Technology & Design 2026-06-12 9 min read

Fire Protection Strategies for CFRP-Strengthened Structures: Design and Materials

Fire is CFRP’s defining limit — and codes handle it by assuming the strengthening vanishes. Protect the FRP with insulation, or design the bare structure to survive without it. Both, ideally.

At a glance

Fire is CFRP’s key limit: the epoxy softens near its glass-transition temperature (Tg 60–82 °C), and service temperature must stay 15 °C below it.
ACI assumes the FRP is lost in a fire: the unstrengthened structure must independently carry a reduced fire-load combination as a backstop.
Two routes: protect the FRP with passive insulation to keep it below Tg, or use inherently fire-resistant FRCM, whose inorganic matrix has no Tg ceiling.

Fire is the defining limit of epoxy-bonded CFRP strengthening — and codes handle it by assuming the strengthening simply vanishes. That single design assumption drives everything about fire strategy: you either protect the FRP so it survives, or you make sure the bare structure survives without it. This guide covers both, grounded in ACI 440.2R. It builds on the resin behaviour in structural epoxy Tg requirements.

Why is fire the key limit for CFRP?

Because the epoxy matrix softens with heat. Every epoxy has a glass-transition temperature (Tg), typically 60–82 °C for room-temperature-cured strengthening systems, above which its stiffness and bond strength fall sharply. ACI 440.2R caps continuous service temperature at Tg − 15 °C for this reason, and in humid service the lower wet-Tg governs. The carbon fiber itself tolerates far more, but the system is only as fire-worthy as the resin holding it to the concrete — and near Tg, that bond lets go.

The CFRP contribution is temperature-limited. As the epoxy approaches its glass-transition band (60–82 °C), stiffness and bond fall and the strengthening effect drops toward the bare-structure baseline. ACI 440.2R conservatively assumes it is lost entirely in a fire — so the unstrengthened member must survive on its own.

How does design handle fire?

Conservatively. ACI 440.2R §9.2.1 assumes the FRP’s contribution is completely lost in a fire unless proven otherwise by testing. What must then hold is the unstrengthened member: its nominal fire resistance must satisfy a reduced fire-load combination — in the form the guide recommends, roughly the existing capacity being at least the full dead plus full live load (built on ACI 216.1 and ACI 562). This sits on top of the everyday redundancy check that already governs any strengthened design: the bare structure carrying 1.1× dead + 0.75× live load if the FRP were lost for any reason. Fire is just the most demanding version of “what if the FRP isn’t there.”

What passive fire protection is used?

Insulation applied over the cured CFRP to keep it below its Tg for the required fire duration. The categories are insulation boards, cementitious or sprayed coatings, and intumescent coatings — each chosen and thickness-designed for the specific fire rating and member. (Specific ratings, thicknesses, and products are project- and system-specific and are set by the protection manufacturer’s tested assemblies, not by the FRP data sheet.) The goal is narrow and clear: buy enough time below Tg that the strengthened capacity survives the design fire.

Two independent safety layers. Passive insulation keeps the CFRP below its glass-transition temperature long enough to preserve its contribution during a design fire. Separately, the bare section is designed to carry a reduced fire load on its own — a backstop that holds even if the protection underperforms.

Is FRCM a fire-resistant alternative?

Yes — and often the cleaner answer where fire governs. Because FRCM binds its fiber mesh in an inorganic cement mortar with no glass-transition ceiling, it is inherently fire-resistant, unlike epoxy CFRP. FidStrong’s PBO mesh option does not decompose until about 650 °C, far beyond any epoxy Tg. Choosing FRCM sidesteps the passive-protection problem entirely rather than adding a layer to solve it — the full comparison is in FRCM mesh vs. CFRP fabric.

Protecting the FRP versus protecting the structure

These are two different jobs, and good fire design separates them. Protecting the FRP means insulation that keeps the laminate below Tg so its contribution is still there when the fire is over — it depends on the protection performing as tested. Protecting the structure means designing the bare section to carry the reduced fire load with zero help from the FRP — a backstop that holds regardless of how the insulation fares. Robust designs do both.

Never let fire safety rest on the FRP alone. ACI 440.2R deliberately assumes the strengthening is gone in a fire, so the unstrengthened member must survive the fire-load combination by itself. Passive protection is what preserves the strengthened capacity; it is not what prevents collapse — that is the bare structure’s job by design.

FAQ

What temperature can CFRP-strengthened concrete tolerate?

ACI 440.2R limits continuous service temperature to Tg − 15 °C, where Tg is typically 60–82 °C for room-temperature-cured epoxy systems. In humid exposure the lower wet-Tg governs, since moisture reduces the effective ceiling further. Above that, design cannot rely on the CFRP’s strength.

Does CFRP strengthening need fire protection?

Only if the design fire load exceeds what the unstrengthened structure can carry alone. ACI 440.2R §9.2.1 assumes the FRP contribution is lost in a fire, so passive protection is used specifically to keep the CFRP below its Tg long enough to preserve that contribution during the fire event.

What happens if CFRP exceeds the epoxy’s Tg?

The resin softens and its bond and stiffness drop, so codes treat the FRP’s contribution as lost once temperatures approach or exceed Tg (60–82 °C typical). Design must not rely on CFRP strength there — which is exactly why the service ceiling is set at Tg − 15 °C.

Can a CFRP-strengthened structure survive a fire without protection?

Only if the bare, unstrengthened member itself satisfies ACI 440.2R’s reduced fire-load combination (§9.2.1). That is separate from the ambient redundancy check (1.1× dead + 0.75× live) that already governs strengthened design — both assume the FRP could be absent.

Is FRCM more fire-resistant than CFRP?

Yes, by matrix chemistry. FRCM uses an inorganic cement matrix with no glass-transition ceiling, so it is inherently fire-resistant, versus FRP’s organic epoxy matrix (Tg 60–82 °C). FidStrong’s PBO mesh option decomposes near 650 °C, making PBO-FRCM a strong choice for fire-rated applications.

What passive fire protection is used for CFRP?

Insulation boards, cementitious or sprayed coatings, and intumescent coatings applied over the cured CFRP to keep it below its Tg during a design fire. Specific ratings and thicknesses come from the protection manufacturer’s tested assemblies, not the FRP data sheet — treat it as a designed protection layer, not a generic add-on.

For how heat fits alongside the other aging factors, see the durability of CFRP strengthening, and what CFRP strengthening is for the fundamentals.

FidStrong manufactures FSC carbon fabric plus FCM carbon mesh, FPM PBO mesh, and FMM mortar for inherently fire-resistant FRCM systems, under ISO 9001, 14001, and 45001 quality systems. Fire-design figures here are drawn from ACI 440.2R (§9.2.1, §1.2.1.3) and fib Bulletin 14; passive-protection design is licensed-engineer work using tested assemblies — confirm against the governing code.