Asphalt (paving)

This article is about Asphalt (paving). For other uses of the term Asphalt, please see Asphalt (disambiguation).

(PD) Photo: The United States Agency for International Development (USAID)
Asphalt-laying machine paving a road.

Paving asphalt is a mix of petroleum asphalt (commonly referred to as bitumen), aggregate and additives that is very commonly used as a top layer of pavement for roads. A typical paving asphalt consists to 90 to 95 percent by weight of aggregate and 5 to 10 % of bitumen.

Paving asphalt is sometimes referred to as asphaltic concrete and, when used to pave airports, may be referred to as tarmac.

History

The recorded use of naturally occurring petroleum asphalt dates back to about 4000 B.C.:

• 3800 B.C.: Asphalt used for caulking boats made of reeds.
• 3500 B.C.: Asphalt used as cement for jewelry.
• 3000 B.C.: Asphalt used as construction cement by the Sumerians of ancient Mesopotamia (now known as Iraq). Also used to seal a bathing pool or water tank in the city of Mohenjo-Daro in the Indus Valley Civilization located in what is now Pakistan.
• 2500 B.C.: Asphalt and other petroleum oils used in ancient Egypt for embalming mummies. (The Persian word for asphalt is mumiyah which may be related to the English word for mummy).
• 1000 B.C.: Asphalt used for waterproofing by lake dwellers in what is now Switzerland.
• 625 – 650 B.C.: The first recorded use of asphalt as a road-building material was in Babylon during the reigns of King Nabopolassar and his son, King Nebuchadnezzar.
• 500 B.C.: Asphalt mixed with sulfur was used as an incendiary device in the Greek wars. (The word asphalt comes from the Greek word asphaltos, meaning secure in English.)
• 300 B.C. – A.D. 250: Reported occurrences of asphalt and oil seepages in Mesopotamia and the use of liquid asphalt as an illuminant in lamps.
• A.D. 750: First reported use in Italy of asphalt as a coloring material in paintings.

Europeans exploring the Americas discovered natural deposits of petroleum asphalt. Writing in 1595, Sir Walter Raleigh described a lake of asphalt on the island of Trinidad, near Venezuela. He used it to recaulk his ships.

In the late 1700s and early 1800s, first Pierre-Marie-Jérôme Trésaguet of France, then Thomas Telford and subsequently John Loudon McAdam (both of Scotland) perfected the leveling, draining and construction of roads using layers of broken stones and gravel. In the period of 1860 – 1880, to reduce road dust and road maintenance, builders began using hot coal tar to bond the stones together. Such roads became named after McAdam and known as tarmacadam roads, later shortened to tarmac.

In 1870, Belgian chemist Edmond J. DeSmedt laid the first true asphalt pavement in the United States of America in Newark, New Jersey. He also paved Pennsylvania Avenue in Washington, D.C. in 1876 using 54,000 square yards (45,140 square metres) of asphalt from Pitch Lake in Trinidad.

During the early 1900s, coal gasification was being widely used to produce town gas and the by-product tar produced during coal gasification was a readily available product. That tar was extensively used in the construction of tarmacadam (or, more simply, tarmac) roads.

By 1907, asphalt from petroleum refineries had outstripped the use of natural asphalt from Trinidad or elsewhere.. Later in the 1900s, when natural gas replaced town gas, asphalt from petroleum refineries dominated the asphalt paving market from that point on.

Properties

Paving asphalt is water repellant due to the bitumen which is part of the asphalt mix.

The physical properties are dependent on the temperature of the material. At high temperatures, paving asphalt has a very distinctive viscous behavior. As the paving asphalt cools, it will solidify into a nearly-rigid solid, though some flexibility is retained at ordinary outdoor temperatures.

As the physical properties are mainly dependent on the aggregates used, they can vary over a large range. Typical values are:

• thermal conductivity: 0.8 - 1.2 W/m•K
• specific heat capacity: 850 - 1050 |J/kg•K

Use in road construction

(CC) Photo: Asphalt Education Partnership
Close-up photograph of asphalt paving.

There are various mixtures of asphalt with other materials that are used in road construction and other paving applications:

• Rolled asphaltic concrete that contains about 95% aggregate and 5% petroleum asphalt binder.
• Mastic asphalt that contains about 90–93% aggregate and 7–10% petroleum asphalt binder.
• Asphalt emulsions that contain about 70% petroleum asphalt and 30% water plus a small amount of chemical additives.
• Cutback asphalt that contains petroleum solvents (referred to as cutbacks).

By the early 1990s, asphalt paving mixture producers in the United States used more than 50 × 10⁶ barrels (7.95 × 10⁶ cubic metres) of petroleum asphalt per year. Of the 2.27 × 10⁶ miles (3.65 × 10⁶ kilometers) of paved road in the United States, 94 percent of them are surfaced with asphalt paving.

Preparation of paving asphalt mixtures

The preparation of paving asphalt mixtures is done in several steps:

• Predose: Depending on the asphalt recipe, the individual components of the aggregate are weighed. Usually this is done using a belt weigher, as the material has to reach the next step in a continuous flow.
• Drying: In a rotary drying drum, the components of the aggregate are dried at a temperature between 140 and 190 °C. After the drying, the components are sifted and separately stored in several silos.
• Weighing: As the drying can change the previous grain-size distribution curve, a final weighing has to take place before adding the individual aggregate components to the mixer. This also allows for quick changing of the recipe. The binder (asphalt) is kept in heated tanks and added to mixer according to a measured rate of flow.
• Mixing: The mixer can be working either continuously or at intervals. An interval mixer is fed with the aggregates using a charging screw and the binder is synchronously injected into the mixing chamber. For very high volumes, a continuously working mixer is used and it can not be adapted quickly to changing volume and recipe needs.
• Dispatching: The mixed material is stored in a heated silo which usually has several chambers to store different recipes.

Transport

Asphalt has to reach the construction site with a reasonably high temperature as it can not be properly placed if it cools to the point where it can no longer be compacted. The transportation has to be set up in a way to ensure the correct temperatures can be maintained until the construction site is reached. For short distances between the asphalt plant and the construction site, ordinary trucks can be used and covered with tarpaulins to maintain the temperatures. If the ambient temperature is too low or the construction site is too far away to use ordinary trucks and maintain the correct temperatures, then special trucks have to be used.