Physical Properties & Safety Tests of Asphalt as a Roofing Material

Asphalt Binders

Asphalt or bitumen is a thick, black or brown, viscous liquid and a by-product of crude oil. Asphalt functions as an inexpensive, waterproof, thermoplastic, viscoelastic adhesive in road construction and some other applications.

Background

The first recorded use of asphalt by humans was by the Sumerians around 3,000 B.C. Statues from that time period used asphalt as a binding substance for inlaying various shells, precious stones, or pearls. Other common ancient asphalt uses were preservation (for mummies), waterproofing, and cementing (used to join together bricks in Babylonia). In more modern times, asphalt paving use first began with footpaths in the 1830s and then progressed to actual asphalt roadways in the 1850s. The first asphalt roadways in the U.S. appeared in the early 1870s.

Refining

In the simplest terms, asphalt binder is simply the residue leftover from petroleum refining. Thus, asphalt binders are produced mainly by petroleum refiners. The composition of base crude oil from which asphalt is refined can vary widely and thus the asphalt yield from different crude oil sources can also vary widely.

Basic Refining Process

Crude oil is heated in a large furnace to about 340° C (650° F) and partially vaporized. It is then fed into a distillation tower where the lighter components vaporize and are drawn off for further processing. The residue from this process (the asphalt) is usually fed into a vacuum distillation unit where heavier gas oils are drawn off. 

Asphalt cement grade is controlled by the amount of heavy gas oil remaining. Other techniques can then extract additional oils from the asphalt. Depending upon the exact process and the crude oil source, different asphalt cement of different properties can be produced. Additional desirable properties can be obtained by blending crude oils before distillation or asphalt cement after distillation. 

Physical Properties

Asphalt binders are most commonly characterized by their physical properties. An asphalt binder’s physical properties directly describe how it will perform as a constituent in road pavement. Some tests are used to characterize key asphalt binder parameters and how these parameters change throughout the life of a pavement.

Durability

Durability is a measure of how asphalt binder physical properties change with age (sometimes called age hardening). In general, as an asphalt binder ages, its viscosity increases and it becomes more stiff and brittle. Age hardening is a result of a number of factors, the principal ones being ;

• Oxidation. The reaction of oxygen with the asphalt binder.

• Volatilization. The evaporation of the lighter constituents of asphalt binder.

• Polymerization. The combining of like molecules to form larger molecules. These larger molecules are thought to cause a progressive hardening.

Viscosity

Viscosity denotes the fluid property of bituminous material and it is a measure of resistance to flow. At the application temperature, this characteristic greatly influences the strength of resulting paving mixes. Low or high viscosity during compaction or mixing has been observed to result in lower stability values. At high viscosity, it resists the compactive effort and thereby resulting in low stability values. And at low viscosity instead of providing a uniform film over aggregates, it will lubricate the aggregate particles.

Safety Tests

Asphalt cement-like most other materials volatilizes (gives off vapor) when heated. At extremely high temperatures asphalt cement can release enough vapor to increase the volatile concentration immediately above the asphalt cement to a point where it will ignite (flash) when exposed to a spark or open flame. This is called the flash point. For safety reasons, the flash point of asphalt cement is tested and controlled. The fire point, which occurs after the flashpoint, is the temperature at which the material (not just the vapors) will sustain combustion.

Flash Point Test

The flash point test determines the temperature to which asphalt can be safely heated in the presence of an open flame. A typical flash point test involves heating a small sample of asphalt binder in a test cup. The temperature of the sample is increased and at specified intervals, a test flame is passed across the cup. The flash point is the lowest liquid temperature at which the application of the test flame causes the vapors of the sample to ignite. The test can be continued up to the fire point – the point at which the test flame causes the sample to ignite and remain 5 seconds.

Penetration Test

It is an empirical test that measures the consistency (hardness) of asphalt at a specified test condition.

• The basic principle of the penetration test, was to determine the depth to which a needle penetrated an asphalt sample under specified conditions of load, time, and temperature.

• The current penetration test, first published in 1959, describes the following basic procedure:

• Melt and cool the asphalt binder sample under controlled conditions.

• Measure the penetration of a standard needle into the asphalt binder sample under the following conditions:

– Load = 100 grams

– Temperature = 25° C (77° F)

– Time = 5 seconds

• The depth of penetration is measured in units of 0.1 mm and reported in penetration units (e.g., if the needle penetrates 8 mm, the asphalt penetration number is 80). Penetration Grading is based on the penetration test.

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