Contributing writer: Bruce W. Uhlman, LCACP, BASF Corporation
Originally published in Pavement Preservation Journal, May 2018.
Pavement preservation — the systematic scheduling of nonstructural maintenance applications to protect engineered road pavements and extend their service life — helps promote better road conditions, increases safe driving by minimizing surface deterioration and the potential for structural failure, and is an efficient use of tax dollars.
In many parts of the United States, government and public agencies mandate the use of eco-efficient asphalt construction materials that are durable, and aid in the speed of construction for pavement preservation projects.
The challenge facing many of these agencies and key material specifiers is how can they decide which technologies and materials are the most eco-efficient? On what basis should they make their comparison, and what metrics truly define the sustainability of road construction materials?
In this article, an eco-efficiency analysis will be presented that compares the relative eco-efficiencies of a pavement preservation technology, namely a cape seal (chip seal covered with a micro surfacing treatment), vs. standard road construction practice, namely a hot mix overlay for roads in two unique regions of the United States.
Utilizing a regional approach allowed for modeling of regional variations such as the chip seal technology, durability and costs. The analysis considered the environmental and economic impacts required to maintain a one-mile stretch of a 12-ft.-wide lane of road using best engineering practices for a 40-year lifetime.
Results of the analysis, which have been third-party verified by NSF International, can be found on NSF’s certified products and systems website1.
Though the specific results vary for each region analyzed, the study shows that cape seals provide both the lowest life-cycle cost, while also contributing the least environmental impact when compared to hot mix overlays.
Cape seals achieve these advantages because of their ability to maintain desired road characteristics and performance using a significantly reduced amount of materials compared to hot mix overlays.
The report also supports through additional scenario analyses, the continued emphasis on advancements in hot mix technologies that reduce application temperatures and increase the usage amount of recycled asphalt pavement (RAP). These advances can lead to significant improvements in the overall sustainability of hot mix overlays.
Finally, the study supports the overall conclusion that doing preventive maintenance, specifically cape seals, early in a pavement’s life cycle, can cost-effectively extend the life of the road as well as reduce the overall environmental impact associated with the maintenance of the road.
The purpose of an eco-efficiency analysis is to harmonize economy and ecology, with the aim of spurring product innovations in chemistry, and to promote sustainable development. The eco-efficiency analysis provides information about the relationship or balance between the economic benefits of a product and its impact on the environment.
BASF has used the eco-efficiency analysis as a strategic tool since 1996, completing over 600 studies for a diverse range of products including chemical intermediates, consumer and personal care products, vitamins, packaging materials, adhesives and renewable-based products.
Not only does the analysis provide the necessary data to support strategic decisions related to new product development, but it also provides customers and key decision makers with a comprehensive comparison of products or processes that is both science-based and comprehensive. Results are also presented in a way that is clear, easily understood and facilitates informed decision making about the sustainable attributes of products. The analysis has helped to increase the market success of products by clear and transparent differentiation from competing technologies.
The eco-efficiency analysis assesses the life cycle impacts of a product or manufacturing process from the “cradle to the grave” with equal importance to environmental and economic impacts. As a life-cycle approach, the unit of comparison for the assessment is defined as the customer benefit (CB). It includes not only the impacts of the starting and intermediate raw materials but also takes into consideration the consumption behavior of the end consumers during the use phase of the product, as well as the various end-of-life options such as recycling or landfill disposal. The comprehensive accounting of environmental impacts for each alternative can be reflected in an environmental fingerprint.
Economic data also are compiled for each alternative for their respective life cycles. All the various costs incurred in the manufacturing, use and disposal of the product are included in the calculation. Using environmental relevance and social weighting factors, the environmental impacts for each alternative are combined into a single environmental score.
Likewise, a single life-cycle cost (total cost of ownership) for each alternative is developed. These environmental and economic scores are plotted on a biaxial graph known as the eco-efficiency portfolio. The graph reveals the relative eco-efficiency of a product or process compared to other products or processes.
