Flexural Life of Unbound Granular Pavements with Chip Seal Surfacings

Transcription

1 Flexural Life of Unbound Granular Pavements with Chip Seal Surfacings

2 Austroads Design Checks vertical strains at top of subgrade, ie primary function is to limit rutting BUT - trigger for rehabilitation of NZ unbound granular pavements is frequently because of mechanisms other than rutting distress. Check for other failure modes??? If AC check horizontal tensile strains If chip seal no check (even if multiple chip seal layers) but should there be some check for flexure or is it always non-critical?

3 Flexural Life Maintenance costs are a principal trigger for rehabilitation Much will be repair of surfacing distress that could probably be classified as flexural ie repeated flexure and tensile strains giving rise to (i) cracking initiation (with subsequent water infiltration quickly followed by pumping and potholing), or (2) basecourse degradation (leading to shoving), or (3)other instability of the surfacing. Therefore a flexural measure may be worth considering for unbound pavements MRWA for several years have been specifying allowable curvature (D 0 -D 200 ) in new unbound chip seal pavements (but less restrictive criterion than for AC). Ie MRWA are essentially limiting horizontal tensile strains in the surfacing

4 1 ESA Transverse Section (Static or Dynamic) Tensile strains at bottom > tensile strains at top

5 1 ESA Longitudinal Section Static symmetric Dynamic asymmetric due to inertia and visco-elastic effects Traction=, 65 kpa (to counter air resistance on level road at 80 km/hr) but greater on inclines. May be -240 kpa with typical braking Check both top and bottom of layer Tensile strain at top > tensile strain at bottom, for a thin seal layer

6 Potential Indicators of Flexural Distress for (Single or Multiple) Chip Seal Layers Tensile horizontal stress at base of layer Tensile horizontal stress at top of layer Function of both (combined) Maximum of tensile stresses at top or bottom of the surfacing layer (for thin seals, top strains may be 3 times larger than at bottom) Or similar function, provided it can be readily determined from RAMM (eg multi-layer elastic analysis of deflection bowls) Load deflection hysteresis loop

7 Hysteresis from Load-Deflection Curve Low dissipated energy High dissipated energy

8 Establishing a Flexural Criterion Backanalyses & case histories Austroads Guide (10 5 to 10 8 ESA) + traction Austroads Light Traffic (10 3 to10 5 ESA) Case histories where flexural distress (especially cracking) is evident. TNZ Long Term Pavement Performance sites

9 Investigating a Flexural Criterion Permissible Tensile Strain in Bituminous Layers 10,000 Austroads Pavement Design Chart (10 5 to 10 8 ESA) + traction Austroads Light Traffic (10 3 to10 5 ESA) 1,000 TRRL1 TRRL2 100 Asphalt Inst. Denmark 10 NAASRA Unknown - Asphalt (1379 MPa) Friction Course Austroads Figure 6.8 Backanalyses of Austroads Design Charts Interim Tensile Strain Criterion for Chip Seals 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 1.E+07 1.E+08 Number of load repetitions

10 Investigating a Flexural Criterion Permissible Tensile Strain in Bituminous Layers 10,000 Austroads Pavement Design Chart (10 5 to 10 8 ESA) + traction Austroads Light Traffic (10 3 to10 5 ESA) 1,000 TRRL1 TRRL2 100 Asphalt Inst. Denmark NAASRA 10 Unknown - Asphalt (1379 MPa) Friction Course Austroads Figure 6.8 Backanalyses of Austroads Design Charts Interim Tensile Strain Criterion for Chip Seals 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 1.E+07 1.E+08 Number of load repetitions Note absolute value applies only if same assumptions are used (correct modelling of subgrade modulus non-linearity as that has significant effect on moduli of upper layers, consistent isotropy)

11 National LTPP Sites

12 Establishing Flexural Criterion TNZ LTPP SITES Max horiz tensile strain vs ESA May be reasonable to assume that on average, the designers expectation for 25 year life will be met!

13 Tensile MicroStrain APPLICATION: 1 Re-seal or Overlay? Facilitate decision on whether a distressed surfacing will perform adequately with just a new chip seal layer, or is structural overlay required. At this stage, suggest consider in relative rather than absolute terms or calibrate in relation to existing distress No.2 Rd (13/09/2006) R Road Name: No.2 Rd (13/09/2006) R1 Parameter: Tensile Strain at the Top of Surfacing (microstrain) Road Name: No.2 Rd (13/09/2006) R1 Parameter: Tensile Strain at the Bottom of Surfacing (microstrain) Chainage (km)

14 Percentage of samples < X APPLICATION: 2 Total Life if Resealed V. Tentative Network Data - MESA - Total Traffic (MESA) - Flexure Model No.2 Rd (13/09/2006) No.2 Rd (13/09/2006) MESA - Total Traffic (MESA) - Flexure Model

15 Percentage of samples < X Percentage of samples < X APPLICATION: 3 Absolute Measure of Pavement Life? Comparison of Minor Rd & Motorway Lifetime MESA (Cumulative Distribution) Network Data - MESA - Total Traffic (MESA) - Flexure Model 01N (2003) (8/09/2003) No.2 Rd (13/09/2006) No.2 Rd (13/09/2006) 90 Road Name: 01N (2003) (8/09/2003) Parameter: Total Traffic (MESA) - Flexure Model MESA - Total Traffic (MESA) - Flexure Model Total Traffic (MESA) - Flexure Model Minor Rd, 5 percentile Flexural Life = 40, 000 ESA Motorway, 5 percentile Flexural Life = 30,000,000 ESA

16 Total Traffic (MESA) APPLICATION: 3 Minor Road Volcanic Subgrade Example Flexural Life compared to Austroads GMP Life Road Name: No.2 Rd (13/09/2006) L1 Parameter: Total Traffic (MESA) - Austroads (GMP-Rigorous) Subgrade Only Road Name: No.2 Rd (13/09/2006) L1 Parameter: Total Traffic (MESA) - Flexure Model Chainage (km)

17 APPLICATION: 4 Motorway Example Flexural Life compared to Austroads GMP Life Total Traffic (MESA) Road Name: 01N (2003) (8/09/2003) Parameter: Total Traffic (MESA) - Austroads (GMP-Rigorous) Subgrade Only Road Name: 01N (2003) (8/09/2003) Parameter: Total Traffic (MESA) - Flexure Model Chainage (km)

18 APPLICATION: 5 AWT where rutting is not terminal or as additional check if there is rutting. Precedent design - same principle as TNZ supplement for rutting. Permissible Tensile Strain in Chip Seal Surfacing 10,000 1,000 - Tensile Strain Criterion Compute tensile microstrain for each point along treatment length. Adopt relevant percentile (90th or 95th) precedent value. (Past ESA, Microstrain) Permissible strain for Future ESA Past ESA Future ESA Number of load repetitions (ESA)

19 CONCLUSIONS Flexural check for chip seals Investigation stage only at present, but even so: Provides rational rehabilitation design based on precedent for pavements that are not subject to significant rutting (especially volcanic subgrades) Will reduce thickness of overlays on subgrades that do not exhibit rutting Will be an aid to judgment for reseal vs overlay Will only increase thickness of a small percentage of overlays, but this has advantages, ie: Will reduce risk of premature distress from that mechanism No different from other multiple checks in engineering design (eg moment and shear in beam design) Easily added to deflection testing output, ready for application in pavement design using conventional layered elastic model.

20 Flexural Life of Unbound Granular Pavements with Chip Seal Surfacings