When you hear the word “Asphalt”, most probably you think of the black tar stuff on roads and highways, right? But that’s not exactly correct. Asphalt is the liquid that is in the road, it’s the binding agent that kind of holds the rocks together. The difference is that the Asphalt comes from crude oil, while tar comes from coal. Therefore that Black Stuff you see on the Road, technically it´s not Asphalt.
Asphalt also is found naturally in the earth, and there are lakes of it where oil from underground has risen to the surface, like the La Brea Tar Pits in Los Angeles and Pitch Lake in Trinidad, which is the largest natural deposit of asphalt in the world. But the most common way the binder is made today is through the oil refining process. Asphalt is the heaviest of materials in a barrel of oil; it’s basically the waste product. Asphalt is the heavy residue that settles to the bottom. It cannot be used for energy, so it takes on new life as the sticky stuff that holds materials together. Combined with various amounts and types of rocks and other substances, it eventually becomes the mixture we drive on. The road is really an asphalt mixture or better termed “asphalt pavement.”
The History of Asphalt
All the talk of oil refining may make asphalt sound like a relatively modern invention, but the 1st recorded use of asphalt in a road was in Babylon in 615 B.C.; The Romans used it to seal structures like baths and aqueducts. In fairness, the Carthaginians are generally credited with being the first to construct and maintain a road system. The Romans eventually decided that their neighbors across the Mediterranean were a bit of a threat to the empire destroying Carthage in 146 B.C. (The Carthage ruins are located in Tunisia (Northern Africa) next door to Algeria (on the left) and Libya (on the right — so to speak).) It is suggested that the Romans took up the practice of a military road system from the Carthaginians. It is estimated that the Romans built about 87,000 km of roads within their empire (about equal to the length of the U.S. Interstate system).
Apparently, there is no record of “traditional” roads in the U.K. prior to the Romans. For the most part, the main Roman roads in the U.K. (total of about 4100 km) was for military purposes in that they connected camps which were about 30 km apart (or about one day’s march). Since the primary purpose of these roads was for foot soldiers, the roads were straight, but virtually without regard to grade. They generated high noise levels, were rough and labor intensive (slave and “statue” labor often used). The Roman design for their primary U.K. roads generally consisted of 4 layers (top to bottom) as follows:
- Summa Crusta (PAVIMENTUM): Smooth, polygonal blocks bedded in underlying layer.
- Nucleus: A kind of base layer composed of gravel and sand with lime cement.
- Rudus: composed of rubble masonry and smaller stones also set in lime mortar.
- Statumen: 2 or 3 courses of flat stones set in lime mortar.
The total thickness was as much as 0.9m and road widths of 4.3m or less. An illustration of Roman pavement structure near Radstock, England, is shown as Figure 1.
Roman roads in some countries have been up to 2.4m thick. These structures had crowned (sloped) surfaces to enhance drainage and often incorporated ditches and/or underground drains. As one might expect, Roman road building was varied to suit local conditions and materials — not unlike today actually. The Romans departed the U.K. about A.D. 406. Road design and construction languished for about 1.200 years thereafter. Roman road construction was not inexpensive. Updated construction estimates of the Appian Way in Italy are about $2.000.000 per km. The oldest known road in the U.K. is near the River Brue in southwestern England. Actually, the “road” is a 6.000 year old walkway which was discovered in 1970 in a peat bog. The construction of the road coincides with the arrival of the first farmers in the U.K. about 4.000 B.C.
When English explorer Sir Walter Raleigh turned up at Pitch Lake in Trinidad in 1595, he used the asphalt for caulking his ships. It’s been used in other non-road functions throughout history. Using it as a binder in roads became more common in the 1800s. John Loudon McAdam, who built the Scottish turnpike, added hot tar to reduce dust and maintenance on roads. This method also improved driving conditions. In the United States, bituminous mixtures (asphalt concrete) first appeared in the 1860s, and the 1st “true asphalt pavement” was laid in Newark in 1870 by Edmund J. DeSmedt, a Belgian inventor. This asphalt was modeled after a natural pavement highway in France. DeSmedt then paved Washington, D.C.’s Pennsylvania Avenue with asphalt from Trinidad, further proving its durability.
