Adfil Ignis – Synthetic Micro Fire Protection Fibers are monofilament polypropylene microfibers formulated for addition to dense, low-permeability cementitious matrices. The product is intended for high-strength concrete, precast tunnel segments, columns, and sprayed concrete where fire resistance depends on limiting moisture-driven explosive spalling. Under heating, the fibers soften and melt, leaving continuous pressure-relief channels through which water vapour can escape before pore pressure exceeds the tensile capacity of the hot concrete. Published product-specific numerical data for Adfil Ignis is limited; the dimensional and performance ranges stated below are therefore representative of EN 14889-2:2006 compliant synthetic micro polypropylene fire-protection fibers and should be confirmed against the manufacturer’s batch-specific technical data sheet and CE certificate for any project.

The explosive spalling mechanism becomes acute in concretes with compressive strength above 60 MPa and water/cement ratio below 0.35, particularly when silica fume or high-range water reducers produce a matrix so dense that vapour migration is restricted. During rapid heating, free water and chemically released water generate pore pressure faster than mass transport can relieve it. Because hot concrete has reduced tensile strength near the exposed surface, the pressure gradient can produce sudden spalling of concrete fragments. Adfil Ignis does not suppress the temperature rise itself; it introduces a controlled discontinuity network that activates at the polymer melt range and links pores, thereby reducing peak internal pore pressure.

What Happens Inside a Low-Permeability Matrix During Rapid Heating?

As the surface temperature rises above 100 °C, free water begins to vapourise and migrates inward, condensing in cooler zones. This moisture front creates a saturated layer with low permeability that blocks vapour escape. The resulting pore pressure can exceed 2 MPa to 4 MPa in dense concretes, while the hot tensile strength remains below 1 MPa near the exposed face. Synthetic microfibers provide a heat-triggered pressure relief mechanism. The polypropylene monofilaments soften between 150 °C and 170 °C, then decompose into capillary-size channels with diameters close to the original fiber diameter. These channels reduce the effective vapour path length and allow pressure equalisation before crack propagation becomes unstable.

Dispersed fiber count is more important than total fiber mass in this mechanism. A finer monofilament yields a higher number of discrete channels per cubic metre for a given dosage. Consequently, spalling resistance depends strongly on uniform distribution through the cementitious matrix, wet-out behaviour, and the absence of fiber agglomeration during batching. The fibers do not act as primary post-crack reinforcement in the same way as steel or macro synthetic fibers; their function is thermal-pressure relief rather than structural crack bridging.

Without a header, the next application context is described directly. In tunnel segment production, the concrete mixture is typically batched in a high-shear twin-shaft compulsory mixer and discharged into carousel molds where compaction is supplied by external vibration. Fiber addition before the aggregate is fully wetted may result in visible clumps on the segment inner surface. The preferred sequence is to charge coarse and fine aggregates first, add the fibers with the initial mixing water, and then introduce cement, silica fume, and superplasticizer only after fiber bundles have opened into individual filaments.

Table 1 provides the representative specification profile for the synthetic micro fire-protection fiber class.

The operational boundary for dosage is not fixed by the fiber alone. Below 1.0 kg/m³, the melt-channel network may be too sparse in very dense concretes exposed to severe hydrocarbon or RWS curves. Above 3.0 kg/m³, workability reduction, pump pressure increase, and fiber balling can occur in low-paste mixtures. In production-scale ready-mix trucks, adding the fiber too quickly into a high-speed drum can create electrostatically bound clumps that survive transport and appear as surface defects after placing.

Thermal Stability, Alkali Resistance, and Mixing Parameters

Polypropylene is hydrophobic and has low water uptake, which allows the fiber to remain dimensionally stable in wet concrete before the fire event. The material is not affected by the alkaline cement paste environment because the polymer backbone lacks hydrolytically sensitive ester linkages. However, storage must avoid temperatures above 50 °C, direct sunlight, and contact with solvents or strong oxidising agents that can degrade the monofilament or cause bundles to fuse. In central mixing plants, the fiber should be introduced through a fiber dispenser or pre-dosed water-soluble bag rather than pneumatically blown into dry aggregate without dust control.

In high-performance concrete, rheology is sensitive to the sequence of admixture addition. When Adfil Ignis is added together with a polycarboxylate ether superplasticizer before the aggregate is uniformly moist, the water-reducing polymer can coat the hydrophobic fiber surface and delay dispersion. The resulting batch-to-batch variance is observed as irregular surface voids or increased mixing time. A more stable sequence is to add fibers after 50 % of the batch water has been incorporated and to maintain wet mixing for at least 90 seconds after filament separation.

