The Greening of Civil Infrastructure
Across many industries, manufacturers turn out the countless products we rely on in our daily lives. These processes also create something else: waste - lots of it. According to the U.S. Environmental Protection Agency, industrial facilities in the United States generate 7.6 billion tons of nonhazardous waste annually.1
To help keep this waste from ending up in landfills, many leading manufacturers are rethinking their approach to recycling and reuse. At General Motors, everything from scrap steel, paint sludge and worn out tires is viewed as a resource that can be reused or sold for recycling. In 2011, GM diverted 2.5 million metric tons of waste from landfills through reuse and recycling - the equivalent of 38 million garbage bags. Worldwide, 90 percent of GM's manufacturing waste is reused or recycled - more than any other automaker.2
Similar progress is being made by manufacturers such as Unilever, a global brand leader in food, personal healthcare and other consumer products. In 2013, 75 percent of the company's manufacturing sites brought their delivery of nonhazardous waste to landfills down to zero. Included among Unilever's wide array of global waste recovery, recycle and reuse initiatives is the use of overflow plant sludge in prefabricated construction products and as an alternative raw material in cement production.3
Increasingly, the waste generated from the manufacturing processes of companies like these and others is also finding its way into major infrastructure projects throughout the nation. The byproducts from manufacturing, as well as other industrial processes such as energy generation, offer a valuable resource for use in civil infrastructure.
Industrial byproducts such as coal combustion materials, construction and demolition debris, spent foundry sands, steel slag and used rubber serve as sustainable alternatives to virgin materials and construction products (see sidebar, "Using Industrial Materials in Infrastructure Construction"). With their unique chemical and physical properties, many of these byproducts can enhance the durability and performance of roadways, decrease energy use and reduce greenhouse gas emissions.
According to the Green Highways Partnership, a public/private network focusing on green transportation innovation, roadway and highway construction in the United States consumes between 500 and 800 million metric tons of virgin crushed rock, gravel and sand annually as aggregates. Replacing these nonrenewable materials, which must be mined and processed, with recycled materials conserves natural resources.
The reuse of industrial byproducts further reduces environmental impact by decreasing the need for the energy-intensive manufacture of new products and extraction of virgin materials. For example, substituting coal fly ash for portland cement in concrete saves the energy consumption and greenhouse gas emissions associated with producing cement.4
For infrastructure project owners, contractors and other stakeholders, using industrial materials makes good economic sense as well. The EPA emphasizes that industrial byproducts are often less expensive than the virgin materials they replace, and recycling or reusing materials onsite can lower transportation and disposal costs.
Industry experts also readily highlight that many recycled materials perform better than the virgin materials they replace. Steel slag, for example, when used as an aggregate for asphalt roadway, increases surface friction and makes driving safer. Using fly ash as a partial substitute for portland cement in concrete enhances the durability of the concrete surface. Even recycled rubber tires have utility: as an asphalt binder modifier, they are sometimes used to replace polymers and may help make asphalt pavements quieter and less prone to cracking.1
Among the most widely used recycled industrial materials in infrastructure construction is coal fly ash. A naturally occurring product of the coal combustion process, fly ash is universally praised for its benefits as a "green" building material that reduces the demand for carbon-intensive portland cement.
Replacing a portion of the cement with fly ash creates a cementitious material that, when used with other aggregates, is well suited to road, airport runway and bridge construction. Over the long run, fly ash concrete has increased strength and durability, resulting in less roadway wear and tear and lower repair costs.5 (See sidebar, "Coal Fly Ash and Slag Enhance California Bridge Span," for an example of its use.)
These features underlie the notable success of coal fly ash in the transportation construction market. According to the American Road and Transportation Builders Association, concrete represents about 15 percent of the total cost of building and maintaining transportation infrastructure in the United States annually. More than 75 percent of that concrete - an economic value of $9.9 billion - uses fly ash as a partial cement replacement blend. ARTBA estimates that coal fly ash use in roads and bridges saves $5.2 billion per year in U.S. construction costs.
"Coal fly ash is a true recycling success story that is used virtually everywhere throughout the transportation construction sector," says John Ward, spokesman for the American Coal Ash Association, Farmington Hills, Michigan. "While using fly ash as an additive in the production of concrete offers the biggest performance and environmental benefits, its chemical and physical properties make it highly effective as a mineral filler in asphalt, in road and embankment base materials, and in other engineered structural fill applications."
According to the National Asphalt Pavement Association, reclaimed asphalt pavement is recycled at a higher rate than any other material in the United States. It is vital to extending infrastructure service life while lowering overall costs.
Reclaimed asphalt shingles from both manufacturer waste and post-consumer shingles provide similar utility. The use of RAP and RAS in asphalt pavements promotes sustainability by decreasing the amount of material going to landfills and reducing the amount of new asphalt binder and aggregate required in mixes.6
Other recycled materials being incorporated into asphalt pavements include ground tire rubber, steel slag, blast furnace slag and others. Steel slag, a byproduct of converting iron to steel, is particularly well suited to civil engineering and infrastructure projects. Used as a substitute for natural aggregates, steel slag promotes a strong bond between the slag and asphalt, resulting in superior skid resistance and durability (see the sidebar, "Vroom Vroom").