Enterprising chemists and inventors soon filed patents for different blends of asphalt mixtures, which appeared under a variety of names. As the industry grew, cities began requiring warranties on workmanship and materials. Until the early 1900s, nearly all asphalt came from natural sources, but with the launch of the first modern asphalt facility in East Cambridge, Massachusetts, in 1901 and the increase in automobiles, requests for better roads invigorated the asphalt industry. By 1907, natural asphalt production was overtaken by refined petroleum asphalt. People started demanding better modes of transportation. The roads where people started using the asphalt to keep the rocks together held up longer than the conventional dirt road that people were used to. Driving on a gravel road versus one that was paved offered a significantly different experience. Finally, the 1956 Federal-Aid Highway Act helped transform the roads in the United States still made of packed dirt and created the 78.660 kilometer Interstate System in the U.S.
Early Thickness Trends
Thus, we have seen pavement structures decrease from about 0.9 m for Roman designs to 350 to 450 mm for Telford designs, to about 250 mm for Macadam designs, to 100 mm at about the turn of the century. (Naturally, the thinnest pavements were not always used.)
The Massachusetts Highway Commission standard cross-section for macadam construction was 150 mm thick as reported by Gillette in 1906. This thickness was also used on New York state roads at about that time. Up to the early 1900s, the design emphasis was placed on the use of fixed standards occasionally modified for local soil conditions. Further, the need for more durable pavements was mandated by the changing vehicle fleet. To better illustrate the problem we can say that the existence of our macadam roads depends upon the retention of the road-dust formed by the wearing of the surface. But the action of rubber-tire motor-cars moving at high speed soon strips the macadam road of all fine material, the result being that the road soon disintegrates.
Early Bituminous Pavement – Tar Macadam
It appears that the first tar macadam pavement was placed outside of Nottingham (Lincoln Road) in 1848. At that time, such pavements were considered suitable only for light traffic (not for urban streets). Coal tar (the binder) had been available in the U.K. from about 1800 as a residue from coal-gas lighting. Possibly this was one of the earliest efforts to recycle waste materials into a pavement! Soon after the Nottingham project, tar macadam projects were built in Paris (1854) and Knoxville, Tennessee (1866). In 1871 in Washington, D.C., a “tar concrete” was extensively used. Sulfuric acid was used as a hardening agent and various materials such as sawdust, ashes, etc. were used in the mixture. Over a 7-year period, 630.000 m2 were placed. In part, due to lack of attention in specifying the tar, most of these streets failed within a few years of construction. This resulted in tar being discredited, thereby boosting the asphalt industry. However, some of these tar-bound surface courses in Washington, D.C., survived substantially longer, about 30 years. For these mixes, the tar binder constituted about 6% by weight of the total mix (air voids of about 17%). Further, the aggregate was crushed with about 20% passing the No. 10 sieve. The wearing course was about 50 mm thick. As a side note, the term “Tarmac” was a proprietary product in the U.K. in the early 1900s. Actually it was a plant mixed material, but was applied to the road surface “cold.” Tarmac consisted of crushed blast furnace slag coated with tar, pitch, Portland cement and a resin.
Sheet asphalt placed on a concrete base (foundation) became popular during the mid-1800s with the first such pavement of this type being built in Paris in 1858. The first such pavement placed in the U.S. was in Newark, New Jersey, in 1870. This pavement can be described being a system of:
- a wearing course 40 to 50 mm thick composed of asphalt cement and sand,
- a binder course (about 40 mm thick) composed of broken stone and asphalt cement, and
- a base layer of hydraulic cement concrete or pavement rubble (old granite blocks, bricks, etc.). Generally, the concrete layer was 100 mm thick for “light” traffic and 150 mm thick for “heavy” traffic. The final thickness was based on the weight of the traffic, the strength of the concrete and the soil support.
In 1901 and 1903, Frederick J. Warren was issued patents for the early “hot mix” paving materials. A typical mix contained about 6% percent “bituminous cement” and graded aggregate proportioned for low air voids. Essentially, the maximum aggregate size was 75 mm ranging down to dust. The concept was to produce a mix which could use a more “fluid” binder than used for sheet asphalt. This material became known as “Bitulithic.”
