The hydrocarbon fire curve (HC curve, also called the hydrocarbon time-temperature curve or ASTM E1529/ UL 1709 curve in some contexts) is a standard nominal time-temperature exposure used in fire-resistance testing of structures, particularly for hydrocarbon pool fires (e.g., petrol, diesel, or HGV loads in tunnels).
It features a very rapid initial temperature rise (reaching ~1,100°C within ~10–20 minutes) and is defined by the analytical equation:
T = 1080(1 − 0.325e^(−0.167t) − 0.675e^(−2.5t)) + 20 (T in °C, t in minutes; ambient +20°C).
Key characteristics of the hydrocarbon fire curve
t = 0 min: 20 °C (ambient)
t = 1 min: ≈ 743 °C (extremely rapid rise)
t = 5 min: ≈ 948 °C
t = 10 min: ≈ 1,034 °C (already exceeds 1,000 °C)
t ≥ 30 min: Stabilises at ≈ 1,100 °C (steady-state plateau)
At 5 minutes the hydrocarbon curve reaches almost 950°C and at 10 minutes it exceeds 1,000 °C. Already at 30 minutes the HC curve stabilises at 1,100 °C.
Hydrocarbon fire curve vs cellulosic (ISO 834) fire curve:
It is significantly more severe than the standard cellulosic ISO 834 curve and is widely applied in tunnel fire protection design for concrete linings, and passive systems to assess spalling, reinforcement overheating, and structural integrity under realistic high-HRR hydrocarbon fires (50–300 MW). Variants include the modified/increased HC (HCM scaled to 1,300°C) used in some European tunnel standards. It appears frequently in the tunnel design guideline documents (e.g., ITA Report, CETU guide, alongside comparisons to RWS, RABT/ZTV, and ISO curves for risk-based tunnel classification and testing.
This post explores hydrocarbon fire curve in the context of tunnel fire protection. HC curve is also used in other areas like petrochemical plants, oil rigs or transformer rooms.
The HC curve originated in the 1970s–1980s for petrochemical, oil & gas, and refinery facilities, where pool fires from process equipment, storage tanks, piping, or spills are a primary risk. It is the foundation of the UL 1709 / ASTM E1529 standard (“Rapid Rise Fire Tests of Protection Materials for Structural Steel”).
Fire board for steel fire protection under hydrocarbon fire curve:
Aestuver T fire board for hydrocarbon fire curve
Aestuver T fire board is a great choice in fire protection of structural steel under HC curve. This board was tested by Efectis according to EN 1363-2 and EN 13381-4. It’s main application is steel beam and column enclosure under hydrocarbon fire curve.
The testing proves that Aestuver T is suitable for HC fire scenarios with fire ratings: R30, R60, R90, R120, R150, R180 and R210. 240 minutes fire protection under HC curve can be provided on request.
It’s a typical steel enclosure system for fire protection with a single layer of Aestuver T fire board. Thickness ranges from 30mm to 60mm with 10mm steps. Boards are fixed to each other with screws in the corners.
As with all Aestuver fire boards, Aestuver T can be used externally. The T board is resistant to water, moisture, freezing, thawing, UV radiation, soaking and drying. All these properties are confirmed in the ETA-15-0531:
More about specific A/V ratios and fire rating can be found here – Beam and column encasement under hydrocarbon fire curve.
Hydrocarbon curve in tunnel fire scenarios
You’ll find the HC curve referenced all over tunnel design literature. The PIARC (World Road Association) technical reports and the ITA (International Tunnelling and Underground Space Association) Working Group 6 guidelines both highlight it as a key exposure for testing passive fire protection in road tunnels.
European standards such as EN 1991-1-2 and EN 1992-1-2 use the hydrocarbon curve (and its modified HCM version) alongside the more severe RWS curve when assessing concrete linings. NFPA 502 in the US also points to hydrocarbon-type exposures for tunnels carrying flammable liquids. Researchers like Haukur Ingason and the UPTUN project team have repeatedly shown in large-scale tests that the HC curve closely matches the temperature profiles of real vehicle fires in confined tunnels. It’s become the go-to benchmark because it forces designers to plan for the worst — not just the average — fire scenario.
Concrete Spalling under hydrocarbon fire curve
High-strength concrete is great for building tunnels, but it has a weakness. When it gets hot fast, the moisture trapped inside turns to steam. The pressure builds up quicker than it can escape, and suddenly chunks of concrete start exploding off the surface — that’s called spalling. It can happen at temperatures as low as 200 °C. Once the outer layer blows away, the steel reinforcement is exposed, heats up, loses strength, and the whole structure can be in serious trouble.
We’ve seen it in real incidents — Mont Blanc, the Channel Tunnel fires, and others. The damage isn’t just cosmetic; it can mean major repairs, long closures, and huge economic hits.
