At a glance
- Tg caps the service temperature of every epoxy CFRP system. ACI 440.2R limits service temperature to Tg − 15°C (Tg − 27°F), and typical structural epoxy Tg is only 60–82°C — so a standard system tops out around 45–67°C.
- Bond and modulus drop sharply near Tg, and in wet service you must use the lower wet-conditioned Tgw, not the dry value.
- HDT is not Tg. A data sheet quoting a heat-distortion temperature (e.g. 65°C) has not told you the glass-transition temperature — ask for both, measured by DMA.
The glass-transition temperature (Tg) of the saturating epoxy or plate adhesive is what sets the temperature ceiling of a CFRP strengthening system — and because typical epoxies have a Tg of just 60–82°C, knowing that number before you specify is essential for any warm or industrial environment. This guide explains what Tg is, the ACI service-temperature rule, why HDT is a different number, and how Tg is verified. For the wider resin picture, see the structural epoxy adhesives guide.
What is Tg, and why does it cap a CFRP system?
Tg is the temperature at which the epoxy matrix transitions from a hard, glassy solid to a soft, rubbery state. Below Tg the resin is stiff and transfers load; as it approaches and passes Tg, its modulus and its bond to the concrete fall away — and in a bonded CFRP system, the resin is the load path from concrete into fiber. So the resin’s Tg, not the carbon fiber (which tolerates far higher temperatures), sets the usable temperature ceiling of the whole system. This is a property of the organic matrix, which is exactly why FRCM — with an inorganic mineral matrix and no glass transition — has no such limit.
What service temperature can a CFRP system actually handle?
ACI 440.2R Sec. 1.2.1.3 sets the rule: service temperature ≤ Tg − 15°C (Tg − 27°F). Most room-temperature-cured structural epoxies have a Tg of 60–82°C (140–180°F) — a figure two separate ACI documents (440.2R-17 and 440.7R-22 §3.3.3) agree on. Combine the two and a standard field-applied system is limited to a continuous service temperature of roughly 45–67°C, depending on the resin. That is fine for most interiors, but it is a real constraint next to ovens, furnaces, steam lines, or on a dark roof in a hot climate — check the specific resin’s Tg first.
Does humidity change the number?
Yes. In continuously wet or humid service, ACI 440.2R directs designers to use the wet-conditioned Tg (Tgw), which is lower than the dry Tg (Luo & Wong 2002). Absorbed moisture plasticises the resin and drops the transition. The code gives the rule but not a universal numeric reduction, so for humid, submerged, or splash-zone work you should request the wet-conditioned Tg from the manufacturer rather than assume the dry value holds.
Is HDT the same as Tg?
No — and conflating them is a common specification error. Heat-distortion temperature (HDT, ASTM D648) measures the temperature at which a resin bar deflects under a set load; Tg (measured by DMA) is the thermodynamic glass transition. They are related but not interchangeable, and a data sheet may quote one without the other. FidStrong’s FSE362 plate adhesive, for instance, publishes an HDT of 65°C (ASTM D648) — a useful number, but it is HDT, not a DMA Tg. When comparing systems for a hot environment, ask every supplier for both, tested by a named method.
| Tg (glass transition) | HDT (heat-distortion) | |
|---|---|---|
| What it measures | Glassy→rubbery transition | Deflection under load at temperature |
| Method | DMA (or DSC) | ASTM D648 |
| Used for | The ACI service-temp ceiling (Tg−15°C) | A related thermal indicator |
Two independent temperature rules to check
Design against both. ACI 440.2R imposes a ceiling: service temp ≤ Tg − 15°C. fib Bulletin 14 (§8.4.1.2) imposes a floor on the adhesive itself: its Tg must be ≥ 45°C, or the maximum service air temperature + 20°C, whichever is greater. And both are separate from fire: ACI 440.2R Sec. 9.2.1 assumes the FRP contribution is completely lost in a fire unless proven otherwise, so the bare structure must satisfy a reduced fire-load check on its own.
How is Tg measured and verified on site?
Tg is measured by DMA (dynamic mechanical analysis); the European (prEN 6032) and American methods run at the same 5°C/min heating rate and are considered equivalent, though instrument differences can matter more than the method. On a project, ACI 440.2R’s inspection checklist (Sec. 7.2) verifies degree of cure by checking the achieved Tg (with field hardness and tack checks alongside) — an under-cured resin shows a depressed Tg. Note that a higher-Tg resin is achievable by process: FidStrong’s factory-pultruded CFRP bar carries Tg ≥ 100°C, well above field-cured wet-layup epoxy — Tg is a resin-and-process property, not a fixed constant of “CFRP.”
FAQ
What service temperature can a CFRP strengthening system handle?
Service temperature must stay at or below Tg − 15°C (ACI 440.2R Sec. 1.2.1.3). Since typical structural epoxies have Tg = 60–82°C, that generally caps continuous service around 45–67°C — check the specific resin’s data sheet before using it near heat.
Does humidity lower the allowable temperature?
Yes. In wet or humid service use the wet-conditioned Tg (Tgw), which is lower than the dry Tg (Luo & Wong 2002). ACI gives the rule but no universal numeric reduction, so request the wet Tg from the manufacturer for humid or submerged work.
Is the HDT on the data sheet the same as Tg?
No. HDT (ASTM D648) is the deflection-under-load temperature; Tg is the glass transition, measured by DMA. They are related but not interchangeable — a sheet may quote one without the other. Ask every supplier for both. FidStrong’s FSE362 adhesive, for example, publishes HDT 65°C, which is an HDT figure, not a DMA Tg.
What happens to a CFRP system in a fire?
ACI 440.2R Sec. 9.2.1 directs designers to assume the FRP contribution is completely lost in a fire unless fire performance is specifically demonstrated, so the unstrengthened structure must satisfy a reduced fire-load check on its own. Tg exceedance and fire are separate checks.
Can a CFRP system be made to tolerate higher temperatures?
Tg is resin-and-process dependent, so higher-Tg systems exist — FidStrong’s factory-pultruded CFRP bar carries Tg ≥ 100°C versus 60–82°C for field-cured wet-layup epoxy. For a genuinely hot environment, an inorganic FRCM system (no glass transition) may be the better answer than epoxy CFRP.
How do I know the resin cured to its full Tg on site?
ACI 440.2R Sec. 7.2 verifies degree of cure by checking the achieved Tg (by DMA), backed by field hardness and tack checks — an under-cured laminate shows a depressed Tg. Follow the manufacturer’s cure schedule; see curing CFRP systems.
FidStrong manufactures saturating epoxies and plate adhesives for CFRP strengthening, with thermal data (e.g. FSE362 HDT 65°C, ASTM D648) on each data sheet, under ISO 9001, 14001, and 45001 systems. The service-temperature rule follows ACI 440.2R Sec. 1.2.1.3; request the specific Tg (and wet Tg) for any hot or humid application, and confirm against the governing code and a qualified engineer.