For tunnel segments with specified residual moisture below 3 % by mass, conditioning of samples at 60 % relative humidity before fire testing is necessary because pore pressure generation is controlled by free moisture content. Specimens dried too aggressively may show unrealistically low spalling damage, while saturated specimens may fail even with fiber addition if the concrete permeability is extremely low.

When Hydrocarbon and RWS Curves Drive Fiber Dosage Selection

In traffic tunnels, the design fire often follows the Rijkswaterstaat RWS curve, which reaches 1200 °C within 5 min and 1350 °C at 120 min. Under such rapid heating, the temperature gradient across a segment lining can exceed 400 °C/mm at the exposed face, producing severe thermal dilatation and moisture pressurisation. For these exposures, fiber dosage should be validated by full-scale or intermediate-scale fire tests on the actual mix design rather than by thermal analysis of the fiber alone.

Published data for Adfil Ignis in specific RWS-tested segment configurations is limited, and the controlling performance data are generated by project-specific tests such as slab furnace exposure under ISO 834-1:1999 or a hydrocarbon curve. In such tests, observed spalling depth, mass loss, and residual cross-section are compared against acceptance criteria defined by the tunnel owner. The fiber contributes one control parameter; it does not replace thermal barrier linings, passive reinforcement, or concrete cover requirements.

In high-rise columns and shear walls, the standard fire curve of ISO 834-1:1999 may be used for verification. Dense high-strength columns containing silica fume can experience spalling after 15 min to 30 min of exposure if macrosteel fibers or micro polypropylene fibers are absent. Adfil Ignis is added to the concrete mix to increase the probability that the element retains its cross-section long enough for the reinforcing steel to remain below the critical temperature. The fiber does not reduce thermal conductivity of the concrete and does not act as an intumescent coating.

For sprayed concrete, fiber addition must account for the different mixing and placement route. Dry-mix shotcrete introduces the fiber into the dry material stream before the nozzle, while wet-mix shotcrete introduces the fiber in the batching plant. In both cases, the fiber should be dispersed before the accelerator is added. If an alkali-free accelerator is injected at the nozzle, the fiber is already embedded in the paste and does not require additional wetting time. Published data on the exact fiber recovery rate in rebound for Adfil Ignis is limited; rebound sampling should be part of preconstruction spray trials.

Table 2 compares the synthetic micro fire-protection fiber class with steel and macro synthetic fibers to illustrate the product differentiation.

Compared with conventional shrinkage-control polypropylene microfibers, the fire-protection grade is distinguished by finer filament diameter and higher filament count per kilogram at the specified dosage. This distinction matters because spalling resistance is governed by the spacing of melt channels, not solely by fiber mass. A poorly dispersed coarse macrofiber cannot provide the same vapour-relief density at 1.0 kg/m³ to 3.0 kg/m³. Conversely, Adfil Ignis is not a replacement for structural steel fibers where post-crack flexural toughness is required under EN 14889-1 or EN 14889-2:2006 Class II macrofiber provisions.

In precast plants, the most common production bottleneck is not mixing but fiber transfer into the mixer. Bulk bag discharge without a controlled screw feeder can deliver irregular fiber mass and produce dosage variation between batches. Automated dosing units with mass loss correction have reduced batch-to-batch variation to below 2 % of target fiber mass in some installations, but published performance data for Adfil Ignis-specific dispensing systems is limited. When dosing is accurate but mixing is shortened below the filament separation time, the unopened bundles remain visible at the form face after demolding.

The fiber is compatible with typical concrete admixtures including polycarboxylate ether, naphthalene-based superplasticizers, and air-entraining admixtures. Freeze-thaw resistance still requires a separate air-void system according to ASTM C260 or EN 934-2; the fiber does not substitute for entrained air. The product should not be used as the sole means of fire protection where the concrete cover is below code requirements or where the element is expected to carry load during fire without supplemental thermal insulation.

For moisture-saturated structures with very low permeability, the fiber reduces but does not eliminate spalling risk. If the matrix has a water/cement ratio below 0.30 and contains more than 10 % silica fume by mass of cement, the vapor pressure may still generate localized spalling even with fiber addition if the heating rate is severe. In such cases, the design should include increased cover, thermal barriers, or higher fiber dosage validated by fire testing.