Fire-induced spalling in dense, low-permeability concrete is a vapour-pressure-driven failure mode. When the cementitious matrix has a water/cement ratio at or below 0.42 and a compressive strength class of C60/75 or higher, capillary porosity is insufficient to vent steam generated at the heated face. Sika Fibermesh 150F, dosed as a monofilament micro-synthetic addition at 1.0–3.0 kg/m³, introduces a temperature-sensitive percolation network. At approximately 160–170°C, the monofilaments melt and leave channels that permit pore pressure dissipation before the tensile capacity of the hot concrete is exceeded. This mechanism is addressed within EN 1992-1-2:2004, Clause 4.5, where spalling is evaluated separately from load-bearing resistance and thermal insulation. For bored tunnel segmental linings, the relevant thermal exposure is commonly the Rijkswaterstaat RWS curve, which reaches 1,350°C and remains above 1,200°C for a prolonged period. Concrete producers evaluate such linings not by calculation alone but by furnace testing because the spalling response is controlled by moisture content, aggregate type, section thickness, load level and heating rate.

Segmental ring production for TBM tunnels typically uses C50/60 to C70/85 concrete with silica fume at 5–10% by mass of cementitious material and a water/cementitious ratio between 0.30 and 0.38. The monofilament fiber is added either onto moist aggregate before cement and silica fume are introduced, or through a vibratory chute onto the wetted mix at the start of mixing. Field observations from carousel precast lines with twin-shaft compulsory mixers indicate that dumping entire bag loads into dry cement produces localized fiber clusters; sequential dosing through a vibratory feeder reduces this batch-to-batch variance. After mixing, the concrete is cast into steel segment moulds, compacted on external vibrators, and steam cured to achieve early demoulding. The terminal component is a precast ring segment with formed gaskets, bolt boxes and tolerance faces. Fire acceptance is generally based on loaded slab or panel specimens tested under the RWS curve in accordance with EN 1363-1:2020 and ISO 834-1:1999, with pass criteria covering specimen integrity, insulation and the absence of explosive spalling. The fiber addition must be verified after curing because prolonged exposure to high ambient temperatures during summer batching can reduce air content control and increase superplasticizer demand, which affects pumpability and mould filling.

Dosage ranges in the matrix are indicative and are derived from commonly published fire-test programmes for dense, low-permeability concretes. Project-specific verification is required because aggregate thermal expansion, moisture condition, section geometry and applied load all shift the spalling threshold independently of fiber addition.

Does 2 kg/m³ Addition Persist Through Extended Mixing and Pumping in High-Rise Core Concrete?

High-rise shear walls and column cores cast with C80/95 concrete combine low water/cementitious ratios, silica fume and high cementitious contents, producing the low permeability profile associated with explosive spalling under the ISO 834-1:1999 cellulosic curve. When 2.0 kg/m³ of monofilament micro-synthetic fiber is introduced, the principal processing concern is not initial slump loss but fiber distribution under long pumped deliveries and extended truck-mixer agitation. Polypropylene has a density near 0.91 g/cm³, which is lower than the paste fraction, and its monofilament geometry resists uniform dispersion if added too rapidly. Production-scale observations in high-rise core pours show that automated addition onto the aggregate conveyor, rather than direct addition into the truck drum, reduces visible fiber agglomeration and improves post-pumping uniformity. Extended agitation beyond 90 minutes at high concrete temperatures can increase air content and cause measurable slump reduction; the mix should be adjusted with a polycarboxylate superplasticizer rather than adding water. The interval between batching and pumping should be recorded, and the air content of the fresh concrete should be checked under EN 12350-7:2019. For fire performance, acceptance is normally based on loaded column or wall tests under ISO 834-1:1999, with limits on spalling depth, residual cross-section and unrestrained strain. Published data for this specific high-rise configuration is limited, so test panels should be conditioned to the same moisture content as the in-service core.

Long-span precast prestressed hollowcore units place prestressing strand in a thin lower flange where 28-day compressive strength often exceeds C60/75. The fire scenario is dominated by heat exposure to the soffit, and delamination of the lower flange can expose strand to temperatures above 400°C, at which point prestressing steel loses a significant portion of its cold-drawn strength. The addition of 1.5–2.0 kg/m³ monofilament micro-synthetic fiber assists in limiting this delamination by creating vapour-release channels in the dense matrix before pore pressure reaches the spalling threshold. In dry-cast hollowcore production, the mixing regime is shorter and the free-water content is low; fiber must be added to moving aggregate before cement slurry develops, otherwise dry clusters become trapped in the augers of the extruder. Slipform compaction frequency may require a reduced fiber addition at the lower bound of 1.5 kg/m³ where the concrete has a zero-slump consistency. The terminal products, including hollowcore slabs and prestressed beams, are cured on heated long-line beds and then cut to length. Fire testing is typically carried out under ISO 834-1:1999 or EN 1363-1, with acceptance based on maintaining cover to the strand and preventing fire-side spalling that would shorten the load path from strand to bearing.

