some significant findings from accelerated … significant findings from accelerated pavement...
TRANSCRIPT
Some Significant Findings fromAccelerated Pavement Testing
Fred Hugo
Emeritus Professor and Research Fellow
Stellenbosch University
South Africa
Pavinars
7 October 2014
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According to NCHRP Synthesis
433 Significant Findings on Full-
Scale APT there are now ca.30
facilites operational in the
United States
5
APT is…….
o controlled application of wheel loading on a
o layered pavement structure to determine
o pavement response & performance under
o controlled, accelerated damage accumulation
o in a compressed time period.
For this discussion my definition of
Accelerated Pavement Testing is -
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The Point of
Departure for the
Presentation -
broadened to cover
all APT (small-scale
and full-scale) and
provide a personal
viewpoint
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Framework for Evaluating APT Findings
XXX•Structural
Designs
XXX•Materials and
Tests
XXX•Management
and Economy
Pavement
Design Guides
Operation of
APT programs
Construction
specifications
Technical
/Economical
EconomicalTechnicalField of
Application
Program
A B and C
Program
A B and C
Program
A
CROSS-CUTTING FINDINGSTOPIC
SPECIFIC
FINDINGS
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Goal is to …..assist engineers, technologists and
technicians in understanding
TOPIC SPECIFIC FINDINGS
will focus onpavement materials,
structures &interaction between
vehicle-pavement-environment
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APT pavements and
trafficking modes
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1. Test Roads with vehicles
2. Circular tracks with APT equipment
3. Linear tracks with APT equipment
4. Static pads with APT equipment
1. Slow , overloaded , uni- & bi-directional2. Faster, unidirectional3. Very fast uni-directional
APT modes
Slow , overloaded, uni- and bi-directional
Faster , uni-directional Very fast , uni-directional
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Structural Designs
Innovative designsSome noteworthy findings:
■ Pavements constructed partial depth with RAP
perform as well as new full-depth new mixes
resulting in 20 percent savings. (KDOT)
■ Thin asphalt pavements may be subject to structural
fatigue cracking as well as considerable surface
cracking. (LINTRACK)
■ Remixed asphalt pavements may be too stiff to
prevent reflected cracking and stripping (TxDOT)
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Structural Designs
Innovative designs (cont)
Inverted pavement
• Uses a strong cement stabilized subbase layer
(CTSB) with high quality unbound crushed rock base
on stabilized layer & asphalt surfacing on top.
• Developed in South Africa & has since then been
replicated or further explored & developed elsewhere
• (PRF-La -Metcalf et al, 1999). PRF-LA reported a five
fold increase in performance life compared to their
conventional pavements
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Structural Designs
Innovative designs (cont)
High Modulus Pavement20/30 Pen binder HMA is used to resist rutting. It has a
thin HMA layer on top for surfacing. Requires relatively
stiff supporting layers & has been used with a foam-
bound aggregate-graded course (cold mix) between the
high modulus layers. Developed in France (LCPC)Effectiveness was dependent on support by a relatively
stiff layer.
(De la Roche et al, 1994; Gramsammer et al, 1999)
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Structural Designs
Innovative designs (cont)
Overlays
tested extensively as primary applications of
APT.
Gap-graded, crumb-rubber-modified AC
overlay outperformed a dense graded mixture
with a conventional binder in terms of
permanent deformation when supported on a
sound support-structure.
(Harvey et al, 1999, 2000)
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Structural DesignsConcrete pavements
Innovative designs (cont)■ Shrinkage and environmental effects on
the performance of FSHCC pavements at
Palmdale, CA - of importance relative to
distress mechanisms
■ Benefits and use of dowels and tie-
bars quantified
■ Minimized concrete slab thickness, is
possible through use of increased flexural
strengths and small coefficients of thermal
expansion (CAL-APT).
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Structural Designs
Innovative designs (cont)
More noteworthy findings:
■ Spring load restrictions policy and winter
overload policy developed to cater for
freeze-thaw conditions. (MnROAD)
■ Thicker/weaker cement treated base courses
proven to be as effective as thinner layers with
higher cement contents (10 percent vs. 4
percent). (PRF-LA)
■ Innovative materials for rapid road construction
developed. (ERDC-GSL)20
Materials and Tests
• Some introductory remarks:
• The important role of non-destructive testing
(NDT) in APT programs cannot be over
emphasized. The FWD, DCP, GPR and SASW
(Spectral Analysis of Surface Waves) have
been widely used as invaluable supplemental
tools.
