Combustion analysis of thermal insulation materials is a critical fire safety requirement for metro and subway systems, where confined tunnels, high passenger density, and limited evacuation routes demand strict control of flammability, smoke production, and toxic gas generation. This technical article explains the sample types, testing items, standard test methods, and acceptance criteria used by accredited third-party laboratories to qualify insulation materials for underground rail applications. It covers the GB 8624 classification framework along with international ISO, EN, ASTM, UL, and JIS test procedures relevant to metro engineering and material certification.
Technical Overview
Metro systems operate in confined underground environments where fires develop rapidly, smoke accumulates quickly, and evacuation paths are long and heavily constrained. Thermal insulation materials are installed extensively throughout these systems, including on heating, ventilation and air-conditioning (HVAC) ducts, chilled-water and heating pipelines, tunnel walls and ceilings, platform screen door assemblies, and cable management routes. Because these materials cover large surface areas, they can represent a significant portion of the potential fuel load. If they ignite, they may contribute to flame propagation, sustained heat release, dense smoke, and the generation of lethal gases such as carbon monoxide and hydrogen cyanide, all of which directly threaten passenger life safety.
Combustion analysis of metro thermal insulation materials is the systematic, standards-based evaluation of their fire behavior in laboratory conditions. Its purpose is to verify compliance with the fire protection provisions of metro design codes, to classify the burning behavior of the material, and to provide quantitative data on heat release, smoke density, and fire effluent toxicity. Classification and testing are typically performed by accredited third-party laboratories against frameworks such as GB 8624-2012 for building products, GB 50157 for metro design, EN 13501-1 for European fire classification, and EN 45545-2 for railway applications, supplemented by ISO, ASTM, UL, and JIS test methods.
- Ignitability: resistance of the material to small-flame ignition under end-use conditions
- Flame spread: tendency of combustion to propagate across exposed surfaces
- Heat release: contribution of the material to fire growth and flashover potential
- Smoke production: reduction of visibility along tunnels and escape routes
- Effluent toxicity: generation of incapacitating or lethal combustion gases
- Dripping behavior: formation of flaming droplets or particles that spread fire downward
Testing Samples
Elastomeric (Rubber-Plastic) Foam Insulation
Closed-cell nitrile-butadiene rubber (NBR) and PVC-blended flexible foams are the most widely used insulation for chilled-water pipes and air ducts in metro stations and tunnels. Typical densities range from 40 to 80 kg/m³ with wall thicknesses of 9 to 50 mm. Their organic polymer matrix makes combustion performance the primary qualification parameter, and B1-grade products are normally required to demonstrate a limiting oxygen index of 32 percent or higher.
Phenolic Foam Insulation
Closed-cell phenolic foam offers low thermal conductivity combined with a high char yield during burning, which limits flame spread and smoke generation. It is commonly used on HVAC ductwork where both fire classification and energy efficiency are specified, and it is tested in the as-supplied board or pipe-section format including any facing film.
Polyurethane and Polyisocyanurate (PIR) Foams
Sprayed and board-form polyurethane or PIR foams provide high insulation value but contain combustible organic content that requires flame-retardant modification. Combustion analysis for these materials focuses heavily on smoke density and toxic effluent behavior in addition to flame spread classification.
Glass Wool and Rock Wool (Mineral Wool)
Inorganic fiber insulation products are essentially non-combustible and are targeted at Grade A applications such as tunnel lining systems, fire-rated ducts, and critical escape route protection. Their qualification relies on the non-combustibility furnace test and gross heat of combustion determination rather than flame spread testing.
Sample Preparation and Conditioning
Specimens are cut to the exact dimensions prescribed by each test method and are representative of the production batch, thickness, density, and facing condition as installed. Where adhesives, jackets, or composite laminates form part of the end-use system, the complete build-up is tested. All specimens are conditioned to constant mass at 23 ± 2 °C and 50 ± 5 percent relative humidity in accordance with ISO 291 prior to testing.
