ASTM E84 is one of the most commonly referenced fire test standards for building materials in North America. It produces two numbers: the Flame Spread Index (FSI) and the Smoke Developed Index (SDI). These values are not physical measurements in engineering units—they are relative ratings derived from a specific laboratory test. For FRP products, the same material can produce very different FSI and SDI results depending on resin chemistry, filler package, and laminate construction. Understanding how the test works, and what the numbers actually mean, helps avoid misreading a test report.

What ASTM E84 Actually Measures

The test method is formally known as the Steiner tunnel test. A sample, typically 24 inches wide and 24 feet long, is mounted horizontally on the ceiling of a ventilated tunnel. A gas burner at one end supplies a controlled flame and draft. During the 10-minute test, the flame front’s progression along the sample is recorded, along with the optical density of the smoke produced. The data is then converted into two index values.

The Flame Spread Index compares the sample’s flame progression to two reference materials: cement board, assigned an FSI of 0, and red oak flooring, assigned an FSI of 100. A sample that behaves like cement board gets a low FSI; one that behaves like red oak gets an FSI around 100. Values can exceed 100 if the flame spreads faster than red oak.

The Smoke Developed Index works on the same relative scale, also referencing cement board (0) and red oak (100). It quantifies the amount of visible smoke generated during the test.

What FSI and SDI Values Mean in Practice

In many building codes, materials are grouped into classes based on their ASTM E84 results. A common classification scheme uses the following general ranges:

  • Class A (or Class I): FSI 0–25, SDI 0–450
  • Class B (or Class II): FSI 26–75, SDI 0–450
  • Class C (or Class III): FSI 76–200, SDI 0–450

These ranges are not part of ASTM E84 itself. They come from model building codes and specification practices, and they can vary by jurisdiction and application. Always check the specific code requirement for the project rather than assuming a certain FSI automatically qualifies for a particular use.

Why FRP Results Depend on Configuration

FRP is not a single material. It is a composite of glass fiber reinforcement and a thermosetting resin matrix, often with fillers and additives. Each component can influence fire behavior. Halogenated resins, for example, tend to produce lower flame spread but may generate more smoke. Non-halogenated systems with alumina trihydrate (ATH) filler can achieve low FSI while releasing water vapor during combustion. The type and amount of filler, the resin-to-glass ratio, the laminate thickness, and the surface veil all affect the tunnel test results.

Because of these variables, a test result from one configuration should not be applied to another. A molded grating panel with a specific resin and filler cannot be assumed to perform the same as a pultruded profile with a different resin system. Each configuration needs its own test data.

Reading a Test Report Correctly

When reviewing an ASTM E84 report for an FRP product, pay attention to what was actually tested: the exact product designation, resin system, filler content, panel thickness, and surface finish. The report should state the test standard and version year—for example, ASTM E84-23. If the report is older or the configuration has changed, the results may not apply to current production.

A statement like “Class A per ASTM E84” is only meaningful when the tested configuration matches the product being specified, and when the test was performed by a qualified laboratory. Without that context, the claim is just a label.

Where ASTM E84 Fits in FRP Selection

ASTM E84 is a comparative screening test, not a full-scale fire endurance test. It provides useful information for interior finish materials and some industrial applications, but it does not predict how a structure will behave in a real fire. For FRP used in outdoor industrial platforms, walkways, or cable trays, other fire characteristics—such as self-extinguishing behavior, limiting oxygen index, or specific code requirements—may be more relevant. Engineers should match the fire test to the application and the governing code.

In short, ASTM E84 gives you two relative numbers—FSI and SDI—that help compare materials under a specific laboratory condition. For FRP, those numbers are heavily influenced by material configuration. Always verify that the test data matches the product you are actually specifying, and treat any classification claim as configuration-specific, not universal.