How Intumescent Coatings Work: Passive Fire Protection for Structural Steel

Intumescent coatings look broadly like a conventional painted finish in normal conditions. When exposed to heat, the coating reacts and expands to form an insulating char. That char slows the transfer of heat to the steel beneath.

The coating does not make steel fireproof, and it does not create the same performance on every steel member. It is one component of a project-specific passive fire-protection design.

Why structural steel may need protection

Steel is non-combustible, but its strength and stiffness reduce as its temperature rises. The fire engineer and structural designer assess the building, the load on each element and the fire scenario to establish the required fire-resistance performance and limiting steel temperature.

Where protection is required, the design may use intumescent coating, boards, sprayed non-reactive materials or another suitable method. The right solution depends on the structure, environment, appearance, access and evidence available for the selected system.

What happens when an intumescent reacts

An intumescent coating contains components that react in sequence when heated. The reaction creates an expanded carbon-rich char with low thermal conductivity. The char acts as a barrier, slowing the rate at which heat reaches the steel.

The timing and amount of expansion vary between products. Performance is established through testing and assessment of the specific coating system, not from the general description of how intumescence works.

SteelConstruction.info's guide to structural steel fire protection describes intumescent coatings and the other common forms of passive protection used for steelwork.

Why there is no universal coating thickness

The coating build required for a steel member depends on connected design inputs, including:

• The required fire-resistance period
• The steel member and its section factor
• The number of sides exposed to fire
• The limiting or design temperature
• The tested performance range of the selected product
• The environmental exposure and any topcoat requirements

The coating manufacturer's loading tables or design software use these inputs to provide a thickness for the selected tested system. A thickness taken from another product, another member or a generic online example should not be transferred to the project.

The coating works as a system

The visible intumescent layer is only part of the assembly. Surface preparation, the primer, the intumescent product and any topcoat must be compatible and applied within the manufacturer's limits.

Primer identity and thickness can be important. A topcoat may be required for appearance or environmental protection, but it must be approved for the chosen intumescent system. Exposure during construction and in service also needs to be considered.

Application and inspection

Intumescent coatings are commonly spray-applied in controlled passes. Application conditions, recoat intervals and maximum thickness per pass are product-specific. The applicator follows the approved specification and current manufacturer data.

Quality checks may include substrate condition, ambient conditions, material batch details and wet and dry film measurements. The inspection plan and acceptance criteria should be agreed for the project. Independent inspection may also be required by the contract or approval process.

The Association for Specialist Fire Protection technical documents provide recognised industry guidance covering the specification, application and inspection of structural steel protection systems.

Newlook's application role

Newlook Protective Coatings prepares and applies intumescent coatings on site for structural steel projects. A firm scope depends on receiving the project-specific fire-protection design, steel schedule, selected product system, primer details, exposure and inspection requirements.

Those design inputs must come from or be approved by the responsible project team and coating manufacturer. Newlook's application work does not replace fire-engineering or structural-design responsibility.

For application enquiries, see Newlook's Intumescent Coatings service.