Adfil Sinta 28/12 (Syntec 12) is specified as a 28 µm nominal monofilament diameter, 12 mm cut length, 100% polypropylene microfibre for cementitious matrices. The parenthetical reference Syntec 12 identifies the 12 mm length series within the supplier’s monofilament range, not a separate polymer system. The base polypropylene homopolymer used in this fibre class has a density of 0.90–0.91 g/cm³ when measured under ISO 1183-1:2019, a melt transition in the range 160–170°C under ISO 11357-3:2018, and a saturated hydrocarbon backbone that is not susceptible to alkaline hydrolysis in the pH 12.5–13.5 pore solution of portland cement. The product is intended for plastic shrinkage crack control in concrete, mortar, screed, precast, and shotcrete; it is not primary reinforcement.

At 0.9 kg/m³, the polypropylene volume fraction is approximately 0.1 vol%, placing the product within the conventional microfibre band for low-volume crack control. The theoretical specific surface area of a 28 µm circular filament at 0.91 g/cm³ is approximately 157 m²/kg. This high surface-to-volume ratio explains the measurable influence on fresh-state water demand, segregation, and dispersion. These values are derived from geometry and density, not from destructive testing, and should be checked against the current manufacturer’s dosage guidance.

What Does the 28/12 Designation Control in Fibre Dispersion?

The 28 µm diameter and 12 mm length generate a theoretical fibre count of approximately 1.5 × 10⁸ fibres per kilogram. A 0.9 kg/m³ dose therefore introduces on the order of 1.3 × 10⁸ discrete fibres into each cubic metre of concrete. This is orders of magnitude higher than the 10⁴–10⁵ fibres per kilogram typical for 0.5–1.0 mm steel macrofibres. High fibre count improves distribution density but does not automatically guarantee crack control; balling, paste volume, and mixing energy are limiting variables. The designation also controls cut length. At 12 mm, the fibre is long enough to bridge fine plastic-shrinkage openings but short enough to be handled in standard aggregate batchers and worm-screw dispensers.

At the same addition mass, the product has a greater number of fibres than longer macrofibres, but each individual fibre has low stiffness. Tensile modulus for unmodified polypropylene filament is typically 3.5–4.5 GPa; tensile strength is typically 300–500 MPa. These are class-typical values for polypropylene monofilament, not lot-specific acceptance limits. A lower aggregate cement ratio or low paste volume can expose the additional surface area as a drop in slump under EN 12350-2; the effect is most pronounced above 1.2 kg/m³ without a plasticizer.

Plastic shrinkage cracking occurs when the evaporation rate from fresh concrete exceeds the bleed-water supply, producing capillary tensile stress while the matrix has little tensile capacity. In restrained slab trials under ASTM C1579-21, monofilament microfibres in the 0.6–1.2 kg/m³ range have been reported to reduce crack width and total crack area relative to plain concrete. Published data for this specific configuration is limited; exact crack-reduction ratios depend on admixture chemistry, cement fineness, wind speed, and form geometry. Trial batching is therefore required before a performance-based specification is fixed. Because the fibres obstruct some bleed channels, bleeding may also decrease; bleed rate should be measured under ASTM C232/C232M-21 when evaporation conditions are severe. This shift can increase plastic shrinkage risk if curing or evaporation control is not adjusted.

The product is also used in dry-mix shotcrete at dosages of 0.9–1.8 kg/m³, where the low density of polypropylene limits pumping and nozzle-blocking tendencies compared with rigid steel fibres. Shotcrete consistency and rebound should be assessed through EN 14487-1. In thin-section precast elements, the fibre reduces surface cracking during demoulding and early curing, but it does not provide edge protection against impact or transport damage.

Controlling Dispersion Without Fibre Balling in Dry-Mix Plants

On twin-shaft compulsory mixers with 30–60 s dry mixing, the addition sequence affects fibre distribution. Field reports indicate that the monofilament is best introduced with fine aggregate or through a calibrated screw conveyor at the start of dry mixing, not after large volumes of mixing water have created a low-shear slurry. Water should be added only after the fibres are distributed through the aggregate. After water contact, a minimum wet mixing time of 60 s is typical for full random orientation, but high-speed pan and planetary mixers may require shorter or longer cycles depending on blade condition and batch size. The material should not be dumped from a torn bag directly into a stationary centre pocket; this creates high local fibre concentration and agglomeration.