The relative life-cycle environmental and economic impacts of multiple cape seal technologies (pavement preservation) were comprehensively compared against those of a standard road construction practice, namely a 1.5-inch hot mix overlay, in two distinct regions of the United States.
The comparison was based on the ability of each technology to maintain a one mile stretch of a single lane road over 40 years to a similar profile and performance. The full study report and findings can be obtained on the NSF website1.
Key study assumptions that formed the basis of the analysis included:
- A full environmental and economic accounting was done for each assessed alternative over their respective life cycle and considered the production, use and disposal phases.
- For both regions assessed, two cape seal technologies were compared against a hot mix overlay. For the southeast assessment, the two cape seal technologies featured a ground tire rubber (GTR) chip seal and a SBR polymer modified emulsion chip seal. The California (West Coast) assessment featured an asphalt rubber (AR) chip seal and a SBR polymer modified emulsion chip seal. Composition ranges for all alternatives generally reflected average industry data.
- A full energy accounting of the processes was considered, and included the energy required to produce and apply each technology, energy required to produce the crumb/ground tire rubber, and energy associated with transportation and disposal.
- The maximum amount of RAP (reclaimed asphalt pavement) allowed in the base case hot mix asphalt overlay was 15 percent for the California (West Coast) model and 20 percent for the southeast model.
- Various literature sources 2,3 as well as input from regional producers and trade associations helped establish the durabilities list for each alternative: Asphalt rubber (AR) based cape seal, California (West Coast): 14 years; Polymer modified cape seal (CRS-2P): eight years; Ground tire rubber (GTR) cape seal: eight years; Hot mix overlay (1.5-in. mill and overlay): 12 years
- Consistent with U.S. DOT FHWA guidelines, constant dollars and real discount rates were considered4 for each alternative’s initial and future cost impacts. Both financial and social discount rates were considered.
Fig. 1 shows the respective global warming potential (carbon footprints) for the alternatives considered in the southeast analysis. The polymer modified emulsion-based cape seal (Cape Seal II) had the lowest carbon footprint: 20 percent lower than the GTR chip seal-based cape seal, and 30 percent lower than the hot mix overlay.
Contributions to CO2 emissions come from both material use and energy consumption during manufacturing and transport. The main contributor to the hot mix overlay was the large amount of energy required during the production, storage and application of the asphalt. The binder contribution to the global warming potential (GWP) for each alternative was roughly the same.
For the California (West Coast) assessment the asphalt rubber-based cape seal had a GWP about 10 percent less than the polymer modified emulsion-based cape seal. The hot mix asphalt overlay had the highest global warming contribution.
Expanding beyond the carbon footprint, the relative impact for each alternative in each environmental category assessed can be seen in Fig. 2, the environmental fingerprint.
Excluding resource depletion and fresh water eutrophication, the Cape Seal II (polymer modified emulsion-based cape seal) alternative scored the lowest in each category, followed by the GTR modified cape seal and then the hot mix overlay (HMA).
Specific to resource depletion, the HMA overlay scored the best while the cape seal technologies had about 30 percent higher impact. Eutrophication is the process by which water bodies become enriched in dissolved nutrients leading to the depletion of dissolved oxygen ultimately resulting in harm to aquatic life. The Cape Seal II technology had the highest impact in fresh water eutrophication.
Applying weighting factors to the normalized results in Fig. 2, the polymer modified emulsion-based cape seal (Cape Seal II) scored the lowest in overall environmental impact, followed by the Cape Seal I (GTR modified) alternative. The hot mix overlay technology had the highest overall environmental impact.
For the California (West Coast) analysis, the asphalt rubber (AR) modified cape seal technology demonstrated reduced overall environmental impacts in all environmental categories, when compared against the competing technologies. The key factor influencing the reduced overall environmental impact is the technology’s longer durability and lower resource consumption.
The life-cycle costs for the southeast cape seal analysis are shown in Table 1. Material unit costs were provided by material suppliers. The pricing data were recent (2017) from multiple sources, specific to the region assessed, and deemed representative of standard industry pricing. Due to their identical durability and low material costs, both cape seal alternatives were the least expensive alternatives, about 25 percent less expensive than the HMA overlay.
