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)
Tyre Rotation +X +Y +Z • Figure 4: SAE sign convention used during SIM measurements. X-Longitudinal, Y-Lateral and Z -Vertical loads/stresses
TOP VIEW 356 mm OF THE VRSPTA TYRE MK II PATCH TYRE Instrumented Pins 755 mm PATCH TYRE Direction PATCH of Traffic pattern1.wmf
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 3D LOADCELL PIN 10 LOADING IN "X" DIRECTION 4 3 VOLTS 2 LOADCELL (VOLTS) 1 0 -1 -2 -3 -4 -800 -700 -600 -500 -400 -300 -200 -100 0 100 200 300 400 500 600 700 800 +/- X - LOAD (kN) ENTRAN LOADCELL (NEWTONS) X Y Z 3D-CELLX/Y/Z.WK3 11/01/1999 3D LOADCELL PIN 10 LOADING IN "Y" DIRECTION 4 VOLTS 3 2 LOADCELL (VOLTS) 1 0 -1 -2 -3 -4 -800 -700 -600 -500 -400 -300 -200 -100 0 100 200 300 400 500 600 700 800 ENTRAN LOADCELL (NEWTONS) +/- Y - LOAD (kN) X Y Z 3D-CELLX/Y/Z.WK3 11/01/1999 3D LOADCELL PIN 10 LOADING IN "Z" DIRECTION 4.5 VOLTS 3.5 LOADCELL (VOLTS) 2.5 1.5 0.5 -0.5 0 100 200 300 400 500 600 700 800 Z - LOAD (kN) ENTRAN LOADCELL (NEWTONS) X Y Z 3D-CELLX/Y/Z.WK3 11/01/1999
315/80 R22.5 HVS TYRE ON SIM MK II
HVS 04 SIM MEASUREMENTS OF 11R22.5 TYRE Z - LOAD INFLATION PRESSURE
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 Single tyre % of rated Comments Inflation Loading loading @ Pressure (kN) 720 kPa[1] (kPa) Under- 1 720 20 - 20 loaded Overloaded 2 720 35 + 45 Extremely overloaded 3 720 50 + 107 [1] For this test tyre the rated load at 720 kPa = 24 kN
(c) Case 1: (a) Case 1: (b) Case 2: 50 kN, 720 kPa 20 kN, 720 kPa 35 kN, 720 kPa, + 107 % + 45 % - 20 % Figure 6: Static Tyre Foot Prints
SIM DATA USED FOR ANALYSIS
SIM DATA : VERTICAL STRESSD (Z)
SIM DATA : LATERAL STRESS (Y)
SIM DATA: LONGITUDINAL STRESS (X)
Table 2: Maximum Stresses Loading Max Max Max Case Vert Stress: Z Lat Stress: Y Long Stress: (kPa) (kPa) X(kPa) 1 (n- 953 191 102 shape) 2 (m- 1189 209 185 shape) 3 (m- 1486 261 210 shape)
NORMALISED CONTACT PRESSURE (NCP) Maximum Contact Stress @ load P NCP = Inflation Pressure (rated tyre load) NCPz - Vertical Stress; NCPx - Long. Stress; NCPy - Lateral Stress; Performance Based Standard ?
Table 3: NCPs Load NCP Z NCP Y NCP X Case 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
NCPs cont. NORMALISED CONTACT PRESSURE (NCP)- FREE ROLLING TYRE 2.5 NCPz-VERTICAL 2 LIMITING NCPz = 1.5 1.5 NCPi 1 NCPy-LATERAL 0.5 NCPx-LONGITUDINAL 0 10 15 20 25 30 35 40 45 50 55 TYRE LOADING (kN)
Interface shear stresses (x,y) Long. (Y) shear stresses Lateral (X)shear stresses
425 /65 R22.5 HVS TIRE ON SIM SYSTEM: EXAMPLE OF FOOTPRINT TO FOLLOW …..
