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26 Understanding the Benefit/Cost Analysis Used in the Online Tool Results This chapter summarizes the methodology behind the benefit/cost results that the online tool produces. There are many ways to measure benefits and costs, including life-cycle costing, cost-effectiveness analysis, and a longevity cost index. GA managers will get the most use out of benefit/cost analysis developed in the tool. Using decision trees/matrices narrows down the list of available treatments to two or three spe- cific treatments for a given set of airport parameters. From there, a decision still needs to be made about the optimal treatment. Ideally, the user will have information on the expected treatment life (which can also vary), general material unit costs, and particular details of the construction procedure (expected downtime, specialized contractor training, seasonal availability, etc.). Applications to GA Airport Managers GA airport managers face a cycle of treating the worst-off pavement first. This is the natural result of underbudgeting. But since the worst-first approach is more costly in the long run, GA airports benefit from using the online tool to see the impacts and cost savings from interven- tion earlier in the pavement life cycle. Pavement Life Span Figure 5 and Figure 6 illustrate, respectively, the life span of typical asphalt and concrete air- field pavement without any intervention. This life span is also known as its deterioration curve. Concrete has a much longer life span than asphalt. Calculation of Relative Benefit Relative benefit is a calculated value used for comparing treatment options. Figure 7 shows the calculation of relative benefit. In the graph, in year 10 of this pavement, the PCI has fallen to the low 80s. By applying a micro/slurry treatment, the PCI returns to 100, and the life of the pavement is extended by a few more years. The shaded area, the relative benefit, is defined as the sum of the condition (expressed as a PCI score) after applying a treatment, minus the condition had the treatment not been applied (expressed as a net PCI increase) for each year until the PCI score with treatment reaches the threshold value of 40, summed over the reported period as PCIâYears (Figure 7). The larger the shaded area, the greater the relative benefit. In understanding the benefit calculations, a higher number means more benefit. Often a higher benefit also means a higher cost, so it is possible that selecting a treatment with a lower benefit may result in a higher (and more desirable) benefit/cost ratio. C H A P T E R 5
Understanding the Benefit/Cost Analysis Used in the Online Tool Results 27 Figure 5. Default PCI versus age for asphalt pavement. Figure 6. Default PCI versus age for concrete pavement.
28 Pavement Maintenance Guidelines for General Aviation Airport Management Calculation of Treatment Life For benefit/cost data used in the online tool, researchers gathered data on the service life of each treatment and calculated a corrected average treatment life to use in relative benefit calculations. Figure 8 through Figure 11 show a range of results for a pavementâs PCI and life span that can be obtained from treatments. Figure 8 graphs a treatment (such as a crack seal) that would extend the life of the pavement but not improve the PCI. In Figure 9, there is an increase in PCI, which extends the pavement life. Figure 10 shows a situation where the distress is eliminated, the PCI is increased to 100, but the treatment does not fully restore the pavement to new. Figure 11 illustrates the impact of complete reconstruction, which fully resets the PCI curve. Development of Cost Data Researchers gathered cost data to use in the online tool for the purpose of benefit/cost compari- son, not for calculating a total cost of a treatment. The repair costs for each distress type/severity/ quantity/treatment type were calculated based on the distresses found within an idealized 5,000 ft2 (100 ft long by 50 ft wide) or 20-slab sample (typical slabs assumed to be 20 ft by 20 ft). Materials and labor costs vary significantly among regions and contractors, and also drasti- cally change over time and with market dynamics. Because of this tremendous potential varia- tion in costs, users of the online tool are allowed and encouraged to obtain quotes from local contractors and input customized cost estimates for the analysis. The tool will allow users to modify the cost data, enter the dimension of the feature (runway, apron, or taxiway) in which they are interested, and, from that, calculate an approximate total cost. Figure 7. Illustration of relative benefit.
Understanding the Benefit/Cost Analysis Used in the Online Tool Results 29 Figure 8. Example of treatment changing deterioration rate only. Figure 9. Example of treatment increasing PCI and changing deterioration rate.
30 Pavement Maintenance Guidelines for General Aviation Airport Management Figure 10. Example of treatment increasing PCI to 100 and changing deterioration rate. Figure 11. Example of reconstruct treatment resetting PCI curve.
