Laura Ingalls Wilder, author of the beloved Little House on the Prairie, tells of her first encounter with an asphalt pavement.  She was on a wagon journey with her parents in 1894 that took them through Topeka. 

"In the very midst of the city, the ground was covered by some dark stuff that silenced all the wheels and muffled the sound of hoofs.  It was like tar, but Papa was sure it was not tar, and it was something like rubber, but it could not be rubber because rubber cost too much.  We saw ladies all in silks and carrying ruffled parasols, walking with their escorts across the street.  Their heels dented the street, and while we watched, these dents slowly filled up and smoothed themselves out.  It was as if that stuff were alive.  It was like magic."

Today, this dark, resilient material covers more than 94 percent of the paved roads in the United States; it’s the popular choice for driveways, parking lots, airport runways, racetracks, tennis courts, and other applications where a smooth, durable driving surface is required.  Called at various times Hot Mix Asphalt, blacktop, tarmac, macadam, plant mix, asphalt concrete, or bituminous concrete, asphalt pavements have played an important role in changing the landscape and the history of the U.S. since the late 19th century. 

But the story of asphalt begins thousands of years before the founding of the United States.  Asphalt occurs naturally in both asphalt lakes and in rock asphalt (a mixture of sand, limestone, and asphalt).

The first recorded use of asphalt as a road building material was in Babylon around 625 B.C., in the reign of King Naboppolassar.  In A Century of Progress: The History of Hot Mix Asphalt, published by National Asphalt Pavement Association in 1992, author Hugh Gillespie notes that  “an inscription on a brick records the paving of Procession Street in Babylon, which led from his palace to the north wall of the city, ‘with asphalt and burned brick.’”

We know that the ancient Greeks were familiar with asphalt and its properties. The word asphalt comes from the Greek “asphaltos”, meaning “secure.”  The Romans changed the word to “asphaltus,” and used the substance to seal their baths, reservoirs, and aqueducts.

Many centuries later, Europeans exploring the New World discovered natural deposits of asphalt. Writing in 1595, Sir Walter Raleigh described a “plain” (or lake) of asphalt on the Island of Trinidad, off the coast of Venezuela.  He used this asphalt for re-caulking his ships.

Laying the Foundation for Asphalt Roads

Despite these early uses of asphalt, several hundred years passed before European or American builders tried it as a paving material. What they needed first was a good method of road building.

Englishman John Metcalf, born in 1717, built 180 miles of Yorkshire roads.  He insisted on good drainage, requiring a foundation of large stones covered with excavated road material to raise the roadbed, followed by a layer of gravel. Thomas Telford built more than 900 miles of roads in Scotland during the years 1803-1821.  “He perfected the method of building roads with broken stones, laid to a depth according to the weight and volume of traffic it would have to carry,” Gillespie writes.
 
Telford’s contemporary, John Loudon McAdam, taught himself engineering after being appointed a trustee of a Scottish turnpike. McAdam observed that it was the “native soil” that supports the weight of traffic, and that “while it is preserved in a dry state, it will carry any weight without sinking.”  To construct his roads, McAdam used broken stone “which shall unite by its own angles so as to form a hard surface.”  Later, to reduce dust and maintenance, builders used hot tar to bond the broken stones together, producing “tarmacadam” pavements.

Asphalt Roads Come to America

Coal tar, which is no longer used in asphalt pavement, was the binder in the first bituminous mixtures produced in the United States.  These mixes were used for sidewalks, crosswalks, and even pavements starting in the late 1860s.  In 1870, a Belgian chemist named Edmund J. DeSmedt laid the first true asphalt pavement in this country, a sand mix in front of the City Hall in Newark, New .Jersey.  DeSmedt’s design was patterned after a natural asphalt pavement placed on a French highway in 1852.

DeSmedt went on to pave Pennsylvania Avenue in Washington, DC, a project that included 54,000 sq. yds. paved with sheet asphalt from Trinidad Lake Asphalt.  The durability of this pavement proved that the quality of the asphalt found in the Americas was as good as that imported from Europe.

Patented Roadways 

Builders, quick to see the advantages of asphalt, tried to stake out claims to the material.  “Looking back from today’s marketplace, where Hot Mix Asphalt (HMA) is compared and classified only by its technical qualifications, it is difficult to comprehend that until 70 years ago, competing proprietary brands of HMA were peddled, touted, and huckstered with all the enthusiasm that now is used to advertise soft drinks,” writes Gillespie.

