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By Deborah Jackman, PhD, PE, LEED AP™ - originally posted on 06/04/2012

The Photograph and its Significance:
The subject work being considered in this essay is not a painting but a photograph taken by the German photographer, Hermann Harz, in 1935. It is one of several in a series of dye-transfer photographs which highlight the German Autobahn system.

The Nazi regime used photographs to document its major construction projects—the Autobahn construction and the building of a number of major bridges—for propaganda purposes. It wished to demonstrate the superiority of the National Socialist system over that of other world governments. However, while the Nazis used the Autobahn as a propaganda tool, the concept of the Autobahn did not originate under the Nazis, but rather in the 1920s during the Weimar Republic.

In 1926, the Planning Association for the Motorway Linking the Hanseatic Towns, Frankfort and Basle (HAFRABA) was created under the leadership of Willy Hof, a prominent German business leader. The HAFRABA was not a governmental body, but rather a private organization dedicated to advocating for the development of modern roadways. By 1926, motorized vehicles were becoming fairly commonplace among regular German citizens, and the HAFRABA wanted to promote the development of an infrastructure to allow motorized vehicles to easily and safely travel across the entire country, thereby connecting major cities and promoting commerce. To this end, the HAFRABA drew up construction plans for the roadway and began to lobby the Weimar Republic to finance the construction of this system of roadways (the Autobahn). The Weimar Republic rejected the project for two basic reasons. First, in order to build the Autobahn, the government needed to secure financing. Since bank credit was not readily available to the German government after the First World War, the only financing option would have been to charge tolls, as Italy did in building its national roadways in the 1920s. However, the assessment of tolls was a breach of German law and was therefore not an option (i.e. under the Financial Adjustment Law of 1926.) The second reason was that the German railroads lobbied against the development of any national roadway system, fearing that it would negatively impact their industry. Thus, it wasn’t until the Great Depression and the rise of the Nazi party to power that the idea of building the Autobahn was reconsidered. Hitler saw value in supporting the construction of the Autobahn for two reasons. First, it was a way to offer thousands of unemployed Germans employment, and second, it could be used as a propaganda tool to showcase the Nazi regime. Therefore, in 1933, Hitler met with Willy Hof to discuss the project. Hitler was able to use the brute force of his dictatorship to compel the railroads to withdraw their objection to the project. The project was financed by taxes on crude oil and petroleum, a levy on the railroads, loans from German banks, and by savings in unemployment compensation, due to increased construction industry employment. Contrary to popular belief, the Autobahn was not primarily built by slave labor. Up until 1939, when most able-bodied German men were conscripted into the military, free labor was used to build the Autobahn. It was only after 1939 that slave labor, made up of concentration camp prisoners and POWs, was used [1], [2].

Today, the building of the German Autobahn remains a highly charged subject, filled with negative political and historical implications. However, from a purely technical viewpoint, the Autobahn represents a significant engineering achievement. Despite its Nazi origins, the building of the Autobahn helped to lay the groundwork for post World War II German road-building technology and design expertise. Modern German highway engineers use advanced engineering techniques to extend the life of pavements, to reduce maintenance costs, and to minimize environmental impacts. Pavement design methods used in the German highway system are being studied in the United States today, in an effort to improve both general pavement performance and in order to make US roads more sustainable.

A Primer on Pavement Design:
Before beginning a discussion of sustainable pavement, we will cover, very broadly, the basics of pavement design, in order to become familiar with the types of pavement and their characteristics. This will allow an informed discussion of how sustainable pavement technologies can be best incorporated into US roads

Pavements can be placed in one of three categories: rigid pavement, flexible pavement, or composite pavement. Rigid pavement includes portland cement concrete pavements, with or without expansion joints and with or without steel reinforcement. Flexible pavement includes asphalt concrete pavement (more commonly known simply as asphalt pavement). Composite pavement consists of rigid (concrete) pavement underneath, covered with an overlay of asphalt. Each type of pavement has its unique advantages [3].

