Principles of Mechanistic Pavement Design · Main hypotheses of mechanistic pavement design •...
Transcript of Principles of Mechanistic Pavement Design · Main hypotheses of mechanistic pavement design •...
Principles of Mechanistic
Pavement Design
prof. Ezio Santagata
ANAS S.p.A. - Politecnico di Torino
Doha, December 11th, 2018
Presentation outline
• Historical perspective – AASHTO Design Method
• Main hypotheses of mechanistic pavement design
• Required data
• Practical implications
• Pavement design in Qatar
• Ongoing activities of the QSD-ANAS team
• Discussion
Historical perspective
• Originally developed in the 60s
• Based on the new concepts of:
• Structural Number (SN)
• Present Serviceability Index (PSI)
• Equivalent Single Axle Loads (ESAL)
• PSI depends mainly on:
• Smoothness (Slope Variance, SV)
• Rutting (Rut Depth, RD)
• PSI depends only marginally on cracking
• Main emphasis on load spreading capacity (stiffness)
AASHTO Design Method - Origin
Historical perspective
• Improvement of procedures
• Minor mechanistic components
• Introduction of reliability
AASHTO Design Method - 1993
• Limitations:
• Extension to extreme environmental conditions
• Extension to non-conventional and regional materials
• Extrapolation beyond actual traffic loadings
• Derived from the old style approach to pavement design:
subgrade protection (rather than true performance)
Need for a “physical” (mechanistic) approach to pavement design
Main hypotheses of mechanistic pavement design
• Vertical loads uniformly distributed on circular areas
• Layers of constant thickness on a semi-infinite half-space
• Full friction (adhesion) between layers
• Materials: homogeneous, isotropic and elastic
Multi-layer elastic model
Evaluation of structural response
(stresses, strains, displacements)
by means of adequate methods/algorithms
• Critical issues:
• Obtain input data (accounting for effects of external variables)
• Ensure consistency with hypotheses (materials and construction practices)
TESTING, MODELLING & QA/QC SYSTEMS
Layer 1
Layer 2
Layer 3
Interface 1
Interface 2
Interface n-1
Layer n
(homogeneous half-space)
h1, E1, ν1
h2, E2, ν2
h3, E3, ν3
hn = ∞, En, νn
Main hypotheses of mechanistic pavement design
• ASPHALT
Viscoelastic (rate- and temperature-dependent)
• Complex modulus (or ITT resilient modulus) formally considered as an elastic modulus
• Representative temperature and loading rate (frequency)
• Direct measurement or calculation by means of composition-dependent models
• Assumed values of Poisson’s ratio
• Dependency upon stress mode (e.g. uniaxial vs. bending vs. indirect tensile)
Assessment of materials properties (stiffness)
• UNBOUND MATERIALS
Non-linear elastic (stress- or strain-dependent)
• Resilient modulus formally considered as an elastic modulus
• Direct measurement for a wide combination of stress states and subsequent modelling
• Assumed values of Poisson’s ratio
Main hypotheses of mechanistic pavement design
Adhesion between layers
• Lack (or loss) of adhesion:
• Affects stress-strain distribution (and consequently fatigue life)
• Can lead to reduction of fatigue life and to debonding/delamination phenomena
• Can lead to surface cracking (top-down)
• Adhesion is strongly dependent upon tack coat and prime coat application
• Need for specific QA/QC and performance tests
Canestrari & Santagata (2005) Raab & Partl (2007)Roque et al. (2017)
Main hypotheses of mechanistic pavement design
Thickness uniformity and materials homogeneity
• Variability of thickness and homogeneity:
• Promotes the onset of relative permanent deformation (rutting and roughness)
• Amplifies dynamic loading which further contributes to relative permanent deformation
• Leads to creation of weak areas with corresponding local damage
• Need for tight control during production and laying
Santagata & Sciamanna (2015)Santagata & Barbati (2006)
Main hypotheses of mechanistic pavement design
Rationale of mechanistic (structural) design
• By means of transfer functions structural response can be related to:
• Limiting conditions = pass/fail approach
N/Nf (terminal damage)
single loading and conditions
• Performance = damage accumulation approach
D(N) = ΣNi/Nfi (progressive damage)
multiple loadings and conditions
• Classical transfer functions:
• FATIGUE CRACKING: tensile strain at bottom of the asphalt layers
• RUTTING: compressive strain on top of the subgrade
TESTING & FIELD OBSERVATION
Santagata et al. (2004)
Main hypotheses of mechanistic pavement design
Assessment of materials properties (transfer functions)
• ASPHALT – Fatigue life
• Generally expressed as a function of strain level (Wohler curve)
• Laboratory assessment to be adjusted to field performance (calibration)
• Direct measurement or calculation by means of composition-dependent models
• Dependent upon stress mode (e.g. uniaxial vs. bending vs. indirect tensile)
• Advanced modelling with Viscoelastic Continuum Damage (VECD) mechanics
Number of loadings to failure
Init
ial st
rain
(µ
m)
Miglietta et al. (2018)
Main hypotheses of mechanistic pavement design
Auxiliary information for mechanistic (structural) design
