Asphalt cements are classified based on their viscosity in poises (centimeter-gram-second units of dynamic viscosity) at 140°F. For example, if AC-5 or AC-10 is specified, the numerical value in these designations indicates the viscosity in hundreds of poises at 140°F. Additional letter designations such as “P” or “TR” as in AC-15P or AC-20-5TR indicate the presence of a polymer or (5 percent) tire rubber, respectively.

There are many requirements in the specifications for asphalt cements. The most important requirements are the viscosity, penetration, and aged viscosity. With these requirements we attempt to control the temperature susceptibility (change of viscosity with change in temperature) of the asphalt and limit the amount of aging expected through the hot mix plant. We do not want an asphalt that gets too viscous at low temperatures or gets too fluid at high temperatures. We also don’t want an asphalt which ages quickly in the hot mix plant. Although asphalt cements used for seal coats and surface treatments are not processed through a hot mix asphalt concrete plant, some test requirements were developed to consider the aging that occurs through the hot mix asphalt concrete plant. The main tests performed for asphalt cement and the reasoning behind them follow.

Viscosity.

Viscosity is defined as a fluid’s resistance to flow. The viscosity test is conducted at two temperatures, 140°F and 275°F. This test indicates how viscous the binder is at approximate maximum road temperatures (140°F) and its relative resistance to deformation (rutting) at summertime road temperatures. The limits on the high-temperature viscosity help ensure the asphalt does not get too fluid at high temperatures. An example of what to avoid is an asphalt that behaves like a wax. A wax will be stiff and hard at low temperatures, but as the temperature is raised, will soften and lose its stiffness.

At 140°F, there is a minimum and maximum viscosity required. At 275°F, there is only a minimum viscosity required.

Penetration.

The standard penetration test indicates the relative stiffness of the asphalt at a temperature of 77°F.

The test measures the distance a standard needle weighted with a mass of 100 grams penetrates into the asphalt in 5 seconds. All testing takes place at a controlled temperature of 77°F.

The specification places a minimum on the penetration. The further the needle penetrates into the asphalt, the softer the asphalt.

Temperature Susceptibility.

If there are limits on the viscosity at two different temperatures, and limits on the penetration, this will effectively limit the temperature susceptibility of the asphalt. (Remember temperature susceptibility is the change in viscosity with change in temperature.)

Flash Point.

The flash point is defined as the lowest temperature at which application of a test flame causes the vapors above the surface of the liquid to ignite. This test is conducted for safety reasons. The test performed is the Cleveland Open Cup Flash Point as described in AASHTO Test Method T-48. An open cup of asphalt is heated at a specified rate. At temperature intervals, a small gas flame is passed over the surface of the asphalt. A minimum flash temperature is required for each type of asphalt cement.

Solubility in Trichloroethylene (TCE).

In this test (AASHTO T-44), asphalt is dissolved in TCE and filtered. The insoluble material is weighed. This test places maximum limits on inorganic materials or carbon residues in the asphalt. These materials, if present, add no binding quality to the asphalt and are considered contaminants.

Spot Test.

To conduct the spot test (Tex-509-C), a small amount of asphalt is dissolved in a solvent and dotted on a filter paper. A positive spot is seen as a ring of dark material deposited as the solvent-asphalt blend is absorbed and spreads out on the filter paper. This test is used to screen asphalts that age excessively in the Thin Film Oven Test (TFOT). Item 300 requires a negative result for the spot test. A positive spot indicates overheating during the manufacturing process.

Specific Gravity.

Specific gravity is not a specification requirement but is a test performed to allow temperature-volume conversions in the field to ensure proper application rates. Specific gravity is the ratio of the mass of a given volume of material at 77°F to that of an equal volume of water at the same temperature.

Thin Film Oven Test.

The Thin Film Oven Test (TFOT) is an aging test. This aging simulates the aging expected in the hot mix asphalt concrete plant.

Viscosity of TFOT Residue.

The viscosity of the TFOT residue is measured to limit the aging of the selected asphalt to a threefold increase over the midpoint of the grade range at 140°F. As an example, AC-10 has a limit of 3 x 1000 = 3000 poise.

Ductility of TFOT Residue.

The TFOT residue is subjected to a ductility test. A dog bone-shaped specimen is cast and pulled apart at a standard speed and temperature in a water bath. The distance to rupture is measured. This test provides an empirical measure of the cohesiveness of the asphalt after aging. It is another limit on the stiffness of the asphalt after aging.