Rigid pavement is built of portland cement concrete, which is comprised of portland cement, fine and coarse aggregates, water, and various chemical additives to improve workability. Air is also sometimes entrained in the concrete to varying degrees, depending on the physical properties desired. In the case of reinforced concrete pavements, steel rebar is also used as part of the roadbed. The chemical reaction between the portland cement and the water drives a curing process that transforms the concrete mixture from a very viscous slurry into a solid with high compressive strength following a curing period. The fine and coarse aggregates used are traditionally various sizes of sand and gravel. However, increasingly, in an effort to reduce the environmental impacts of rigid pavements, various recycled materials are being substituted for sand and gravel as the aggregate. Such recycled materials include ground glass and ceramics, foundry ****, and crushed portland cement concrete, recovered from demolished roadways and construction projects and recycled back into the concrete mix as aggregate.[3] Fly ash from power plants has been used since the 1980′s to replace a portion of the portland cement component in concrete. (US EPA guidelines have mandated since the mid 1990s the use of fly ash in concrete.) The use of such recycled materials does more, from an environmental perspective, than merely keep these materials out of landfills. Viewed from a Life Cycle Assessment standpoint (discussed in greater detail below), using recycled materials in rigid pavements saves significant amounts of energy and raw materials and reduces the overall amount of green house gases generated over the lifecycle of the highway. This is primarily because virgin sand and gravel does not need to be mined and transported to the construction site. In the case of old highways being demolished and rebuilt, even greater environmental benefits can be achieved if the old concrete to be used as aggregate can be reground and reused on-site, thereby saving the energy that would have been expended to transport the recycled material to the road site.

Flexible pavement consists of asphalt cement, a mixture of asphalt ( a tar-like product derived from bituminous coal), fine and coarse aggregate, and various chemical fillers and additives to improve performance and workability. Asphalt paving operations are very energy intensive both because of the energy needed to mine the coal used to create the asphalt cement and because the asphalt has to be heated in order to liquefy it sufficiently to allow it to be mixed with the aggregate and laid down on the roadbed. Efforts to make flexible pavement more sustainable include 1) the use of recycled materials in place of the virgin aggregate, 2) the use of various recycled materials which contain asphalt ( such as roofing shingles) melted down and used in place of some of the virgin asphalt cement, and 3) the use of various additives to increase the workability of the asphalt cement at low temperatures, thereby reducing energy demands associated with mixing and laying down the pavement.

Composite pavement consists of a bed of rigid pavement overlaid with asphalt concrete. It combines the environmental impacts of both rigid and flexible pavements. Its primary advantage over either of the other two types is improved ride-ability and noise reduction characteristics [3].

Regardless of the pavement type, the key to optimal durability and performance is a well-prepared road bed, including a properly designed drainage system and a properly designed and compacted sub-base (usually comprised of various grades of dirt, sand, and gravel).

Decisions on which pavement type to use have been driven by cost, and have differed based on location and societal viewpoints. In the US, first-cost considerations have often dominated road building decisions. Many US governmental bodies (federal, state, local) seem willing to tolerate more on-going maintenance costs in favor of lower first costs. This philosophy has favored the use of flexible and composite pavements in many (although not all) areas of the US. However, in Europe (in Germany and Austria in particular), the philosophy favors building roads that are extremely durable, which have low maintenance requirements, and which have very long life spans. This philosophy tends to favor rigid pavement designs, which can have significantly higher first costs, but lower maintenance costs. The Autobahn itself is an example of this.

While cost has historically been the main driver in pavement design, sustainability is now an additional design parameter considered by highway engineers. Because of the German emphasis on rigid pavement design, much of the research on how to make rigid pavements more sustainable has come out of Germany. It is based on the premise that by increasing life span and minimizing maintenance requirements, one inherently lowers environmental impacts because over the life span of the road, fewer interventions involving the expenditure of energy and raw materials are needed. Because of the greater emphasis on flexible pavements in much of the US, US engineers have emphasized the recycling of materials and various energy conservation strategies more in their quest to develop more sustainable pavements. Part of this emphasis is driven by governmental mandate. (The US federal government passed the Intermodal Surface Transportation Efficiency Act (STEA) in 1994, which mandated the use of recycled tire content in asphalt paving projects which receive federal funding.) Both approaches have validity, but ultimately, neither provides the full picture on how to maximize pavement sustainability. The big picture can only be understood in the context of a Life Cycle Assessment, discussed below

Life Cycle Assessment and the Attributes of Sustainable Pavement:
In trying to determine which of the pavement types (rigid or flexible) is more sustainable, and in trying to develop new strategies for minimizing the environmental impacts of roads and pavement, a life cycle assessment must be conducted. Life Cycle Assessment (LCA) is a technique which views the entire life cycle of an engineered system as a single control volume. It looks at energy and mass inputs and outputs from the control volume and quantifies the environmental impacts based on how much energy and resources are used to create the system and on how much hazardous waste and green house gases are generated. It must include impacts ranging from the energy required to extract and transport the raw materials during initial construction, to the impacts which occur during routine system maintenance, to the impacts created during final disposal of residues and wastes from the system following demolition. Researchers have created databases and software packages which help to catalog and calculate environmental impacts.

