ASTM E84 for FRP: Flame Spread Index and Smoke Developed Index Explained

What ASTM E84 Measures

The method is commonly known as the Steiner tunnel test. In the standard arrangement, a nominal 24 ft long by 20 in wide specimen is installed in the tunnel with the exposed surface facing downward in a ceiling position. Controlled airflow and a flaming fire exposure are used while flame progression along the specimen and smoke density are measured during the 10-minute test.

The Flame Spread Index describes the relative progression of flame along the specimen. Its reference scale is based on selected reference surfaces, including fiber-cement board and select-grade red oak. Because FSI is an index rather than a direct physical measurement, the number should be interpreted as a comparative result from the test method. FSI values can also exceed 100.

The Smoke Developed Index is derived from smoke-density measurements recorded during the test. SDI is also a relative index, and it should not be interpreted as a direct measurement of smoke mass or as a standalone measure of overall fire hazard. ASTM E84 also notes that FSI and SDI do not necessarily have a direct relationship.

How Class A, B and C Ranges Are Used

Building codes and specification practices commonly group materials into three classes using ASTM E84 results:

• Class A: FSI 0–25 and SDI 0–450
• Class B: FSI 26–75 and SDI 0–450
• Class C: FSI 76–200 and SDI 0–450

These class ranges are not classifications created by ASTM E84 itself. They are used within code and specification frameworks, and the applicable requirements can differ by jurisdiction, occupancy, location, or application. A material should therefore be evaluated against the classification requirement that governs the specific project.

Why FRP E84 Results Vary by Configuration

FRP is a composite rather than a single uniform material. Many industrial FRP products combine glass-fiber reinforcement with a thermosetting resin matrix, together with fillers, additives, surface layers, or other construction features. Changes in resin formulation, filler content, fiber-to-resin composition, thickness, surface condition, or construction can affect the observed flame spread and smoke behavior.

This is why an ASTM E84 result from one FRP configuration should not automatically be transferred to another product or construction. A molded grating made with one resin and filler system, for example, cannot be assumed to have the same E84 result as a pultruded profile made with a different formulation and construction.

For an engineering review, the useful question is not simply whether an FRP material has an FSI or SDI value, but which tested product and configuration produced that result.

What to Check in an FRP ASTM E84 Test Report

When reviewing an E84 report for an FRP product, first identify the exact material or product designation that was tested. Then check the available information on the resin system, filler or flame-retardant formulation, thickness, surface condition, and specimen construction. The report should also identify the applicable ASTM E84 edition used for the test.

Support conditions deserve particular attention. ASTM E84 permits specimens to be self-supporting or to be held in position by supporting materials or other means. The standard notes that underside supports can lower the reported FSI compared with testing without that support. As a result, results obtained with different support arrangements are not automatically interchangeable.

FRP behavior during the test also matters when interpreting a low FSI. If a material melts, drips, or delaminates enough to interrupt the continuity of the flame front, the resulting low flame spread index may not be directly comparable with a material that remains in place throughout the test.

Important Limits When Interpreting ASTM E84 for FRP

ASTM E84 provides comparative information about surface flame spread and smoke density under controlled laboratory conditions. It does not measure heat transmission through the tested surface, and an FSI by itself does not classify a material as noncombustible. The result also does not incorporate every factor that can influence fire hazard or fire risk in an actual fire.

For FRP, this distinction is particularly important because the tested surface, construction, support arrangement, and material formulation all form part of the test context. A low index should therefore be read as a result of the tested configuration, not as a universal statement about the fire behavior of every product using the same broad FRP material category.

Where ASTM E84 Fits in FRP Selection

ASTM E84 can provide useful comparative fire-performance information where surface burning characteristics are part of the project requirement. For industrial and infrastructure FRP applications, however, the relevant fire-performance requirements depend on the product, its intended use, the installation context, and the governing code or specification.

The practical value of an ASTM E84 result is therefore highest when the test configuration and the specified product are clearly matched. If those details differ, the E84 value should not be treated as automatically transferable.

In summary, ASTM E84 reports two relative indices—FSI and SDI—that describe flame spread and smoke density under a defined test method. For FRP, the meaning of those numbers depends on the configuration that was actually tested. The most reliable way to use an E84 result is to identify the tested product, understand the test context, and compare the result with the requirement that applies to the specific project.