COMPARACIÓN DE COSTOS

The layer thickness required to achieve at least 85% reliability with increasing annual average temperature rise is shown in Figure 5-7. The required HMA-layer thickness (holding the base- layer thickness constant) under current conditions is simulated to be 145 mm, approximately 4% higher than the existing thickness (140 mm). With a 5°C temperature rise, a 46% HMA thick- ness increase (from 140 to 205 mm) is required. The required base-layer thickness (holding the HMA thickness constant) under current conditions is 432 mm (6% thicker than the existing 406 mm thickness). The required thickness increases steeply to 508 mm with 2°C temperature rise, levels off between 2 and 3.5°C temperature rise and increases steeply again from 513 to 623 mm (53% increase) between 3.5 and 5°C temperature rise.

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Figure 5- 7. Layer thickness required for pavement to achieve its design life with at least 85 percent reliability: (a) HMA-layer thickness required assuming 406 mm gravel base, (b) gravel- base-layer thickness required assuming 140 mm HMA. The boxes represent the 95% confidence interval of temperature rise projected during the period indicated.

The 95% confidence intervals for temperature rise from the RCP 4.5, 6.0 and 8.5 scenar- ios are shown with rectangles in Figure 5-7. The interval is less than 0.5°C up to 2040 but in- creases to 1.5°C in the late-mid-century period (2060 to 2080). The required HMA and base layer thicknesses are relatively insensitive within the mid-century period’s temperature

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indicating that differences among models does not result in projected differences in pavement management. In contrast, required layer thickness is much more sensitive to the potential range of the late-mid-century temperatures. If the base-layer thickness is left unchanged at 406 mm, the corresponding HMA thicknesses required to maintain 85% reliability are 150, 170, 178, and 185 mm for the four periods considered.

The corresponding HMA material costs are $40,000/km by early-mid-century and in- crease to $60,000/km by late-mid-century. If the HMA layer needs to be removed and replaced to rebuild a failing base layer, the required base-layer thicknesses are 437, 488, 510, and 533 mm, respectively. The corresponding gravel material-costs are $21,000/km by early mid-century and increase to $35,000/km by late-mid-century. Replacing the HMA layer would cost up to $350,000/km for 140 mm thick HMA material. In addition to these material-only costs, there will be substantial agency and user costs, such as project planning, design and construction qual- ity-assurance costs, and user delays costs. Furthermore, the environmental impacts of increased greenhouse gas production due to increased pavement roughness and frequent maintenance needs will have potential to accelerate climate change and will result in other increased agency and user costs [Valle et al., 2017]. This suggests that considering seasonal change and long-term temperature rise in pavement design and rehabilitation can help to avoid premature pavement failure, increased environmental impacts, and high road reconstruction costs.

5.4 SUMMARY AND CONCLUSIONS

Seasonal and long-term effects of climate-change-induced temperature rise on pavement life were investigated. Changes in pavement season length and seasonal average temperatures were determined at a pavement evaluation site in coastal NH using downscaled daily average

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temperatures [ICF International, 2016; Reclamation, 2016] and MnDOT season definitions [Tanquist, 2001]. Pavement material MR changes with incremental temperature rise were deter-

mined using MR and temperature relationships from existing laboratory testing data [Janoo et al.,

1999]. MR,eff was determined using the AASHTO empirical pavement design damage factor ap-

proach following protocols used by NHDOT[NHDOT, 2014].

MnPAVE software [Tanquist, 2001]was used with the projected season durations, sea- sonal average temperatures and seasonal MR values to calculate pavement damage and seasonal

contributions to total damage for 0.5°C incremental annual average temperature increases from 0 to 5°C. Many pavement layer thicknesses were simulated to determine the thickness required to achieve 85% reliability with each 0.5°C temperature increase. Temperature rise projections for early century, early-mid-century, mid-century and late-mid-century were then superimposed on the pavement thickness and temperature rise curves to identify the timing of plausible tempera- ture changes. The following conclusions have been reached:

• Pavement season length in coastal NH will change with climate-change-induced tempera- ture rise. The summer season increases steadily at a rate of approximately 8 days per de- gree of temperature rise. The winter season will cease to exist when the temperature rises by more than 2.5°C; predicted to occur by mid-century. The fall season will initially lengthen as temperature rises due to shorter winters but, with continued warming, will shorten as summer lengthens.

