Are AUSTROADS Pavement Design Performance Models Adequately Calibrated for New Zealand? - PowerPoint Presentation

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Are AUSTROADS Pavement Design Performance Models Adequately Calibrated for New Zealand?

Dr Bryan PidwerbeskyGeneral Manager - TechnicalFulton Hogan Ltd

AUSTROADS PTF Workshop, Wellington

04

December

2014

Slide2

Outline

Asphalt fatigue strain criterionSubgrade strain criterionTerminal rut depthBack-calculation from FWD deflection bowlsConclusion/summary

Slide3

AUSTROADS Pavement Design Guide

1

Horizontal tensile

strain

in

bottom of asphalt

–

fatigue cracking

2

Horizontal tensile

strain in bottom of cemented material - cracking3 Vertical compressive strain in top of subgrade - rutting & shape loss

Subgrade

3

Asphalt

1

Cemented Material

2

Unbound

subbase

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Inadequate load

supporting

capacity:

Loss of base,

subbase

or subgrade support

(

eg

water ingress)

→ high deflection and/or deformationInadequate thickness of the pavement to take the loads Increase in loading

Poor constructionCauses of Cracking in Asphalt

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Reflective cracking (from underlying asphalt, stabilised base or subgrade)

Causes of Cracking in Asphalt

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Brittle FailuresOld oxidized asphalt

Asphalt too stiff for environmental conditions

Outside Wheel Path

Little Shape loss

Causes of Cracking in Asphalt

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Thermal-induced cracking

Causes of Cracking in Asphalt

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Causes of Cracking in Asphalt

Classic fatigue-induced cracking is rare in New Zealand

Cracks normally start at top of asphalt

Start as very fine cracks created during roller compaction

Largest tensile strain is at top of asphalt

rarely bottom up

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Fatigue strainVery small strains (~100

με) per loadingFlexure strainLarger strains exceed maximum tensile strain capacityThermal-induced strain

Environmental factors

Causes of Cracking in Asphalt

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The History of Asphalt Fatigue Criterion

Asphalt fatigue criterion

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The History of Asphalt Fatigue Criterion

Pell

, P.S. (1962) Fatigue Characteristics of Bitumen and Bituminous Mixes. Int’l

Conference

on Structural Design of Asphalt Pavements, Ann Arbor, USA.

Asphalt fatigue criterion

Fatigue lives for different mixes at

0°C showing derived bitumen strain

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Asphalt

Fatigue Relationship1960’s - Laboratory-derived fatigue relationship1970’s - Adjusted to predict

fatigue life

in pavements using

a

shift factor

F

N = allowable number of load repetitions

µ

ε = tensile microstrain

produced by the loadVB = % by volume of binder in asphaltSmix =mix stiffness modulus (MPa)F = range of values

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Shift Factors

Shell Pavement Design Manual (1978) : F=10Saunders, L.R. A Modern Basis for Pavement Design (

1982)

: F = 10

AUSTROADS Pavement Design Guide (1992)

Ignored shift factor (F = 10 was considered)

Baburamani

,

ARR 334 Asphalt Fatigue Life Predictions Models (1999)

F = 10 to 20AUSTROADS Pavement Design Guide (2001 draft) : F = 5

Saleh (2012) : F = 5.7

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AUSTROADS Guide (2014) Table 6.15

Suggested Reliability Factors for Asphalt Fatigue

 

Desired project reliability

80%

85%

90%

95%

97.5%

RF

2.5

2.0

1.5

1.0

0.67

Desired

project

reliability has

two components:

a

shift factor relating mean laboratory fatigue life to a mean in-service fatigue life, taking account of

differences between

laboratory test conditions

and conditions applying to in-service pavement;a reliability factor relating mean in-service fatigue life to in-service predicted life

at a desired project reliability, taking into account factors such as construction variability, environment and traffic loading“

for lightly-trafficked roads load-induced fatigue cracking is uncommon

.”

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R

eliability factor/shift factor is too low

Confusion

about what constitutes fatigue

cracking

Fatigue

cracking is

result

of millions of very

small resilient strains under wheel loadings, at significantly less than horizontal strain capacity of

bound materialIn majority of cases, crack-induced failures are actually due to excess deflection/flexure of the underlying pavement &/or subgrade, causing significant tensile strain in asphalt that exceeds its tensile strain capacityFatigue criterion not applicable to thin asphalt surfacings

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Appropriate shift/reliability

factorSaunders (1982) 10Saleh (

2012) 5.7

Experience 5-10

Recommended Reliability Factors

 

Desired project reliability

80%

85%

90%

95%

97.5%

RF

10

5

4

3

2.5

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“...the primary function of a road structure is to protect the underlying soil from excessive stresses produced by traffic loads

....”“It is therefore necessary to limit the deformation in the soil and this may be done by limiting the value of the vertical compressive stress reaching the top of the subgrade....”“… the value of the vertical stress in the subgrade is one of the critical quantities determining the performance of a flexible pavement.”