Why concrete spalling can be a severe phenomenon under the HC fire curve? Rapid temperature increase causes thermal shock on the concrete surface. Based on fire tests in testing laboratories, such spalling occurs very early into the test, in some cases even within the first 30 minutes.
Passive fire protection in hydrocarbon fire in tunnels
There are several methods of protecting tunnel concrete structures against devastating effects of hydrocarbon fire and spalling. Designers consider utilisation of fire resistant concrete mixes by adding polypropylene (PP) fibres into the mix. However, such mix needs to be fire tested in order to provide the actual fire resistance evidence.
What is more, application of PP fibres helps only once. In case of fire, PP fibres melt and the concrete area exposed to fire requires costly and time consuming repair. This can lead to long tunnel closures, detouring issues and potential loss of revenue from tolls.
Alternatively, tunnel owners may benefit from specifying installation of fire boards. Such boards in the form a secondary skin and work as a thermal barrier in tunnel protecting the main tunnel structure. In case of fire, the fire boards can be replaced fairly quickly during a couple of nightshifts and tunnel is ready for opening much earlier.
The full economic review of fire protection cladding in tunnels was presented in the research published together with STUVA. The paper concluded that damage caused by a single serious fire can be 15-20 times more expensive than the costs of installation of fire boards.
How Aestuver Tx Boards help in hydrocarbon fire scenario in tunnels
This is where Aestuver Tx passive fire protection comes in, lightweight cement-bonded, glass-fibre-reinforced boards made specifically for tunnels.
The boards are fixed directly to the tunnel walls and ceiling (either during new construction or as a retrofit). The fire boards act as a thermal shield. When tested under full HC, HCM, and even the tougher RWS fire curves, they keep the concrete surface well below the temperature where spalling kicks in — usually staying under 380 °C on the concrete face and keeping the rebar temperatures safely low (around 250 °C or less).
Aestuver Tx board have been tested for HC120 and HC180 fire protection. They’re stable at 1,100 °C — no cracking, shrinking, or falling off. They’re also tough enough to handle the everyday tunnel environment: road salts, exhaust fumes, pressure washing, and freeze-thaw cycles. At around 21 kg per square metre for a 25 mm board, they’re relatively easy and quick to install without closing the tunnel for weeks.
The HC curve isn’t some abstract lab thing — it’s the closest we can get to replicating the fire you’d actually face if a fuel tanker went up inside a tunnel. Using it as the benchmark for passive protection means the structure stays standing long enough for people to get out and firefighters to do their job, and it cuts down on the nightmare repair bill afterwards.
If you’re involved in tunnel design, refurbishment, or asset management, it’s worth taking a close look at how passive systems tested to the HC curve can fit into your next project.
Do you need any support with selecting the right fire board for the hydrocarbon fire curve?
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References:
- International Tunnelling and Underground Space Association (ITA) Working Group 6 (2017). Structural Fire Protection For Road Tunnels. ITA Report No. 18, April 2017.
- Centre d’Études des Tunnels (CETU) (2017). Passive Fire Protection Systems – Justification of performance for road tunnel structures. Updated version, March 2017.
- Tarada, F. & King, M. (2009). Structural Fire Protection of Railway Tunnels. Railway Engineering Conference, University of Westminster, UK, 24–25 June 2009.
- Ingason, H. (2008/2009). State of the Art of Tunnel Fire Research (and related UPTUN project reports, 2006).
- Li, Y.Z. & Ingason, H. (2012). The maximum ceiling gas temperature in a large tunnel fire. Fire Safety Journal
- PIARC (World Road Association) – Multiple technical reports on tunnel fire safety, structural protection, and fire curves (various years, including recommendations on HC, HCM, and RWS curves), i.e. Design fire characteristics for road tunnels, Fire Resistance of structures.
The maximum temperature during the hydrocarbon fire reaches 1100°C – 2012°F
The rise time is extremely rapid and within approximately 30 minutes the hydrocarbon curve reaches it’s max. temp. of 1,100°C (2012°F)
Hydrocarbon fire curve is used to simulate hydrocarbon pool fires (e.g., petrol, diesel, or HGV loads in tunnels) in fire resistance design of structures.
It’s often used at petrochemical plants, transformer rooms containing oil or in tunnels.
EN 1363-2
ASTM E1529
UL 1709
The temperature raises quickly and hydrocarbon fire curve temperature at 5 minutes is ≈ 948 °C
Aestuver T fire board is used for hydrocarbon fire curve. It can be applied for fire protection of concrete and steel structures.
1 Comment
What are the design fire curves for tunnels? · 1 April 2026 at 1:17 pm
[…] in the 1970s, the hydrocarbon curve was initially developed to model fires in industrial and offshore settings, particularly […]