Testing Items
Accredited third-party laboratories evaluate metro insulation materials against the following combustion performance parameters, and the combined results determine the final classification used for project acceptance.
- Combustion performance classification under GB 8624-2012 (Grades A, B1, B2, B3) or EN 13501-1 (Euroclasses)
- Single burning item parameters: FIGRA (fire growth rate index), THR600s (total heat release), SMOGRA (smoke growth rate index), TSP600s (total smoke production)
- Ignitability under small-flame impingement, including flame spread distance and flaming duration
- Limiting oxygen index (LOI) as a quality-control and formulation indicator
- Smoke density rating (SDR) and specific optical density (Ds) in flaming and non-flaming modes
- Heat release rate, peak heat release rate, total heat release, and MARHE by cone calorimeter
- Gross heat of combustion (PCS) and non-combustibility parameters for Grade A determination
- Toxicity of fire effluents, expressed as AQ grading or the conventional index of toxicity (CIT)
- Formation of flaming droplets and particles within the observation period
- After-flame time, after-glow time, and residual mass loss
- Vertical and horizontal burning characteristics, including UL 94 ratings for component-level assessment
Testing Methods
Classification Framework Under GB 8624-2012
GB 8624-2012 establishes a hierarchical classification for building materials: Grade A (sub-grades A1 and A2) for non-combustible and limited-combustibility products, Grade B1 (sub-grades B and C) for flame-retardant products, Grade B2 for ordinary combustible products, and Grade B3 for readily combustible products. Additional classifications cover smoke production (s1 to s3) and flaming droplets (d0 to d2). Metro technical specifications generally require insulation to be Grade B1 or above, with smoke class s1 and droplet class d0, together with an AQ1 smoke toxicity grade.
Single Burning Item (SBI) Test
Performed to GB/T 20284 or EN 13823, the SBI test exposes an L-shaped specimen assembly consisting of a 1000 mm × 1500 mm long wing and a 495 mm × 1500 mm short wing to a 30 kW propane burner. Exhaust gas analysis yields FIGRA, THR600s, SMOGRA, and TSP600s. For Grade B, typical acceptance limits are FIGRA not exceeding 120 W/s and THR600s not exceeding 7.5 MJ; for Grade C, FIGRA not exceeding 250 W/s and THR600s not exceeding 15 MJ. Smoke class s1 corresponds to SMOGRA not exceeding 30 m²/s² with TSP600s not exceeding 50 m², and d0 requires the absence of flaming droplets within 600 seconds.
Small-Flame Ignitability Test
Conducted to GB/T 8626 or EN ISO 11925-2, this test applies a 15-second or 30-second diffusion flame to vertically or horizontally oriented specimens. The material passes when the flame tip does not reach 150 mm above the ignition point within the prescribed time and when flaming droplets are absent or self-extinguishing, supporting the B, C, D, and E classification levels and the droplet sub-classification.
Smoke Density Testing
The smoke density chamber test to GB/T 8627 measures the smoke density rating (SDR) of the material under flaming and non-flaming decomposition, with B1 elastomeric foams commonly required to achieve an SDR of 75 or lower. The equivalent ASTM E662 method reports maximum specific optical density (Ds max) and is frequently specified for projects referencing North American criteria. For EN 45545-2 compliance, EN ISO 5659-2 provides the smoke generation data used in both opacity and toxicity assessment.
Cone Calorimeter Testing
ISO 5660-1 testing at a 50 kW/m² irradiance level uses 100 mm × 100 mm specimens to quantify time to ignition, peak and average heat release rate, total heat release, MARHE, and smoke production rate. These results feed directly into EN 45545-2 requirement sets for railway components and provide essential input data for fire modeling of tunnel and station scenarios.
Limiting Oxygen Index Testing
The oxygen index test to ISO 4589-2, GB/T 2406.2, or JIS K 7201 determines the minimum oxygen concentration required to support candle-like combustion. It serves as a sensitive quality-control indicator for flame-retardant formulation consistency, with B1 elastomeric insulation products typically specified at LOI values of 32 percent or greater.
