TOWARDS THE APPLICATION OF

STRESS-IN-MOTION (SIM)

RESULTS IN PAVEMENT DESIGN

AND

INFRASTRUCTURE PROTECTION

Morris De Beer, Colin Fisher (CSIR, Transportek)

and

Louw Kannemeyer (SANRAL)

Layout of Presentation

• Introduction;

• Stress-In-Motion (SIM) system;

• Pavement Modeling using 3 loading cases;

• Concept of Normalised Contact Stress (NCP);

• Conclusions and recommendations.

Introduction

• Protection of road infrastructure – major challenge to RA’s;

• Africa: Inter - regional traffic – as much as 70 % over loading;

• World wide: Road user charges - complex;

• Current tensions: Road Authorities vs private sector interests.

• CHALLENGE TO US “TECHNOCRATS” FOR SOLUTIONS !

Stress-In-Motion (SIM) system

• Description;

• Calibration and data acquisition;

• Typical Data and outputs;

– Normalised Contact Pressure (NCP);

– Effect of tyre speed;

– Pavement response-top down cracking;

Figure 1: Single SIM pad configuration (used under Heavy

Vehicle Simulator (HVS) with single test tyre)

Figure 4: SAE sign convention used during SIM measurements.

X-Longitudinal, Y-Lateral and Z -Vertical loads/stresses

•

Tyre Rotation

+Z

+X

+Y

Direction

of

Traffic

Instrumented

Pins

356 mm

755 mm

pattern1.wmf

TOP VIEW OF THE VRSPTA MK II

TYREPATCH

TYREPATCH

TYREPATCH

425 /65 R22.5 HVS TYRE ON SIM SYSTEM:

EXAMPLE OF FOOTPRINT TO FOLLOW…..

5-Axial Load Cell:

[+/- X; +/- Y; Z]

SIM Measuring Pins- 3-Axial

Laboratory Calibration Issues

“Pin by Pin ” calibration- Jig A

Laboratory Calibration Issues

“In-Situ” calibration - Jig B

Typical Pin Calibration data

-800 -700 -600 -500 -400 -300 -200 -100 0 100 200 300 400 500 600 700 800

-4

-3

-2

-1

0

1

2

3

4

ENTRAN LOADCELL (NEWTONS)

LOAD

CELL

(VOL

TS)

X Y Z

3D LOADCELL PIN 10LOADING IN "X" DIRECTION

3D-CELLX/Y/Z.WK3

11/01/1999

-800 -700 -600 -500 -400 -300 -200 -100 0 100 200 300 400 500 600 700 800

-4

-3

-2

-1

0

1

2

3

4

ENTRAN LOADCELL (NEWTONS)

LOAD

CELL

(VOL

TS)

X Y Z

3D LOADCELL PIN 10LOADING IN "Y" DIRECTION

3D-CELLX/Y/Z.WK3

11/01/1999

0 100 200 300 400 500 600 700 800

-0.5

0.5

1.5

2.5

3.5

4.5

ENTRAN LOADCELL (NEWTONS)

LOAD

CELL

(VOL

TS)

X Y Z

3D LOADCELL PIN 10LOADING IN "Z" DIRECTION

3D-CELLX/Y/Z.WK3

11/01/1999

+/- X - LOAD (kN)

+/- Y - LOAD (kN)

Z - LOAD (kN)

VO

LT

SV

OL

TS

VO

LT

S

315/80 R22.5 HVS TYRE ON SIM MK II

HVS 04 SIM MEASUREMENTS OF 11R22.5 TYRE

INFLATION PRESSURE

Z -

LOAD

Figure 2: Twin (or dual) SIM pad configuration (used under

Heavy Vehicle Simulator (HVS) with dual test tyres)

Figure 2: Quad (full) SIM pad configuration at a typical

weighbridge site on National Road 3 (N3), near Heidelberg

in Gauteng.

In operation: Quad (full) SIM pad configuration at a typical

weighbridge site on National Road 3 (N3), near Heidelberg

in Gauteng.

