Intumescent Fire-Protective Coating for Structural Steel — Fire Rating per EN 13381-8, ASTM E119 in Thailand

Structural steel is strong at room temperature, but in a fire it loses about 50% of its strength at ~550°C and softens until the structure can collapse within minutes. This is why building-control law specifies a fire-resistance rating for primary structure — so people have time to escape and firefighters can respond.

Intumescent coating is the most popular passive fire protection for exposed steel because it is thin, aesthetic, and follows the steel profile. This article explains the principle, the standards, and how to choose the right thickness.

1. The Principle — A Thin Paint That Swells into Insulation

An intumescent coating is a thin film (measured in microns) that, when heated, undergoes a chemical reaction and intumesces — expanding many times into a porous char layer that insulates:

• Delays the steel reaching its critical temperature (often ~550°C) within the required time
• Provides a rating in minutes: 30 / 60 / 90 / 120 / 180
• Thin and profile-following, unlike thick/heavy boards or spray cementitious (vermiculite)

2. Test and Certification Standards

Thailand work typically references EN 13381-8 or UL/ASTM results + states minutes per the Ministerial Regulation, with an engineer/specialist certifying.

3. The Heart of It — DFT Depends on Section Factor

flowchart TD
  A[Structural steel] --> B[Compute section factor Hp/A
per section]
  A --> C[Set required rating
30-120 min per regulation]
  B --> D[Open manufacturer loading table
EN 13381-8]
  C --> D
  D --> E[Get required DFT
per member]
  E --> F[Select fire curve
cellulosic vs hydrocarbon]
  F --> G[Apply + verify actual DFT
per member + topcoat if needed]

DFT (dry film thickness) is not fixed:

• Section factor (Hp/A) = heated perimeter / cross-section area — higher (thin steel) heats faster and needs more DFT
• Manufacturers provide a loading table from EN 13381-8 results to select DFT per section factor + minutes
• Verify actual DFT on every member — under-applied = fails the rating

4. Choose the Fire Curve to Match the Risk

• Cellulosic fire (ASTM E119/EN 1363) — fire from typical building contents → offices/warehouses/general factories
• Hydrocarbon fire — petroleum-fuel fire, far hotter/faster → refineries/petrochemical/offshore require a hydrocarbon-grade intumescent

Choosing the wrong curve = no real protection in that building's fire event.

5. Fire-Coating Spec Checklist

1. State the rating (minutes) per regulation/building type
2. Choose the fire curve (cellulosic vs hydrocarbon)
3. Compute section factor per section → set DFT from the loading table
4. Surface prep + primer compatible with the intumescent
5. Verify actual DFT per member + keep test results/cert for acceptance

We supply and coordinate intumescent fire-protection systems for structural steel — computing section factor → DFT from the loading table (EN 13381-8), selecting the fire curve to suit the building type, compatible primers, and verifying actual DFT with test results for acceptance and permitting.

Talk to our engineering team to set passive fire protection that meets the requirements — call 02-096-2118 or LINE OA @406rrgvm.

Summary

• Steel loses ~50% strength at ~550°C → law specifies a fire-resistance rating for structure
• Intumescent = a thin paint that swells into an insulating char, delaying heat, giving 30-120 minute ratings
• Standards: EN 13381-8 (assessment) + ASTM E119/UL 263 (test)
• DFT depends on section factor (Hp/A) + minutes from the manufacturer's loading table — verify per member
• Choose cellulosic vs hydrocarbon curve to match the building's risk

Fire coating is not "just paint" — it is an engineered system with per-member DFT calculation, accepted on test evidence.