• Changing season duration impacts the seasonal contribution to total pavement damage. The late spring and summer seasons currently contribute more than 90% of the total pavement damage at this site. Rutting damage becomes more prevalent during other times of the year, especially during the expanding fall season as the winter season

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shortens. For temperature increases greater than 3.5°C, early spring also contributes to the total pavement damage. At 4.5°C temperature rise and above, the damage is spread much more evenly throughout the year.

• Downscaled climate model output for RCP 4.5, 6.0 and 8.5 was used to identify plausible temperature increases and the timing of the effects [Taner et al., 2017]. Temperature rise ranging from 0.7 to 0.8°C is projected to occur during years 2000 through 2020, 1.3 to 1.6°C for 2020 to 2040, 2.0 to 2.8°C in 2040 to 2060 and 2.5 to 4.0°C in 2060 to 2080. For all periods except 2060 to 2080, the adaptation needs are relatively insensitive to these temperature ranges.

• A straightforward, preliminary adaptation analysis shows that if the existing base-layer remains structurally sound with 85% reliability, required HMA-thickness increases range from 7% for early century to 32% (costing $60,000/km) by late mid-century. If the base layer fails and needs to be replaced, additional base-layer thickness costing approxi- mately $35,000/km (material costs) plus HMA replacement, agency, and user costs will be needed to maintain 85% reliability through late-mid-century. If actions are not taken to prevent premature pavement failure from temperature rise the costs of widespread base- layer reconstruction will be much higher than the costs of asphalt overlays.

• The hybrid bottom-up/top-down approach is an effective investigatory method for ana- lyzing pavement response to climate-change-induced temperature rise. While computa- tionally-intensive, the bottom-up portion of the analysis shows the effects of incremental temperature rise on season length, seasonal average temperatures, pavement material properties, and pavement life. It also reveals trends in pavement damage and projected pavement response to rehabilitation actions. A more complete understanding of the

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pavement’s climate-stress response supports more effective adaptation strategies. The top-down portion, in which the temperature change is defined, is used to determine the timing of impacts to support staged-adaptation planning and budgeting.

This research can be expanded both nationally and globally because the methodology is applicable to other regions (inland and coastal) where temperature rise and climate-change-in- duced seasonal changes will impact pavement performance. The adaptation approach, that is calculating the pavement layer thickness required to maintain a specified reliability level, pro- vides practitioners with actionable guidance to address rising temperatures and changing seasons. We are currently building on this research to develop a staged-adaptation approach in which multiple adaptation pathways will be evaluated. Rehabilitation and reconstruction costs will be accumulated along these pathways and robust solutions will be sought to address uncertainties in climate-change projections (26, 28). Other adaptation options should also be evaluated includ- ing, but not limited to, different base materials (crushed stone, reclaimed stabilized base, crushed gravel) and HMA binder grades.

Future research on coastal-road infrastructure will build on previous research showing pavement-life reduction with sea-level-rise-induced groundwater rise [Knott et al., 2018b; Knott et al., 2017]. The hybrid approach will be used to determine the combined effect of temperature and groundwater rise on coastal roads. Adaptation pathway analysis (26, 28) will be used to identify combinations of HMA and base-layer thicknesses that minimize damage costs caused by increased moisture content, temperature and traffic. Future research should utilize the latest pavement analysis and design tools to better capture pavement response mechanics and durabil- ity (moisture damage and aging) in pavement performance evaluation.

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CHAPTER 6: DESIGNING A CLIMATE-READY COASTAL ROAD IN THE