All Things Gypsum
Fire Resistance Testing
Two important aspects of fire resistance testing are frequently misunderstood. This article will clarify the misperceptions surrounding the meaning of fire resistance ratings and the role of the hose stream test in assessing fire resistance. Understanding the basic concepts behind fire resistance and hose stream testing will provide the background needed to identify and correct misunderstandings and misinformation that may be perpetuated in the competitive arena regarding fire resistance ratings and gypsum board systems.
What does a fire resistance rating mean?A fire resistance rating is one of many tools used by designers to assess relative fire risk. In addition to fire resistance, other properties of the construction materials to be considered include burning characteristics, fuel load of the space, and the proposed use of the structure or occupancy. All of these features must be considered before an assessment of the actual fire risk can be made. Additional factors such as building location, distance to fire services, and the presence or absence of other fire protection systems are also part of this complex assessment process.
A fire resistance rating alone cannot predict the performance of a system or building in an actual fire. In fact, no fire test method that is conducted under laboratory conditions can predict what will happen in a real structure fire. Fire tests are simply convenient ways of classifying materials and establishing a ranking of performance among different materials so designers can compare and select materials and systems for specific projects.
Fire test results-including fire resistance ratings-enable code officials to compare materials and systems against code requirements to determine compliance. It may help to think of a fire resistance rating in the same way one thinks of the mileage sticker on a new car. The mileage on the sticker is determined under very specific test conditions and actual mileage will likely be either more or less than the mileage on the sticker.
How is fire resistance measured?
The fire resistance test method used throughout the United States is ASTM E 119, Standard Test Methods for Fire Tests of Building Construction and Materials. This test procedure was first published by ASTM in 1918 as ASTM C 19-18. That first ASTM version prescribed two tests:
1. A fire endurance test.
2. A fire and fire stream test (commonly referred to as the hose stream test).
Similar test methods are published by Underwriters Laboratories and by the National Fire Protection Association.
The fire endurance test subjects a specimen to a prescribed fire until certain conditions are met that indicate the end of the endurance test. This period of time is known as the "resistance period" of the fire endurance test. All fire resistance rated systems, regardless of the materials from which they are built, are tested using this test.
The hose stream test is divided into:
1. A "primary" or "standard" method.
2. An "optional program" method.
The optional program is referred to as an "exception" in the NFPA version. The standard method states: "A duplicate specimen (is) subjected to a fire exposure test for a period equal to one half of ? the resistance period of the fire endurance test, but not for more than one hour." The duplicate specimen is then immediately subjected to the impact, cooling and erosion effect of a stream of water from a fire hose at a pressure and for a duration of time specified in the test method. If no significant amount of water passes through the test specimen, the fire endurance time of the first specimen becomes the fire rating for the system.
The duration of the hose stream exposure is a function of the fire endurance period of the original specimen and is keyed to the fire resistance rating of the system being tested (i.e., the longer the rating, the longer and more severe the hose stream exposure). The "optional" program, which can only be used if both the testing laboratory and the test sponsor agree, is to administer the hose stream to the same specimen used for the fire endurance test without the need for-and added cost of-constructing and burning a duplicate specimen as required by the standard method.
Role of hose stream testingAlthough the standard method and the optional program have remained essentially unchanged over the years as ASTM C 19-18 evolved into the current version of ASTM E 119, the value of the hose stream test has been a subject of debate for years. It was eliminated from testing requirements for floor-ceiling and roof-ceiling systems in the '50s because years of tests had resulted in no failures of systems because of the hose stream test. The test was removed from European and International test methods years ago, and the British test method has not contained a hose stream test for more than 40 years. The test is now found only in the ASTM, UL and NFPA test standards, but even here, it is applied only to walls.
In spite of the fact that the international fire-safety community discontinued the hose stream test years ago, the masonry industry would prefer to see fire testing in the United States take a step backward by eliminating the standard hose stream test method and requiring what is now the optional program. The masonry industry often misrepresents the origin and purpose of the hose stream test in an attempt to strengthen its technically deficient arguments in hopes of gaining a competitive advantage.
The ASTM E 119 procedure, including both the fire endurance test and the hose stream test, is intended to evaluate the performance of systems during the period of fire exposure. It is not, nor has it ever been, the intent of the tests to evaluate a system's suitability for continued use after the fire exposure. Section 1.2 of the scope of ASTM E 119 states clearly, "It is the intent that classifications shall register performance during the period of exposure and shall not be construed as having determined suitability for use after fire exposure."
Questions surrounding why the hose stream exposure is conducted on a duplicate specimen that has undergone a fire exposure for a period of less than the full fire rating were raised as long ago as 1930 when a representative of the gypsum industry asked fire test experts at the National Bureau of Standards to explain the rationale for the procedure. In a letter dated January 2, 1930, Mr. Fitzhugh Taylor, a fire test expert from NBS and Secretary of the ASTM Committee responsible for developing and maintaining the fire resistance test method, responded with the following explanation:
"The second test of the prescribed pair, the fire and fire stream test (hose stream test), in which the fire exposure is shorter than that of the fire endurance test, is a means of investigating stability of the test subjects, including essential parts thereof, during the period in which they function effectively as fire barriers; this being the intent of the second test of the pair, it is prescribed that the fire hose stream shall be applied well in advance of the time when the fire resistance or the test subject has been taxed to its ultimate."
Mr. Taylor's comments confirm that it was never the intent of the hose stream test to evaluate a system's performance after a fire of the full duration of the rating. In fact, the validity and value of the hose stream test, in general, has been challenged by Mr. S. H. Ingberg, recognized as one of the father's of modern fire testing, whose understanding of the rationale for the hose stream test is:
"In general, the justification of the hose stream test is predicated on the following sequence of events:
1. The application of hose-streams to a construction exposed to fire causing damage making the construction vulnerable to fire exposure.
2. The fire-fighting force leaves the scene or is driven away by the fire.
3. The fire recurs in sufficient severity to overtax the damaged construction.
The occurrence of consecutive events 1), 2) and 3) is apparently of the order of a fairly remote probability, considering particularly that 3) is unlikely to occur after the amount of water application required under 1) by the test procedure. This formed the basis for the omission of the hose-stream test for columns in the present standard, and ¿ in a forthcoming revision of the British Standard ¿ on fire test methods, the hose stream is not required for any type of construction."
Although the validity and value of the hose stream test is questionable and it has been removed from most international fire test methods, the hose stream test remains a part of the fire resistance test methods used in the United States today. Its purpose is, and has always been, to evaluate physical stresses encountered during the period of fire exposure, not to evaluate performance after fire exposure. There is no added significance to the fire rating from using the "optional program" method, nor is the significance of the fire rating diminished by using the "standard" method.
Copies of ASTM E 119 are available from ASTM International, 100 Barr Harbor Drive, West Conshohocken, Pa. 19428-2959. W&C