Combustion Performance Analysis of Thick-Type Fireproof Coatings: Testing, Standards, and Applicatio

Thick-type (non-intumescent) fireproof coatings are cementitious protective systems applied to steel structures at dry film thicknesses of roughly 7–50 mm, achieving fire resistance ratings of up to 3.0 hours through thermal insulation rather than chemical expansion. This article provides a professional analysis of their combustion performance, covering composition and protective mechanisms, test specimens, evaluation items, standardized test methods under GB, ISO, ASTM, EN, and UL frameworks, and typical engineering applications. It is intended as a technical reference for specifiers, coating manufacturers, and fire safety engineers.

Technical Overview

Thick-type fireproof coatings, also referred to as non-intumescent or cementitious fire-resistive coatings, are passive fire protection materials designed primarily for structural steel members. Unlike thin and ultra-thin intumescent coatings, which swell to form an insulating char at temperatures of a few hundred degrees Celsius, thick-type coatings protect steel by physical means: the applied layer itself acts as a durable, low-conductivity thermal barrier. In the classification framework of the Chinese national standard GB 14907, they correspond to the non-expansion type, with typical dry film thicknesses in the range of approximately 7 mm to 50 mm, and achievable fire resistance ratings from 0.5 h up to 3.0 h depending on coating thickness and the section factor of the protected member.

The typical formulation consists of an inorganic binder system, such as Portland cement, calcium aluminate cement, or gypsum; lightweight aggregates such as expanded vermiculite or perlite; reinforcing fibers; and functional additives. During fire exposure, the coating protects steel through a combination of low thermal conductivity, endothermic release of chemically bound and free water, and the formation of a stable porous ceramic-like residue that resists cracking and spalling. Because the organic content of these systems is negligible, most qualified thick-type coatings are classified as Class A (non-combustible) building materials under GB 8624, equivalent to classes A1 or A2 under EN 13501-1.

Combustion performance analysis of these coatings therefore has two complementary objectives. The first is to verify reaction-to-fire behavior, confirming that the material does not ignite, flame, or contribute heat to a developing fire. The second is to verify fire-resistance performance, demonstrating that the coating delays the temperature rise of the protected steel so that load-bearing capacity is maintained for the required rating period under standardized furnace conditions.

• Governing product standards: GB 14907 (fire resistive coating for steel structures), GB 8624 (classification for burning behavior of building materials), EN 13501-1
• Fire-resistance test standards: GB/T 9978 series, ISO 834 series, ASTM E119, UL 263, EN 1363-1, EN 13381-4, JIS A 1301
• Reaction-to-fire test standards: GB/T 5464 / ISO 1182 (non-combustibility), ISO 1716 (gross heat of combustion), GB/T 20284 / EN 13823 (single burning item), ISO 5660 (cone calorimeter, screening)

Testing Samples

Coated Steel Beam Specimens

Beam specimens typically comprise rolled or welded I-section beams of structural grades such as Q235 or Q355, with test spans commonly in the range of 4.5 m to 6.0 m. The coating is spray-applied to the specified nominal thickness on all exposed surfaces, including edges and corners where coating geometry must be maintained. Specimens are conditioned for the manufacturer-declared curing period before testing. In loaded configurations, beams are subjected to four-point bending or uniformly distributed loading consistent with the design limit state, allowing both the insulation performance and the load-bearing capacity of the protected member to be evaluated simultaneously.

Coated Steel Column Specimens

Column specimens, generally approximately 3 m in height, may be H-shaped, box-shaped, or circular hollow sections. They are coated over the full exposed perimeter and tested under axial compression. Column tests are particularly important for thick-type coatings because the required thickness is strongly influenced by the section factor, defined as the heated perimeter divided by the cross-sectional area of the steel member; slender sections with high section factors require thicker coatings to achieve the same rating.

