Determine Theoretical Service Life (ESA)Not OK
CIRCLY
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Mechanistic Design Process
Design Traffic (ESA)
Proposed Pavement Model
Calculate Critical Strains
Apply Performance Criteria
Determine Theoretical Service Life (ESA)
Input to Decision
Not OKOK
CIRCLY
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Subgrade Evaluation
• One strategy is to compact subgrade at a
Equilibrium Moisture Content (EMC)
• Number of methods to establish design subgrade
strength• e.g measure on nearby pavement with similar materials &
conditions
• vertical modulus = 10xCBR & horiz modulus = 5xCBR
• poissons ratio = 0.45
• Potential to reduce costs by using a designstrength other than soaked CBR
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Rutting Resistance of AsphaltRutting Resistance of Asphalt
• Larger mix size
• Angular/textured aggregates
• Stiffer/plastomeric binders
• Coarser grading
• Reducing air voids (min 3%)
• Increase filler
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Fatigue Resistance of AsphaltFatigue Resistance of Asphalt
• Elastomeric binders
• Increase binder content
• Reduce air voids (min 3%)
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Durability of AsphaltDurability of Asphalt
• Reduce air voids
• Softer binders
• Increase binder film thickness
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Skid Resistance of AsphaltSkid Resistance of Asphalt
• Larger mix size
• Coarser grading
• Angular/rough aggregate
• Higher Polished Stone Value (PSV) of
larger aggregate component only
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Workability of AsphaltWorkability of Asphalt
• Increase VMA
• Higher binder content
• Softer binders
• Reduce filler
• Rounded aggregate
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Moisture in the Pavement
A small amount is okay, and isbeneficial
Too much - disastrous!Everything in Moderation!
Pavement stress dissipation
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Radius of contactabout 100 mm
Pressure > 100 psi
Granular layer(s)
Subgrade
Radius of stress
about 500 mmPressure ≈≈≈≈ 4 psi
Radius of stressabout 300 mm
Pressure ≈≈≈≈ 10 psi
Pavement saturation
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Pavement saturation
Numerous contact points each,under high stress in a wellcompacted granular material
Aggregate particles
In a saturated system the applied loadis transmitted equally in all directions,forcing the aggregate particles apart
Saturated pavement rutting & heavingSaturated pavement rutting & heaving
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HeaveHeave
SubgradeSubgrade
Distance of heave from wheelpath isDistance of heave from wheelpath is
relative to depth/extent of failurerelative to depth/extent of failure
HeaveHeave
RutRut
Subgrade shear failureSubgrade shear failure
due to saturationdue to saturation
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3 0 0 m m r a i n i n 3 d a y s !
LoadLoad
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Model of asphalt interfaces and potential hydraulic debonding stress
If the voids at the interface become
near saturated the hydraulic stress willtend to debond the surfacing which isnot confined by the load.
Air voids will be concentrated at the layerinterface because the overlay mix will notconform identically to the texture of the
substrate material.
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Pavement Construction Quality• Density - % of Max Density & Optimum
Moisture
– Nuclear Density Meter – sand cone displacement
• In situ CBR and Plate Bearing tests
• Dynamic Cone (Scala) penetrometer• Performance properties - Field tests
– NOT Clegg Hammer - measures consistency ofthe surface finish
– Deflection bowls - including Benkelman Beam
• Laboratory tests
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Deflection Testing Equipment
• Full scale, heavy load devices:
– Benkelman Beam
– Deflectograph
– Falling Weight Deflectometer
• Single point portable devices:
– Clegg hammer
– Prima
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Prima 100
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PortableFWD Device
AC; Run 1 Ch 10 Pt 37
-100
-50
0
50
100
150
0 5 10 15 20 25 30Time (ms)
F o r c e ( k P a ) & D e f l e c t i o n
( m i c r o n s )
Force (kPa) D(1) (µm)
Force (kPa) D(1) (µm)
Force (kPa) D(1) (µm)
TYPICAL PLOT PRIMA LOAD & DEFLECTION v LOAD DURATION
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Deflection Bowl Analysis
-3
-2.5-2
-1.5
-1-0.5
0
0 0 . 2 0 . 4 0 . 6 0 . 8 1 1 . 2 1 . 4 1 . 6 1 . 8
Distance from Centre of Load (m)
D e f l
e c t i o n ( m m
)
Curvature Function (CF) of a
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( )deflection bowl
• D0 = maximum deflection for a test point
• D200 = deflection measured where the test load is200 mm from the point of maximum deflection (inthe direction of travel).
Characteristic Deflections &
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Curvatures
Determined for each sub-section
Homogeneous sub-sections have:
• Coefficient of Variation (CoV) < 0.25
• Characteristic deflection (CD) or curvature(CC) is equal to average deflection orcurvature (µ) + a factor (f) x standarddeviation (σ):
CD or CC = µ + f x σ – f is selected based on reliability required
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Coefficient of Variation (CoV)
0
5
10
15
20
25
30
P e r c e n t a g e ( % )
0.5 0.7 0.9 1.1 1.3 1.5
Deflections (mm)
• Statistical measure of
consistent quality ofconstruction
• Std dev of values divided by
average value x 100%– ± 1 Standard deviation includes
68% of values
• Typical maximum values for
CoV are < 25 or 30%– If deflection high, reduce to 20%
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Coefficient of Variation
0
5
10
15
20
25
30
P e r c e n t a g e ( % )
0.5 0.7 0.9 1.1 1.3 1.5
Deflections (mm)
• Previous distribution:
– Average = 1.0 mm
– Std. Dev. = 0.1 mm
– CoV = 10%
• This distribution:– Average = 1.0 mm
– Std. Dev. = 0.2 mm
– CoV = 20%• Greater variation!
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Factor ‘f’ for Characteristic Values
RoadFunctional
Class
f % of all deflectionmeasurements which will
be covered by theCharacteristic Deflection*
1 and 6
2, 7, 8 and 9
3, 4 and 5
2.00
1.65
1.30
97.5
95
90
CD or CC = µ + f x σ
* after identifying areas to be patched/reconstructed
Example Calculation of
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Characteristic Deflection (CD)
Sub-section: Average Deflection µD
= 1.0 mm
Standard Deviation σD = 0.1 mm
Require 95% confidence, for f
CD = µD + f x σD