CHRYSO Fibre S25 is a polypropylene structural synthetic macro fibre supplied for concrete and sprayed-concrete reinforcement. Under EN 14889-2:2006, the material is classified as a Class II polymer macrofibre, meaning that its geometric and dosage characteristics are intended to provide post-crack residual capacity rather than only plastic shrinkage crack control. The S25 suffix is commonly associated with a nominal length of 25 mm in manufacturer nomenclature; however, the certified equivalent diameter, tensile strength, elastic modulus, embossment pattern, melt temperature, and dosage range for this specific model must be obtained from the current CHRYSO technical data sheet and Declaration of Performance. The product is specified in ground-supported industrial slabs, external hardstandings, composite metal-deck slabs, precast concrete elements, tunnel linings, and fibre-reinforced sprayed concrete where distributed crack bridging and improved post-crack toughness are required.

Class-level data for polypropylene macro fibres are available from EN 14889-2:2006, ACI 544.1R-96, and ISO 527-1:2019; published property values for the exact CHRYSO Fibre S25 configuration are limited in open technical literature. Consequently, the numerical ranges presented in this document are furnished as reference characteristics for the polypropylene structural macrofibre class, not as certified product values. The manufacturer’s DoP issued under Regulation (EU) No 305/2011 remains the controlling source for batch-level specifications.

What Distinguishes Structural Macro Fibres from Micro Fibres and Steel Fibres?

The distinction between micro and macro synthetic fibre is not defined by polymer type alone but by fibre diameter and concrete function. EN 14889-2:2006 uses a 0.30 mm diameter boundary: Class Ia microfibres have diameters below 0.30 mm and are dosed at 0.6–1.8 kg/m³ to mitigate plastic settlement and early-age shrinkage, while Class II macrofibres have diameters above 0.30 mm and are dosed at 3.0–9.0 kg/m³ to create measurable post-crack flexural capacity. CHRYSO Fibre S25 falls into the structural macrofibre class. Compared with cold-drawn steel fibres, polypropylene macro fibres have a much lower density and elastic modulus but a much higher fibre count per kilogram; this shifts the reinforcement mechanism from high-stiffness crack pinning to high-count frictional pull-out over a distributed crack surface.

Steel fibres provide higher stiffness and are often specified where structural crack-width control requires high residual strengths at small crack openings. Polypropylene macro fibres offer lower density, inertness in chloride-containing environments, and absence of magnetic or electrical interference. The difference in modulus means that a direct mass-for-mass substitution of steel fibre with polypropylene macro fibre is not valid. Design conversion must compare equal fibre counts or equal residual flexural performance, not equal mass dosage. The lower modulus also produces a lower slope in the load-crack mouth opening displacement curve after first crack; therefore, the product is generally used for crack-width control, impact resistance, and shrinkage stress redistribution rather than as full structural steel replacement without explicit design verification.

Microfibres with diameters below 0.30 mm and lengths below 20 mm are dosed at 0.6–1.8 kg/m³ and are intended to reduce plastic settlement and early-age shrinkage cracking before the concrete matrix has developed significant strength. They do not provide measurable post-crack residual flexural capacity at structural crack widths. CHRYSO Fibre S25 is distinguished by its larger geometry and higher dosage range, which allow the fibres to remain anchored on both sides of a crack after the cement matrix begins to fail. This functional distinction is important in specifications: a microfibre dosage cannot be substituted for a structural macro fibre dosage unless the design explicitly addresses post-crack residual flexural strength.

The crack-bridging behaviour of a polypropylene structural macro fibre is evaluated by notch-beam residual flexural tests. EN 14651:2005+A1:2007 uses a three-point prismatic beam with a sawn notch and measures the load at crack mouth opening displacement of 0.5 mm and 2.5 mm. The resulting residual flexural tensile strengths, fR,1 and fR,3, are not fibre properties alone; they are composite properties dependent on the full concrete mix design, aggregate type, curing history, fibre geometry, and fibre dosage. In ASTM C1609/C1609M-19, a similar four-point flexural procedure records residual strengths at net deflection points to characterize toughness.

A reference geometry of 25 mm length and 0.50 mm equivalent diameter yields an aspect ratio of 50:1 and a fibre volume of 4.91×10⁻⁹ m³ per filament. At a polypropylene density of 0.91 g/cm³, the mass per filament is approximately 4.47×10⁻⁶ kg, corresponding to roughly 223,500 fibres/kg. At a dosage of 5.0 kg/m³, the resulting fibre count is approximately 1.1×10⁶ fibres/m³. This high fibre count contributes to crack distribution even though the individual fibre modulus is low. The specific fR,1 and fR,3 values for CHRYSO Fibre S25 are declared in the manufacturer’s performance documentation and must not be taken from class-level ranges.

