Identification of the product as SikaFiber Micro-1 – Ultra-fine Anti-Spalling Micro Fiber establishes its position as a synthetic monofilament addition for concrete matrices that require passive fire protection against moisture-induced explosive spalling. The model designation Micro-1 distinguishes an ultra-fine filament geometry intended to disperse into discrete fibres under standard concrete mixing energy. In high-strength concretes with water-to-cement ratios at or below 0.40 and compressive strengths exceeding 60 MPa, the cement paste densifies beyond the point at which capillary percolation can relieve internal vapour pressure during rapid heating. The product is therefore specified where fire exposure is defined by the hydrocarbon curve, RWS curve, or modified HCM curve, and where concrete cover to reinforcement is governed by structural fire design rather than ambient-temperature durability alone.

Specifiers typically invoke this material in tunnel linings, cast-in-place columns, precast segmental liners, and high-rise core walls where spalling of the concrete section could expose reinforcement to temperatures above the critical 500 °C threshold for yield strength loss. The product is added at the batching plant and is compatible with polycarboxylate-ether superplasticizers, air-entraining admixtures, silica fume, fly ash, and ground-granulated blast-furnace slag systems. No additional water demand is introduced when the fibre is dosed correctly, and the as-delivered collated bundles require only the mechanical shearing and impact action of a compulsory mixer to separate into monofilaments. The function of the fibre is physical, not chemical: it does not react with hydration products, does not alter the stoichiometry of cement hydration, and does not contribute to the pore solution chemistry.

Why does a sub-30 µm monofilament form a percolated pressure-relief network during heating?

At ambient temperature, an ultra-fine polypropylene microfibre is hydrophobic and chemically inert in the alkaline pore solution. During a fire, the cement matrix undergoes differential thermal expansion, free and adsorbed water desorbs, and capillary water flashes to steam. In a dense concrete with permeability below roughly 10^-16 m² to 10^-17 m², the steam front cannot migrate toward the exposed surface quickly enough; local pore pressure rises at a rate governed by the heating curve and the moisture content of the concrete. When the polypropylene filament reaches its melting range, approximately 160 °C to 170 °C, it melts and leaves behind an empty channel. Because ultra-fine fibres are distributed at high filament counts per unit volume, these channels form a continuous network once the dosage reaches the critical percolation threshold. The result is a temporary permeability increase in the heated zone that allows vapour to escape before the tensile stress exceeds the hot tensile strength of the concrete.

The mechanism is thermal and geometrical, not hydraulic at ambient temperature. The fibre does not perform as a capillary wick prior to melting; its presence contributes little to moisture transport during normal service. After melting, the channels are microscale, and the residual polymer may undergo oxidative decomposition at higher temperatures rather than block the channels. The fibre length, diameter, and dosage must be selected together because percolation depends on the number of individual filaments per cubic metre, not on fibre cross-section alone. If the fibres are too coarse or the dosage too low, the channel network remains discontinuous and vapour pressure is not relieved. If the fibres are too long at a given workability class, dispersion becomes non-uniform and localised spalling may still occur in fibre-poor zones.

Percolation threshold calculations for this class of fibre can be approximated by the product of fibre volume fraction and aspect ratio, but direct percolation is not the only requirement. The melt-induced channel must remain open long enough for vapour to escape. For this polymer class, melt viscosity is approximately 10² Pa·s to 10³ Pa·s at the melting point, which is sufficiently low for the melt to coalesce on aggregate surfaces rather than fill the channel. At temperatures above 350 °C, polypropylene thermally decomposes through chain scission and volatilisation; the reaction is endothermic and produces low-molecular-weight hydrocarbons, but the heat absorption is negligible compared with the thermal mass of the concrete. Therefore, the anti-spalling function is attributed to the physical channel network, not to any chemical heat sink effect.

The supplied grade is routinely characterized by a filament diameter in the ultra-fine class below 30 µm, a fibre length in the 6 mm to 12 mm range depending on batch configuration, a polymer density of approximately 0.91 g/cm³, and a melt point near 160 °C. The base polymer is a high-purity polypropylene with no chloride content, and the fibre is non-corrosive to steel reinforcement. Because the product is hydrophobic, water absorption is lower than 0.1% by mass. The fibre count per kilogram for an 18 µm by 6 mm monofilament is on the order of 7.2×10⁸, which illustrates why even a dosage of 1 kg/m³ can place millions of potential pressure-relief channels within a cubic metre of concrete. Published batch certificate values for this specific product should be obtained from the manufacturer for the actual lot, because dimensional tolerances and melt-flow characteristics can vary between production campaigns.

Dosage thresholds for moisture-induced spalling resistance in hydrocarbon fire exposure

Dosage is the primary variable controlling anti-spalling efficacy. In project specifications, the addition rate is generally selected between 0.9 kg/m³ and 2.0 kg/m³ for concretes with a maximum aggregate size of 20 mm and a compressive strength class above C60/75. The lower value is often adequate under the ISO 834-1 cellulosic curve for moderately dense concrete with residual moisture below 3% by mass. The upper value is frequently required under the RWS or HCM hydrocarbon curves, where heating rates of 200 °C/min can occur in the first minutes and where concrete moisture content may exceed 4% due to curing conditions or groundwater contact. Above 2.0 kg/m³, the fibre volume fraction begins to affect fresh-state rheology: the increased surface area of the fibres absorbs a portion of the paste film and can reduce the slump flow of self-compacting concrete by 50 mm to 150 mm depending on the superplasticizer dosage and mixing sequence. Below 0.9 kg/m³, the linear density of fibres may not be sufficient to create percolation, particularly in concretes with a paste volume below 300 L/m³.

