Transcript PPSX, 4.7MB

Are AUSTROADS Pavement Design Performance
Models Adequately Calibrated for New Zealand?
Dr Bryan Pidwerbesky
General Manager - Technical
Fulton Hogan Ltd
AUSTROADS PTF Workshop, Wellington
04 December 2014
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Outline
•
•
•
•
•
Asphalt fatigue strain criterion
Subgrade strain criterion
Terminal rut depth
Back-calculation from FWD deflection bowls
Conclusion/summary
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AUSTROADS Pavement
Design Guide
Asphalt 1
Cemented Material 2
Unbound subbase
Subgrade
3
1 Horizontal tensile strain in bottom of asphalt – fatigue cracking
2 Horizontal tensile strain in bottom of cemented material - cracking
3 Vertical compressive strain in top of subgrade - rutting & shape loss
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Causes of Cracking in Asphalt
• Inadequate load supporting capacity:
– Loss of base, subbase or subgrade support (eg
water ingress) → high deflection and/or
deformation
– Inadequate thickness of the pavement to take
the loads
– Increase in loading
– Poor construction
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Causes of Cracking in Asphalt
• Reflective cracking
(from underlying
asphalt, stabilised
base or subgrade)
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Causes of Cracking in Asphalt
• Brittle Failures
– Old oxidized asphalt
– Asphalt too stiff for environmental conditions
Outside Wheel
Path
Little Shape loss
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Causes of Cracking in Asphalt
• Thermal-induced cracking
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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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Causes of Cracking in Asphalt
• Fatigue strain
Very small strains (~100 με) per loading
• Flexure strain
Larger strains exceed maximum tensile strain capacity
• Thermal-induced strain
Environmental factors
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Asphalt fatigue criterion
The History of Asphalt Fatigue Criterion
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Asphalt fatigue criterion
The History of Asphalt Fatigue Criterion
Fatigue lives for
different mixes at 0°C
showing derived
bitumen strain
Pell, P.S. (1962) Fatigue Characteristics of Bitumen and Bituminous Mixes. Int’l Conference on
Structural Design of Asphalt Pavements, Ann Arbor, USA.
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Asphalt Fatigue Relationship
• 1960’s - Laboratory-derived fatigue relationship
• 1970’s - Adjusted to predict fatigue life in pavements using
a shift factor F
 6918(0.856 VB  1.08) 
N  F

0.36
S

mix


•
•
•
•
•
5
N = allowable number of load repetitions
µε = tensile microstrain produced by the load
VB = % by volume of binder in asphalt
Smix =mix stiffness modulus (MPa)
F = range of values
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Shift Factors
5
 6918(0.856VB  1.08)
N F

0.36
S

mix


• Shell Pavement Design Manual (1978) : F=10
• Saunders, 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 20
• AUSTROADS 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
RF
80%
2.5
Desired project reliability
85%
90%
95%
2.0
1.5
1.0
97.5%
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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Reliability 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 material
• In 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 capacity
• Fatigue criterion not applicable to thin asphalt surfacings
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Appropriate shift/reliability factor
• Saunders (1982)
• Saleh (2012)
• Experience
10
5.7
5-10
Recommended Reliability Factors
RF
80%
10
Desired project reliability
85%
90%
95%
5
4
3
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97.5%
2.5
Subgrade strain criterion
“...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
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Subgrade strain criterion
“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.
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160
Granular Overlay (mm)
140
120
100
80
60
40
20
0
11.4
11.9
12.4
12.9
Chainage (km)
Granular Overlay (mm) - Austroads (GMP-Rigorous)
13.4
Granular Overlay (mm) - TNZ Precedent Method
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Subgrade strain criterion
For unbound or stabilised granular pavements, subgrade
strain criteria is conservative
Actual 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 procedure
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Terminal rut depth
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
measured
Assumed relationship between resilient & permanent
subgrade strain
Accumulation of permanent subgrade strain manifests itself
as pavement rutting
Thickness designs assume terminal rut depth is 20-25 mm
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Terminal rut depth
“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. APT33/04
Severity Level
Low
Moderate
High
Rut (mm)
6 – 12.5
12.5 - 25
>25 mm
Typical Definitions of Rutting
(FHWA, 2011)
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Terminal rut depth
30
Actual 1
25
Extrapolate 1
20
Actual 2
15
Extrapolate 2
10
Terminal Rut
5
Maintenance
Intervention
0
0
2
4
6
8
10 12 14 16 18 20 22 24
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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 variables
• CBR’s derived from back calculation only intended to be
relative & approximate, & used only in the context of
pavement design overlays
• Derived 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
Example data from actual projects shows variability in
subgrade CBR values derived from different techniques
Subgrade Bearing Capacity
Parameter
CBR inferred from in-situ Scala Penetrometer
Rehabilitation Project
A
B
C
4%
4-5%
4%
Isotropic Modulus Backcalculated
69 MPa
35 MPa
86 MPa
Anisotropic Modulus Equivalent(1)
Laboratory soaked Subgrade CBR
Subgrade CBR assumed for design
100 MPa 52 MPa
15%
25%
5
4
113 MPa
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)
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Deflection & Back-calculation
Subgrade strain criterion was only ever intended to be used for
design purposes & provides reasonable values, given cumulative
effect of assumptions made during design process
Predictions of material properties and remaining life from backcalculation 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 site
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Conclusion/Summary
For fatigue cracking in bitumen-bound layers, project reliability factors should be
in range of 2.5 to 5 (at least) for New Zealand
Asphalt 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 mm
Back-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 site
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