One of the most comprehensive LCA databases is the US Department of Energy’s LCI Database, available on-line [4]. The DOE LCA database quantifies the environmental impacts of a number of basic industrial and construction processes in terms of the amount of energy they consume and the amount of greenhouse gases and toxic emissions produced. Using DOE LCI, one can find the energy cost and emissions for the production of unit quantities of portland cement, asphalt, steel, and other raw materials used in pavements. Using this data along with estimates of the energy and emissions costs to transport and install them, researchers are able to quantify the relative sustainability of different pavement types and systems.

Horvath and Hendrickson [5] conducted an LCA comparing asphalt pavement to steel reinforced concrete pavements (RCP). The LCA considered energy consumption, ore and fertilizer requirements, toxic emissions, and the hazardous wastes generated during extraction, transportation, mixing, and construction of both asphalt and RCP pavements. Their study initially assumed no recycled content in either type of pavement; the analysis was based upon the use of virgin raw materials. Their conclusions were that RCP required less energy and generated lower amounts of hazardous wastes, but had higher ore and fertilizer requirements and higher toxic emissions, than did asphalt pavements. But, if one subsequently accounted for the fact that there is currently more recycled content in asphalt pavement than in RCP, asphalt pavements can be concluded to be marginally more sustainable.

The results of this study can be understood best if one understands the primary environmental impacts for both RCP and asphalt pavements. Portland cement production is extremely energy intensive and the production of the portland cement used in RCP pavement production is arguably the single largest negative environmental impact associated with rigid pavements. The largest negative environmental impacts in the production of flexible (asphalt pavements) are the large amounts of energy needed to mine the bituminous coal from which the asphalt is produced, and the energy required to warm the asphalt mix prior to installation. To the extent that a portion of virgin asphalt is being successfully replaced with recycled asphalt shingles, with recycled tires, and with recycled asphalt cement pavement re-melt, total energy costs to produce flexible pavements can be driven down. Since the portland cement component in concrete is chemically changed during the concrete curing process, a similar opportunity to recycle this component of RCP, and thus save energy is not possible. Hence, other strategies, mainly aimed at extending the life of concrete pavements, must be employed to make rigid pavement design more sustainable.

Recent Developments to Enhance the Sustainability of Pavements:
Keeping in mind the principles of LCA and the attributes of sustainable pavements discussed above, a number of interesting avenues to minimize the environmental impacts of both rigid and flexible pavements are being researched.

In the category of rigid pavement, German engineers continue to lead the quest to reduce the environmental impacts of portland cement concrete pavements.

One of the major directions this research is taking is in the development of two-layer concrete pavements. These pavements consist of two separate PCC layers—a thick sub-layer, covered with a relatively thin wear layer. Such pavements have been shown to have durability that is comparable to traditional single layer PCC pavement, yet because they are comprised of two layers, it is possible to more readily use higher amounts of recycled materials in the aggregate of the sub-layer. Lower quality aggregate—such as recycled, ground PCC pavement, ground glass, and **** are used in the sub-layer. These recycled aggregates do not appear to reduce the structural performance of the pavement. A higher quality, more expensive aggregate, such as pea gravel, is reserved for the wear layer. This aggregate is exposed as part of the surface finishing process during construction. The exposed aggregate improves roadway safety (by improving pavement friction characteristics) and reduces road noise characteristics. In addition to reducing environmental impacts, and improving friction and noise characteristics, the two-layer PCC pavement is also cheaper [6]. Another German innovation in two-layer PCC pavement is the use of a 3 millimeter thick, polymeric geotextile as an interlayer between the two PCC layers [7[. This interlayer has been shown to lengthen the life of the pavement through 3 mechanisms: 1) the interlayer keeps cracks and other discontinuities in the lower layer from propagating to the wear layer; 2) the interlayer, if properly installed, can promote drainage of any water that enters the wear layer away from the bottom structural layer, thereby increasing roadbed life by reducing cracking due to freeze-thaw cycles; and 3) the interlayer absorbs some of the dynamic stresses caused by heavy traffic, thereby reducing stresses on the structural sub-layer and thus extending its life. The primary disadvantages of the use of the geotextile are its added cost and the need for careful installation to ensure proper performance. This necessitates having a highly proficient and well-trained construction team.

Another German road design practice that is making its way into the US is a movement away from steel reinforced concrete to plain concrete pavements. In a seminal, 15 year longitudinal study in Michigan, comparing standard US concrete pavement design (the control) to standard German concrete pavement design along a stretch of Michigan highway near Detroit, one conclusion has been that steel reinforcements can actually promote transverse cracking in the pavement, thereby shortening its life [8]. Since eliminating steel from PCC pavement removes one material input to the LCA, while potentially also lengthening its overall life and reducing maintenance costs, this change can make PCC pavements more sustainable as well.