"The use of recycled materials has enabled the asphalt pavement industry to improve the quality of infrastructure in ways that are more sustainable than ever before," notes Kent Hansen, director of engineering at NAPA and chairman of ASTM Subcommittee D04.23 on Plant-Mixed Asphalt Surfaces and Bases, part of Committee D04 on Road and Paving Materials. "Through innovation, our industry continually finds ways to develop products that are environmentally friendly and economically sound, while still maintaining or enhancing performance. This includes reclaiming old asphalt pavements and rejuvenating their component parts for use in new pavements, to the incorporation of recycled materials and the adoption of energy-saving warm-mix asphalt technologies."
1. U.S. Environmental Protection Agency, Guide for Industrial Waste Management.
2. Muller, Joann, "How GM Makes $1 Billion A Year By Recycling Waste," Forbes, Feb. 21, 2013.
3. Unilever, "Reducing Waste from Manufacturing."
5. Transportation Development Foundation, American Road and Transportation Builders Association, "The Economic Impacts of Prohibiting Coal Fly Ash Use in Transportation Infrastructure Construction," Sept. 2011.
6. National Asphalt Pavement Association, "Annual Asphalt Pavement Industry Survey on Recycled Materials and Warm-Mix Asphalt Usage: 2009–2013."
7. National Slag Association, "Indianapolis Motor Speedway Chooses Steel Slag For Its Durability, Strength and Friction."
Coal Combustion Products
-Concrete and recycled asphalt pavement
-Flue gas desulfurization material
Recycling Uses: Portland cement and concrete, flowable and structural fills, soil stabilization
Construction and Demolition Debris
-Concrete and recycled asphalt pavement
-Gypsum and drywall
-Brick and block
Recycling Uses: Added to asphalt and concrete paving, concrete mixes, road base and sub-base
-Discarded sand used in metal casting
Recycling Uses: Road embankments, manufactured soils, flowable and structural fills,
road base and sub-base
Steel Production Materials
-Electric arc and blast furnace slag
Recycling Uses: Added to concrete as aggregate for road base and sub-base
-Ground tire rubber
Recycling Uses: Added to asphalt and as aggregate for road material mixes
Source: Green Highways Partnership4
Among the landmark transportation projects across the U.S. that have utilized coal fly ash is the ongoing construction work on the new east span of the San Francisco Oakland Bay Bridge. According to the California Department of Transportation (Caltrans), the eastern span is one of the largest public works projects in California history, with an estimated cost of $6.4 billion.
The project is taking advantage of the unique properties of fly ash and ground granulated blast furnace slag to enhance the durability and strength of the concrete used. Caltrans started the new span in 2002 to replace the old, seismically vulnerable span that was damaged in a 1989 earthquake and subsequently repaired. The chemical and physical properties of fly ash concrete help mitigate the corrosive effects of seawater and salt fog and meet the structural requirements of an earthquake zone.
Caltrans uses over 30 concrete mix designs in the new bridge, some containing more than 50 percent fly ash.1
Steel slag has a prominent place not only in the asphalt roads we navigate every day, but also the raceways where professional drivers compete. At the Indianapolis Motor Speedway, home of the renowned Indianapolis 500 race, the asphalt track must be in peak condition to support drivers who reach speeds of over 200 miles per hour. In 2004, when the speedway's main racing oval required resurfacing, the IMS organization paved the track completely with steel slag. Ten years later, the asphalt track continues to provide excellent skid resistance and durability with at least five years additional life span anticipated.7
Through a diverse range of standards development activities, several ASTM International technical committees are helping to advance the beneficial use of industrial byproducts and recycled materials in infrastructure construction. Among the many notable activities and standards are the following.
o Committee C09 on Concrete and Concrete Aggregates has developed several standards to assist industry stakeholders in the selection, testing and blending of fly ash in concrete production, ultimately supporting greater concrete usage and furthering sustainable practices. These standards include C311 for sampling and testing fly ash for use in Portland cement concrete, and C618 for using coal fly ash in concrete.
o Subcommittee D18.14 on Geotechnics of Sustainable Construction, part of ASTM Committee D18 on Soil and Rock, offers standard D7765 for the use of foundry sand in structural fill and embankments, and it is developing other standards for using industrial byproducts together with earth materials for sustainable infrastructure construction.
o Subcommittee E50.03 on Beneficial Use, part of Committee E50 on Environmental Assessment, Risk Management and Corrective Action, guides industry stakeholders in the use of coal ash through E2277 for design and construction of coal ash structural fills.
o Subcommittee D04.99 on Sustainable Asphalt Pavement Materials and Construction, part of ASTM Committee D04 on Road and Paving Materials, is developing a trio of new standards involving sustainable bituminous materials and construction that consider and improve environmental impact. These include a specification for the use of blast furnace and steel furnace slag and guides for the use of recycled asphalt shingles and recycled asphalt pavement in bituminous mixtures.
Doug Clauson is a freelance writer based in Wynnewood, Pennsylvania.