estimating remaining service life
DESCRIPTION
Estimating Remaining Service LifeTRANSCRIPT
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Estimating Remaining Service LifeRobert Rodden, P.E.www.robertrodden.com
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Network-Level ImpactEstimating Remaining Service Life
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Some Perspective on Asset Management Strategic asset allocation is a well established theory that is most notably applied in personal portfolio management
This method adheres to the base policy mix principle, in which a combination of asset classes exists and the combined return is based on a proportionate combination of each asset
For example, for an investment portfolio:
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% of
Portfolio
Annual
Rate of
Return Return
Stocks 75 10% 7.5
Bonds 25 5% 1.25
Combined 100 8.75% 8.75
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The Pavement Asset Management Cycle Application of strategic asset allocation to a pavement system allows the system to maintain the network in the highest overall condition possible at the lowest constant level of dollar flow into the pavement network
Such a system is inherently dynamic, so reallocation is necessary at regular intervals to deliver a continuously optimized system
Thus, the asset allocation mix will reflect the strategic goals for the system at any given time
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Network Health The Federal Highway Administration (FHWA) publication A Quick Check of Your Highway Network Health says:
By viewing the network in this manner [with each pavement as an asset in a collected network], there is a certain comfort derived from the ability to match pavement actions with their physical/functional needs. However, by only focusing on projects, opportunities for strategically managing entire road networks and asset needs are overlooked.
This approach might conflict with the traditionally used worst first approach
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Remaining Service Life Approachto a Network Level PMS It is critical to know if a proposed action (e.g., preservation or reconstruction) will produce a net improvement of the overall condition of the roadway network
The framework presented in this FHWA document assumes that a remaining service life (in years) is estimated for every section in the roadway network
Note that the remaining service life (RSL) may not be the point of ultimate failure of the roadway but, possibly, a pre-determined trigger at which preservation or some other activity will be considered
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Current Network Condition 10
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Network Condition in 1 Yearif Nothing is Done 1
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The % of the network with 0 years remaining will begin to increase with time
The total pavement life lost each year is the # of lane mi (km) in network x 1 year to keep system at the current condition, you have to add that much life to the system each year!
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The Worst-First Approach The traditional approach has been to do something about the pavement sections with 0 years of service life remaining
When a pavement has 0 years of service life remaining, reconstruction likely is necessary; reconstruction is the most expensive approach to add significant service life to a section
When reconstructed, the % of roadways with 0 years of service life remaining will have a new service life that might originally be in-line with the intended design life
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Impact of New Construction Service Life on the Network RSL Consider a simple, hypothetical network of 3,000 mi (km)
1/3 of network requires reconstruction NOW!
1/3 of network requires work in 5 years
1/3 of network requires work in 10 years
Current average remaining service life (RSL) of the network = (1,000 * 0 yrs + 1,000 * 5 yrs + 1,000 * 10 yrs)/3,000 = 5 yrs
The options for reconstruction are:
Long-Term = 30 year service life
Short-Term = 15 year service life
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Hypothetical Network Results
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mi (km) of Network
Years toNext Fix
Years of Servicein Segments
1,0001,0001,000
51030
Average Remaining Service Life for Network = 45,000/3,000 = 15 yr
Total = 45,000 yr-mi (yr-km)
5,000 yr-mi (yr-km)10,000 yr-mi (yr-km)30,000 yr-mi (yr-km)
(1000 x 5)
mi (km) of Network
Years toNext Fix
Years of Servicein Segments
1,0001,0001,000
51015
5,000 yr-mi (yr-km)10,000 yr-mi (yr-km)15,000 yr-mi (yr-km)
Total = 30,000 yr-mi (yr-km)
Average Remaining Service Life for Network = 30,000/3,000 = 10 yr
(1000 x 5)
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Deciding What, When, Where Evaluation of an existing PMS and consideration of alternative new construction options is relatively simple once the PMS is developed
Making the decision on what pavement preservation activity is most appropriate and at what time and location on a specific project is much more significant to the success of an effective pavement allocation program
Remaining Service Life (RSL) is the tool we need to apply.
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Looking into the Future The primary problem with RSL on a network level is that it focuses exclusively on performance, thus ignoring the cost impacts
In the U.S., attempts are being made now to try and tie RSL and $ together in PMS to develop metrics like $ / lane mi (km) / yr this will server as a much more pure metric of the cost effectiveness of both new construction and preservation alternatives
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Project-Level EstimationEstimating Remaining Service Life
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RSL Design Life Years of Service Some of the many things that might impact how long a pavement will be serviceable:
Rounding in design
Over- or underbuilding pavement layer thicknesses during construction
In-place strength of materials
Construction defects
Poor traffic projections (e.g., if 50% of the design traffic is realized in the first 5 years the pavement is in service, 50% of the cracking fatigue capacity should be expected to remain, regardless of what other performance metrics like FWD at joints or surface profile [IRI] might indicate)
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Accuracy Improves asPMS Builds Upon Itself Predicted performance is compared to actual on a regular basis to improve predictive models in the future
Continuous revisiting of models is crucial because designs, materials, construction practices, etc. all change with time and new design, material, and preservation methods come out all the time!
