The Portland Cement Association (PCA) has published a research and development report that provides a critical review of existing literature and modeling tools related to life-cycle assessment (LCA) applied to pavements. The report, titled “Life Cycle Assessment of Pavements: A Critical Review of Existing Literature and Research,” covers 12 published LCA studies and three LCA models. It finds that the existing body of work is flawed, preventing the widespread use of LCA by pavement engineers and policymakers.

The review identifies five of those key deficiencies:

  • Inconsistent functional units — A drawback of pavement LCAs is the lack of consensus of a single functional unit that can be used to assess pavements. Some LCA studies have examined pavements that support a wide range of traffic and design lives for projects subjected to different climatic conditions. The variability of these elements creates a situation in which two pavements of equal lengths may have fundamentally different characteristics. A better functional unit would need to include the same traffic level, vehicle characteristics, climate, and period of analysis. Compared alternatives such as asphalt versus concrete must also provide equivalent service over their lives.
  • Improper System Boundaries — None of the existing LCAs has included all phases of the life cycle. Differing data sources for the processes involved in the extraction and production of the pavement materials account for much of the disparity in results. Perhaps the largest source of discrepancy regarding energy consumption is the use of feedstock energy to calculate asphalt pavements. Although this phase is considered in each study, the use phase is omitted from nearly all of the pavement LCA literature. It includes potentially influential components such as fuel consumption attributed to pavement roughness, urban heat island effect, radiative forcing, concrete carbonation, leachate, and tire wear.
  • Imbalanced data for bitumen and cement — The depth and sources of existing research on binders used in pavement applications differ greatly. The few LCAs that investigate bitumen manufacturing are generally focused on impact allocation of products within the oil refining sector, of which bitumen is only a small part. However, there are several studies that have documented the environmental impact of cement. The imbalanced understanding between cement and bitumen is potentially a significant source of error within pavement LCAs.
  • Use of limited inventory and impact assessment categories — The common output between all of the reviewed pavement LCA studies is energy consumption. Some have added an inventory of conventional air pollutants. Absent from the existing literature are thorough impact assessments that evaluate the magnitude and significance of the potential environmental impacts such as human toxicity, eutrophication, and acidification.
  • Poor overall utility — Inconsistencies in the functional unit, system boundaries, data quality, and environmental indicators have created a situation where the results of different studies are largely incompatible and therefore cannot be aggregated to draw any broad conclusions from the literature. There is no agreement on which material consumes more energy during the pavement life cycle.
  • The report identifies common data and modeling gaps in pavement LCAs that should be addressed in future work. These gaps include the use phase (rolling resistance, albedo, carbonation, lighting, leachate, and tire wear and emissions), asphalt fumes, feedstock energy of bitumen, traffic delay, the maintenance phase, and the end-of-life phase.

    A comprehensive list of recommendations for future research includes the need to improve the understanding of the environmental impact of bitumen and secondary materials through research and collection of data specific to various regions. Additionally, the report recommends more research into how major factors — including concrete carbonation, urban heat island effect, and leachate — affect the sustainability of concrete while it is in use.

    — Abdo (fabdo@cement.org) is the market manager for pavements at the Portland Cement Association.