A review of the associated patent claims reveals that Warren, in effect, patented asphalt concrete, the asphalt binder, the construction of asphalt concrete surfaced streets and roads, and the overlayment of “old” streets. It seems that he covered “all the bases” with these patents. Thus, asphalt concrete mixes thereafter were more oriented to the smaller maximum aggregate sizes. A “fine aggregate” or “modified Topeka asphaltic concrete” is mentioned in a 1926 Standard Oil Co. of California publication. The mixture consisted of:
- 30% graded crushed rock or gravel (all passing 12.5 mm sieve,
- about 58% to 62% sand (material passing 2.0 mm and retained on 75 µm),
- 8% to 12% filler (material passing 75 µm sieve).
- This mixture required 7.5% to 9.5% asphalt cement.)
Warrenite-Bitulithic was invented in 1910 by a retired employee of Warren Brothers. It consisted of a thin, approximately 25 mm thick layer of sheet asphalt placed on top of the hot, uncompacted Bitulithic. The advantage of this system is that the large aggregate of the Bitulithic mixes were not exposed directly to heavy, steel rimmed wheels which cracked the aggregate, eventually resulting in mix degradation.
Early Asphalt Cements – Trinidad Lake asphalt.
As I´ve already mentioned an early natural source of asphalt binder in the U.S. was from Trinidad (near the coast of Venezuela). Trinidad supplied about 90% of all asphalt (worldwide) from 1875 to 1900. The asphalt was produced from a “lake” with a surface area of 465,000 m2 and a depth of about 24 m. It was estimated by George W. Tillson in 1900 that this “lake” contained about 8.000.000 metric tons of “asphalt” (compare this against 1990 consumption in Europe and the U.S. of approximately 40.000.000 metric tons of asphalt binder). This asphalt, once free of water, was too “hard” to use in paving.
In fact, Trinidad lake asphalt, when loaded bulk into a ship, would fuse to the point that removal required chopping. It appears that the earliest use of asphalt binder in the U.S. was about 1874 for a project built in Washington, D.C. This binder was a combination of Trinidad lake asphalt and a flux oil distilled from crude oil. Without question, these early asphalt binders were quite variable making structural design somewhat challenging. By the 1880s, asphalt binders were regularly produced in California and by 1902 in Texas as well. It was not until 1907 that crude oil-based asphalt surpassed “natural” asphalt production. An early Standard Oil Co. of California asphalt cement specification contained 4 original penetration ranges (at 25 °C) of 31-40, 41-50, 51-60, and 61-70. Thus, it appears that some of the early asphalt cements were a bit “harder” than generally used today.
Early Portland Cement Concrete Pavements
At the turn of the century (1900), cements were categorized as “natural” or “artificial.” Natural cements were made directly from specific rock. Artificial cement was made from proportioned ingredients and became known as “Portlands” (named after the natural limestone rock found on the Portland Bill, which is a small projection of land into the English Channel near Weymouth on the southern U.K. coast). The first true Portland cement was produced in the U.K. about 1824 (actually Portland cement was patented in 1824 by Joseph Aspdin, a bricklayer in Leeds, U.K.) and in the U.S. about 1865. Interestingly, Portland Cement Concrete (PCC) was not used as a pavement wearing course much until after about 1910; however, it was regularly used as a “stiff” base to support other wearing courses such as wooden blocks, bricks, cobble stones, etc. One likely reason for this was the lack of a consistent specification for the early cements.
In 1900 George W. Tillson summarized over 109 separate specifications on Portland cement fineness. Add to this confusion the fact that natural cements were widely used as well (about 60% of total cement consumption in 1898). Further, PCC hand mixing was still common in 1900 which undoubtedly restricted productivity and accurate proportioning. By 1900 the motorized traffic was in its early stage too, hence for cost evaluation it was estimated that if motor traffic alone were to be considered, a road built entirely of cement concrete might prove the most satisfactory and economical form for the future. But for mixed traffic (horses and motor vehicles), however, such a road is by no means ideal and as, in spite of the increase in motor vehicles, the number of horse drawn vehicles did not seem to be decreasing. To further illustrates some of the issues which held back the use of PCC as a wearing course, I would mention:
- Low compressive strengths.
- Lack of understanding about the need for longitudinal and transverse joints. Expansion and contraction may cause longitudinal cracks but generally, for the ordinary width pavement, placed on a flat subgrade, the liability of cracking from this cause, is remote.