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Estimation of RSL of an Existing Pavement Could use an available performance prediction model on as-built pavement sections and compare to in-service conditions to estimate remaining capacity or life (step 3 below) this static approach provides a relatively low reliability in the prediction
To have a reliable prediction of RSL, the process requires:
1. Deciding performance triggers
2. Setting performance threshold limits
3. Selecting/developing performance prediction curves
4. Developing data collection protocols
5. Establishing a process for strategy selection
6. Performing regular field measurements and updates
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FHWAs Pavement Health Track (PHT) Software FREE Engineering software that can determine and report the health of pavement networks in terms of RSL based on IRI and various pavement distresses
Uses pavement performance models developed by FHWA
Simplified versions of the complex models used in the AASHTO MEPDG/ DARWin-ME/Pavement ME
Uses FHWAs National Pavement Cost Models (NAPCOM)
Pavement health can be assessed for different pavement types and under various environmental and administrative conditions
Can be utilized on a single corridor or applied across a broad network such as a district, state, or even country
RSL can be expressed in years or number of load applications
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FHWAs PHTSoftware 1
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19FREE at: http://www.fhwa.dot.gov/pavement/healthtrack/
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FHWAs Pavement Health Track (PHT) Software Necessary data includes:
Pavement distress data
Material properties
Climate
Loading
Of course, performance metrics and thresholds must be defined by the owner of the network
Provides standardized charts and reports and GIS mapping of data (with groupings) and allows for report customization
Modular design allows expansion (e.g., bridges, new techniques)
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FHWAs Pavement Health Track (PHT) Software Default Terminal Values
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FHWAs PHT Software:Prediction Examples
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FHWAs PHTSoftware 1
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Questions?Robert Rodden, P.E.www.robertrodden.com
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Step 1: Deciding Performance Triggers Measureable pavement performance conditions that can be used to trigger a need for corrective action
Might be network-level:
Leverages automated data collection efforts
Metrics like IRI, faulting, cracking, friction, etc.
Might be project-level:
Requires more subject interpretation and detailed field surveys
Metrics like material-related distress, lane-to-should separation, lane-to-lane opening, etc.
Might be a combination of network-level and project-level
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Step 2: Setting Performance Threshold Limits Consider the various thresholds we discussed for the different structural and functional distresses but local thresholds are needed and might be different for different roadway classifications, for example:
Tolerable roughness of the pavement varies from country to country and for low-speed versus high-speed roadways
Faulting, once initiated, typically will develop more quickly on roadways with larger numbers of heavy trucks
Friction loss might occur more quickly in an area where aggregates are more prone to polishing
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Step 3: Selecting/Developing Performance Prediction Curves Ideally, these performance prediction curves are developed locally for the metrics employed and using past field measurements of distress development
If such historic performance data is not available locally, well-supported existing performance prediction models might be used as a starting point in the system, to then be updated in the future
Because active preservation is intended as part of a comprehensive system, models are also necessary on preservation activities themselves (e.g., dowel bar retrofit, full-depth path, joint resealing, etc.)
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Step 4: Developing Data Collection Protocols These data collection protocols need to be developed to provide the metrics being considered as performance triggers
The data collected will be used to:
Provide a point-in-time to assess a single road section
Validate and update models
Identify trends in measurement variability, testing frequency and sampling interval deficiencies, etc.
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Step 5: Establishing a Process for Strategy Selection The proper preservation activity (or consideration of reconstruction) requires many different variables to be considered, including the pavement condition and available $
Life cycle cost analysis (LCCA) is used to help balance various alternate actions, future considerations, and cost
On the network level, the process for strategy selection is used more to characterize the overall state of the network and to identify section that require action so that.
On the project level, the process for strategy selection is used to support decisions on the corrective action necessary on individual projects
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Step 6: Performing Regular Field Measurements and Updates A programmatic approach is necessary to schedule regular automated condition surveying both to identify sections that require action and to build a performance database to improve performance predictions in future cycles
The process itself should be reviewed periodically as well to incorporate new advancements in pavement distress measurements, modeling of performance predictions, and other considerations
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Role of RSL
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Role of RSL
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