In operation – SIM N3-TCC

In operation: SIM N3-TCC

Table 1 Example of loading cases

Case Cold

Inflation

Pressure

(kPa)

Single tyre

Loading

(kN)

% of rated

loading @

720 kPa[1]

Comments

1 720 20 - 20

Under-

loaded

2 720 35 + 45

Overloaded

3 720 50 + 107

Extremely

overloaded

[1] For this test tyre the rated load at 720 kPa = 24 kN

(a) Case 1:

20 kN, 720 kPa

(b) Case 2:

35 kN, 720 kPa,

(c) Case 1:

50 kN, 720 kPa

Figure 6: Static Tyre Foot Prints

- 20 %+ 45 %

+ 107 %

SIM DATA USED FOR ANALYSIS

SIM DATA : VERTICAL STRESSD (Z)

SIM DATA : LATERAL STRESS (Y)

SIM DATA: LONGITUDINAL STRESS (X)

Loading

Case

Max

Vert Stress: Z

(kPa)

Max

Lat Stress: Y

(kPa)

Max

Long Stress:

X(kPa)

1 (n-

shape)

953 191 102

2 (m-

shape)

1189 209 185

3 (m-

shape)

1486 261 210

Table 2: Maximum Stresses

NORMALISED CONTACT PRESSURE

(NCP)

Maximum Contact Stress @ load P

Inflation Pressure (rated tyre load)NCP =

NCPz - Vertical Stress;

NCPx - Long. Stress;

NCPy - Lateral Stress;

Performance Based Standard ?

Load

Case

NCPZ NCPY NCPX

1 (n-

shape) 1.32 0.27 0.14

2 (m-

shape) 1.65 0.29 0.26

3 (m-

shape) 2.06 0.36 0.29

Table 3: NCPs

NCPs cont.

NORMALISED CONTACT PRESSURE(NCP)- FREE ROLLING TYRE

NC

Pi

0

0.5

1

1.5

2

2.5

10 15 20 25 30 35 40 45 50 55

TYRE LOADING (kN)

NCPz-VERTICAL

NCPy-LATERAL

NCPx-LONGITUDINAL

LIMITING NCPz = 1.5

Interface shear stresses (x,y)

Lateral (X)shear stresses

Long. (Y) shear stresses

425 /65 R22.5 HVS TIRE ON SIM SYSTEM:

EXAMPLE OF FOOTPRINT TO FOLLOW…..

SHEAR STRESS (X-Y) EXCURSION - Single Tyre: 20 kN, 720 kPa (Test T472A)

-300

-200

-100

0

100

200

300

-200 -150 -100 -50 0 50 100 150 200

X-Longitudinal Stress (kPa)

Y-L

ate

ral S

tress (

kP

a)

Pin 3

Pin 5

Pin 9

Pin 12

Pins: 3 5 9 12

Tyre Tread

LATERAL (X-Y) STRESS EXCURSIONS

SHEAR STRESS (X-Y) EXCURSION - Single Tyre: 35 kN, 720 kPa (Test T772A)

-300

-200

-100

0

100

200

300

-200 -150 -100 -50 0 50 100 150 200

X-Longitudinal Stress (kPa)

Y-L

ate

ral S

tre

ss (

kP

a)

Pin 3

Pin 5

Pin 9

Pin 12

Pins: 3 5 9 12

Tyre Tread

LATERAL (X-Y) STRESS EXCURSIONS

SHEAR STRESS (X-Y) EXCURSION - Single Tyre: 50 kN, 720 kPa (Test T1072A)

-300

-200

-100

0

100

200

300

-200 -150 -100 -50 0 50 100 150 200

X-Longitudinal Stress (kPa)

Y-L

ate

ral S

tress (

kP

a)

Pin 3

Pin 5

Pin 9

Pin 12

Pins: 3 5 9 12

Tyre Tread

LATERAL (X-Y) STRESS EXCURSIONS

Effects of Tyre Speed on contact stress

• Not studied in detail with SIM;

• Preliminary work done in 1996 on car tyres;

• Changes in shape of stress regime expected;

• Axle lift may result in smaller contact patch

(6 % @ 100 km/h);

• Some pavement response parameters

decrease with increased speed.

