Standard Test Method for Surface-Burning Characteristics of Building Materials ASTM E-84/UL 723

INTRODUCTION
Throughout history, structural fires have caused massive destruction and countless injuries and fatalities. Although the flammability characteristics of interior finish within these structures has played a major role in many of these losses, prior to the middle of the 20th century, fire protection of buildings focused primarily on: 1) the prevention of fire occurrence, 2) early detection and warning, 3) automatic or manual extinguishment, and 4) confinement with fire-resistant structural components, such as floors, ceilings, walls and partitions, columns, roofs, and doors.

The occurrence of major fires in individual buildings, distinguished by the rapid flame spread of interior finish materials, aroused public concern and demonstrated the need to address and regulate the burning characteristics of these materials. Specific material characteristics of concern included the spread of flame and the amount of heat generated and smoke developed. This led to the research and development of various testing protocols, most of which were small, laboratory-scale tests. However, based on work conducted by Albert J. Steiner at Underwriters Laboratories Inc., from the early 1920s through the 1940s, the 25 ft. (7.6 m) long Steiner tunnel emerged as the predominant method to characterize and regulate the surface-burning characteristics of interior finish materials.

The Steiner tunnel is a furnace chamber that measures flame spread and smoke development. Its prominence in the fire protection community was based on its ability to provide cost-effective, repetitive testing and use a sample size that could better characterize interior finish materials used in actual installations. This method is currently described in UL 723,1 Test for Surface Burning Characteristics of Building Materials, as well as ASTM E-842 and NFPA 255.3

SUMMARY OF TEST METHOD
The Steiner tunnel is used to assess the comparative surface-burning characteristics of building material samples with the exposed area measuring 18 in. (460 mm) wide by 24 ft. (7.3 m) long, up to a thickness of approximately 5-6 in. (125-150 mm). The test is conducted with the sample mounted in the " ceiling" position of an enclosed tunnel furnace measuring 18 in. (460 mm) wide by 12 in. (300 mm) deep by 25 ft. long (7.6 m). A nominal 5000 Btu/min. (88 kW), 4-1/2 ft. (1.4 m) flame provides an ignition source to the underside of the mounted specimen for a 10-minute duration. A controlled inlet draft of 240 feet per minute (1.2 meters/second) facilitates horizontal flame propagation throughout the test. A light and photoelectric cell mounted in the exhaust duct record smoke obscuration during the test. Flame-spread and smoke-developed indices are reported in comparison with calibration materials of red oak lumber and inorganic reinforced cement board. Red oak propagates flames to the end of the tunnel in 5 minutes 30 seconds ± 15 seconds and generates a flame-spread index of approximately 90. A smoke-developed index of 100 is assigned for red oak. Inorganic reinforced cement board generates flame-spread and smoke-developed indices of zero.

EARLY HISTORY AND DEVELOPMENT
The initial version of the tunnel furnace was developed in 1922 when Mr. Steiner, an engineer in UL's Fire Protection Department, assessed the effectiveness of a "fireproof" paint. The prototype test method consisted of a long wooden bench measuring approximately 18 in. (460 mm) in width and depth and 16 ft. (4.9 meters) long with a noncombustible top. The interior of the tunnel was coated with the paint under investigation and ignited with a given quantity of wood at one end. The extent of the spread of flame was compared with an unpainted replica, and the flame retardancy of the coating was thus evaluated.

In the late 1920s, the development of pressure-impregnated fire-retardant lumber, in conjunction with further research at UL, led to modifications to the test method in which the test sample formed the top of a 36 in. (91 mm) wide by 13 in. (330 mm) deep by 23 ft. (7.0 m) long chamber. The use of untreated red oak and maple flooring in this investigation was a major factor in the selection of red oak as one of the calibration materials for the test method.

By the beginning of World War II, there was growing interest in reducing the combustibility of existing materials through various treatments and in measuring the flammable properties of new materials. In addition, by the mid-1940s, a number of disastrous fires occurred, including the Cocoanut Grove nightclub fire in Boston in 1942 and the Chicago LaSalle Street Hotel and Atlanta Winecoff Hotel fires, both in 1946. In all, 670 people perished in these three fires alone. The magnitude of the fire fatalities in each of these fires was directly related to the rapid flame spread and smoke development of the interior finish materials. These findings highlighted the need to test and classify materials on a scale that would measure the three essential material characteristics previously identified: flame spread, fuel contributed, and smoke developed. All these factors led to the evolution of the current tunnel apparatus. It was at this time that the Surface-Burning Characteristics Classification Scale was first defined. It was essential that, in order to classify materials according to the properties of flame spread, fuel contributed and smoke developed, as well as to have this information be of value, a comparative scale was required. Accordingly, the test initially developed a classification for each of these properties for a sample material on a comparative scale with a combustible (red oak lumber) defined as 100 and a noncombustible cement board as zero.

The current physical version of the tunnel was completed in the late 1940s. Many controls were implemented to enhance repeatability and reproducibility. The standard specimen size became 20 in. (510 mm) wide by 25 ft. (7.6 m) long. The ignition source was adjusted to obtain a nominal 4-1/2 ft. (1.4m) long, 5000 Btu/min. (88 kW) test flame that generates gas temperatures of approximately 1200°F to 1600°F (650°C-870°C) near the specimen surface at the ignition end of the test sample. The inlet draft was established at 240 feet per minute (1.2 meters/second).

