Some Significant Findings fromAccelerated Pavement Testing

Fred Hugo

Emeritus Professor and Research Fellow

Stellenbosch University

South Africa

Pavinars

7 October 2014

1

Stellenbosch University

South Africa

2

APT in the

South African

Desert 1966

3

APT Vicksburg - Ned Burns 1967

4

According to NCHRP Synthesis

433 Significant Findings on Full-

Scale APT there are now ca.30

facilites operational in the

United States

5

APT as an element of pavement engineering

(after Hugo et al, 1991 –TRR 1293)

Model APT

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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 vs. Time vs. Degradation

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Variety of Traffic vs. Time vs. Degradation

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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

14

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.

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Moisture damaged HMA cores due to wet MMLS3

trafficking at NCAT. Damage much greater than under

truck trafficking

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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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SASW stiffness vs Cracking in Fatigue

studies in North Carolina USA

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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)

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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

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Downward deformation mm

Equivalent E80s millions

Comparative Deformation of Crush Stone Bases

under different degrees of Saturation in SA

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Benalla Trials

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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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Stress – In – Motion (SIM) Technology – 4

PAD – N3 – TCC De Beer et al CAPSA 2004

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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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Example:

7 Axle Truck

(1:2:2:2)

fully loaded

with cement

Note Contact

Stresses on

Steering Tires

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Example:

7 Axle Truck

(1:2:2:2)

Note missing

tire

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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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Deflection vs. speed under full-

scale truck traffic

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Selected extracted notes from

NCHRP

SYNTHESIS 433

FULL- SCALE APT

by

WJvdM Steyn

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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

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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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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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