Flat Panels and Molded Coupon Specimens

Flat panels and molded coupons are prepared for the full range of physical and durability characterization. These include composite panels for bond strength (adhesion) testing, cubes or cylinders for compressive strength, slabs for thermal conductivity measurement, and sheets for initial drying crack resistance evaluation. Cementitious systems are typically conditioned for 7 to 28 days in a standard atmosphere before testing to ensure full hydraulic curing.

Combustion Classification Specimens

For reaction-to-fire testing, specimens are prepared in the geometries required by each method: cylindrical specimens of approximately 45 mm diameter and 50 mm height for the non-combustibility furnace test per GB/T 5464 and ISO 1182, and pressed pellets or powdered samples for oxygen bomb calorimetry per ISO 1716. Where a classification route requires it, larger panel specimens are prepared for single burning item testing per GB/T 20284 and EN 13823.

Testing Items

The complete evaluation program for thick-type fireproof coatings, as commonly specified in GB 14907 and equivalent international frameworks, integrates reaction-to-fire, fire-resistance, mechanical, thermal, and durability items:

• Combustion classification and non-combustibility, including gross heat of combustion and furnace temperature-rise, mass-loss, and flaming-duration criteria
• Fire resistance under the standard temperature-time curve, covering integrity, insulation, and load-bearing capacity for ratings of 0.5 h, 1.0 h, 1.5 h, 2.0 h, 2.5 h, and 3.0 h
• Bond strength (adhesion) between the coating and the steel substrate, commonly required to be at least on the order of 0.04 MPa for thick systems, verified by pull-off testing before and after durability conditioning
• Compressive strength of the cured coating matrix, typically specified at levels on the order of 0.3 MPa or higher
• Dry density, generally limited to values in the range of 500 kg/m³ or below for lightweight insulating grades
• Thermal conductivity at reference and elevated temperatures, commonly limited to approximately 0.1 to 0.12 W/(m·K) at 40 °C for qualifying products
• Water resistance under prolonged immersion, verified by absence of cracking, delamination, or loss of adhesion
• Freeze-thaw cycling resistance, typically assessed over multiple cycles between sub-zero and ambient conditions
• Chemical resistance to acidic and alkaline solutions, and salt-fog resistance for outdoor-grade products
• Initial drying crack resistance, appearance in container, drying time, and application workability
• Optional screening of heat release rate and smoke production using cone calorimetry per ISO 5660

Testing Methods

Fire resistance testing is performed in gas- or oil-fired horizontal or vertical furnaces per GB/T 9978, ISO 834, ASTM E119, UL 263, or EN 1363-1. The furnace atmosphere follows the standard temperature-time curve defined by the logarithmic relationship T = 345 log10(8t + 1) + 20, where T is the furnace temperature in degrees Celsius and t is the elapsed time in minutes. This curve reaches approximately 841 °C at 30 minutes and approximately 945 °C at 60 minutes, and furnace control thermocouples must keep the measured curve within the tolerances specified by each standard. Sheathed or embedded thermocouples monitor the steel substrate temperature at multiple positions, while displacement transducers track deflection or axial deformation of loaded specimens.

Failure criteria differ slightly between standards. In loaded tests, failure occurs when the member can no longer sustain the applied load, manifested by excessive deflection, deflection rate, or axial contraction. In unloaded tests, failure is defined by reaching the limiting steel temperature, typically an average of about 538 °C with a point maximum of about 649 °C under ASTM E119 and UL 263, with comparable temperature limits applied under GB/T 9978 and EN 1363-1. For petrochemical and offshore exposures, coatings may additionally be qualified against hydrocarbon fire curves, such as the hydrocarbon curve of EN 1363-2 or the rapid temperature rise regime of UL 1709, in which furnace temperatures approach 1100 °C within minutes.

Reaction-to-fire behavior is established through a tiered approach. Non-combustibility testing per GB/T 5464 and ISO 1182 stabilizes cylindrical specimens in a 750 °C furnace and evaluates furnace temperature rise, mass loss, and sustained flaming duration. Gross heat of combustion per ISO 1712-equivalent bomb calorimetry confirms that the calorific potential remains below the limit, commonly 2.0 MJ/kg, associated with the highest classification classes. Where required, single burning item testing per GB/T 20284 and EN 13823 quantifies fire growth rate, lateral flame spread, and smoke production for the definitive classification under GB 8624 or EN 13501-1.