Fibre Dispersion and Batching Sequence in Twin-Shaft and Planetary Mixers

Dispersion quality in production-scale mixing is controlled by fibre addition position, mixer energy, and concrete flow state. In twin-shaft compulsory mixers with batch volumes between 2.0 m³ and 4.0 m³, the fibre is typically introduced after coarse aggregate and part of the mixing water, but before fines and the remaining water, so that the aggregate stream assists in separating filaments. Dumping the full bag into a low-shear drum mixer at the end of batching creates fibre balling and non-uniform distribution. On planetary mixers used for precast elements, the mixing time after fibre addition is commonly extended by 30–60 s beyond the plain concrete baseline, with discharge inspection for clumps. Fibre addition directly into a high-velocity water jet is avoided because it forms agglomerates that survive normal mixing. Batching operators typically check the discharge chute and truck drum ribs for fibre clusters after every batch when dosage exceeds 6.0 kg/m³.

Concrete pumping of structural macro fibre mixes is generally maintained at nominal line diameters of 100 mm or larger and slump values controlled to EN 12350-2:2019. Reductions in line diameter below this value can amplify jamming at reducers and hose couplings when fibre clusters are present. In precast plants, the fibre is often introduced through a vibrating or rotary feeder directly onto the aggregate weigh belt to minimize manual bag dosing variability.

In ground-supported slabs and external hardstandings, the fibre dosage is usually selected by the slab designer to replace light steel mesh in crack-width control or to improve moment redistribution. The specification may refer to TR34 Concrete Industrial Ground Floors or ACI 360R-10. The product must not be substituted for structural reinforcement in load-bearing elements unless the design calculations explicitly account for residual flexural strength under EN 1992-1-1:2004 or ACI 318-19. In fibre-reinforced sprayed concrete, the polypropylene macrofibre is added at the batching plant to the wet-mix and may influence pump pressure, rebound, and nozzle placement. Trial spray tests under EN 14487-1:2022 are required to confirm the fibre dosage and equipment settings before production spraying.

In precast concrete elements such as box culverts, manholes, and tunnel segments, the fibre can provide improved resistance to handling damage and edge chipping, but the design should use the measured residual flexural strength rather than assuming a simple equivalence to welded wire mesh. The combination of CHRYSO Fibre S25 with conventional steel reinforcement is permitted where the design accounts for the combined effect on crack-width control and post-crack stiffness. The product is not intended to increase compressive strength; its effect on compressive strength is generally neutral or slightly negative depending on air entrainment and compaction. For projects under North American specifications, fibre use is classified under ASTM C1116/C1116M-10a Type III synthetic fibre-reinforced concrete.

When Addition Rates Approach the Critical Upper Bound of 9.0 kg/m³

Above 6.0–9.0 kg/m³, polypropylene macro fibres increase the paste and mortar demand of the mix and produce measurable reductions in workability. The effect is nonlinear: a dosage increase from 3.0 kg/m³ to 6.0 kg/m³ may reduce slump by 20–40 mm in a C30/37 reference mix, while an increase from 6.0 kg/m³ to 9.0 kg/m³ can reduce slump by an additional 40–80 mm depending on aggregate grading and superplasticizer content. Trial batching under EN 12350-2:2019 is mandatory at the upper bound because fibre clumping and non-uniform slump can develop before the residual strength plateau is reached.

The residual flexural capacity measured by EN 14651:2005+A1:2007 does not increase linearly with dosage above a certain fibre count. Published studies of equivalent polypropylene macro fibres show that packing and dispersion limitations can create a plateau or local decrease in fR,3 if the mixer cannot distribute the fibres uniformly. This upper bound is not a fixed property of CHRYSO Fibre S25 but is a mix-design threshold observed across the class. The tendency to balling is also influenced by aggregate shape, moisture state, and the presence of other fibres; when manufactured sand with high fines content is used, fibre clustering can occur at lower dosages than in naturally rounded aggregate mixes.

For projects requiring CE marking under the Construction Products Regulation (EU) No 305/2011, the fibre manufacturer must supply a Declaration of Performance linked to EN 14889-2:2006. The conformity assessment includes fibre geometry, tensile behaviour, alkali resistance, and effect on concrete properties. The table below summarizes the controlling verification matrix for structural macro fibres rather than product-specific certified values.

CHRYSO Fibre S25 is not a direct volumetric substitute for conventional steel bar reinforcement in primary structural members unless the design is based on residual flexural test results and the relevant concrete design standard. In aggressive chemical environments, polypropylene is resistant to chloride-induced corrosion and does not produce surface staining, but its lower elastic modulus means that creep under sustained load is more significant than with steel fibre reinforcement. Published data for the specific CHRYSO Fibre S25 configuration is limited; therefore, project qualification testing of the actual concrete mix at the production plant or spray rig remains the controlling verification method before a fibre class or dosage is fixed.

Polypropylene macro fibres should not be exposed to direct flame or used at temperatures above their melting range, approximately 160–170 °C, in applications where thermal stability of reinforcement is required. They may be used with most admixtures, but compatibility with air-entraining agents and high-range water reducers must be confirmed during trial batching because fibre addition can alter air void stability. These constraints do not negate the use of CHRYSO Fibre S25 but define the operational boundary within which the fibre performs as a Class II structural macrofibre under EN 14889-2:2006.