Effect of Tyre Speed: 27.6 km/h

Effect of Tyre Speed: 58.1 km/h

Top Down Cracking in (“thick”) AC layers

• MAJOR REASON:Non-

Traffic associated-

shrinkage, temperature,

construction, etc.;

• Traffic associated- tyres

and stresses – but not

solely responsible for this

type of cracking;

• Working conjointly - most

probable scenario;

Circular load - Tyre Model (Blab, 2001)

AC

G3

Subgrade

VERTICAL STRESS: TYRE CENTRE AND EDGE

0

200

400

600

800

1000

1200

1400

Case 1-std Case 1 Case 2-std Case 2 Case 3-std Case 3

Vert

ical S

tress (

kP

a)

Vertical Stress - Centre Vertical Stress - Edge

Case 1: 20 kN; 720 kPa Case 2: 35 kN; 720 kPaCase 3: 50 kN; 720 kPa

VERTICAL ELASTIC DEFLECTION ON SURFACE:

TYRE CENTRE AND EDGE

0

200

400

600

800

1000

1200

1400

Case 1-std Case 1 Case 2-std Case 2 Case 3-std Case 3

Ve

rtic

al D

efle

ctio

n (

um

)

Deflection - Centre Deflection - Edge

Case 1: 20 kN; 720 kPa Case 2: 35 kN; 720 kPaCase 3: 50 kN; 720 kPa

HORISONTAL STRAIN BOTTOM OF 20 mm ASPHALT

SURFACING: TYRE CENTRE AND EDGE

0

100

200

300

400

500

600

700

800

Case 1-std Case 1 Case 2-std Case 2 Case 3-std Case 3

Horisonta

l S

train

(um

)

Tensile Strain - Centre Tensile Strain - Edge

Case 1: 20 kN; 720 kPa Case 2: 35 kN; 720 kPa Case 3: 50 kN; 720 kPa

FACTOR OF SAFETY (FoS) - GRANULAR BASE TYRE CENTRE AND EDGE

0

0.1

0.2

0.3

0.4

0.5

0.6

Case 1-std Case 1 Case 2-std Case 2 Case 3-std Case 3

Case

Fa

cto

r o

f S

afe

ty (

Fo

S)

FoS-Centre FoS-Edge

Case 1: 20 kN; 720 kPa Case 2: 35 kN; 720 kPa Case 3: 50 kN; 720 kPa

VERTICAL STRAIN ON TOP OF SUBGRADE: TYRE CENTRE AND EDGE

0

500

1000

1500

2000

2500

3000

3500

Case 1-std Case 1 Case 2-std Case 2 Case 3-std Case 3

Ve

rtic

al S

tra

in (

um

)

Vertical Strain - Centre Vertical Strain - Edge

Case 1: 20 kN; 720 kPa Case 2: 35 kN; 720 kPaCase 3: 50 kN; 720 kPa

PAVEMENT LIFE : MECHANISTIC –

EMPIRICAL: CRITICAL LAYER APPROACH (me-PADS)

0.00E+00

2.00E+04

4.00E+04

6.00E+04

8.00E+04

1.00E+05

1.20E+05

Case 1-std Case 1 Case 2-std Case 2 Case 3-std Case 3

“La

ye

r L

ife

”

Surfacing-Centre G3-Base-Centre Subgrade-Centre

Layer Life

> 1.00+06

1<NCPc<1.5

NCPe > 1.5

NCPc =

NCPe ~ 1.0

PERFORMANCE BASED STANDARD (PBS)

NCPz (for Vertical Stress) < 1.5

….for rated tyre loading on

thinly surfaced pavements.. ?

IN SUMMARY…….

• Use of SIM technology demonstrated;

• Tyre contact stress shapes: Vertical: typically

“n-shapes and “m”-shapes;

• X-Y Shear excursion plots- useful concept ?;

• Three-circle modeling in MLLE analyses;

• NCP useful concept recommended for PBS;

RECOMMENDATIONS…..

• Continued R & D with SIM:

– Higher speeds;

– Braking and acceleration (down hill);

• Analysis of a wider range of pavement

types;

• Improved modeling (tyre contact patch non-

uniform & non-circular);

• NCPz, NCPx, NCPy to be further

investigated for PBS applications.

Thank you