- Poor inspection.
- Poorly prepared subgrade.
- Inadequate mix design, mixing, consolidation and curing of the PCC.
- Failure to achieve adequate strength gain prior to opening streets to traffic.
The “lean” PCC base – for example, a 1:4:7 (cement: sand: gravel) mix as used by City of Seattle- U.S.A. – actually was beneficial for brick surfaced pavements having at least 3 advantages as follows:
- less affected by temperature changes,
- due to a lower tensile strength, the cracks which do form are more numerous but narrower,
- expansion joints are not needed (or recommended).
By the 1930s, several PCC pavement design features began to evolve in the U.S.A. First, typical slab thicknesses were about 200 mm with several states using a thickened edge design (maximum of about 225 mm). Second, it became clear that longitudinal joints should be used every 3.0 to 3.7 m and transverse contraction joints the same. Clearly, not until after 1910 did PCC begin to receive widespread use.
As stated earlier, pavement structural design was achieved by standards or catalogs from the 1800s well into the 1900s. The focus of this review has been on those pavement types which led to asphalt pavement design. It should be noted that Portland Cement Concrete (PCC), up until 1909, was largely used as a base or “foundation” layer for surface course materials such as bricks, wood blocks, sheet asphalt, etc. The year 1909 is noteworthy in the U.S. since this is the time which is generally used to mark the beginning of PCC as a structural wearing course. By 1949 [Public Roads Administration], the following characteristics of PCC pavement design can be summarized:
- Thickness: The thickened edge cross section was less used since a better understanding of temperature induced stresses was made. Almost all slabs ranged in thickness from 150 to 250 mm.
- Joints: Expansion joints were not needed if contraction joints were frequently spaced (short panels). Before, they were provided every 9.1m to 30.5 m. Expansion joints were expensive to construct properly and difficult to maintain. Contraction joints in plain PCC slabs were recommended every 4.6 to 6.1 m. Load transfer devices (dowel bars) were used by about one-half of the states.
- Reinforcement: The usual practice in 1949 was to place the reinforcement (if used) about 50mm to 75 mm below the PCC surface. Generally, transverse joint spacings were increased to 18.3 to 30.5 m. Experiments were under way in New Jersey and Illinois examining the potential for continuously reinforced PCC (0.3 to 1.0% of the cross section area for the longitudinal reinforcement).
For hot-mix construction, the typical surface thickness was about 50 mm; however, this ranged from as thin as 19 mm to as thick as 125 mm. The base course types ranged from PCC to asphalt-treated base to gravel. The stabilized bases were generally 150 to 200 mm thick. A typical state such as Kansas applied 50 mm of asphalt concrete over a 175 mm thick PCC slab (urban areas).
The kinds of loads pavements have been subjected to have varied significantly over the last 200 years. In the U.K. during the 1600 and 1700s, restrictive legislation was passed to adapt vehicles to the available pavements (which apparently were in very poor condition). An Act of 1751 prohibited wagons on turnpike roads with wheel rims less than 225 mm wide (a bit narrower than today’s heavy truck tires). To suggest the speed of travel during that era, in the mid-1700s, the average speed between London and Bristol was about 13 kph (a distance of 187 km).
Meantime wheels continued to evolve. In the early 1800s, Goodyear discovered the hot vulcanization of rubber which made it possible to manufacture solid rubber tires. Specifically, the Roads Act of 1920 increased the maximum empty weight of a “heavy traction engine” from 14 to 15.5 tons and, if equipped with rubber types instead of steel wheels, could travel at legal speeds of 19 kph instead of 8 kph. The pneumatic tire was patented in the U.K. in 1845, but did not come into widespread use until about 1925 (80 years later). The earliest pneumatic tires had major shortcomings in strength and durability. John Dunlap, U.K., greatly improved the design of the pneumatic tire in 1888 although only used on cycles until about 1900. The first pneumatic tire used on a motor vehicle was in 1895 in a race from Paris to Bordeaux.