The life-cycle costs for the California (West Coast) cape seal analysis also showed a life-cycle cost advantage for the cape seal technologies when compared to the hot mix overlay.
Due to its low material costs and high durability, the AR-based cape seal was the least expensive alternative—about 50 percent less expensive than the most expensive alternative, the HMA overlay. The Cape Seal II alternative was about 30 percent less expensive than the HMA overlay.
To determine the relative eco-efficiency of each alternative, the life-cycle costs and environmental impact of each alternative is plotted on the eco-efficiency portfolio. The portfolio allows for the balancing of benefits and trade-offs between products economic value proposition and environmental impact.
As can be seen in Fig. 3, for the southeast analysis, due to their slightly better performance in both the environmental and economic assessments, the cape seal alternatives were the more eco-efficient alternatives when compared to the HMA overlay.
Both cape seal technologies scored an eco-efficiency index score within 10 percent of one another, and thus were deemed of similar eco-efficiencies. The Cape Seal II alternative combined the lowest overall environmental impact and lowest life-cycle cost, giving it a 12 percent eco-efficiency advantage over the HMA overlay (worst performing alternative) and around a five percent advantage over the GTR-modified cape seal.
For the California (West Coast) assessment, the asphalt rubber (AR) modified cape seal alternative was the most eco-efficient alternative — over 30 percent better than the least eco-efficient alternative, the HMA overlay. The polymer modified, emulsion-based cape seal (Cape Seal II) trailed the AR-modified cape seal, but was still about 15 percent more eco-efficient than the HMA overlay.
This case study also looked at conducting various scenario analyses that influenced the eco-efficiency of the assessed alternatives. One specific group of assessments focused on the standard road construction practice of hot mix overlays (HMA) and evaluated technologies that could reduce the HMA production and application temperatures, while additionally allowing for the increase in RAP.
Research5 has shown that it is possible to reduce HMA production temperatures by as much as 100 deg F while also allowing for increased amounts of RAP, ultimately leading to lower overall resource and energy consumption. The assessment showed significant eco-efficiency improvements for hot mix overlays if these temperature reductions and RAP increases are possible without compromising any technical or performance characteristics.
The eco-efficiency analysis tool facilitates strategic decision making along the entire value chain, enabling companies to drive innovative product development focused on bringing more sustainable products to the marketplace.
Specific to this study, the eco-efficiency analysis showed both a significant life-cycle environmental and cost advantage for cape seal pavement preservation technologies over hot mix overlays for two regions in the United States.
Finally, the author would like to acknowledge Mark Ishee, vice president, Pavement Preservation & Specialty Products at Ergon Asphalt & Emulsions Inc., and 2018 president of AEMA; Sally Houston, technical manager at VSS Emultech and 2018 president of WRAPP; and Arlis Kadrmas, technical development lead, Construction and Fiber Bonding, BASF Corporation, for their significant contributions to the study.
- 1. Cape Seal Eco-Efficiency Analysis Final Report; January 2018. http:// info.nsf.org/Certified/Sustain/listings. asp?standard=P352
- 2. Cornell Local Roads Program. What is a Cape Seal and how is it compared to other Surface Treatments, http://www. clrp.cornell.edu/q-a/075-cape_seal.html
- 3. Hajj, Elie Y., Sebaaly Peter E. and Habbouche, Jhony, Laboratory Evaluation of Thin Asphalt Concrete Overlays for Pavement Preservation. Report WRSC-UNR-TAO-201602. Table 3 Treatment Life of and Cost Estimate for Selected Preventive Maintenance Treatments. Page 14. 2016
- 4. Life Cycle Cost Analysis in Pavement Design, Publication No. FHWA-SA-98-079. Pages. xii-xiii
- 5. Hansen, Kent R. and Copeland, Audrey; Asphalt Pavement Industry Survey on Recycled Materials and Warm-Mix Asphalt Usage; Information Series 138; NAPA (National Asphalt Pavement Association); 2015