The basics of asphalt used in roofing
Putting roofing asphalt to the test
By Mike Anderson, P.E.
In the first article, we discussed the history of asphalt roofing and the make-up of composition roofing systems (shingles). To reset the stage, the roofing market is divided into two general categories: steep-slope and low-slope.
Steep-slope roofing includes coverings installed on slopes exceeding 14 degrees. This is usually associated with residential roofing and, by connection, shingles. Low-slope roofing includes water-impermeable or weatherproof systems installed in slopes that are less than or equal to 14 degrees. This is usually associated with commercial roofing systems.
Standard specifications for asphalt shingles can be found through the American Society for Testing and Materials (ASTM) in ASTM Standard D3462 “Standard Specification for Asphalt Shingles Made from Glass Felt and Surfaced with Mineral Granules.” Although a number of properties are listed in the specification, only two properties are specifically related to the coating asphalt used in the shingle manufacture: penetration at 77°F (25°C) and softening point (ring and ball).
These two properties provide some general indication of the consistency (stiffness) of the asphalt at intermediate and high temperatures, but don’t tell the whole story about whether or not a particular asphalt will make a good shingle coating.
Properties of good roofing asphalt for steep-slope applications
What kind of properties does a good roofing asphalt need? For shingles, a roofing asphalt must have the following properties:
- resistance to flow at high temperatures,
- proper adhesion (neither overly sticky or brittle) for mineral granules,
- resistance to water intrusion (waterproof),
- resistance to fire (fire retardant),
- proper tensile strength, flexibility and
- low potential for staining.
These properties can be affected by the crude source and process used to produce the roofing asphalt flux before it is oxidized into a roofing asphalt.
Resistance to flow at high temperature
Resistance to flow at high temperatures means that the roofing asphalt used in the shingle should be stiff enough at the temperatures encountered on the roof that it won’t slough off of the shingle. Depending on the geographical area in which it is used, an asphalt roofing material could experience temperatures as high as 80°C (176°F).
For roofing asphalt, resistance to flow has been most often characterized through the use of the softening point test – usually the Ring & Ball Softening Point test (ASTM D36). In this test, an asphalt sample is poured into two small brass rings, trimmed, and loaded with a steel ball in the center of each ring. After the samples are prepared, the assembly is suspended in a beaker of water, glycerin or ethylene glycol at a height of 25 millimeters (1 inch) above a metal plate. The liquid is then heated at a rate of 5°C (9°F) per minute. As the asphalt softens, the balls and the asphalt gradually sink toward the plate. At the moment the asphalt ball touches the plate, the temperature of the liquid is determined, and this is designated as the ring-and-ball (R&B) softening point of the asphalt binder sample.
It is important to note that for asphalt materials, the softening point is not the same thing as the melting point. Asphalts do not have an exact melting point since they are neither crystalline nor chemically homogeneous. As such, the softening point is simply an arbitrary indication of the resistance to flow at rooftop temperatures.
Typically, unmodified paving grade asphalt binders will have softening point temperatures in the range of approximately 35-55°C (95-130°F) depending on the grade of the asphalt binder. A modified paving grade asphalt binder could have softening point temperatures in the range of approximately 50-75°C (122-167°F) – again, depending on the grade of the modified asphalt binder as well as the type of modification. Roofing asphalt binders, particularly those used for shingle coating, typically have softening point temperatures in the range of approximately 88-105°C (190-221°F).
Proper adhesion characteristics
In addition to resistance to flow at high temperatures, a roofing asphalt should have proper adhesion characteristics so that granules will stick to the shingle without it being overly sticky (at high temperatures) or brittle (at intermediate-low temperatures). This characteristic has typically been measured by using the penetration test (ASTM D5) at three temperatures: 4, 25 and 46°C (41, 77, and 115°F).
To conduct the penetration test, an asphalt sample is heated to an appropriate pouring temperature and poured into a test container — usually a 3-ounce tin. After a specified conditioning period which is tightly controlled to minimize the effects of steric hardening, the asphalt sample is brought to 25°C (77°F) in a temperature-controlled water bath. The sample container is then placed in the penetrometer equipment. A needle of prescribed dimensions is attached to the penetrometer and suspended directly over the asphalt binder sample. A 50-gram weight is attached to the needle’s loading platform so that the total weight used for loading is 100 grams (50-gram weight plus the needle assembly weight of 50 grams). The penetrometer is lowered until the needle tip just contacts the surface of the asphalt. The load is then released, allowing the weighted needle to penetrate the asphalt for 5 seconds. The distance that the needle penetrates into the asphalt is reported as the penetration value. This distance is reported in units of 0.1 millimeters, or decimillimeters (dmm).