Actually, many historians have given Goodyear credit for launching the interstate trucking industry in 1917. Of course, there were trucks before then, but virtually all trucking was confined to intercity hauling because of the solid tires of the day which gave a bone-rattling ride and consequent slow speeds. In 1917, Goodyear was convinced that trucks would be tremendously more efficient if they rolled on pneumatic tires. Therefore in the first quarter of 20th century Goodyear pioneered in the development of several important trucking concepts including tandem axles, dual wheel assemblies, and the 5th wheel system for trailering. All of this came about because Goodyear was fighting to prove the value of pneumatic tires for trucks. In 1926 the sale of pneumatic truck tires topped solids for the first time.
The development of the balloon tire has so reduced load concentrations that even though traffic volumes have greatly increased, the damaging effects of impact formerly experienced have practically disappeared. Measurements were made on solid rubber, cushion and pneumatic tires. The solid rubber tires had measured contact pressures up to 1050 kPa. The pneumatic tires had contact pressures of about 700 to 800 kPa for a tire inflation pressure of 620 kPa. Data summarized for aircraft in the early 1940s suggests inflation pressures of no more than 350 to 590 kPa.
A 1948 paper showed that the thickness of surfacing and base was proportional to the tire pressure and the number of load applications. This concept evolved into Equivalent Wheel Loads. The work was based on the relative destructive effects of wheel load groups ranging from 20 to 42 kN. The Equivalent Wheel Load (EWL) constants were developed to more easily summarize the available truck traffic (classified by the number of axles). Eventually, in the late 1950s and early 1960s, the AASHO (American Association of State Highway Officials) Road Test produced the equivalent single axle load concept. Axle load equivalency has been one of the most widely adopted results of the AASHO Road Test. A variety of equivalency factors can be used depending on the pavement section which is defined by a Structural Number and the terminal serviceability index (for flexible pavements).
Some of the earliest recorded information about materials which could be (and were) used in pavements concern hydraulic cement and the Romans; however, in fairness, the earliest known use of hydraulic lime was in Syria about 6.500 B.C. (over 6,000 years before the Romans — give or take a few hundred years). The Romans “discovered” that grinding volcanic tuff with powdered hydraulic lime produced a hydraulic cement (“hydraulic” in that it hardened in the presence of water). The hydraulic lime was produced by heating limestone above 850 °C thereby driving off CO2 and converting the limestone to CaO.
The 1st known use of hydraulic cement by the Romans occurred at about 120 B.C. (in Rome oddly enough). The “best” variety of volcanic tuff was found near the town of Pozzuoli (near Naples on the southwestern coast of Italy) and the material acquired the name of pozzolana. Further, the Romans learned a bit about the use of other additives such as blood, lard and milk. Apparently, blood (hemoglobin actually) is an effective air-entraining agent and plasticizer (given the mild Mediterranean climate the primary use was likely for workability). With regard to material testing, the year 1898 was significant: the American Society for Testing and Materials (ASTM) was established.
With regard to PCC compressive strength, Austin Thomas Byrne reported in 1896 that PCC air cured for six months for a St. Louis bridge had a compressive strength of 8.3 MPa. Other compressive strengths he reported (presumably pre-1890s) ranged from 1.4 MPa (1 month age) to 9.7 MPa (1 year age). Compare these results to contemporary compressive strengths of 34.5MPa to 137.9 MPa. Thus, in 100 years, PCC strength has increased about 100 times (ratio of extreme low to extreme high). Material characterization up to the 1930s tended to focus on basic material parameters such as liquid limit, plasticity index and gradation. The strength or bearing capacity of unstabilized materials was usually discussed in terms of cohesion and friction angle.
The one material characterization test which had a tremendous impact on design and to some extent still does, was the development of the California Bearing Ratio (CBR). Some of the positive aspects of the CBR laboratory test are as follows:
- Improvement over field static load tests which must overcome “consolidation deformation” since the CBR specimen is compacted to a density expected in the field.
- The soaking of the laboratory specimens with a surcharge (which represents the weight of the pavement) permits the material to swell and reach the adverse state of moisture which can exist in the field.
- The penetration test determines the material’s resistance to lateral displacement resulting in a combined measure of the influence of cohesion and internal friction.
- The test provides a quick method of comparing base and subgrade materials.
- By investigating and testing the associated materials, an empirical relationship can be established between CBR values, pavement thickness, and performance.