Finally, another growing trend to improve the sustainability of PCC pavement has been to use scrap tires to fuel portland cement kilns instead of coal. Since the production of portland cement remains highly energy intensive, one way of mitigating the environmental impacts across the life cycle of the pavement is to recover the embodied energy in scrap tires, rather than use “virgin” coal to fuel the kilns. Not only is this more sustainable, but it saves energy costs during production. In 1996, 23 cement plants across the US used tires as a supplemental fuel. Air emissions using scrap tires as fuel are no worse than air emissions from burning coal. And, given that 250 million tires are discarded in the US per year, which possess 15,000 BTU per pound, this represents a potentially significant energy and material savings [9].

Recent developments to improve the sustainability of flexible pavement (in addition to the increasing reuse of asphalt concrete as re-melt, and the use of used asphalt shingles, both discussed above) include Cold-in-Place recycling (CIR) and Cold-in-Place recycled expanded asphalt mix (CREAM) [10]. Both of these technologies involve the on-site reclamation of used asphalt pavement using specialized machinery that can demolish the existing pavement, regrind it on-site, re-melt it and lay it down to create a new pavement surface. CIR uses various chemical additives and conditioners to allow the recycled pavement to be melted and reworked at lower temperatures than normal, thus saving energy. CREAM uses a similar technology, but also adds air to create an asphalt foam, that can be reworked at lower temperatures. The combination of on-site reclamation (thereby removing transportation impacts from the LCA and reducing the impacts associated with the use of virgin asphalt cement) with the reduced energy costs of re-melting and remixing the asphalt places both CIR and CREAM at the forefront of sustainable technologies for flexible pavements.

Final Thoughts:
Increased mass transportation and the development of vehicles that minimize the use of fossil fuels are the ideas that have often dominated our national discussion of ways to make our transportation system more sustainable. Indeed, both are important strategies in the overall effort to reduce the environmental impacts of transportation. But, the use of the personal automobile is not likely to vanish any time soon. And, our economy relies on semi tractor-trailers to haul large amounts of freight. Given these facts, the need for well-constructed highways will continue into the foreseeable future. However, as this essay is intended to show, the existence of our national highway system can be compatible with responsible environmental stewardship. Highway engineers have made considerable progress through innovative designs and increased recycling to reduce the environmental impacts of highway construction and maintenance. And, just around the “bend in the road,” there will undoubtedly be even more interesting and exciting developments in sustainable pavements in the future.

References:

  1. “The Autobahn Myth”; Oster, Uwe; History Today , November, 1996, p. 39- 41. (Translated from German by Judith Hayward).
  2. “The Third Reich’s Concrete Legacy”; Boser, Ulrich; U.S. News and World Report, Volume 134, Issue 23, p. 45, June 30, 2003.
  3. The Highway Engineering Handbook—Building and Rehabilitating the Infrastructure, 3rd Edition; Roger L. Brockenborough, P.E., editor; 2009; ISBN: 978-0-07-159763-0.
  4. DOE LCI Database, https://www.lcacommons.gov/nrel/search.
  5. “Comparison of Environmental Implications of Asphalt and Steel-Reinforced Concrete Pavements”; Horvath, A. and Hendrickson, C., Transportation Research Record: Journal of the Transportation Research Board of the National Academies; Volume 1626, 1998.
  6. “Design and Construction of Sustainable Pavements: Austrian and German Two-Layer Concrete Pavements”; Tompkins, D., Khazanovich, L., Darter, M., and Fleischer, W.; Transportation Research Record; Volume 2098, p. 75-85, 2009.
  7. “Nonwoven Geotextile Interlayers for Separating Cementitious Pavement Layers: German Practice and U.S. Field Trials”; Rasmussen, R. and Garber, S.; Research Report prepared by the International Scanning Study Team for the Federal Highway Administration, U.S Department of Transportation, May 2009.
  8. Fifteen Year Performance Review of Michigan’s European Concrete Pavement, Smiley, D., Report Number R-1538, Michigan Department of Transportation, Construction and Technology Division, February, 2010.
  9. A Comparison of Six Environmental Impacts of Portland Cement Concrete and Asphalt Cement Concrete Pavements; Gadja, J., and VanGeem, M., PCA R&D Serial No. 2068, Portland Concrete Association, 2001.
  10. “Sustainable Pavements: Environmental, Economic, and Social Benefits of In-Situ Pavement Recycling”; Alkins, A., Lane, B., and Kazmierowski, T.; Transportation Research Record: Journal of the Transportation Research Board of the National Academies, Volume 2084, 2008.