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Paving Materials

July 13, 2020

US President, Dwight D. Eisenhower (1890-1969), thought that highways were an essential part of national infrastructure. While serving as an army general, he realized that the rapid movement of troops and their supplies was essential to national defense. That's why the US Interstate Highway System, initially known as the Dwight D. Eisenhower National System of Interstate and Defense Highways, was created in 1956.

Since roadways are ubiquitous, we hardly give them much thought. When we do, our thoughts tend to either complaints about heavy traffic, or some of the many roadway themed jokes. My favorite of these is the advice of baseball great, Yogi Berra (1925-2015), "When you come to a fork in the road, take it." He apparently said this while giving driving directions to his New Jersey home to fellow baseball player, Joe Garagiola (1926-2016).[1]

There are also some science themed roadway jokes of the pedestrian chicken variety. For physicists, we have a reference to Newton's first law of motion; viz., "Chickens at rest tend to stay at rest. Chickens in motion tend to cross the road." For biologists we have, "Why did the dinosaur cross the road? Because chickens hadn't evolved yet."

Synthetic materials are a recent development, so early roadways were constructed from easily sourced natural materials. The Romans, with a few exceptions,[2] did little science and just incorporated the scientific ideas of the Greeks. They did, however, excel at engineering. While the aqueducts are usually presented as the prime example of Roman engineering prowess, the Romans were also road builders who weren't hesitant to cut tunnels and build bridges to attain the straightest possible path. The most famous Roman roadway was the Appian Way (Via Appia, "via" being the Latin word for road). This roadway, begun in 312 BC, was 196 kilometers (122 miles) in length, and it connected Rome to Capua.

Extant portion of the Appian Way (Via Appia) in Rome

Extant portion of the Appian Way (Via Appia) in Rome.

All roadways must be carefully maintained to repair damage from weather and heavy truck traffic. In Tikalon's neighborhood, and elsewhere, the many potholes show the fragility of roadways.

Portions of the Via Appia, such as the one shown here, have existed for more than 2,000 years. It's estimated that the construction cost of the Appian Way was about two million dollars per kilometer ($3.2M per mile) in today's money.[3] This is nearly equal to today's cost of construction for two lanes of an interstate highway.

(Via Wikimedia Commons.)

The larger Roman roads had a standard width of 4.2 meters, which was sufficient to allow two vehicles to pass each other. Road construction used the proven technique of layering in which a coarse rubble layer (statumen) was first applied in an excavated trench, sometimes atop a leveling layer of sand, then overlaid with a coarse concretion of small pebbles and a lime mortar (rudus). Above that was a concretion of smaller stones and lime (nucleus), followed by a top layer of paving slabs slightly recessed into that lower layer and grouted with lime (summum dorsum). The roadway had a crown or pitch that allowed water flowflow to the sides.

Structure of a Roman road

Roman builders used locally-sourced materials, so there are some variations in composition of the Appian Way along its route. The small stones used in the "rudus" layer above the rubble of the "statumen" were usually a type of small, fine-grained sedimentary rock called oolites. (Created with Inkscape from data in ref. 3.[3] Click for larger image.)

The layered roadway concept was improved by John Macadam (1756-1836), who realized that angular materials would offer a mechanical advantage over rounded gravel. His type of road construction, called Macadam, specified the size of rock that could be used. Macadam said that “no stone larger than will enter a man’s mouth should go into a road.”[3]

An important variant of a Macadam was tarmacadam, which combined sand and tar with Macadam surface materials. This was patented in 1902, and the term, tarmac, is used to describe the paved apron of an airport, although these are not really made from tarmacadam. Tarmac is also incorrectly used to describe asphalt roadways. Asphalt pavement is now the preferred roadway surface covering, and it's used on more than 94 percent of the paved roads in the United States.[4]

Asphalt is a natural material that can be found in asphalt lakes and in mixture with sand and limestone called rock asphalt.[4] Asphalt was used by the Babylonians for a road material around 615 BC.[4-5] It was named άσφαλτος (asphaltos) by the ancient Greeks, and the Romans used asphalt as a sealant for aqueducts, reservoirs, and public baths.[4] It was used as a road material in the United States in the mid-19th century, when Belgian chemist, Edmund J DeSmedt, used lake asphalt mixed with sand to pave the street in font of the Newark, New Jersey, city hall, and to pave Pennsylvania Avenue in Washington, DC.[4-5]

Road Crews Paving

An all too familiar sight, road paving in the United States. This photo shows road workers paving a street in Wise County, Virginia, in 2017.

There's a joke that the road signs that state, "End Construction," are not an advisory, but rather a driver's slogan.

(Wikimedia Commons image by Famartin.)