LATERAL (X-Y) STRESS EXCURSIONS SHEAR STRESS (X-Y) EXCURSION - Single Tyre: 20 kN, 720 kPa (Test T472A) 300 Tyre Tread 200 Y-Lateral Stress (kPa) Pins: 3 5 9 12 100 Pin 5 Pin 3 Pin 9 0 -100 Pin 12 -200 -300 -200 -150 -100 -50 0 50 100 150 200 X-Longitudinal Stress (kPa)
LATERAL (X-Y) STRESS EXCURSIONS SHEAR STRESS (X-Y) EXCURSION - Single Tyre: 35 kN, 720 kPa (Test T772A) Tyre Tread 300 200 Pins: 3 5 9 12 Y-Lateral Stress (kPa) Pin 5 100 Pin 3 0 Pin 9 -100 Pin 12 -200 -300 -200 -150 -100 -50 0 50 100 150 200 X-Longitudinal Stress (kPa)
LATERAL (X-Y) STRESS EXCURSIONS SHEAR STRESS (X-Y) EXCURSION - Single Tyre: 50 kN, 720 kPa (Test T1072A) Tyre Tread 300 200 Pins: 3 5 9 12 Pin 5 Y-Lateral Stress (kPa) Pin 3 100 0 Pin 9 -100 Pin 12 -200 -300 -200 -150 -100 -50 0 50 100 150 200 X-Longitudinal Stress (kPa)
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 1400 Case 3: 50 kN; 720 kPa Case 2: 35 kN; 720 kPa Case 1: 20 kN; 720 kPa 1200 Vertical Stress (kPa) 1000 800 600 400 200 0 Case 1-std Case 1 Case 2-std Case 2 Case 3-std Case 3 Vertical Stress - Centre Vertical Stress - Edge
VERTICAL ELASTIC DEFLECTION ON SURFACE: TYRE CENTRE AND EDGE 1400 Case 3: 50 kN; 720 kPa Case 2: 35 kN; 720 kPa Case 1: 20 kN; 720 kPa 1200 Vertical Deflection (um) 1000 800 600 400 200 0 Case 1-std Case 1 Case 2-std Case 2 Case 3-std Case 3 Deflection - Centre Deflection - Edge
HORISONTAL STRAIN BOTTOM OF 20 mm ASPHALT SURFACING: TYRE CENTRE AND EDGE 800 Horisontal Strain (um) Case 1: 20 kN; 720 kPa Case 2: 35 kN; 720 kPa Case 3: 50 kN; 720 kPa 700 600 500 400 300 200 100 0 Case 1-std Case 1 Case 2-std Case 2 Case 3-std Case 3 Tensile Strain - Centre Tensile Strain - Edge
FACTOR OF SAFETY (FoS) - GRANULAR BASE TYRE CENTRE AND EDGE 0.6 Case 2: 35 kN; 720 kPa Case 3: 50 kN; 720 kPa Case 1: 20 kN; 720 kPa Factor of Safety (FoS) 0.5 0.4 0.3 0.2 0.1 0 Case 1-std Case 1 Case 2-std Case 2 Case 3-std Case 3 Case FoS-Centre FoS-Edge
VERTICAL STRAIN ON TOP OF SUBGRADE: TYRE CENTRE AND EDGE 3500 Case 3: 50 kN; 720 kPa Case 1: 20 kN; 720 kPa Case 2: 35 kN; 720 kPa 3000 Vertical Strain (um) 2500 2000 1500 1000 500 0 Case 1-std Case 1 Case 2-std Case 2 Case 3-std Case 3 Vertical Strain - Centre Vertical Strain - Edge
PAVEMENT LIFE : MECHANISTIC – EMPIRICAL: CRITICAL LAYER APPROACH ( me-PADS) Layer Life > 1.00+06 1.20E+05 NCPc = NCPe ~ 1.0 1.00E+05 “Layer Life” 8.00E+04 1<NCPc<1.5 NCPe > 1.5 6.00E+04 4.00E+04 2.00E+04 0.00E+00 Case 1-std Case 1 Case 2-std Case 2 Case 3-std Case 3 Surfacing-Centre G3-Base-Centre Subgrade-Centre
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
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