All Things Gypsum: The Iceman Cometh Redux
If you can flash back to your youth, doubtless you remember one of the fondest days of elementary school-Science Day.
Two aspects of Science Day in my 1960s elementary school were always consistent: Mr. Wilson always taught it and it always occurred on Friday.
Every year, Mr. Wilson would visit from the local campus of the New York State University system and show us a vast array of tricks like nitrogen-frozen bananas that he used to pound nails with and eggs that he could magically transport into milk bottles. His display always got the boys in the class so jacked up that they were essentially unteachable for the next three days; thus, he always gave his show on Friday to spare the teachers the agony of trying to deal with us for the next two days.
During my last year of elementary school, Mr. Wilson showed up on Science Day with a block of ice and a miniature blowtorch. He placed the ice on a pan, lit the torch, and applied the flame to one side of the ice. After a few minutes, and some melting, he put his hand on the side of the ice opposite from the torch. To the amazement of the class, nothing bad happened to his hand: no burns, no pain. His hand was far colder than it was hot and the flame had not penetrated through the other side of the ice.
This continued until the core of the ice was quite well melted and the torch flame was a substantial distance into the center of the block. Even at this point, Mr. Wilson’s hand came out of the process unscathed. Proof that he wasn’t cheating or using magic was provided by the class volunteers who were afforded the opportunity to touch the ice themselves; a process that in the 21st century would certainly cause a major liability insurance nightmare at the local school board.
In rural 1960s New York State, however, it was OK to do this and it taught the class a simple scientific property: the notion that a material could gradually sacrifice itself to prevent the passage of heat and flame.
WOULDN’T IT BE ICE
Like the block of ice that Mr. Wilson brought to school, gypsum also sacrifices itself to prevent the passage of heat and flame: a characteristic that contributes to its ability to be classified as a noncombustible building material.
Gypsum board is used throughout the country where noncombustible construction is required in buildings; it complies with building code requirements whenever it is used in this fashion. This may seem unusual at first blush, since gypsum board typically has paper on the front and back surfaces, and everyone knows that paper can burn. However, primarily because of the unique properties of the gypsum core, gypsum board is typically the building material of choice in fire-rated construction. It has long been recognized for its inherent fire resistant qualities.
The basic test that building officials consider when determining noncombustibility of most building materials is ASTM E136, Standard Test Method for Behavior of Materials in a Vertical Tube Furnace at 750 degrees Celsius. This test method exposes the material being tested to a stream of air heated to 750 degrees Celsius (1,382 degrees Fahrenheit). To qualify as noncombustible, four specimens must be tested, and three must pass the following criteria:
The surface or interior temperature must not exceed the furnace temperature by more than 30 degrees Celsius;
After 30 seconds into the test, no flaming of the specimen is allowed; and
If the sample loses more than 50 percent of its weight during the test, its temperature cannot exceed 750 degrees Celsius and it cannot flame.
This is an extremely difficult test to pass; very few materials make the grade. Because of its paper facing, gypsum board does not meet the stringent requirements of the test.
However, it is recognized that the gypsum board core does pass the three basic criteria contained in ASTM E136 as described above when the face paper is removed. Therefore, alternate criteria (with which gypsum board easily complies) have been established and accepted by building code officials to determine noncombustiblity.
The International Building Code allows layered building materials to be considered noncombustible if:
The core of the material passes the test procedure set forth in ASTM E136;
The material surfacing is not more than 1/8 inch thick; and
The composite material has a flamespread rating not greater than 50 when tested in accordance with ASTM E84, Standard Test Method for Surfacing Burning Characteristics of Building Materials.
Gypsum board meets these criteria because its core complies with the E136 requirements, the paper surfacing is less than 1/8 inch thick, and the flame spread on regular gypsum wallboard is typically in the 5 to 15 range, considerably less than the maximum of 50 that is required.
LIMITED COMBUSTIBLE MATERIAL
In addition to the concept of a noncombustible material, mechanical codes or standards also occasionally incorporate the concept of a “limited combustible material.” The definition of a limited combustible material is very similar to the alternate building code definition for noncombustible materials outlined above. The term is typically used to define a material that requires a specific amount of functional clearance from a heat-producing or -conveying mechanical system element.
As an example, NFPA 90A, Standard for the Installation of Air-Conditioning and Ventilating Systems, incorporates the term and uses it to define requirements for materials that may be exposed to heated airflow in a mechanical plenum. The standard allows the use of noncombustible and limited combustible materials in the air plenum, but does not permit the use of combustible materials.
In a couple of specific instances, mechanical codes also incorporate the limited combustible material concept, primarily in situations where clearance to a range hood or grease duct is an issue. In those specific cases, only noncombustible materials that comply with the E136 test may be in close contact with the heat produced by the mechanical element.
The beauty of Mr. Wilson’s Science Day was that it showed a group of rambunctious elementary school students that there really is a practical relationship between all the things that they teach you in school and the “real world.” Thirty-five years ago, he showed my class that products and materials don’t simply work by magic; they undergo physical and chemical changes that allow them to function properly because of, not in spite of, the science that was a part of their development. Gypsum board is such a material and its ability to resist the passage of heat and flame is one reason why it can be used in noncombustible construction designs. W&C