The History of Asphalt
Hot mix asphalt (HMA) pavements consist of a mixture of aggregates (about 95 percent) and an asphalt binder (about 5 percent) and have existed in their present form since the beginning of the 20th century.
Road Pavements: Then and Now
Road surface pavements can be traced back to Roman times, dating from 312 B.C.E. At its height, the Roman road network totaled more than 62,000 miles. By law, the public was entitled to use the roads, but their maintenance was the responsibility of the inhabitants of the district through which the roadway ran. Although Roman roads did not use asphalt as a binder, they often used lime grout and other natural pozzolans as binders.
The first recorded use of liquid asphalt cement by humans was by the Sumerians around 3,000 B.C.E. Around 1500 A.D. the Incas of Peru were using a composition similar to modern bituminous macadam to pave parts of their highway system.
The modern history of road pavements began in England in the late 1700s with Thomas Telford, who constructed roads using unbound aggregate to help transport horse-pulled cargo.
By the early 1800s, Scotsman John McAdam (from whose name the word macadam derives) used a sloped surface to improve road drainage. Onto it he placed two layers of angular aggregate, on top of which he put a further layer of smaller aggregate to act as a wearing or surface course. At this stage, no asphalt binder was used and the layers of broken stone eventually became bound together by the fines generated by traffic.
The first tar macadam pavement was laid outside Nottingham in the United Kingdom in 1848, using coal tar as the binder.
In more modern times, asphalt paving first came into use with footpaths in the 1830s and then progressed to asphalt roadways in the 1850s. The first asphalt roadways in the United States appeared in the early 1870s.
In North America, hot mix asphalt pavement began to assume its modern form around the beginning of the 20th century, when Frederick J. Warren was issued patents for a "hot mix" asphalt paving material and process, which he called "bitulithic." A typical bitulithic mix contained about 6 percent "bituminous cement" and graded aggregate proportioned for low air voids.
What Is HMA?
HMA consists of a mixture of aggregates (about 95 percent) and an asphalt binder (about 5 percent).
The aggregate is typically crushed to certain size or gradation specifications. In HMA, gradation helps determine almost every important property, including stiffness, stability, durability, permeability, workability, fatigue resistance, frictional resistance, and resistance to moisture damage.
The fundamental requirements for aggregates to be used in asphalt manufacture are defined by a combination of their engineering and physical properties.
The engineering properties required of the aggregate are that it must be durable, strong, and resistant to the polishing effects of traffic (obviously not important in the lower pavement layers) and exhibit good adhesion to the asphalt cement binder.
The main physical properties required for an aggregate to be used in asphalt manufacture are proper gradation and shape, absorption and water content, and cleanliness (the absence of any deleterious material and/or contaminants).
Asphalt or bitumen is a dark brown to black, highly viscous hydrocarbon produced from petroleum distillation residue. This distillation can occur naturally, resulting in asphalt lakes, or take place in a petroleum refinery. Asphalt cement refers to asphalt that has been prepared for use in HMA.
Asphalt can be classified by its chemical composition and physical properties and is a measure of rheology and durability (hardening). In North America, almost all asphalt cement is characterized by the Superpave performance grading (PG) system. Superpave performance gradings are reported using two numbers: the first is the average seven-day maximum pavement temperature (in degrees Celsius), and the second indicates the minimum pavement design temperature likely to be experienced (also in degrees Celsius).
Asphalt cement can be further modified to meet certain additional performance characteristics; namely, higher stiffness at high temperatures to reduce rutting, and lower stiffness at low temperatures to reduce thermal cracking. Asphalt cement can also be modified to increase the adhesion between the asphalt binder and the aggregate in the presence of moisture.
Lafarge and HMA
The Lafarge Group created its aggregates and concrete division following the company's 1999 acquisition of U.K.-based Redland, which operated asphalt production and paving and construction businesses in the United Kingdom, the United States, and France.
The company's asphalt business was further strengthened in 2001 with the merger between Lafarge and Canadian-based Warren Paving & Materials Group, enhancing Lafarge's asphalt, paving, and construction operations in Canada to complement its existing U.K., U.S., and French businesses.
Today, Lafarge operates more than 50 asphalt sites and is one of the top 10 global HMA producers.
As a leader in asphalt development, Lafarge offers a wide range of proprietary materials that provide enhanced performance, improved environmental credentials, and a pleasing aesthetic result.
A network of regional laboratories and central research and development is centered in both Western Canada and the Western United States. Lafarge's innovations in North America are supported by the company's global research facility in Lyon, France-the largest center of its kind in the world.