Physical and thermal properties are measured using established laboratory techniques. Bond strength is determined by direct tensile pull-off on coated steel panels. Compressive strength is measured on molded cubes or cylinders using a constant-rate compression machine. Thermal conductivity is measured by the guarded hot plate method per GB/T 10294 and ISO 8302, or the heat flow meter method per GB/T 10295 and ISO 8301. Durability evaluation subjects specimens to immersion, freeze-thaw, and chemical conditioning sequences, after which bond strength and, where required, fire resistance are re-verified to confirm retained protective performance. All testing is performed by accredited third-party laboratories, with reported results including coating thickness, steel temperature histories, failure mode, and the achieved fire resistance period.

Applications

Thick-type fireproof coatings are selected wherever long fire resistance durations, high durability, or exposure to severe environments make intumescent systems less suitable. Their principal application areas include:

• High-rise and long-span steel buildings, including airports, stadiums, and exhibition centers, where ratings of 2.0 h to 3.0 h are frequently demanded for columns and transfer members
• Petrochemical and refinery structures, where outdoor-grade coatings qualified against hydrocarbon fire curves protect pipe racks, vessel supports, and structural frames
• Power generation facilities, including boiler house steelwork and turbine pedestals, where mechanical robustness and moisture resistance are essential
• Tunnel and underground works, where dense, impact-resistant grades resist washing, vibration, and freeze-thaw action
• Offshore platforms and marine structures, where salt-fog and cyclic wet-dry durability governs coating qualification
• Retrofit and repair projects, where thick coatings restore or upgrade the fire resistance of existing steelwork with minimal surface preparation

In design practice, the required coating thickness is derived from the member section factor and the target fire resistance period, in accordance with structural fire design codes such as GB 50016, GB 51249, and EN 1993-1-2, and confirmed by furnace test data or listing documents associated with the qualified product system.

FAQ

What is the difference between thick-type, thin-type, and ultra-thin fireproof coatings?

Thick-type coatings are non-intumescent, inorganic systems applied at 7 mm to 50 mm that insulate steel through their inherent low thermal conductivity and thermal mass. Thin and ultra-thin coatings are intumescent systems applied at fractions of a millimeter to a few millimeters; they rely on chemical expansion to form an insulating char during fire. Thick systems generally achieve longer ratings and superior durability, while intumescent systems offer lower weight and architectural finish advantages.

Are thick-type fireproof coatings themselves combustible?

Qualified thick-type coatings are classified as Class A non-combustible materials under GB 8624, equivalent to class A1 under EN 13501-1, because their inorganic binder and aggregate content results in negligible calorific value and no sustained flaming in the ISO 1182 furnace test. They neither ignite nor contribute heat to a fire.

How is the required coating thickness determined?

Thickness is determined from the fire resistance target, the steel section factor, and the validated performance data of the specific product. Members with higher section factors heat faster and require thicker coatings. Qualified products publish thickness tables for each rating period, validated by furnace testing at accredited third-party laboratories.

Do thick coatings require maintenance or re-inspection?

Yes. Although properly formulated systems are highly durable, periodic inspection is recommended to check for mechanical damage, cracking, water ingress, or corrosion beneath the coating. Damaged areas should be repaired with the original or a compatible qualified system, and bond strength spot checks can be used to verify long-term adhesion.

Can thick-type fireproof coatings be used outdoors?

Yes, provided the product is qualified as an outdoor type. Outdoor grades must demonstrate resistance to water immersion, freeze-thaw cycling, chemical exposure, and salt-fog corrosion, and in hydrocarbon exposure scenarios must additionally satisfy the relevant rapid temperature rise fire curves such as UL 1709.