What material carries most of the nation’s cars and trucks, is a multi-million-dollar equipment market, is America’s most recycled product, and can be “hot-mixed” in hundreds of combinations for smoother rides and longer-lasting highways?
The answer, of course, is asphalt, which covers 94 percent of the nation’s highways, according to some estimates.
We all drive on it. Yet few realize how this pavement material has developed over the years, and how asphalt scientists are improving it. Even though asphalt has been used in the United States for more than 125 years, its benefits for the transportation industry are more significant than ever — especially “perpetual pavements,” lasting indefinitely except for their top surfaces, which may dramatically change highway construction in the United States.
Dates Way Back
The first recorded use of asphalt as a roadbuilding material was 625 B.C., in Babylon. Asphalt occurs naturally in both asphalt lakes and rock asphalt. The ancient Mesopotamians used it to waterproof temple baths. Egyptians used it as mortar to prevent erosion along the Nile. Some even believe asphalt waterproofed the basket that hid the infant Moses from Pharaoh in the Nile’s rushes.
Later, the Romans used asphalt to seal their baths and aqueducts (but not for their famous roads).
In modern times, John McAdam used crushed rock, packed into thin layers to form a hard surface, to build a turnpike in Scotland in the early 1800s. Later, to reduce dust and maintenance, builders added hot tar to bond the broken stones together.
An asphalt road was built in France in 1852. Here in the United States, Belgian chemist Edmund DeSmedt laid the country’s first true asphalt pavement in Newark, NJ, in 1870. He also paved Pennsylvania Avenue in Washington, D.C., with 54,000 sq. yds. of sheet asphalt from Trinidad Lake on the island of Trinidad.
“When they first used asphalt here, sometimes all they did was spray some oil over sand,” said Margaret Cervarich, vice president for marketing and public affairs at the National Asphalt Pavement Association (NAPA) in Lanham, MD.
Asphalt got a lot better. The first asphalt patent was filed in Brooklyn, NY, in 1871 and the first central hot mix production facility was soon opened, followed by the first modern asphalt facility in 1901 in Cambridge, MA. As automobiles grew in popularity, the demand for more and better roads led to many improvements in both producing and laying this material.
The entire New Jersey Turnpike, dating from 1951, is asphalt. NAPA said its chief engineer expects it to last another half century.
The ’Hot Mix’
Asphalt pavers move slowly along construction zones, releasing blacktop pavement material. Compaction equipment, the familiar double-drum steel rollers, follow the paver, smoothing out the final highway surface.
This is the climax of a very careful process that often begins with milling machines removing surface material from an existing roadway. This material is loaded into a truck and taken back to a plant for recycling.
New asphalt starts with a mixture of sand and small and large rocks (called aggregate). Next, asphalt cement “binder” is heated and combined with the rocks — thus the name “hot mix asphalt” (HMA). A typical asphalt pavement is 95-percent aggregate and five percent binder. These are heated and mixed in an asphalt plant.
Many states also add different modifiers enabling the mix to perform better under different temperatures, or to carry heavier loads.
Hot-mix plants are usually stationary, though portable plants, on temporary sites close to the workzone, are common in remote areas. (Eighty years ago, some of the earliest plants were actually on railroad cars, which would take them from town to town.)
Hot mix manufacturing plants are usually valued at millions of dollars.
Although plants are often owned by the contractor, they also may be operated by independent entities.
Batch plants make asphalt in batches as needed to go straight to the work sites. Drum plants make asphalt continuously and can store the asphalt for several days in heated silos.
Applying the Mix
The plant disgorges the hot asphalt into trucks that carry it to the site, usually dumping the material into the hopper of the paver, which agitates the asphalt mix to prevent the aggregate from separating and to help ensure a uniform temperature. The paving work can then be done in low-traffic periods (often at night), reopening the lanes a short time later.
That’s the usual process. But asphalt is more a science than an art. By changing the types or amounts of aggregate, adding certain modifiers (even shredded roof shingles or tiny pieces of rubber from old tires), asphalt engineers can change the pavement’s performance.
