5.1 Evaluation Studies Undertaken in the USA
In 2015, the FHWA developed a reference document, Towards Sustainable Pavement Systems (Van Dam et al. 2015). The aim of this document is to provide guidance on sustainability considerations in pavement systems throughout the entire pavement life-cycle (from material extraction and processing, through to the design, construction, use, maintenance/rehabilitation, and EOL phases.
Table 5.1 provides an evaluation of sustainability impacts of treatments for asphalt- and concrete‑surfaced pavements used in the USA. It provides a brief description of the technique and then indicates its effect on a number of preventative and restorative benefits (‘↑’ indicates a positive impact, ‘↓’ indicates a negative impact, and ‘↔’ represents both positive and negative impacts). Information on the relative life expectancy and cost, and relative environmental and social impacts is also provided. It is also noted that relative comparisons also vary depending on the traffic levels and climate region. These treatments are further detailed in FHWA (Van Dam et al. 2015).
|Treatment||Description||Preventative||Restorative||Performance and costs||Relative environmental impact based on life cycle energy use and GHG emissions, materials (low, medium, high, variable)||Societal impact|
|Seal pavement||Rejuvenate surface||Addresses surface distress||Eliminate stable ruts||Improves texture for friction||Improves ride quality and surface profile||Improve texture for noise||Relative treatment life (✓ to ✓✓✓✓)||Relative cost ($ to $$$)|
|Crack filling||Placement of adhesive material into and/over non‑working cracks, minimal crack preparation, lower‑quality materials used||↑||
|↓ Longitudinal over-banding can negatively impact friction||Over-banding may increase roughness; sealing cracks may slow development of roughness||
Over-banding increases noise
|✓||$||Low||Reduced traffic delays, less pleasing aesthetics, potential roughness and noise issues|
|Crack sealing||Placement of adhesive material into and/over working cracks, good crack preparation, high‑quality materials used||↑||
Longitudinal over-banding can negatively impact friction
Over-banding increases noise; filling can reduce noise
|✓||$||Low||Reduced traffic delays, less pleasing aesthetics, potential roughness issues|
|Asphalt patching||Used to treat localised distresses; partial‑depth patches address surface distresses and full‑depth patches address structural distresses||↑||↑||↑||✓✓||$$||
Depends on amount of patching and improvement gained in structural life and ride quality
|Reduced traffic delays compared to other treatments; negative impact on ride quality and noise, poor aesthetics (if patching is substantial)|
|Fog seal/ rejuvenators||Very light application of asphalt emulsion on pavement surface to seal the existing asphalt surface||↑||↑||
(May negatively impact skid resistance)
Depends in part on materials
|Reduced traffic delays; improves aesthetics|
|Chip seals||Sprayed application of asphalt (usually emulsion, heated asphalt cement and cutbacks also used) followed by aggregate chips roller to achieve 50 to 70% embedment. Cost and performance depends on whether it is single or multi‑course, as well as binder type and aggregate quality||↑||↑||↑||
Depends largely on number of courses placed
Depends on chip size
Medium to high|
Depends on number of courses and binder type
|Increases safety by improving friction, reduced traffic delays due to faster construction and opening to traffic; reduced ride quality due to rough surface, potential vehicle damage due to loose aggregate chips|
|Slurry seals||Mix of well‑graded aggregate (fine sand and mineral filler) and asphalt emulsion spread over entire pavement surface||↑||↑||↑||✓✓||$$||Medium||Increases safety by improving friction, reduced traffic delays due to faster construction and opening to traffic; improves aesthetics. Lower albedo may negatively impact urban heat island (UHI) effect|
Source: Van Dam et al. (2015).
For example, asphalt patching, which is also applied in Australia, is used to treat localised pavement distress such as potholes and severely cracked areas. Patching can also be used in preparation or in conjunction with other maintenance and preservation techniques, or as a pre-treatment for an asphalt overlay. This maintenance technique can be applied to different depths with little preparation and uses a cold-mix material, or can involve more intensive practices such as milling, saw cutting, application of a tack coat and asphalt concrete patching materials. In terms of positive environmental benefits, this technique provides:
- improved structural integrity and ride quality
- little material usage for isolated repairs
- the re-use of some construction waste generation from the removal of material, recycled as RAP
- reduced traffic disruptions and delays, as patching can be completed in a short timeframe.
Additionally, potential environmental and social impacts have been identified, which include:
- reduced ride quality and tyre-pavement noise, if applied poorly
- increased costs and environmental impacts if the density of patching increases, instead of using other more appropriate techniques for a large-scale task
- negative impacts on overall aesthetics of the pavement, if large quantities of asphalt patching is undertaken (Van Dam et al. 2015).
The report notes that, whilst information is limited on the effects of various maintenance and preservation treatments on the sustainability of the pavement systems, there are identified environmental and social impacts associated with the application of some techniques for asphalt and concrete-surfaced pavements. It is suggested that further investigation in the areas of:
- full life-cycle inventories to assess the environmental and social value of some techniques
- assessment of life-cycle costs in terms of treatment and materials selection, and timing
- consideration of offsetting higher economic costs of more frequent treatments with larger reductions in environmental impacts due to vehicle operations on smoother pavements
- treatment and material selection in terms of reduced transportation costs by: the use of local materials (which must meet performance requirements); reduced traffic delays on high-volume roads (balanced with high levels of smoothness); the application of new materials which enhance performance or reveal low energy consumption and emissions; and, further investigation of the environmental footprint during the manufacture of certain materials (Van Dam et al. 2015).