• Instrumentation such as strain gauges and
MDD’s are equally important - providing
information for analysis
• The problem of specimen preparation to link field
and lab remains a concern worth noting.21
Materials and Tests
Surfacing materials (cont)
MMLS3 testing, in conjunction with full-scale
APT trafficking, validated that surface distress
of asphalt mixes in terms of rutting, fatigue and
stripping, is dependent on stress, rate of load
applications, material characteristics and
environmental conditions, apart from load.
Field tests were conducted in the US at WesTrack,
NCAT, US281 Tx, Virginia and also in Switzerland
and a variety of sites in southern Africa.
22
Moisture damaged HMA cores due to wet MMLS3
trafficking at NCAT. Damage much greater than under
truck trafficking
23
Materials and Tests
Surfacing materials (cont)WesTrack and others reported Superpave coarse
graded mixes to be less rut resistant than fine graded
Superpave mixes. Design recommendations amended
as follows:
■ N-design for any mix should always be based on a
20- year design life;
■ Once volumetric mix design is completed, a
laboratory trafficking performance validation test
should be conducted at appropriate high
temperature for critical Superpave mix designs
■ Guidelines relating to the dust-to-binder ratio, VMA
and control of binder quantity to be used24
Materials and Tests
•Bases and Subbases (cont)
■ Multiple lifts of stabilized layers susceptible
to debonding hence thicker layers are
preferable provided uniform density can be
achieved
■ Stabilized layers found to have limiting
thickness beyond which performance did not
gain significantly. The same was found true
of unbound aggregate where the limit
appears to be 150 mm
■ In-place mixing of soil-cement & plant
mixing had similar pavement performance.25
Mechanism of DistressCross-cutting Topic 1
Moisture DamageStripping / Surface Fatigue
Strength loss and softening of pavement structure
Pumping/erosion and deformation/chemical disintegration
Freeze/thaw
Chemical Distress
TraffickingTemperature high
Temperature low
Load intensity and equivalency
Suspension
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Mechanism of Distress (cont).
Trafficking (cont)Tire pressure and type; configuration and contact
stressAxle configuration
Trafficking direction (APT specific)
Lateral wander
Speed and frequency
Braking and incline trafficking
Structural Flaws / DiscontinuetiesCurling of concrete slabs
Diurnal patterns
Seasonal effect
Voids due to erosion
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Mechanisms of Distress
Stripping/
surface
fatigue
■ Wet trafficking causes distress resulting in
stiffness loss & fatigue life in asphalt measured
by spectral analysis of surface waves (SASW)
and residual tensile fatigue life using semi-
circular bending test (SCB)
■ GPR is useful for finding layer thickness,
stripping, & subsurface moisture.
■ Dependent on tire pressure rather than load
■ Asphalt treated permeable basis (ATPB) often
block. Frequently better not to install, but to
prevent ingress of water. If used, geotextile
filters can prevent blockage.
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Applications (cont)
Models most frequently used with APT are
based on elastic layer analysis, although
FE analyses are used as much. Four
aspects are most frequently modeled:
• stress–strain
• deformation
• deflection, and
• back-calculation of moduli
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Mechanisms of Distress
Cross-cutting Topic 1 (cont)
• Trafficking causes pore
pressure & loss of shear
strength in granular base
• Deformation of lower layers
causes surface rutting
• Back-calculated stiffness using
FWD, useful tool for
determining of seasonal
environmental change
Strength loss
and softening
of pavement:
Base and
subbase;
Subgrade
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Mechanisms of Distress (cont)
Trafficking
YThreshold temperature below which rate of
rutting in asphalt is at a much slower rate, if
not insignificant lies between 40°C to 45°C.
Appears to be close to the Ring and Ball
softening point of the binder.
YARRB recommend 50°C for rut testing. Some
programs prefer relating to critical
temperature determined from environmental
conditions.
YRate of deformation approximately doubles
every 4°C for a conventional AC-30 binder
compared to 7°C for EVA modified binders.
At 60°C difference is significantly greater.
Temperature
high
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Mechanisms of Distress (cont)
Trafficking (cont)YDeformation due to temperature primarily in
upper HMA layer (100 to 150 mm). High
binder (rich bottom), fatigue resistant asphalt
could therefore be included below this level.
YAggregate type significantly affects rutting;
slag and limestone mixtures rutted 50% less.