The first such patent was filed in 1871 by Nathan B. Abbott of Brooklyn, New York.  In 1900, Frederick J. Warren filed a patent for “Bitulithic” pavement, a mixture of bitumen and aggregate; despite vigorous efforts by the Warren Brothers Company to defend its patent (and the name of the material), “bitulithic” was often used to describe any asphalt pavement.  Other trade names for asphalt mixes included Wilite, Romanite, National Pavement, Imperial, Indurite, and Macasphalt.  Many of these patented mixes were successful and technically innovative.

The fierce competition among asphalt producers, however, allowed cities to require more stringent requirements for their asphalt roadways.  In 1896, for example, New York City adopted asphalt paving in place of brick, granite, and wood block.  But it also required 15-year warranties on workmanship and materials.  The long-term warranties, which did not recognize pavement failures caused by factors beyond the asphalt contractor’s control, bankrupted many builders.  The result was fewer and higher bids for asphalt pavements.

The patents for “Bitulithic” pavement expired in 1920, and subsequent improvements in pavements by Federal and state engineers forced most of the remaining patented pavements from the market.
 
Changing Techniques in Production and Construction

Until about 1900, almost all asphalt used in the U.S came from the natural sources of Lake Trinidad and Bermudez Lake in Venezuela. Refined petroleum asphalts, used initially as an additive to soften the natural asphalt for handling and placing, made an appearance in the mid-1870s and slowly gained acceptance.   By 1907, production of refined asphalt had outstripped the use of natural asphalt.

Meanwhile, as the automobile grew in popularity, local and state governments were besieged by requests for more and better roads.  This demand led to innovations in both the production and laying of asphalt.  Roadway testing gradually became an accepted practice thanks to the efforts of Logan Waller Page, who had studied the procedures in France.

The earliest HMA production units consisted of shallow iron trays heated over open coal fires. The operator dried the aggregate on the tray, poured hot asphalt on top, and stirred the mixture by hand.  The quality of the mix usually depended on the skill and experience of the operator.  The first mechanical mixers were used in Paris in 1854, but they were crude and required four hours to produce just one batch of asphalt.

The Cummer Company opened the first central hot mix production facilities in the U.S. in 1870; by the end of the century, builders on both sides of the Atlantic were producing mixers and dryers in a variety of forms.  While some asphalt producers tried to develop portable machines, and others turned to units mounted on railroad cars, most production facilities were costly and cumbersome, limiting the areas in which HMA was readily available.
 
The first asphalt facility to contain virtually all the basic components of those of today was built in 1901 by Warren Brothers in East Cambridge, Mass.  (It lacked only a cold feed and pollution control equipment.) The first drum mixers and drum dryer-mixers, which came into use around 1910, were Portland cement concrete mixers that were adapted for use with HMA.  Mechanization took another step forward in the 1920s with the improvement of cold feed systems for portable and semi-portable systems.  Vibrating screens and pressure injection systems were added in the 1930s.

Construction methods improved at an even faster pace.  “The earliest items of equipment for laying asphalt could be easily transported in one hand,” Gillespie notes.  “Brooms, lutes, squeegees and tampers were used in what was a highly labor-intensive process.  Only after the asphalt was dumped, spread, and smoothed by hand did the relatively sophisticated horse-drawn roller, and later the steam roller, move in to complete the job.”

Builders in the early 1920s used modified portland cement concrete mechanical spreaders for the first machine-laid asphalt. Later, they added tailgate spreaders and concrete strike-off screeds.  In the 1930s, Sheldon G. Hayes was the first to use a Barber-Greene finisher, which consisted of a tractor unit and a screed unit with a vertical tamping bar.  Barber-Greene introduced the floating screed a few years later, and its design dominated the market until the patent expired in 1955.

Production facilities could not at first keep up with the additional demand for HMA that resulted from paving improvements, but the gap was finally closed around 1930.  J.S. Helm, President of the Asphalt Institute, noted in 1939 that “Asphalt is an essential material in nearly every form of highway construction and maintenance.  In the four years from 1934 to 1937, asphalt entered into the construction of more than four-fifths of the mileage of highways completed in those years under state highway direction.”

Building Boom

During World War II, asphalt technology improved at a great pace, spurred in part by the need of military aircraft for surfaces that could stand up to heavier loads. After the war ended, and families moved to the suburbs, road building became a huge industry.  In 1956, Congress passed the State Highway Act, allotting $51 billion to the states for road construction.

Contractors needed bigger and better equipment.  Pavers added electronic leveling controls in the 1950s, and automated screed controls in the early 1960s. Extra-wide finishers, capable of paving two lanes at once, made their debut in 1968.

