Why concrete needs protection
Concrete does not burn and in itself resists fire reasonably well. The fire resistance of a reinforced concrete member is governed mainly by how quickly the main reinforcement heats up, and that depends on the depth of the concrete cover. If the cover is sufficient, no additional fire protection is needed. In practice, however, there are quite a few situations where it has to be added:
- refurbishment and change of use of a building, where the fire resistance requirements have increased;
- where a survey has shown the actual cover to be less than designed;
- hollow-core and thin-walled slabs, ribbed floors and precast units with minimal cover;
- tunnels, underground car parks and plant rooms, where an intense fire is possible;
- composite slabs on profiled steel decking.
What happens to concrete in a fire
When heated, reinforcement loses strength like any other steel, and the concrete cover is its only shield. Concrete is also prone to explosive spalling: moisture in the pores turns to steam, pressure builds up and the surface layers break away, exposing the reinforcement. The risk is higher for dense and high-strength concretes, for wet members and where the temperature rises rapidly. A fire protection coating tackles both problems: it slows down heating and reduces the temperature gradient at the surface, lowering the risk of spalling.
Materials used
Boards
Calcium silicate boards are mechanically fixed to the soffits of slabs and beams. This is a predictable solution with a factory-controlled thickness, well suited to tunnels and car parks. Mineral wool boards are used where thermal insulation is needed as well — for example, on soffits above unheated spaces.
Spray-applied plasters
Cement-vermiculite and gypsum-vermiculite compounds are spray-applied to the concrete; smooth surfaces sometimes require a mesh or a bonding primer. For more detail, see the “Fire protection plasters” page.
Intumescent paints
Some thin-film paints have approved solutions for concrete. They are used where a relatively small increase in fire resistance is needed and the appearance of the surface must be preserved — for example, on floor slabs in occupied buildings.
How to choose a solution
- Required rating. For floor slabs this is usually REI, for beams and columns R. The required value comes from the design.
- Actual cover and concrete strength class. These are established from the design documents or a survey. The smaller the cover, the greater the need for additional protection.
- Type of member. Solid or hollow-core slab, ribbed floor, beam, column, tunnel lining — manufacturers have specific solutions for each.
- Fire scenario. Tunnels and facilities handling petroleum products call for materials tested against more severe time–temperature curves.
- Environment and access. Humidity, vibration, fumes, and whether wet trades can be carried out in an operational building.
- Weight and headroom. In car parks and basements every centimetre of headroom counts, which favours thinner solutions.
Common mistakes
- Applying fire protection without assessing the actual cover — the calculation then has no basis.
- Using materials tested only on steel for concrete structures.
- Plaster applied to a dusty, oily or painted surface without preparation.
- Board fixings not specified by the system — in a fire, the boards come away.
- Ignoring the risk of explosive spalling in high-strength concrete.
Materials from leading manufacturers
- PROMATECT-H — Promat — calcium silicate boards for fire protection of concrete structures, shafts and partitions.
- PROMASPRAY-P300 — Promat — vermiculite-gypsum plaster for concrete, steel and profiled steel decking inside buildings.
- Promat FENDOLITE MII — Promat — cement-vermiculite coating that protects concrete, including against explosive spalling.
- CAFCO 300 — Isolatek International — gypsum-based spray-applied fire protection for floors and roofs.
- Pyrocrete 241 — Carboline — cementitious fire protection, also used to upgrade the fire resistance of concrete.
- Nullifire SC804 — Nullifire (Tremco CPG) — water-based intumescent paint with solutions for concrete elements.
- Perlifoc T — Perlita y Vermiculita (Carboline) — cementitious plaster for the fire protection of concrete in tunnels.
- PROMATECT-T — Promat — fire protection board for tunnel linings and concrete structures, up to RWS curve.
- Sikacrete-213 F — Sika — cementitious fire protection mortar for concrete and tunnels, wet spray or hand applied.
- HENSOMASTIK B 3000 — Rudolf Hensel — water-based ablative coating upgrading concrete beams and columns up to 120 minutes.
- AESTUVER Tx — AESTUVER — glass-fibre reinforced lightweight concrete board protecting tunnel linings, RWS and HCM curves.
- PROMATECT-TF-X — Promat — monolithic calcium silicate board for tunnel linings, RWS and MOAC curves.
Frequently asked questions
Can the concrete cover simply be increased instead?
In new construction, often yes — this is resolved at the design stage. On existing structures, additional concrete cover adds weight and is not always technically feasible, so boards or plasters are used.
What is explosive spalling?
The break-up of the surface layer of concrete under rapid heating, caused by steam pressure in the pores. It exposes the reinforcement and sharply reduces fire resistance.
Can a paint designed for steel be used on concrete?
Only if it has an approved solution specifically for concrete. Steel and concrete heat up differently, and test data obtained on steel cannot be transferred to concrete.