YAn increase of 0.25% in the binder content
resulted in as much as a 40% increase in
rutting of gravel mixtures
YLCPC found 100,000 to 200,000 load
repetitions at critical temperature sufficient to
define rutting performance. Several million
load applications needed for fatigue testing.
Temperature
high (cont)
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Mechanisms of Distress (cont)
Trafficking (cont)
Global research indicates damage
exponents varying between 2 and 12.
Apparent that the damage exponent
depends on factors such as: Materials;
Extent of damage; Subgrade conditions;
Environmental conditions. Accordingly, use
of standard equivalent axles (ESAL’s) needs
to be qualified for the site/location otherwise
conclusions are questionable.
Conflicting results found between APT
entities participating in “cross-testing”;
ascribed to differences in pavement
configuration, material characteristics,
subgrade strength, climate, & type of test.
Load
intensity
and
equivalency
(cont)
34
Mechanisms of Distress
Trafficking (cont)
Tire pressure affects the compressive stress
within HMA more than wheel load. Tensile
strain at the bottom of HMA layer and
compressive strain at base and subgrade
level more affected by load.
Wide-based single tires resulted in 1.0 to
2.4 times more rutting than dual tires with
majority of rutting in the aggregate base.
Fatigue cracking approximately four
times greater.
Tire pressure
and type;
configuration
and contact
stress
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Mechanisms of Distress
Trafficking (cont)
Reversed performance results are possible
under APT loading (Nottingham; FDOT)!
Nature of distress is dependent on
pavement composition, material type,
temperature,
This is a significant finding that has to be
taken into account in planning APT
investigations or studies.
APT
Trafficking
Direction:
uni-
directional
versus
bi-
directional
36
Downward deformation mm
Equivalent E80s millions
Comparative Deformation of Crush Stone Bases
under different degrees of Saturation in SA
37
Benalla Trials (cont)Performance Distress
Mechanism
ALF •~ 22 million equivalent E80s.
Small deflections, minor rutting
and some surface ravelling
Minor rutting and
some surface
distress in form of
ravelling
Hwy •After ~ 12 million E80s (2002)
good correlation between
maximum deflections, rutting
and surface condition found
with ALF; expected life is
more than applied ALF loading
Similar to ALF
section
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Example:
7 Axle
EMPTY
Truck
(1:2:2:3)
Note Variation
in Vertical
Contact
Stresses on
all 30 tires
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10 20 30 40 50
Across the SIM pads
60 70 80
450
400
350
300
250
200
150
100
50
500
550
Co
unts
TEST 768-09/10/2003: DDT235N AXLE 2
189 mm;
0.037 Ton
297 mm;
4.312 Ton202 mm;
1.874 Ton194 mm;
1.223 Ton
TIRE barely in contact with surface
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Main conclusions
Overall finding • judicious use of f-sAPT contributes to and supports the
body of knowledge regarding the way that pavement materials and structures react to controlled traffic and environmental loads
• f-sAPT work conducted over the last decade provide important information to pavement engineering community to ensure sustainable and efficient supply of cost effective pavement-related infrastructure
47
Main conclusions
• f-sAPT perceived as important with a major role to be
played in pavement structure and basic materials research
• many programs share their facilities and data in order to
expand their database
• improved characterization of loading conditions mirrored
by use of more complicated materials models
48
Main conclusions
Major benefits of f-sAPT
improved structural and material design methods evaluation of novel materials improved performance modeling development of performance related specifications
evaluation of economic benefits of f-sAPT came to
forefront during past decade
more programs reporting attempts at performing Benefit
Cost Ratio (BCR) type evaluations of research programs
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Main conclusions
f-sAPT perceived as important with a major role to be
played in pavement structure and basic materials research
many programs share their facilities and data in order to
expand their database
improved characterization of loading conditions mirrored
by use of more complicated materials models
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Closing Remarks
We now have ca.twenty years of well documented
data and findings from APT in three Synthesis
Reports.
Looking back I surmise that we have achieved the
ability to bring practice and theory of pavement
engineering very close together.
The paper by Harvey et al in the 2012 fourth Intl
Conf on APT presented how they were able to use
mechanistic-empirical (ME) analysis to simulate
APT performance under actual conditions by
calibration.
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Closing Remarks cont....At the same conference Wynand Steyn made the
following remarks about topics that were still in need
of APT attention and applications:
• more detailed focus on vehicle-pavementinteraction and environment-pavementinteraction
• development of and improvements in performance related specifications
• improved MEPDG validation• evaluation of sustainable pavement solutions• improved reliability in pavement design
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