• Traffic loading - MONITORING & MODELLING
• Intensity, distribution and time growth
• Operative speed
• Models of various levels of complexity:
• Network-based models
• Relationship with social factors
• Environmental factors - MONITORING & MODELLING
• Temperature
• Moisture
• Models of various levels of complexity:
• TEMPERATURE: single temperature vs. continuous modelling
• MOISTURE: prevalent conditions vs. continuous modelling
Main hypotheses of mechanistic pavement design
Key points for the best use of mechanistic design methods
• The value of a mechanistic design method is mainly a function of its VALIDATION
• Quality of predictions depends on quality of input data
• “Cross-breeding” of different methods can jeopardize their predictive capacity
• Fundamental role of (laboratory) testing
• Necessary support of good construction practices and adequate QA/QC systems
Pavement design in Qatar
Reference documents
• Qatar Highway Design Manual 1997 (QHDM 1997)
• Qatar Strategic Transport Model (QSTM 2013)
• Interim Advice Note No. 016 (IAN 016, 2013) – Expressways → Stiffness ranges, no transfer function
• Interim Advice Note No. 019 (IAN 019, 2013)
• Qatar Construction Specifications 2014 (QCS 2014)
• Supplementary Guidelines for Pavement Design (2015) – Local Roads
• Qatar Highway Design Manual 2015 (QHDM 2015) → General “transition document” (refers to MEPDG)
• Interim Advice Note No. 101 (IAN 101, 2016)
More recent documents do not explicitly introduce a Qatar-specific procedure
Pavement design in Qatar
QHDM 1997
• Catalogue format but based on mechanistic principles
• Reference values of materials properties
• Explicit transfer functions (Austroads, adapted from Shell)
• Guidelines for traffic monitoring and modelling
• ESAL loading – suggested truck factor ranges (including overloading)
• Single reference temperature
• Lack of true validation
FATIGUE transfer function
RUTTING transfer function
Ongoing activities of the QSD-ANAS team
Assessment of relevant input data - MATERIALS
• ASPHALT STIFFNESS
• Calculation from binder rheology and volumetrics
• Effects of short-term and long-term ageing
• Calibration against direct measurements
• Neat and modified binders
1.E-02
1.E-01
1.E+00
1.E+01
1.E+02
1.E+03
1.E+04
1.E+05
1.E+06
1.E-07 1.E-05 1.E-03 1.E-01 1.E+01 1.E+03 1.E+05
G*
[kP
a]
ω [rad/s]
Bitumen A - Original
Bitumen A - RTFOT
Bitumen A - PAV
Bitumen B - Original
Bitumen B - RTFOT
Bitumen B - PAV
Ongoing activities of the QSD-ANAS team
Assessment of relevant input data - MATERIALS
• ASPHALT STIFFNESS AND FATIGUE
• Direct measurement and modelling
• Multiple configurations (uniaxial, 4-point bending, indirect tensile)
• Introconversion and adjustment for compaction level, rate of loading and temperature
• Mixes with neat and modified binders, including RAP and CR
1
10
100
1000
10000
100000
1.0E-05 1.0E-04 1.0E-03 1.0E-02 1.0E-01 1.0E+00 1.0E+01 1.0E+02 1.0E+03 1.0E+04 1.0E+05 1.0E+06
|E*|
MP
a
Reduced Frequency, fr=f*a(T)
E*(raw) - aT (fit)
E* (fit) - aT (fit)
Ongoing activities of the QSD-ANAS team
Assessment of relevant input data - MATERIALS
• STIFFNESS OF UNBOUND MATERIALS
• Direct measurement and modelling
• Effect of moisture content and level of compaction
• Correlation with index properties
• Functional for the sub-layering of sub-base and subgrade
Santagata et al. (2001)
Sub
-bas
e
Mean stress (kPa) Deviatoric stress (kPa)
First stress invariant (kPa)
Sub
grad
e
Deviatoric stress (kPa)
Re
silie
nt
mo
du
lus
(kP
a)
Re
silie
nt
mo
du
lus
(kP
a)
Ongoing activities of the QSD-ANAS team
Assessment of relevant input data - TRAFFIC
• Follow-up of data processing from classified traffic counts
• Reference: Qatar Strategic Traffic Model (QSTM)
• North Road (3 stations), Dukhan Road (1 station), Salwa Road (6 stations)
Ongoing activities of the QSD-ANAS team
Assessment of available design methods - TRIALS AND IMPLEMENTATION
• In the absence of field data for calibration/validation:
• Back-analysis of QHDM 1997 and sensitivity evaluation
• Adaptation of Austroads 2017 (in continuity with QHDM 1997)
• Trial use of AASHTO MEPDG (U.S.A., 2008) and SAPEM (South Africa,2013)
• With field data: preliminary implementation
0.00
0.20
0.40
0.60
0.80
1.00
1.E+05 1.E+06 1.E+07 1.E+08 1.E+09
Term
inal
dam
age
, N/N
f
ESALs
Reference
Reduced E1
Reduced E1, Increased E2
Reduced E1, Increased E2, Reduced h1
T1 T2 T6T5T4T3 T6+
Ongoing activities of the QSD-ANAS team
Challenges
• Characterization of non-conventional materials
• Recycled components (e.g. RAP and CR)
• Fine asphalt mixtures (ref. QCS 2018)
• Incorporate countermeasures for rutting and bleeding
• Include perpetual pavement option
• Retrieve and model reliable/ unbiased traffic data for all categories of roads
• Calibration and validation of transfer functions with field data
• Consistency with other reference documents adopted in Qatar
• Link mechanistic design to Life Cycle Cost Analysis (LCCA) and Pay Factors
• Dissemination and training
All stakeholders are invited to cooperate and share available data/information
For further information please visit
the “Qatar Future Roads” website
http://www.q-roads.com.qa/