Shotcrete Fire Linings for Existing Tunnels and the RWS Thermal Gradient

Wet-mix shotcrete used for fire protection linings in existing road or rail tunnels is pumped under pressure and sprayed onto rock or secondary lining surfaces. The mix design often specifies C50/60 concrete with silica fume, an alkali-free accelerator, and 2.0 kg/m³ of monofilament micro-synthetic fiber to address spalling under the severe RWS temperature-time curve. The exposed surface receives a near-instantaneous heat flux that creates a steep thermal gradient across the lining thickness. Polypropylene fibers closer to the heated face melt at 160–170°C, but those deeper within the section remain solid until the thermal front advances, which means the vapour-relief network forms progressively rather than simultaneously throughout the full thickness. In shotcrete application, fibre loss through rebound is a recognized uncertainty; the in-place dosage can differ from the batched dosage, and published data for specific in situ fiber retention in micro-synthetic shotcrete is limited. Trial panels should be sprayed, cored and tested for density and void structure. The finished lining may be finished by hand or left as-sprayed. Fire verification follows EN 1363-1 and EN 14488-3, with additional requirements for bond to the substrate under EN 14488-4. The use of alkali-free accelerators does not degrade polypropylene fiber at the dosage relevant to fire protection, but compatibility with air-entraining admixtures and pumping aids must be confirmed in the actual mix because air void spacing factor can shift after long pump campaigns.

When Hydrocarbon Pool Fires Govern the Design of Industrial Slabs and Secondary Containment

Industrial slabs exposed to hydrocarbon pool fires present a lower compressive strength class than tunnel linings, but the thermal shock can be severe because a retained pool of burning liquid raises the surface temperature within a very short time. Secondary containment areas in petrochemical plants frequently use C40/50 concrete with a water/cementitious ratio below 0.45, and the slab may be saturated during operational leaks. Sika Fibermesh 150F can be incorporated at 1.5 kg/m³ to create meltable channels that reduce the risk of shallow surface spalling and delamination. The mechanism is the same as in high-strength concrete, but the driving vapour pressure is lower because the matrix is less dense; however, the high moisture content of a wetted slab can offset that difference. The fiber does not affect the cast-in-place finishing of a power-floated industrial floor if added as part of a properly sequenced batch. Hydrocarbon fire testing for such applications may use the hydrocarbon curve in EN 1991-1-2, and the acceptance criterion is usually the absence of progressive spalling at the exposed surface. A limitation is that polypropylene fibers do not improve the abrasion resistance of the finished slab, and their use in secondary containment should be accompanied by verification of crack widths and coating adhesion if a chemical-resistant coating is applied after curing. Because the slab is often unreinforced or lightly reinforced, spalling control also supports the passive fire resistance of supporting steel columns and vessel skirts located within the same pool-fire zone.

Multi-storey parking structures use C60/75 concrete in double-tee beams, spandrels and load-bearing precast columns. Vehicle fires in open or partially enclosed decks are less severe than hydrocarbon tunnel curves, and the governing fire test is usually the ISO 834-1:1999 cellulosic curve. For this exposure, 1.0–1.5 kg/m³ of monofilament micro-synthetic fiber is generally sufficient to control fire-induced spalling in dense precast elements. The lower bound is preferred where the precast product is extruded on a long-line bed at very low workability, because overdosing increases compaction effort and may create visible fiber clusters at the top surface. In prestressed elements, the fiber addition does not affect strand bond development at this dosage, provided the embedment length and release strength are maintained. The terminal components are then transported and erected as part of a hybrid or all-precast structural frame. Fire acceptance is typically demonstrated on representative deck units, and the limiting criterion is that the vertical deflection of the heated member does not exceed the span/30 limit used in many fire test standards. Where the parking deck is subjected to de-icing salts, the concrete cover and mix design still govern chloride resistance; the fire-protection fiber is not a durability measure under EN 206.

Spalling Resistance in Cast-in-Place Rail Station Boxes

Cast-in-place rail station boxes combine thick walls, high-strength concrete and long casting sequences. The structure is often formed in 10–20 m lengths with horizontal construction joints, and the concrete is specified as C70/85 to support soil and rail loads. Fire exposure from a train fire inside the box can follow the RWS or HCM curve depending on the network operator. A 2.0 kg/m³ dose of monofilament micro-synthetic fiber is introduced to limit explosive spalling at the intrados of the roof slab and upper wall sections, where vapour pressure builds behind a dense, heated face. In these sections, reinforcement congestion is high, and concrete placement is performed by boom pump. The fiber addition requires a consistent batching sequence and verification by slump and air content tests before discharge. Because the concrete is exposed to high pumping pressure over distances that may exceed 150 m, the fiber must survive the pump and remain distributed in the placed concrete. Core sampling after construction should include visual inspection for fiber clumping at formwork edges and around embedded conduits, since vibration can locally reorient monofilaments. Fire assessment of rail station boxes is frequently performed on scaled wall and roof panels under EN 1363-1:2020, with additional insulation and integrity limits. The absence of spalling in the test coupon does not automatically prove the in-situ response, because thermal restraint and moisture migration in the full structure are different from an unloaded furnace specimen.

The normative matrix identifies the primary standards used during qualification and production monitoring. Additional requirements from national tunnel authorities, rail operators or fire authorities may impose stricter acceptance limits, and the fire curve selection must be confirmed before the concrete mix is frozen.