"The highway network is one of the most significant achievements in the history of the civil engineering field. Perceived as an imperative measure for the development of a robust economy, the highway network has become a primary mode of transportation and driver of the economy in the United States" (Energy Consumption of Asphalt and Reinforced Concrete pavement materials and Construction, Zapata and Gambatese, 2005). nearly 83% of all roads and streets in the United States are of flexible type (asphalt wearing surface), 7% are of rigid type (Portland cement concrete with or without a bituminous wearing surface) and approximately 10% are of composite type (bituminous surface on PCC base) (Zapata and Gambatese 1).
Humans have been using bitumen (the glue component of asphalt) since at least 3000 BC for small waterproofing tasks. It was harvested from places where petroleum had seeped to the surface. By 2500 BC it was being used to provide an impervious surface from which water could be collected. (Ways of the World: A History of the World's Roads and of the Vehicles That Used Them, M.G. Lay, pg 50). It is mentioned in the Bible as being used for waterproofing Noah's Ark, "Make yourself an ark...and cover it inside and out with pitch" and for building the Tower of Babylon. (Gen 6:14 as mentioned in Ways of the World pg 50). Asphalt has been around for a long time, but only since the early 1900's has the production of asphalt exploded to the level we use it today.
Asphalt is the 'bottom of the barrel' product from the distillation and fractioning of crude oil, and accounts for 0.95% of each barrel of oil (42 gallons in a barrel). 80% of this asphalt is being consumed in the United States for asphalt-concrete road pavings. Roofing Shingles account for most of the remaining 15% of asphalt, with a little being used for waterproofing of other objects. Asphalt used for roads is composed of 5 percent asphalt cement and 95 percent aggregates (stone, sand and gravel). The process of manufacturing this product requires a huge amount of heating energy in order to mix the aggregates and keep it in a liquefied state while storing, resulting in 4,000 mixing plants in the U.S.
While the Energy Information Administration reports that cement production ranks seventh among the most energy-intensive manufacturing industries (EIA 2002) in term of energy use per dollar of output, it is also the most widely recycled material. According to the Federal Highway Administration and the United States Environmental Protection Agency, 80% of asphalt removed each year is reused as part of new roads.
What will happen when oil is no longer cheap enough to use so extensively for road paving? It may be recycled for a while until it is unusable, but ultimately, we will have to find alternatives. Concrete roads are one that has also been in service for a long time, but today is not predominately used. The following graphic depicts an energy analysis of the asphalt throughout it's lifecycle and compares this with concrete roads. Information for graphic interpretation found in (Energy Consumption of Asphalt and Reinforced Concrete pavement materials and Construction, Zapata and Gambatese, 2005).
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There do exist some alternatives asphalt which can be made from non-petroleum based resources such as sugar, molasses and rice, corn and potato starches. It can also be made from distillation of waste motor oils which would otherwise be burned or dumped into land fills. These products are typically lighter colored, which result in roads with less heat from solar radiation reducing potential heat island effect.
