Environmental Impacts of Road Pavement Rehabilitation

Road pavement generates significant environmental impacts through the production, transportation, construction, and maintenance stages. Recycling methods can be used to reduce the demand for virgin materials, but these alternatives are not environmentally benign either. Based on a life cycle assessment of a real case study near Chatham, Ontario, we modelled the trade-offs of a road rehabilitation project over a 30-year service life, subject to three scenarios. The three studied pavement rehabilitation scenarios are the standard mill and fill, and two cold recycling techniques called cold in-place recycling (CIR) and full-depth reclamation (FDR). These scenarios use differing quantities of resources and blends of reclaimed asphalt pavement (RAP). Results show use of RAP with cold in-place recycling substituting virgin materials improves the environmental performance of most indicators, including climate change. These gains are only slightly decreased by the additional transportation of machinery, which we show through sensitivity analysis, and is likely to improve as the method becomes more commonplace.

Impact Assessment

Using a Life Cycle Assessment, Climate change (GWP), fossil depletion (FDP), metal depletion (MDP), and water depletion (WDP) results are reported per functional unit (one lane-km over a 30-year estimated service life). Maintenance and rehabilitation cycles vary between scenarios during the 30-year period (Figure 1). The impacts are assessed in three life cycle stages: production (including cradle-to-factory gate of resources), transportation, and construction (including maintenance activities) of the pavement.

Figure 1. Rehabilitation schedule for each scenario over 30 years

Specific Impacts Related to Three Road Pavement Rehabilitation Scenarios

Environmental impacts vary across the three scenarios. RAP100 (with CIR) yields the lowest GWP, FDP and MDP, while the MF scenario has the lowest WDP. RAP50 (with FDR) fares worst in all impact categories.

Production is the life cycle stage which plays the largest role in all scenarios. That is unsurprising due to the large amounts of diesel, bitumen, and gravel used in all scenarios, as well as water in the RAP100 and RAP50 scenarios. Transportation plays a large role, especially as to climate change impacts. Construction plays a larger role in the impacts of climate change for the RAP50 scenario due to the combustion of diesel. Construction also generates a larger portion of impacts as to water depletion in RAP100 and RAP50 scenarios due to the demand for water in both CIR and FDR processes. The other processes involved in the construction stage are largely the flow of reused materials (RAP), which transfers little in the way of material depletion due to construction. Maintenance generates relative low impacts in all scenarios as the maintenance processes only include impacts associated with crack-sealant production and application. In general, environmental impacts are strongly associated with the production of input resources (materials and energy) and, as such, the difference between the scenarios is that RAP100 is superior due to its lighter demand on virgin materials. This holds true for climate change, fossil and metal depletion, but not for water depletion. This is because the higher demand for water in the CIR construction process used for the RAP100 scenario outweighs savings in water depletion during production of avoided virgin resources. Figure 2 highlights these comparative trade-offs between absolute impacts at different life cycle stages and scenarios for each of the four impact categories.

Figure 2. Comparison of environmental impacts under each scenario associated with production (including resource extraction), transportation, construction, and maintenance life cycle stages

Overall, production of bitumen contributed to large environmental impacts in all three scenarios. Production of diesel also contributed to sizeable portions of these impacts. Transportation activities generated lower impacts in all categories in the MF scenario, largely due to the shorter distances required to source heavy equipment. These distance-related results will likely change over time as CIR becomes more commonplace, and the specialist equipment is expected to be available within shorter service areas. In our full paper, we demonstrate the relevance of transport distances by sensitivity analysis. The environmental impacts tallied to the life cycle stages for each scenario are shown in Table 1.

Table 1. Selected environmental impact results for one lane-km pavement in each scenario, shown in absolute impacts and impacts per tonne of materials over 30-year service life

Reducing Environmental Impacts

Impacts associated with the RAP scenarios could be decreased if transport distances were reduced. Currently, large masses of materials travel long distances, which contribute to sizeable impacts. Our sensitivity analysis shows that reduced distances in the transportation life cycle stage could lower impacts in the transportation stage by 66% in the RAP100 scenario, contributing to a 14% GWP decrease overall in the RAP100 scenario (Figure 3). On the other hand, swapping the portable HMA plant to a stationary plant has mixed, but generally poor outcomes. Savings in transportation of the HMA plant and its fuels to the construction site reduce climate and fossil impacts, but give mixed results as to metal depletion. Most importantly, substituting fuels increases water depletion impacts by much larger proportions than any other savings. Overall, the sensitivity analyses suggest that decreasing transport distances improves environmental performance, especially for RAP100 which is already less environmentally intensive in three out of four categories, indicating this method is preferable in the longer term as it becomes more mainstream. However, the largest portion of impacts is in the production stages for all scenarios (especially bitumen production), meaning efforts should focus on improving other processes in the upstream production systems.

Figure 3. Sensitivity of impacts due to transportation and HMA plant changes

Two striking lessons have emerged from this analysis. First, the environmental performance of RAP50 is hindered due to the need for an MF process during the 30-year service life, which negates most of their anticipated benefits of pavement recycling. Secondly, the high demand for water when manufacturing bitumen emulsion in both RAP100 and RAP50 results in a contest between climate and water impacts. RAP100 achieves the lowest GWP score over the 30-year service life while having a poor WDP score. Local judgment will prevail in the selection between methods; water-scarce environments may select MF for future road rehabilitation, while locations where water availability is not an issue may prefer to reduce the carbon footprint of road projects by choosing RAP100 rehabilitation.

Conclusions

While materials and processes used to make HMA are relatively environmentally intensive compared to RAP, the dependence on supplementary HMA in the RAP50 scenario, and the demand for water and bitumen in RAP100, highlighted trade-offs between impacts and life cycle stages across the three scenarios. RAP100 results in the lowest global warming potential (GWP), making it the least burdensome for climate change, as well as performing best as to fossil (FDP) and metal depletion (MDP). However, the demand for water used to make bitumen emulsion in both RAP scenarios made them a poor choice as to water depletion (WDP).

While the results highlight the complexity of comparing conventional and emerging pavement recycling methods, RAP100 is a superior approach if climate change and material scarcity are the main environmental criterion. In water-rich regions of Canada such as Ontario, water depletion is likely to be less relevant for decision-makers selecting their preferred road rehabilitation method. In other (e.g., water-scarce) regions, however, or as environmental objectives shift, these priorities are likely to differ.

Associated article

Elliot, T., Carter, A., Ghattuwar, S., and A. Levasseur. (2023). Environmental impacts of road pavement rehabilitation. Transportation Research Part D, 103720.