Rationalisation of Pavement Design Methods. Graham Salt, Sam Wong & Dave Stevens

Transcription

1 Rationalisation of Pavement Design Methods 4 NOVEMBER 2014 NZIHT CONFERENCE 2014 Graham Salt, Sam Wong & Dave Stevens N to P

2 High Speed Data (surface & structural): IT Overload Network Overview, but also allows Project Level decisions Available to each region. All historic FWD, updated for age and/or rehab. Next year TSD infill (smaller symbols) Application Overlay requirements Mouse-over for details

3 High Speed Data (surface & structural): IT Overload Application Load Damage Exponents Now much improved. NZ 1 Regional PP (HPMV use) Mouse-over for details

4 Predicted Life by Distress Mode Model determines which mode of distress occurs first for each test point. Legend Flexure Excessive maintenance costs Shear Roughness Rutting Calibration preferable for each region.

5 Predicted Life by Distress Mode Cumulative plot allows ten percentile life to be readily assessed along with principal distress mode. Legend 10 percentile life => 0.04 MESA

6 Contents Reliable Prediction of Forward Work Programmes Pavement Performance Model IAL Mechanistic Pavement Analysis Pavement Structural Rehabilitation : Resurfacing vs Rehabilitation Structural Evaluation of Pavement Maintenance Cost Implications of Increased Weight Limits Net Present Value Method

7 Current Practice A wealth of pavement structural data has been collected in RAMM (Road Assessment and Maintenance Management software) Empirical approach: often only empirical parameters are adopted: Central Deflection (D0s) poor indicator of rutting distress Curvature (D0s-D200s) fair indicator of cracking in bound layers SNP very poor indicator for any distress mode and not consistent with Austroads Even if all 3 of the above are used, still using a small proportion of the structural data collected. Alternative Mechanistic approach (consider stress, strains and layer stiffnesses, using the majority of collected data

8 Forward Work Programme (FWP) A proposed timetable for network maintenance Allow pavement network to sustain the expected traffic levels Keep pavement condition above a certain standard Ensure works stay within approved budgets This requires a Pavement Performance Model (PPM)

9 Pavement Performance Model To predict the rate of deterioration Allow timing of future interventions Important: all prediction systems are limited by quality of input data and availability of comprehensive condition data Often FWPs are calculated at a network general level Gives a picture of level of deterioration in the network No specific roads identified

10 RELIABLE PREDICTION OF FORWARD WORK PROGRAMMES

11 Pavement Performance Model - IAL Structural parameters have been deduced from: Accelerated Pavement Testing (APT at CAPTIF) Long Term Pavement Performance (LTPP) sites in NZ Specific studies of networks and Interim APT-LTPP (IAL) Model: Uses a rational multi-layer elastic pavement model developed from NZ structural (FWD) data Describes remaining life (ESA) of a pavement under each primary structural distress modes

12 Pavement Performance Model Initially addressed four primary structural distress modes: Distress Mode Description Rutting Vertical surface deformation of pavement Results primarily from one dimensional compaction of the pavement layers and the subgrade. Some lateral movement possible in early life of the pavement Shear Lateral deformations or shoving within the pavement layers primarily related to shear. Associated increase in rut depth (likely to increase with ongoing load repetitions) Shear instability will commonly lead quickly to cracking of the surfacing, pumping and potholing.

13 Pavement Performance Model Initially addressed four primary structural distress modes: Distress Mode Description Roughness Loss of shape longitudinally along each wheel path Primarily governed by structural non-uniformity (longitudinally) causing variations in rut depth Also a secondary effect of shear instability Cracking The initiation of conventional bottom-up cracking in a bound layer Results from horizontal tensile strain at the bottom of the layer