Specifiers should not transfer dosage values between different fire curves without verification. A concrete that passes a 120-minute ISO 834-1 spalling test at 1.0 kg/m³ may exhibit explosive spalling under a 30-minute RWS test if the same moisture condition is retained. The relevant moisture threshold is not the free-water content of the fresh concrete but the moisture state in the cover zone at the time of fire. Pre-drying of test specimens or real structures at relative humidity above 60% for extended periods can produce conservative test results; conversely, testing a dry specimen can mask insufficient dosage. Validation should therefore include conditioning according to the test standard and measurement of internal moisture in the cover zone. Published independent data for this specific fibre configuration under all fire curves is limited; therefore, project-specific testing remains the only reliable basis for dosage confirmation.

Moisture content in the cover zone can be measured by destructive gravimetric methods on drilled cores or by relative humidity sensors cast into the concrete. In precast segments cured by elevated temperatures, moisture content may be lower than 2% at the time of fire, which reduces the required dosage but does not eliminate spalling risk. In cast-in-place tunnel linings protected from drying by formwork and high ambient humidity, the moisture content can remain above 4% for years. The fire protection specification must therefore account for the long-term moisture state, not only the water-to-cement ratio. The presence of silica fume at 8% to 10% by mass of cementitious material lowers the permeability and can increase both strength and spalling susceptibility; fibre dosage may need to be raised toward the upper bound under these conditions.

The fibre is normally added to the aggregate fraction before cement or after the batching water, depending on mixer type. In twin-shaft compulsory mixers with paddle geometries intended for high-intensity mixing, the fibre bundles are broken by shear against the aggregate; a post-fibre wet mixing time of 4 minutes to 5 minutes is typical to achieve monofilament separation. In planetary or pan mixers, the fibre should be added as a continuous stream over the aggregate surface rather than as a single mass. Pneumatic dosing systems should be set to avoid blow-back from fines and to prevent the fibre from accumulating on the mixer walls. Batch-to-batch variance in concrete plants with worn scraper blades or insufficient mixing intensity has been observed as fibre balling at the centre of the mixer, particularly when the entire bag is introduced in less than 10 seconds. Fresh concrete control tests should include slump or slump-flow measurement by EN 12350-2, air content by EN 12350-7, and washed-out fibre distribution verification when spalling resistance is critical. The addition of the fibre does not replace the need for standard curing; however, it may reduce early-age plastic shrinkage cracking in slabs and liners when judged by ASTM C1579.

At the production scale, fibre feeding through automatic dosing equipment should be verified against a mass-flow check. Fibre loss through dust extraction systems in central mix plants can be significant if the fibre is added as a dry component with no aggregate coverage. A mass balance check on the first three batches should compare the as-batched fibre mass with the recovered fibre count from washed-out fresh concrete. Discrepancies above 10% indicate fibre segregation or loss and require adjustment of the addition sequence or mixer venting. When fibres are added by whole-bag manual charging, the total mix time after fibre addition should be extended by at least 30 seconds beyond the standard programme to compensate for non-continuous addition.

If the specification contains both plastic shrinkage crack control and spalling resistance, this fibre class can satisfy both service-state criteria without structural macrofibre addition

The same ultra-fine monofilaments that provide post-melt permeability channels also contribute to the suppression of plastic shrinkage cracking in the first 24 hours after placement. The fibre does not increase the flexural toughness of hardened concrete in the sense of EN 14651 or ASTM C1609 because the filaments are too short and too flexible to bridge macro-cracks after the matrix has fractured. Therefore, the product is not interchangeable with steel macrofibres, polypropylene macrofibres, or structural fibres that are specified to provide post-crack residual strength for structural design. A comparison is provided in Table 1.

The comparison highlights the central distinction: ultra-fine polypropylene microfibres function as a sacrificial pressure-relief medium under thermal load, whereas macrofibres and steel fibres function as mechanical inclusions that maintain load transfer across cracked sections. A specification that confuses these roles can produce a concrete that passes a flexural toughness test but still spalls under fire, or a concrete that resists fire-induced spalling but fails the required structural residual strength criterion. The two functions are not mutually exclusive, but they cannot be assumed to be interchangeable simply because both materials are called fibres.

Operational boundaries are defined by the fibre geometry and the concrete matrix. The product does not compensate for insufficient cover, poor curing, or inadequate aggregate interlock. It is not a fireproofing material and does not reduce the thermal conductivity of the concrete section. It should not be specified as the sole measure for structural fire protection where spalling is caused by load-induced cracking rather than moisture-driven pore pressure. In concretes with a maximum aggregate size above 32 mm, the fibres tend to follow the paste fraction and may not distribute uniformly through the full section unless the paste volume is increased or the aggregate grading is adjusted. The fibre should be stored in original bags away from sources of ignition and prolonged exposure to ultraviolet light; opened bags should be resealed and protected from condensation in high relative humidity environments above 75%. No chloride contribution is present, but the product is not a rust inhibitor and does not replace corrosion-resistant mix design measures in chloride-bearing service environments.

Product conformity for polymer fibres for concrete is assessed under EN 14889-2:2006, which defines requirements for geometry, tensile strength, alkali resistance, and effect on concrete consistency. The product class falls within fibres for non-structural use, and the manufacturer’s declaration should list the specific fibre length, diameter, and dosage for each batch. When the product is used in fire-resistant concrete, the applicable fire test standard may be EN 1363-2:1999 for element testing, with the heating curve selected from ISO 834-1, RWS, or HCM according to the project. No single fibre dosage can be considered universally validated across all concrete mix designs, and the fire engineering report should be based on full-scale or intermediate-scale spalling tests rather than on fibre dosage alone.