How is the flame spread index (FSI) determined in ASTM E84?

In ASTM E84, the flame spread index (FSI) is determined by conducting a test called the Steiner Tunnel Test or Tunnel Test. This test measures the flame spread characteristics of building materials when exposed to a controlled fire source.

Here is a general overview of the test procedure:

1. Test Sample Preparation: The material being tested is cut into specimens that are at least 20 inches wide by 24 inches long. The thickness of the specimen depends on the specific material being evaluated.

2. Test Apparatus: The specimens are mounted vertically inside a test chamber, which is a tunnel-like structure made of noncombustible material. The chamber has adjustable draft control to maintain a specified airflow during the test.

3. Ignition: A gas burner is positioned at the bottom of the tunnel and ignited. The flames from the burner create a controlled fire source.

4. Exposure: The specimen is positioned at the entrance of the tunnel, and the burner flames impinge on the bottom surface of the specimen. The exposure duration is typically 10 minutes.

5. Measurement: During the exposure, several measurements are taken to determine the flame spread characteristics. These include the flame front advancement, the time taken for the flame to reach specific measurement points on the specimen, and the extent of charring or damage to the material.

6. Calculation: Based on the measurements obtained, the FSI is calculated using a formula that takes into account the flame front advancement and the time it takes for the flame to reach certain positions on the specimen.

7. Rating: The FSI calculation results in a numerical value that represents the material's flame spread index. This value is then compared to a scale to assign a specific flame spread rating, typically Class A, B, or C. Class A represents the lowest flame spread, while Class C indicates a higher propensity for flame spread.

It's important to note that ASTM E84 also measures other parameters, such as smoke developed index (SDI), which evaluates the smoke generation characteristics of the material. The FSI and SDI ratings are often reported together to provide a comprehensive assessment of a material's surface burning characteristics.

The ASTM E84 test and the resulting FSI rating are widely used in building codes and regulations to assess the fire performance of materials and ensure compliance with fire safety standards.