Two recent innovations also helped increase paver productivity.  Pickup machines have allowed bottom-dump trailers to windrow the material in advance of the paver so it can operate continuously. The Material Transfer Vehicle (MTV), introduced in 1987, receives mix from trucks and feeds it to the paver as required. 

Until the mid-1950s, most HMA pavements were compacted with a combination of a light tandem roller and a heavy three-wheel roller.  Later, the use of pneumatic-tire rollers became more common.  The most commonly used rollers today are vibratory steel-wheel rollers. 

Better Plants

The asphalt plants of the early ''50s might have included a dryer, a tower with a screed, and a mixer.  They were dirty, dusty operations. But by the mid-''60s, with air pollution a serious concern across the country, many had added wet scrubbers, and a few had baghouses.

“The other major change in the mid to late ''60s was the addition of surge bins and storage bins,” says NAPA member John Spangler, chairman of Milestone Contractors, L.P., Indiana.  “Prior to that, everything was loaded right from the plant into the truck.  Then somebody got the idea that we could put mix in bins for short periods of time, so we had surge capacity. The technology grew to where we could store mix for three or four days.”

Concern for the environment has encouraged the construction of better plants.  “When I first started, state-of-the-art was a batch plant that was dirty, that you could usually see three miles away,” notes NAPA member Don Brock, chairman of Astec Industries, Tennessee.  “We’ve gradually progressed from there to cleaning them up with wet washers and baghouses to being invisible today.  Today, we need to build a plant that you can’t see, you can’t hear, and you can’t smell.”
 
Trends in Quality Improvement

In the 1950s, state and federal highway engineers controlled almost every aspect of road building, from the mining of aggregates to the mixing of asphalt to its placement on the roadway.  Today, however, many jurisdictions have taken a new partnering approach that allows the contractor to take more responsibility (and risk) for the finished roadway.  Pavement warranties are also becoming more common.

The trend toward continuous improvement has also allowed the industry to embrace recycling.  Recycling was common in the early 20th century, but the practice fell out of use as new asphalt refineries were built, increasing supplies and dropping prices.  The energy crisis of the 1970s, however, demonstrated the need for conservation of natural resources; since that time, both base and surface courses have incorporated an increasing amount of recycled asphalt in their mixes. Over 70 million metric tons of asphalt paving material is recycled each year.  Today, asphalt pavement is America''s most recycled material.

The industry itself has led the drive for innovation and quality in design and construction of asphalt roadways.  The National Bituminous Concrete Association (the first name of NAPA; the organization changed its name to National Asphalt Pavement Association in 1965) was established in 1955.  One of the first activities undertaken by the fledgling organization was a Quality Improvement Program, which sponsored asphalt testing at universities and private testing labs, then shared the results with members. 

In 1986, NAPA established the National Center for Asphalt Technology (NCAT) at Auburn University, Alabama, providing a centralized, systematic approach to asphalt research.  NCAT recently opened a new research center and test track and is now the world''s leading institution for asphalt pavement research.

 New developments

Over the last 30 years, the versatility of asphalt has led to its increased use in other applications.  HMA airport runways are finding increased acceptance in the United States, since they provide passengers more comfortable takeoffs and landings, dramatically cut back on runway maintenance, and allow for much faster construction time.  Pavements constructed of high-durability HMA mixes are used increasingly for freight yards, where they stand up to heavy static loads.

HMA is also used worldwide as a practical solution to water storage, flood control, erosion, and conservation problems.  Asphalt has been approved by the Environmental Protection Agency and used successfully as a primary liner for both sanitary and hazardous-waste landfills.  It is also used to line drinking water reservoirs and fish hatcheries in California and Washington.

Meanwhile, the industry continues to benefit from improvements in design and production.  Since the mid-1980s, the industry has developed advanced pavement materials including Open Graded Friction Course (OGFC), Superpave, and Stone Matrix Asphalt (SMA), also called gap-graded Superpave.  Engineering control systems placed on asphalt pavers beginning in 1997 have improved conditions for workers at the paving site. 

Pavements being built today can be engineered to meet a variety of needs – for less noise, greater durability, enhanced skid resistance, reduced splash and spray in rainy weather, and a smoother ride than ever before.

The asphalt street laid 130 years ago in Newark bore little resemblance to today’s asphalt superhighways. But with the constant improvements in production and equipment, and with the continued emphasis on quality, we, too, may look back before too many years have passed and realize how much progress we have made in increasing the durability, safety, and smoothness of asphalt roadways.