How Asphalt Is Used Today
Although it’s most often associated with roads, asphalt is used for many purposes, though roads account for its most extensive use. Of the more than 4.3 million kilometers of paved roads in the U.S., 94% are surfaced with asphalt, according to National Asphalt Pavement Association (NAPA). Interestingly, though, all of that includes a mixture of about 95% stone, sand and gravel, and just 5% asphalt cement. Asphalt also is used for parking lots, airport runways and racetracks. Asphalt is a really flexible and versatile product, it can be used to line fishponds and water reservoirs or for sporting purposes like tennis courts. A couple of years ago, it was chosen as the base surface for the field at the Minnesota Vikings stadium in Minneapolis. Since the early days of asphalt production, the industry has continued to innovate new products, becoming more scientific and rigorous.
Meantime we’ve changed the way we engineer the mixes, we’re at an era today where you are seeing a giant shift in how the industry and how states work. Using advanced testing methods, asphalt researchers have been aiming to improve performance. Incorporating new materials, additives and technologies, they are seeking to learn how various recipes will perform in different temperatures and climates. One major update has been the creation of Warm-Mix Asphalt (WMA), which reduces the production temperature of asphalt at a plant, thereby reducing energy usage and saving time in both production and road surfacing. WMA also improves working conditions with lowered exposure to fuel emissions, fumes and odors, according to the U.S. Department of Transportation Federal Highway Administration. WMA is technology that did not exist in U.S. in 2002 and now accounts for about 40 % of the market.
Criticisms of Asphalt
Asphalt probably isn’t something you think of as eco-friendly; it could be partly guilt by association because asphalt is naturally aligned with major polluters — driving automobiles and oil production. And some of the negativity is warranted: Because asphalt has low reflectivity, it has been determined to be a significant contributor to the Urban Heat Island (UHI) effect. Anyone who has sat in a highway traffic jam on a hot summer day can attest to that. As far as asphalt’s contributions to the UHI, the Environmental Protection Agency states that conventional asphalt pavements can be modified with materials or treated after installation to raise reflectance. For decades, this has been sometimes implemented on surfaces like parking lots and highways. The EPA includes porous asphalt and rubberized asphalt as examples of permeable pavements.
Asphalt has also earned bad marks for being impermeable, for the gases it produces when melted and the fumes it exposes workers to during paving and roofing. Occupational Safety and Health Administration (OSHA) says those fumes can lead to headache, skin rash, fatigue and even skin cancer. While OSHA’s standards do not specifically address asphalt fumes, the administration recommends that controlling exposure can be done through “engineering controls, administrative actions and personal protective equipment.” And of course, there’s still the fact that asphalt is made from petroleum. But asphalt does have positive eco-qualities too.
100% Recycling Efforts
What a lot of people don’t know is all of the environmentally friendly things the asphalt industry is actually doing. For starters, asphalt is 100% recyclable, and more importantly, it actually does get recycled. In 2018, 74.5 million metric tons of Reclaimed Asphalt Pavement (RAP) was put back into new mixes. That means every asphalt mix put down in the U.S. included about 21 % RAP. In fact, the combined weight of all items people recycle annually in the U.S. — paper, plastic and aluminum — totaled a fraction of (about 68 %) of the weight of RAP the asphalt industry recycles annually. That’s just one material we recycle, the asphalt industry is the most active recycling industry in the country. It is also one of the biggest recyclers of tire rubber, which is used as a modifier for mixtures in some states. Roof shingles also are recycled into new asphalt mixtures, and the industry is looking into how plastic might become part of the discussion. A lot of engineering and material science goes into constructing a road. Today, asphalt roads are designed around the concept of “perpetual pavement,” or at least to last 40 years or more. Routine maintenance consists of “milling” the surface — taking off the top inch or so — every 12 to 20 years and replacing it with a new overlay. That top inch can be recycled, and the periodic overlays significantly improve the ride quality and fuel consumption of vehicles traveling on these roads.
Asphalt of the Future
Until it’s time for hover cars, asphalt roads are likely to stick around. And the industry plans to keep innovating in product and production. For example the recent breakthroughs like autonomous rollers and equipment, as well as the increased use of virtual reality for training. As asphalt experts get better at handling big data, they can use it for production and placement to improve efficiencies in real time. One day, he could even see intelligent pavements with nano-sensors in the roads providing feedback on how the pavement is behaving and lasting. Our roads are going to get a lot smarter. We’ve got the technology to really improve the experience of the riding public.