Prior to the 20th century, most asphalt came from natural tar pits, such as Lake Bermudez in Venezuela.[4] Asphalt derived from petroleum, sometimes used as an additive to natural asphalt, displaced natural asphalt by 1907.[4-5] Inventor, Frederick J. Warren, was granted eight US patents for "hot mix" paving materials in 1903, thereby taking control of the market of asphalt as a road material.[3] The first claim from US patent no. 727,505, "Pavement," May 5, 1903, reads,
"A street-pavement mixture composed of mineral ingredients ranging in grades from three inches down to impalpable powder, from fifty to eight per cent, of such mineral ingredients lying between one-fourth inch and three inches in diameter, in combination with a bituminous binder."[6]
A 1910 legal ruling declared that mineral sizes outside the range specified did not infringe, thereby opening the asphalt paving market.[3]

The purpose of a roadway is to allow vehicular transportation. Our energy-conscious and global warming civilization is always looking for ways to reduce the energy expended by vehicles and their greenhouse gas emissions. While fuel efficiency is an important factor, there's also the interaction between the vehicle and the pavement.

Recent research by engineers and materials scientists at the Massachusetts Institute of Technology (Cambridge, Massachusetts) has examined how an increase in the elastic modulus of pavement surface layers will lower vehicle fuel consumption by reducing the bending-induced pavement–vehicle interaction.[7-8] Their research is published in a recent issue of the Transportation Research Record.[7]

The MIT theoretical study shows that even a small change in the mechanical properties of roadway pavement would eliminate a half-percent of the total greenhouse gas emissions from transportation at very little cost.[8] This would be a consequence of improved fuel mileage for heavy vehicles that would reduce greenhouse gas emissions.[7-8] This improvement would come about by increasing the elastic modulus of resurfaced roadways of the interstate highway system, thereby reducing greenhouse gas emissions by 440 megatons carbon dioxide equivalent over a 50-year period.[7]

The weight of heavy vehicles causes a slight deflection in the road surface, and this deflection is enough to change a truck's overall fuel efficiency.[8] The computer model by members of the MIT Concrete Sustainability Hub examined climate conditions, road lengths, materials properties, and road usage, and it examined changes from pavement resurfacing.[8] The research found that stiffer pavements reduce the deflection that forces trucks to climb out of the small hill that results from their weight.[8] Says MIT principal research scientist and co-director of MIT's Concrete Sustainability Hub, Randolph Kirchain,
"When we as individuals walk on pavements, they seem like perfectly rigid things. They're not responding to us... But for trucks, that is not the case. There is enough of a deflection in that surface that some amount of energy is expended to overcome the little divot that you create as you drive along."[8]
One way to reduce pavement deflection is through use of concrete, rather than asphalt. While concrete is a common pavement material in the Northern United States, much asphalt is used in the South, where the higher temperatures cause even higher deflection.[8] Additives can have a major affect, and a very small amount of synthetic fibers or ​carbon nanotubes, about a tenth percent, dramatically improves asphalt stiffness.[8] One problem is that asphalt with additives might require modification of paving machines.[8]

A mirage over a hot asphalt road (Yuri Khristich)

A mirage over a hot asphalt road. These apparitions are caused by the refraction of air, and they were a frequent sight on family summer vacation road trips of my youth. What appears as water on the roadway is actually an inverted image of the sky.

(Wikimedia Commons image by Yuri Khristich.)

References:

  1. Yogi Berra: When You Come to a Fork in the Road, Take It, Quote Investigator.
  2. The most prominent authors of Roman science are as follow:
    • Lucretius (99 BC-55 BC), who wrote De rerum natura, a book about the Greek atomists, optics, and biology.
    • Nigidius Figulus (c.98 BC-45 BC), who wrote about astronomy, zoology, and meteorology.
    • Vitruvius (c.75BC - c.15BC), who wrote De architectura), a book about architecture, surveying, astronomy, mechanics, materials, and mathematics.
    • Pliny the Elder (23-79), whose multi-volume Naturalis Historia (77 AD) contained many scientific facts, including many about minerals.
    • Frontinus (c.40-103), a Roman general who wrote De aquaeductu about Roman aqueducts.
  3. Pavement History at pavementinteractive.org.
  4. History of Asphalt, National Asphalt Pavement Association.
  5. History Of Asphalt Roads In The US, Dykes Paving blog.
  6. Frederick John Warren, "Pavement," US Patent no. 727,505, May 5, 1903 (Via Google Patents).
  7. Hessam AzariJafari, Jeremy Gregory, and Randolph Kirchain, "Potential Contribution of Deflection-Induced Fuel Consumption to U.S. Greenhouse Gas Emissions," Transportation Research Record, Advanced Online Publication, June 10, 2020, https://doi.org/10.1177/0361198120926169.
  8. David L. Chandler, "Stiffer roadways could improve truck fuel efficiency," Massachusetts Institute of Technology Press Release, June 11, 2020