“That’s the beauty of hot mix,” said Pat Parsons, executive director of the Asphalt Pavement Association of West Virginia in Charleston, WV. “You can use most local materials, different grades of asphalt and different modifiers in dozens or even hundreds of different mix designs. You’re tailoring the mix to the actual conditions on the highway, including climate and traffic loads. As traffic increases, you can structurally increase the strength of the road with additional overlays.”
Conventional “full-depth’ asphalt (8 in. or more thick) can last 30 or 40 years, though surfaces need to be replaced after 15 to 25 years. Today, new types of asphalt “perpetual pavements” — a major, even revolutionary, innovation for the industry — are expected to last indefinitely. These pavements often use three layers of durable asphalt: a strong, flexible bottom layer that prevents bottom-up structural fatigue cracks, a load-bearing intermediate layer, and a rut-resistant surface, which can be resurfaced as needed.
“The perpetual pavements concept is where all the excitement is,” said Parsons. “We’re building pavements that for my lifetime, and my child’s lifetime, won’t require anything except renewing the surface. With growing traffic, concerns for driver safety, and increasing taxes, it’s important to be able to build a highway requiring less frequent maintenance and lasting much longer. Asphalt highways will last generations while you only have to worry about the surface. They are much like your home. When the roof needs to be replaced, you don’t tear down the house; you just replace the shingles.”
Some believe that states will eventually adopt the perpetual pavement concept for almost all highways, including urban expressways now built from concrete.
Jim Huddleston, executive director of the Asphalt Pavement Association of Oregon in Salem, OR, told Construction Equipment Guide (CEG):
“The basic purpose of a perpetual pavement design is to make sure that you never develop a crack at the bottom of the asphalt that would propagate up through and result in structural failure. The original concept is to do that through a three-layer system. The bottom layer is designed to be as resistant to cracking as much as possible because, just as when you bend a beam, the maximum fatigue is at the bottom. Then the center layer is designed to be load bearing to minimize bending under heavy loads. The top layer provides surface attributes like frictional properties and rut resistance. The idea is to optimize properties with minimum thickness.
“In a more-conventional perpetual pavement design, more widely used, you can still make the pavement perpetual, but you end up being a little thicker. You prevent cracking by making sure the thickness at the bottom is adequate so that you have a low strain level. We’ve discovered that asphalt will never crack if you can keep below a certain strain level, which is 70 microstrains.”
States Give a Look
Perpetual pavements usually have a greater asphalt content in the bottom layer than conventional asphalt, but the main difference, Huddleston said, is in the structural design, so that all the stress is at the surface, “which you can perpetually manage by milling the stress when it occurs and replacing it with new asphalt.”
He added that the new pavements are “gaining in popularity, with many states trying them to see how they go.
“The concept is being applied differently in different states,” Huddleston added. “As more research is done, states will hone in on a finer set of tools. Actually, a lot of perpetual pavements are out there, but people didn’t know they were building them when they made them thick. When you pull up the pavements, you find they are thick enough so that cracking doesn’t ever start. All the stress is at the surface. We found that roads that were thought to be 20-year designs were 30 or 40 years out there and yet the base was still sound. We thought that, if we did it by accident, let’s figure how to do it on purpose.”
If the concept is so simple, why aren’t more states utilizing it?
“That’s a great question. I think the answer is that, in the transportation business, engineers are slow to change. Many are beginning to, however.”
How long will a perpetual pavement actually last?
“Indefinitely is what we’re saying, if you take care of the surface, just as the siding on your house will last indefinitely, 50 or 60 years, if you don’t let it rot. If the strain level is below 70, theoretically, the asphalt could last forever. A thickness of 12 in. to 14 in. on a high-traffic interstate is about all it takes because you reach a point where the strain level is below this.”
Asphalt is America’s most recycled product. Asphalt pavements are 100-percent recyclable and can be made to perform better the second or third time around. According to NAPA, 73 million tons of asphalt pavements are recycled every year, more than the combined total of glass, paper and plastics from the municipal waste stream.
NAPA is sponsoring an international conference on asphalt at its National Center for Asphalt Technology in Auburn, AL, June 7 to 9.
Further information on asphalt is available on the Web site beyondroads.com.