What the three indices measure

Flame spread index describes how fast flame travels across the surface, and lower is better. Smoke developed index describes how much smoke is produced, also lower is better. Oxygen index describes the lowest oxygen concentration that will sustain combustion, and higher is harder to burn.

The three measure different things and do not substitute for one another. A material can have low flame spread but heavy smoke, or a high oxygen index and still spread quickly once alight. Selecting on one number alone tends to fail on another.

None of them is a fire resistance rating. Fire resistance is the time a component keeps its load capacity, integrity and insulation in a furnace test, which is a test of the whole assembly and a different question from the reaction to fire of the material.

Reading ASTM E84

ASTM E84 is the American surface burning test. A sample sits in a tunnel furnace, flame is introduced at one end, and the distance the flame travels and the density of smoke are recorded and converted into a flame spread index and a smoke developed index.

The class follows the flame spread index: zero to twenty-five is Class A, twenty-six to seventy-five is Class B, and seventy-six to two hundred is Class C. Class A usually also requires a smoke developed index of no more than 450, and B and C carry their own smoke limits.

Read three things in the report: the edition of the standard, the sample description and whether a substrate was used. The same resin can give different results at a different thickness, with a facing or with the underside closed, so the sample has to match what will actually be supplied.

How GB 8624 classifies

GB 8624 divides building materials into A, B1, B2 and B3: A non-combustible, B1 difficult to ignite, B2 combustible and B3 easily ignited. FRP grating normally reaches B1 or B2, depending on the resin and the flame retardant system.

B1 is not a single index but a combination of sub-tests covering heat release rate, lateral flame spread, smoke production and burning droplets. The report carries a string of additional classes such as B-s1, d0 and t1, each tied to one sub-test.

The letters B1 alone are not enough; the full additional classes and the test methods matter. Different editions of GB 8624 use different classification structures, so the edition number has to be on the report.

EN 13501 and the Euroclass

The EN 13501 Euroclass system places materials in A1, A2, B, C, D, E or F, with A1 non-combustible and F unrated. FRP usually falls between B and D, and phenolic resin can reach B or better.

Additional classes use s1 to s3 for smoke and d0 to d2 for burning droplets. s1 is the lowest smoke and d0 means no droplets. For escape routes and enclosed spaces those two often matter more than the main class.

European and some international projects require CE marking and a Declaration of Performance stating the Euroclass and the test basis. Without a DoP the class is only the maker's claim and cannot be used for acceptance.

Choosing the resin system

Isophthalic polyester with a retardant filler is the common entry-level route: low cost, capable of B2 or B1, but with more smoke and only moderate long-term chemical resistance. Vinyl ester with a retardant system is better balanced, with lower smoke, and suits chemical and offshore duty.

Phenolic resin gives the best fire performance, with low flame spread and low smoke, at the cost of brittleness, a dark colour, moderate alkali resistance and harder processing. For crowded platforms and escape routes, phenolic is often the first choice.

Retardant fillers reduce chemical resistance, so the two have to be assessed together. Where the medium is aggressive, resin and retardant choice are covered in chemical resistance of FRP grating

What oxygen index does and does not say

The oxygen index test burns a small specimen in a controlled oxygen concentration and finds the lowest level that sustains combustion. Air is about twenty-one percent oxygen, so an index above twenty-one is needed before a material can self-extinguish in air.

Fire-retardant grating commonly shows an index between twenty-six and thirty, and higher means it self-extinguishes more readily. But the test uses a small specimen, one orientation and no external heat source, so it cannot be extrapolated to a full-size component in a real fire.

A high index does not mean non-combustible and does not mean low smoke. Some formulations raise the index with halogenated additives and produce more toxic and corrosive smoke, which is exactly what underground car parks and tunnels try to avoid.

The limits of fire-retardant grating

Retardant does not mean non-combustible. FRP is an organic material and will eventually decompose and burn under sustained heat; the retardant system only delays that, buying time for evacuation and firefighting.

Reaction to fire is a material property, not a component property. Grating on steel beams, above cable trays and beside walls forms a system whose fire behaviour has to be assessed by a test of the assembly, not by a material report.

Mechanical properties fall quickly at high temperature, and even without flame the load capacity drops as the resin approaches its transition temperature. Load-bearing platforms need a check of residual capacity in fire, not just ambient data. How the two processes compare is covered in molded versus pultruded grating

Specification and documentation

The specification should state the standard, the required class, the additional classes for smoke and droplets, the sample description and the validity of the report. A requirement of fire retardant or B1 alone lets a supplier answer with almost any report.

Ask for the original documents: the full test report, the laboratory accreditation, the sample formulation and thickness, and the edition of the standard behind the class. They must match the batch actually supplied, not be an old report from several years ago.

Where insurance or fire acceptance applies, confirm that the local authority recognises the report. A foreign report is not automatically valid for a domestic project, and that question belongs before purchase. Full parameter wording is in the FRP grating specification guide

The inquiry checklist

The inquiry should list the medium, whether there is an ignition risk, occupant density, escape requirements, the governing standard and the documents required. A higher class is not automatically better: it narrows the resin choice, raises cost and often lowers chemical resistance.

Ask the supplier to explain the resin recommendation and how that resin behaves in the project medium over time. Where fire and chemical resistance are both required, they have to be verified together rather than assembled from two separate reports.

• Confirm the governing standard and class system first.
• Read flame spread, smoke and droplets together.
• Assess fire and chemical resistance as one choice.
• Request the full report and check the sample description.