Peattie

, K.R. (1962) A Fundamental Approach to the Design of Flexible Pavements. Proc. Int’l

Conference

on the Structural Design of Asphalt Pavements,

Ann Arbor

Subgrade strain criterion

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“Deformations of the surface under the action of repeated loadings by traffic is controlled by limiting the vertical compressive stress or strain in the subgrade, and

if necessary on the other granular layers in the structure.”“…irrespective of the construction, the maximum vertical compressive strain in the top of the subgrade is 9 x 10-4, and for roads carrying greater traffic volumes, a permissible compressive strain should be 6.5 x 10-4

.”

Dormon

, G.M. (1962) The Extension to Practice of Fundamental Procedure for the Design of Flexible Pavements. Proc. Int’l

Conference

on the Structural Design of Asphalt Pavements, Univ. of Michigan, Ann

Arbor

.

Subgrade strain criterion

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For unbound or stabilised granular pavements,

subgrade strain criteria is conservativeActual measured strains are greater than permissible strains calculated according to the criteria.Vertical

compressive strains in the

basecourse

can be as large (in magnitude) as

vertical

compressive strains in the

subgrade

Recommendation

Strains in the basecourse should be explicitly considered in

the AUSTROADS pavement design procedureSubgrade strain criterion

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Permanent subgrade strain/load is too small to measure

Subgrade strain criterion based on resilient subgrade strain because that is a much larger magnitude & can be measuredAssumed relationship between resilient & permanent subgrade strainAccumulation of permanent subgrade strain manifests itself as pavement ruttingThickness designs assume terminal rut depth

i

s 20-25 mm

Terminal rut depth

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“Implicit in the design procedure for these pavements (Section 8.3 and, specifically, Figure 8.4 of the Guide) is a terminal condition which is considered to be unacceptable and, hence, signifies the end of life for the pavement

.” “The view of the MEC Review Committee at the time was that, in terms of rutting, it represented an average rut depth of about 20 mm.”AUSTROADS

(2004) Technical Basis of

AUSTROADS

Pavement Design Guide.

AP-T33/04

Terminal rut depth

Severity Level

Rut (mm)

Low

6 – 12.5

Moderate

12.5 - 25

High

>25 mm

Typical Definitions of Rutting (FHWA, 2011)

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Terminal rut depth

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Deflection & Back-calculation

Back-calculation techniques based on FWD deflection bowls inaccurate for

estimating

pavement &

subgrade

properties:

Transfer

functions

are based on regression analyses & are never calibrated for specific projects

Transposition of independent & dependent variablesCBR’s derived from back calculation only intended to be relative & approximate, & used only in the context of pavement design overlaysDerived CBR value is only for modeling requirements & cannot accurately reflect actual subgrade CBR - it has to be measured in lab or inferred from in situ tests

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Deflection & Back-calculation

Subgrade Bearing Capacity

Rehabilitation Project

Parameter

A

B

C

CBR inferred from in-situ Scala Penetrometer

4%

4-5%

4%

Isotropic Modulus Backcalculated

69 MPa

35 MPa

86 MPa

Anisotropic Modulus Equivalent

(1)

100 MPa

52 MPa

113 MPa

Laboratory soaked Subgrade CBR

15%

25%

 Subgrade CBR assumed for design

5

4

5

(1) Modulus back-calculated from FWD deflection bowl: 10th

percentile isotropic subgrade stiffness converted to practical equivalent anisotropic stiffness (EISO=0.67xEANISO(vert

)) (Tonkin & Taylor, 1998)

Example data from actual projects shows variability in subgrade CBR values derived from different techniques

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Subgrade strain criterion was only ever intended to be used for design purposes

& provides reasonable values, given cumulative effect of assumptions made during design processPredictions of material properties and remaining life from back-calculation procedures (based on FWD deflection bowls)

poorly

correlated with actual

performance

Recommendation

To use back-calculation

procedures based on FWD

deflections for estimating remaining

life of a specific pavement contractually, models &

algorithms used in procedure must be robustly validated for specific conditions of each siteDeflection & Back-calculation

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For fatigue cracking in bitumen-bound

layers, project reliability factors should be in range of 2.5 to 5 (at least) for New ZealandAsphalt fatigue criterion is not applicable to thin surfacings

Vertical compressive &

shear strains within

unbound & modified

pavement layers should be explicitly considered as a critical parameter in flexible pavement

design

Terminal rut depth for

unbound granular/

stabilised flexible pavements is 20 mmBack-calculation procedures based on FWD deflection data may be used to estimate remaining life of a pavement ONLY after models & algorithms have been robustly validated for specific conditions of each siteConclusion/Summary