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Loading Frequency and FatigueIn Situ Conditions & Impact on Test Results

Konrad Mollenhauer*, Michael Wistuba* and Rolf Rabe**

**Bundesanstalt fürStraßenwesen, www.bast.de

* Braunschweig Pavement Engineering Centre,www.tu-braunschweig.de

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• Loading frequency• Loading conditions in Full Scale Pavement Tests• Correlating traffic frequency to frequency of

cyclic loading in laboratory testing

• 4PB calibration tests at ISBS• Test device• Derivation of correction factors in the framework

of calibration• Reduction of moving masses

Contents

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Phase 1: Instrumented test track

Phase 2: Measuring mechanical response

non-destructive

Phase 3: Accelerated load testing

destructive

BASt Full Scale Pavement Testing

4

48

20

58

48

22

56

48

48

1418

20

15

4940

SMA 0/8 SABI 0/16 S

Asphaltbase course 0/22 CS

Cementstabilized sand

Cementbound gravel

Sand /Gravel 0/32

Sand 0/2

Frost blanketSand /Gravel 0/32

Section 1 Section 2 Section 3 Section 4

SV SV SV V

9,5 m 9,5 m 9,5 m 9,5 m

100 MPa

97 MPa

126 MPa

96 MPa

126 MPa

93 MPa

126 MPa

104 MPa

136 MPa

-90 cm

BASt Full Scale Pavement TestingTest track: Cross section

5

410

76

45

13

68

45

45

99

2015

5752

SMA 0/8 SABI 0/16 S

Crushedstone base 0/45

Section 5 Section 6 Section 7 Section 8

III III III V9,5 m 9,5 m 9,5 m 9,5 m

AT 0/22 CS

Sand /Gravel 0/32

144 MPa

128 MPa

114 MPa

158 MPa

126 MPa

99 MPa

135 MPa

104 MPa 105 MPa

116 MPa

Frost blanketSand /Gravel 0/32

-90 cm

BASt Full Scale Pavement TestingTest track: Cross section

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AD 4cmAB 8cm

AT 22cm

FSS 56cm

y

xz

Direction of travel

FzTemperature

Asphalt strain

Soil pressure

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gross weight: 40.1 tspeed: 2.7 km/hasphalt-temperature: 16.7 °C

3.12.7 2.3 1.7 2.1

tension/compression ratio:

8

9

ε/σz

ε/σy ε/σx

1 3+

1 3

2

ε/σz

ε/σy ε/σx

2

x

z

y

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Pavement: 4PB beam:

z

x

σz,1

σz,2

σy,1

σy,1

σy,2

σx,1

σx,2

σx,1

x

z

y

σx,2

σx,1

σx,2

σx,1

compression

tension

11

661 ms

70 ms

12

-40

-20

0

20

40

60

80

100

2000 2500 3000 3500 4000 4500

long

itud

inal

str

ain ε x

[µm

/m]

time [ms]

asphalt strain base course, static wheel load: 7075 kg, v = 7,5-7,7 km/h

section 3 (34 cm asphalt)section 7 (22 cm asphalt)section 8 (14 cm asphalt)

0,47 m

34 cm

22 cm

section 3 section 7

14 cm

section 8

0,96 m

0,60 m

13

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Full Scale Tests @ EPF Lausanne

[Wistuba 2005]

The strain signal at the bottom of the base course can befitted in good approximation by means of a sinusoidal function.

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Frequency of traffic loading

[Wistuba 2005]

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Temperature [°C] Frequency [Hz]Stiffness testing 20 8Fatigue testing 20 30

30 Hz ~ 60 km/h

Type testing of AC acc. to DIN EN 13108-20

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• Loading frequency• Loading conditions in Full Scale Pavement Tests• Correlating traffic frequency to frequency of

cyclic loading in laboratory testing

• 4PB calibration tests at ISBS• Test device• Derivation of correction factors in the framework

of calibration• Reduction of moving masses

Contents

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4PB tests at ISBSTest device

• servohydraulic Dyn-IT (Freundl, D)• static/cylic axial load : ± 50 kN• load amplitude: ± 0,5 mm @ 10 Hz• frequency: 30 Hz; 10 Hz @ ± 0,5 mm• sampling rate: 2 ms • climate chamber: -25 to +65°C • displacement controlled mode• specimen: segment roller compactor,

cut on all sides:– wearing course: 40*40*320 / 50*50*320 mm– base course: 60*60*500 / 70*70*600 mm

• LVDT (± 5.0 mm)• LVDT deflection

control directlyon specimen(play of innerclamps remainsunconsidered)

slide bearings

rotational bearings

specimen

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4PB calibration tests at ISBS

ϕcor = ϕmeasured - f ∙ 360 ∙ Δt

• device-dependent time-lag Δt between force and deflection

• correction of phase angle by

0,0028 s

Correction of phase angle:

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• aluminum beams40*40*280 mm³Eeff = 1.475 MPaEeff = 11.804 MPaEeff = 39.838 MPa

• correction of measured deflection Z(x)(play of outer clamps zPlay, stiffness of bearing frame)from back-calculationZ(x)corr = Z(x) – (zPlay + F0 · KFrame)

Correction of deflection:

4PB calibration tests at ISBS

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4PB calibration tests at ISBS

steel frame

aluminium frame

specimen

•up-to-date test devices with self-acting correction of mass inertia (accelerometer in load cell)

•Force-controlled mode: problems @ f > 10 Hz due to moving masses

•Deflection-controlled mode: correction acc. EN

Moving mass:

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4PB calibration tests at ISBS

• reduction of moving mass:

• no slide bearings for inner clamps

• aluminium frame of12 kg for inner clamps (instead of 35 kg)

• reduction (meq = 15 kg) relevant for f > 25 Hz (represented by the ratio of mass inertia and force acting on the beam)

Correction due tomass inertia:

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Summary•Full scale testing:

• 2 main stress/strain situations determined under the passing wheel

• asphalt strain frequency depends on layer thickness

• test frequency in 4PB correspondsto traffic speed by equationf [Hz] ~ 0,5 v [km/h]

•Calibration procedure:• correction of measured deflection• correction of device-dependent

time-lag between force and deflection• consideration of moving masses

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Thank you !

www.tu-braunschweig.dewww.bast.de

Muito obrigado !