The method used to calculate Flame Spread Index (FSI) has undergone some modifications over the years. Originally, the FSI was based on the ratio of the time at which flames traveled the full tunnel length or the partial flame travel distance relative to that of red oak. In 1976, the FSI was changed to a timeflame spread distance area basis. The current method is still based on a timedistance area calculation but incorporates a rate of flame travel as well.

Prior to 1978, a Fuel Contributed Index was reported. This index was based on air temperatures developed within the tunnel furnace during testing. In 1978, the Fuel Contributed Index was deleted from the method since it was recognized that the value did not provide a valid measure of fuel contribution.

COMMUNITY ACCEPTANCE
Just as the test method developed gradually over a period of years, so did its acceptance. The test method was first published in 1950 by Underwriters Laboratories Inc. as Standard UL 723. ASTM followed by publishing the test method as a tentative standard in 1950 and as a formal Standard, ASTM E-84, in 1961. NFPA adopted the test method as NFPA 255 in 1955. It was adopted by ANSI in 1963 as American National Standard A2.5. Although the tunnel test provides for a Classification protocol and is recognized by standards-developing organizations, it does not establish limitations for building codes. The intent of the test method is to provide a tool for those with the responsibility of regulating materials used as interior finish in buildings. Widespread reliance on the tunnel test method by the regulatory community as an acceptable criterion to assess interior finish and other materials has been in place for decades. Factors that have contributed to this reliance include:

• Support by standards-developing organizations, including UL, ASTM, and NFPA.
• The test method utilizes a large sample size and an ignition source representative of a moderately developed fire scenario.
• The ability of the test method to characterize both high and low flame spread materials.
• Research that demonstrates a relationship between tunnel test results and certain large-scale test protocols. ⁴

Class A: Flame Spread 0-25; Smoke Developed 0-450. Class B: Flame Spread 26-75; Smoke Developed 0-450. Class C: Flame Spread 76-200; Smoke Developed 0-450. Prior to 1960, the tunnel test method was used primarily for the evaluation of the surface-burning characteristics of homogenous compositions of ceiling and wall finishes, such as acoustical tiles, wall coverings, coatings, and various types of decorative paneling. Through inclusion of the Guide to Mounting Methods Appendix in the late 1960s, the method was expanded to include the evaluation of composites and assemblies. Sample mounting techniques can have a significant influence on the fireperformance indices developed by the test method. While the Appendix is not considered a mandatory part of the standard, the Guide has proven useful in promoting more-consistent results by various laboratories. Recently, a more comprehensive approach toward the standardization of mounting practices has led to the development of ASTM E2231, Standard Practice for Specimen Preparation and Mounting of Pipe and Duct Insulation Materials to Assess Surface-Burning Characteristics. Similar practices for other material types are currently being considered under the ASTM standard-development process.

ADVANTAGES AND LIMITATIONS

• Certain relationships have been observed between Steiner tunnel test results and performance of some materials during building fires. ²
• The test method provides for a realistic fire scenario by presenting a sample of sufficient size to allow for progressive surface burning over a large exposed area.
• A wide range of materials may be tested, including composite constructions, coatings, faced products, loose-fill materials, sandwich panels, and many others. UL currently classifies over thirty different product types in accordance with the test method.
• The test method provides a means to discriminate products yielding a wide range of flame-spread and smoke-developed characteristics, allowing for the development of codes and standards.
• Some research conducted has demonstrated useful relationships between Steiner tunnel flame-spread values and fire performance of materials in large-scale corner configurations using a 20-pound ignition source wood crib. ⁴
• The horizontal specimen orientation places some limitation on the type of material that can be realistically mounted. Depending on the particular material being tested, specimens requiring additional support may yield low flame-spread values due to the supporting material restricting flame propagation or high-flame spread values because the additional support retains the specimen in the ceiling position rather than allowing the specimen to fall away from the area of flame impingement. • Some materials, such as faced composite samples, may delaminate during testing, which may result in one of two possible responses: the material may expose two or more surfaces to the flame, thereby increasing the flame spread index; or the material may sag or drop to the furnace floor, which may impede further flame propagation.
• Thermoplastic materials may be difficult to evaluate in this as well as other standardized fire test procedures and require careful interpretation of the test results. These materials tend to melt and drip to the floor of the furnace, and may generate potentially misleadingly low flame-spread values.
• Some research has indicated that some types of thermosetting cellular plastics yielding low flame-spread values may generate flameover conditions during certain large-scale room test scenarios, when utilizing igniting sources of sufficient heat flux levels. ⁴

Randy Laymon with Underwriters Laboratories Inc.

REFERENCES

1. UL723, Test for Surface-Burning Characteristics of Building Materials, Underwriters Laboratories, Inc., Northbrook, IL, 2001.
2. ASTM E-84, Standard Test Method for Surface-Burning Characteristics of Building Materials, American Society for Testing and Materials, West Conshohocken, PA, 2003.
3. NFPA 255, Standard Method of Test for Surface-Burning Characteristics of Building Materials, National Fire Protection Association, Inc., Quincy, MA, 2000.
4. "Flammability Studies of Cellular Plastics and Other Building Materials Used For Interior Finishes," Underwriters Laboratories Inc., Northbrook, IL, 1975
5. International Building Code, International Code Council, Falls Church, VA, 2003.
6. NFPA 5000, Building Construction and Safety Code, National Fire Protection Association, Quincy, MA, 2003.