14 Pavement Performance Model Recent advances encompasses additional modes including: Distress Mode Description Flexure The reversal of curvature of a thin surfacing Leads to the initiation of surface cracking and/or top-down cracking in bound layers associated with horizontal strains Excessive Maintenance Cost Strain reversal will occur as the deflection bowl passes along the wheel path; (tensile- compressive tensile) at the top of the surfacing and generally the reverse sequence at the bottom Although not a really a distress mode this is the reason for a large proportion rehabilitation decisions in many regions Where a regional precedent performance study has been carried out, recent developments have established an effective method for predicting pavement life until this cause dominates

15 Pavement Performance Model In practice, all the mechanisms for pavement structural deterioration can be inter-related. E.g. Cracking Initiated Water Ingress Accelerated Rutting/ Pumping/ Shallow Shear/ Potholing Roughness Deterioration Correct prediction of first significant failure mechanism will allow timely and appropriate intervention and significantly prolonged pavement life

16 Mechanistic Forward Work Programme Treatment length: Defined by NZTA as a uniformly performing contiguous section of road, which is performing differently from the adjacent sections For rational roughness progression modelling, treatment lengths should also be further separated into lanes Mechanistic analysis allows relevant sub-sectioning as the structural behaviour of the pavement is revealed E.g. Structurally weak sections are clearly differentiated from the sound sections Treatment lengths are therefore confined to relevant sub-sections

17 Network Maintenance Pavement treatment length without sub-sectioning

18 Network Maintenance Pavement treatment length with sub-sectioning

19 Mechanistic Approach to Pavement Management Advantages of Mechanistic Approach: Uses readily available structural data (FWD data) Consistent with most of the Austroads principles for analysis and design Allows rational insight into pavement performance Identifies relevant distress mechanisms Identifies the critical layer & treatment length and hence Allows decision on ideal alternative for effective remediation Can be readily improved as further information becomes available from pavement performance observations of specific distress modes within a given network or LTPP sites RESULT: MORE EFFECTIVE AND LOWER COST TREATMENTS

20 RESURFACING OR REHABILITATION?

21 Structural Evaluation of Pavement Correct assessment of distress mode is required to assess if either: Simple smoothing is sufficient, or Structural capacity needs to be increased (overlay or reconstruction required) E.g. To extend life of pavement with flexure distress: If existing surfacing is thin (and relatively uncracked) => resurfacing If thick or aged surfacings => require replacement or other structural rehabilitation. Structural data can be used to: Determine rehabilitation treatment of distressed pavements Determine remaining life of working pavement and governing distress mode (as shown in the example below)

22 Performance Model By comparing predicted and observed performance: Project Level Facilitates understanding of critical distress modes experienced and hence informed design of effective and targeted form of rehabilitation Network Level allows validity of the modelled FWP to be determined and recalibrated

23 MAINTENANCE COST IMPLICATIONS OF INCREASED VEHICLE WEIGHT LIMITS

24 Increase in Vehicle Weight Limits Allowances for heavier vehicle limits (e.g. HPMVs) will change rate of wear of pavements Depends on the change in number of standard axles incurred on a road Each distress mode (discussed earlier) has a different load factor or load damage exponent Increase in axle weight leads to exponential increase in wear Flexure most sensitive to increase in weight limits For FWP, it is important to be able to assess actual maintenance cost of allowing either increased numbers of heavy vehicles or increased axle load The most pragmatic way -> the Net Present Value (NPV) approach

25 NPV for FWP To carry out NPV assessment: 1. Determine treatment lengths rationally from structural analysis 2. FWP is assessed under the standard traffic scenario 3. FWP is assessed under the increased traffic loading scenario 4. The NPV is the net difference between the cost of maintenance

26 NPV for FWP Figures show the number of lane kilometres experiencing terminal condition from given distress modes for a pavement under standard and increased loadings Standard Traffic Increased Traffic

27 NPV for FWP Figures show cost of structural treatment, and the NPV to carry increased vehicle weights For most roads, the effect of increased loading does not make a large difference to amount of maintenance done in the long term NPV is substantially affected by the time cost of earlier expenditure as rehabilitation is brought forward Present value is calculated using present day construction costs and discount rate established by client

28 Pavestate app Smartphone or Desktop

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