Roofing technologists have found that penetration values for shingle coating asphalt should be in the range of 16-22 dmm to provide good adhesion characteristics while not being overly sticky or brittle. The penetration test provides an empirical measure of the consistency of the roofing asphalt binder at moderate temperatures.
Resistance to water intrusion (waterproof)
By its nature, asphalt is a waterproofing material. No standard tests are used to validate this property.
Resistance to fire (fire retardant)
As a petroleum product, an asphalt will release combustible fumes when heated to sufficiently high temperatures. The flash point provides an indication of the temperature at which a heated asphalt sample will instantaneously flash in the presence of an open flame. It is important to note that this temperature is usually well below the temperature at which the material will support combustion. The temperature at which the material supports combustion is called the fire point. It is rarely, if ever, used in asphalt specifications.
The most common test method used to determine the flash point of an asphalt is the Cleveland Open Cup (COC) flash point test. In this test, a brass cup is first filled with a specified volume of asphalt and is heated at a constant rate. A small flame is passed over the surface of the asphalt at timed intervals. When the flame passing over the sample surface causes an instantaneous flash, the temperature is recorded as the material’s flash point.
The purpose of the flash point test is to measure the tendency of the asphalt to produce flammable vapors when heated under controlled laboratory conditions. As stated in the test method, it (flash point) shall not be used to appraise the risk of a fire hazard, but instead shall be one component in an overall strategy for fire risk management. The results of the test method are related to safety, and not the quality of the asphalt.
To produce roofing asphalt, it is important to recognize that the oxidation process typically occurs at 260°C (500°F). As such, asphalts that will be oxidized to produce roofing asphalt products should have a minimum flash point of 260°C (500°F) in order to safely conduct the oxidation process.
Proper tensile strength
Shingles are required to have a minimum tear strength as described in ASTM D3462 “Standard Specification for Asphalt Shingles Made from Glass Felt and Surfaced with Mineral Granules.” The roofing asphalt provides some assistance in meeting this requirement by being sufficiently flexible so the shingle can be handled at extreme temperatures without cracking or deforming. The ductility test (ASTM D113) provides a method that can be used for empirically determining the ductility of an asphalt at 25°C (77°F).
Low potential for staining
Shingle staining occurs in service when the lighter oils in the roofing asphalt transfer from the shingle coating to the granule surface. This phenomenon, also referred to as “tobacco juicing”, can happen during the initial weathering period of the shingle when in service. The exudation of oils onto the granule surface likely doesn’t have a significant impact on the long-term performance of the shingle, but it can result in an unaesthetic appearance on the roof.
Staining tendency can be determined by using ASTM D2746, Standard Test Method for Staining Tendency of Asphalt (Stain Index). In this test, roofing asphalt is poured into a brass ring with an inside diameter of 16-mm placed on a sheet of filter paper supported by a brass plate. The sample is then heated at 80°C (175°F) for 120 hours. The diameter of the resulting stained circle on the filter paper is then compared to the inside diameter of the brass ring to determine the stain index – calculated as the difference between the diameter of the stained circle and the inside diameter of the brass ring, expressed in 0.4-mm increments (oddly enough, but thanks to the conversion from inches to millimeters). Regardless of the odd units, a higher stain index indicates lower thermal stability and greater tendency for staining. Generally speaking, a stain index less than 10 indicates low staining tendency while a value greater than 20 indicates high staining tendency.
Resistance to weathering
A good roofing asphalt must also have sufficient resistance to weathering so that the shingle can remain durable for many years in service. Most roofing technologists classify the weathering ability of a roofing asphalt by using one of two weatherometer test procedures (described in ASTM D4798 and D4799). Unfortunately the weatherometer test, which mimics thermal and UV aging of an asphalt shingle, can take 2,000-3,000 hours of continuous operation (or approximately 83-125 days) to characterize the weathering ability of a roofing asphalt.
Acknowledgments and references
The information in this article was derived from the “Roofing 101” presentation conducted at the Asphalt Institute’s Annual Meeting in 2012 and AI’s “Asphalt Binder Handbook (MS-26).” Special thanks go to the members of the Asphalt Institute Roofing Technical Advisory Committee – specifically Andrew Ford, Mike Franzen, Greg Malarkey, Andrew Parker and Keith Stephens for their input.