Performance testing at high service temperatures of asphalt mixtures for airfield pavements

Nicol Botha

1.00

1.10

1.20

1.30

1.40

1.50

1.60

1.70

1.80

10

15

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25

30

35B7

27-2

00

B737

-300

B767

-300

ER

A300

-C4

B777

-200

B737

-700

A320

-200

B777

-300

ER

A320

NEO

A380

-800

B747

-8

B747

-400

ER

B787

-9

B737

MAX

B787

-8

A340

-600

A380

-800

F

A350

-900

A321

neo

A350

-100

0

A330

-900

neo

B787

-10

B777

-9

80's 90's CURRENT FUTURE

Tire Pressure (MPa)W

heel

Loa

d (t

on)

Wheel Load

Tire Pressure

Developments in aircraft industry

1.00

1.10

1.20

1.30

1.40

1.50

1.60

1.70

1.80

10

15

20

25

30

35B7

27-2

00

B737

-300

B767

-300

ER

A300

-C4

B777

-200

B737

-700

A320

-200

B777

-300

ER

A320

NEO

A380

-800

B747

-8

B747

-400

ER

B787

-9

B737

MAX

B787

-8

A340

-600

A380

-800

F

A350

-900

A321

neo

A350

-100

0

A330

-900

neo

B787

-10

B777

-9

80's 90's CURRENT FUTURE

Wheel Load

Tire Pressure

Airfield Pavements – Structural Performance of Asphalt Mix

• Permanent deformation (rutting) of airfield pavements was never a big concern • Conventional (traditional) asphalt mixes performed satisfactorily• Changes to the aircraft manufacturing industry is playing a role in current asphalt

performance• More and more airfield pavements fail in permanent deformation • Rutting of airfield pavements has a serious impact on airport operations

0

50

100

150

200

250

0 0.5 1 1.5 2 2.5 3

Dep

th b

elow

sur

face

(mm

)

Depth Confinement Factor (kz)

Airfield Pavements – Structural Performance of Asphalt Mix

Known rut prediction models (e.g. MEPDG) hints that the biggest risk for

permanent deformations are approximately in the upper 100mm of

the asphalt layers

These models may be valid for typical highway

pavement loading conditions

This is a misperception based on actual field performance of

airfield pavements

Considers sensitivities relating to temperature and loading time – but not airfield loading

conditions

Majority of rutting

Airfield Pavements – Structural Performance of Asphalt Mix PM

APE

N20

5mm

Airfield Pavements – Structural Performance of Asphalt Mix PM

APE

N31

5mm

Traditional vs. Performance Based Asphalt Specifications

TRADITIONAL PERFORMANCE BASED

Chocolate Fudge Cake

200g plain chocolate, broken into chunks (use one with a low cocoa content - we used Waitrose plain Belgian chocolate)

200g butter

200g light brown muscovado sugar100ml soured cream

2 eggs, beaten200g self-raising flour5 tbsp cocoa powder

hundreds and thousands, to decorate

For the icing100g plain chocolate

170g can condensed milk100g butter

Adoption of Performance Based Asphalt Specifications

Performance Based Asphalt Specifications and Testing

European experience shows that satisfactory pavement performance is more likely obtained with performance-based asphalt requirements

FAA is moving towards asphalt mix performance based specifications (FAA AC 150/5370-10H, 2018)

South Africa is moving towards performance based specifications (Sabita Manual 35 – Level II and III mix design)

Structural Mix Performance

Resistance to permanent

deformation

Traffic related densification

Mix durability

FatigueMix stiffness

Fracture toughness

(resistance to crack

propagation)

Low temperature performance

Asphalt Mix Characteristics - Volumetrics

•Improved aggregate packing

•Aggregate shape•Filler content and type

•Proper classification in terms of PG

•Polymer modification•Bitumen content•Bitumen film thickness

•Voids filled with asphalt

•Voids at refusal density

BITUMENAIR VOIDS

AGGREGATES

Asphalt Mix Creep PerformanceCu

mul

ativ

e Ax

ial S

trai

n

Number of Load Repetitions

AVOID

Performance Tests Available

Typical asphalt performance test

• Flow Number• Dynamic Modulus• Triaxial Cyclic Compression Testing

• Hamburg Wheel Tracking Test• Asphalt Pavement Analyzer• Wheel path rutting (EN) - Small Device• Wheel path rutting (EN) - Large Device• Model Mobile Load Simulator (MMLS3)

• Fatigue

Triaxial tests

Wheel tracking tests

Four-point beam fatigue test

Wheel Tracking TestsHamburg Wheel tracking Test (HWTT)

Wheel path rutting (EN) - Large Device

Asphalt Pavement Analyzer (APA)• Steel wheel• Dependent on benchmark performance

(i.e. not an accurate reflection of actual field performance)

• Rigidity of the blank steel wheel tends to overestimate permanent deformation

• Pressurized hose• Tire pressures: 0.7 MPa - 1.7 MPa• Dependent on benchmark performance

(i.e. not an accurate reflection of actual field performance)

• Current benchmark value for airfield pavements mainly based on US conditions

• Pneumatic (inflated) tire• Tire pressure: 600 kPa• Wheel load: 5 000 N• Various temperatures and

loading conditions• produces rut profiles similar to

field performance

Model Mobile Load Simulator (MMLS3)• Pneumatic (inflated) tire• Various temperatures and loading conditions• Superior flexibility• Tire pressure: Adjustable• Wheel load: Adjustable• Lateral wander simulated

Flow Number

Flow number = Number of load repetitions to reach the start of tertiary creep

The creep slope could vary for different asphalt mixes having the same flow number

Not considered a true performance based criterion

Merely an indicator of the risk of permanent deformation

Provides no information about the creep slope (Stage 2)

Cum

ulat

ive

Axia

l Str

ain

Number of Load Repetitions

Triaxial Cyclic Compression Testing

Controlled confinement Controlled (dynamic) vertical

stress Controlled test temperature

Can be used to specify testing temperatures for different regions

Confinement with asphalt depth can be varied

Cum

ulat

ive

Axia

l Str

ain

Number of Load Repetitions

Concluding Remarks

Changes in the aircraft manufacturing industry

Field performance of current airfield asphalt pavements demands a shift in how the mixes are specified

Airfield vs. highway pavement performance

Using benchmark values obtained from different regions

Performance testing

More demanding loading conditions

Performance specifications closes the gap between field and laboratory performance

Loading conditions varies significantly – do not use highway pavement performance models for airfield pavements

Asphalt performance varies for different climatic regions and loading conditions

Know test limitations and boundary conditions

QUESTIONS?