Toxicity of Fire Effluents
GB/T 20285 evaluates the toxic hazard of combustion products and assigns safety grades from AQ1 (safest) to AQ3 (dangerous), with metro specifications requiring AQ1. For European rail projects, EN 45545-2 combines EN ISO 5659-2 smoke generation with FTIR gas analysis to calculate the conventional index of toxicity (CIT), which must remain below limits defined for hazard levels HL1 through HL3.
Non-Combustibility and Heat of Combustion
Grade A qualification relies on the non-combustibility furnace test to ISO 1182 or GB/T 5464, which measures furnace temperature rise, mass loss, and sustained flaming at 750 °C, together with the bomb calorimeter determination of gross heat of combustion to ISO 1716 or GB/T 14402, where PCS values of approximately 2 to 3 MJ/kg or lower are required for A1 and A2 classification. Supplementary methods such as ASTM E84 or UL 723 flame spread and smoke developed indices, and UL 94 vertical burning ratings, may be applied for international project specifications.
Applications
Combustion analysis of metro insulation materials supports every phase of the asset lifecycle, from design and procurement through operation and renewal. Typical engineering applications include:
- Design-stage material selection and fire safety compliance verification for new metro lines and extensions
- Supplier qualification, factory audit programs, and batch conformance testing prior to installation
- Construction acceptance testing and third-party certification reports for handover documentation
- Periodic fire safety audits and re-evaluation of aged or weathered insulation installed in tunnels and stations
- Research and development of low-smoke, halogen-free, and highly flame-retardant insulation formulations
- Input data for computational fire dynamics modeling of tunnels, platforms, and evacuation scenarios
- Post-incident investigation and failure analysis following fire events or abnormal thermal exposure
- Export compliance where projects require parallel classification under GB, EN, ISO, ASTM, UL, or JIS frameworks
FAQ
What combustion classification is required for insulation materials in metro projects?
Under GB 50157 and typical metro technical specifications, insulation installed in tunnels, stations, and ventilation systems must achieve Grade B1 (flame retardant) or higher under GB 8624-2012, normally with smoke class s1, droplet class d0, and AQ1 smoke toxicity under GB/T 20285. Grade A materials such as rock wool are preferred for tunnel lining, fire-rated ductwork, and critical escape routes where the highest level of fire performance is specified.
Why are smoke density and toxicity emphasized so strongly in underground rail environments?
In the confined volume of a tunnel or station, a small quantity of burning material can rapidly reduce visibility to near zero and raise carbon monoxide and hydrogen cyanide concentrations to incapacitating levels before evacuation is complete. Historical underground rail fires have demonstrated that the majority of casualties result from smoke and toxic gases rather than direct flame contact, which is why smoke and toxicity criteria are often the controlling acceptance limits for insulation materials.
How many samples are required and how long does testing take?
Sample quantities depend on the test scope. A full B1 classification typically requires three complete SBI specimen wing sets, multiple smoke density chamber specimens, approximately fifteen oxygen index bars, and additional specimens for toxicity and small-flame testing. Including conditioning, a complete GB 8624 assessment is usually completed within two to four weeks by accredited third-party laboratories, who confirm exact specimen schedules based on the product configuration.
Can installed or aged insulation materials be re-tested?
Yes. Field samples can be cut from installed systems and re-evaluated for classification, oxygen index, and smoke performance. Aging, moisture exposure, and thermal cycling can deplete or migrate flame-retardant additives over time, so periodic re-verification is recommended as part of metro fire safety management and asset renewal programs.
How does the GB 8624 classification compare with EN 13501-1?
The two frameworks are closely harmonized in methodology, both using the single burning item test, small-flame ignitability, non-combustibility furnace, and calorimetric methods, and both applying smoke (s) and droplet (d) sub-classifications. GB 8624-2012 additionally integrates the AQ smoke toxicity grading that metro specifications in China require. International projects may require parallel reporting under both systems, plus EN 45545-2 requirement sets for rolling stock and fixed railway installations.