Fibre balling on site is often misinterpreted as a material defect when the true cause is addition sequencing. The product’s high theoretical fibre count, approximately 1.5 × 10⁸ filaments/kg, means that a 25 kg bag contains a very large population of discrete filaments. If the mixer has insufficient shear or the batch is overloaded, the fibres cannot separate into individual units. Dispersibility should be checked at the plant by washing a 1 kg composite sample over a 0.5 mm sieve and counting agglomerated fibre balls. For wet-cast flowable mixes, addition after coarse aggregate and before final water adjustment often reduces drawdown because the aggregate assists distribution without requiring continuous high-energy mixing. In transit-mixer operations, adding fibres to the conveyor with the aggregate or through the water ring is preferable to dropping bags into the drum after all materials are wet. A dry aggregate pre-mix of 2 min at low drum speed followed by 5 min at mixing speed is a common starting point, but this cycle is mixer-dependent and must be confirmed by wash-out inspection.

Water-soluble or kraft paper bags, if supplied, should be added unopened only when the packaging material is confirmed to disintegrate at the available mixer water temperature. Packaging that does not dissolve can produce large agglomerates that are difficult to separate later.

When Steel Macrofibre Substitution Is Structurally Invalid

Design substitution of steel macrofibre is not permitted for load-bearing post-crack performance. Steel macrofibres with 0.5–1.0 mm diameter and 30–60 mm length are specified when residual flexural tensile strength is required through EN 14845-1:2006 or ASTM C1609/C1609M-19a. Polypropylene monofilament has a much lower modulus, typically 3.5–4.5 GPa versus 200 GPa for steel, and creeps under sustained load; it cannot maintain post-crack opening resistance in hardened concrete. The product may reduce plastic shrinkage crack visibility, but it shall not be used as a substitute for reinforcing bars, structural wire mesh, steel macrofibre, or engineered synthetic macrofibre in slabs designed under EN 1992-1-1.

The difference also appears in hardened flexural testing. In ASTM C1609/C1609M-19a, a beam containing only micro-PP typically exhibits a residual strength ratio close to zero after first crack because the low-modulus fibres pull out without contributing significant load. Steel macrofibres are able to bridge cracks up to several millimetres. Therefore, project specifications that quote f_R,1k or f_R,4k values from EN 14845-1 should exclude this product unless the structural designer has explicitly accepted a non-structural crack-control function.

Polypropylene microfibre also differs from fibrillated polypropylene microfibre. The fibrillated form is produced from oriented film and opens into an irregular netlike cross-section during mixing. The circular monofilament in the 28/12 class has a more predictable cross-sectional area and may disperse more readily in low-slump mixtures, but it provides fewer mechanical interlock sites than an opened fibrillated network. Compared with PVA microfibre, polypropylene is hydrophobic and does not compete for mixing water, whereas PVA has higher tensile strength and modulus and is used in strain-hardening cementitious composites at much higher volume fractions. Compared with alkali-resistant glass fibre, polypropylene requires no alkali-resistant coating for use in portland cement, but its low modulus excludes it from thin-section applications requiring high first-crack strength.

Comparative differences are summarized in the matrix below. The values shown are class-typical ranges drawn from public polymer and fibre literature for the fibre types, not lot-specific acceptance limits.

The most consequential difference in formulation is hydrophobicity. Polypropylene does not absorb mixing water, unlike PVA or natural cellulose fibres, and therefore does not compete with cement for water during hydration. This reduces early-age mix-water adjustment but also means that the fibre relies on mechanical dispersion and surface finish for suspension, not on swelling or hydrophilic interaction.

Operational Boundaries, Storage, and Incompatibilities

Pre-drying of the product is not normally required because polypropylene is hydrophobic; however, wet packaging or exposure to rain can create fibre clumps that resist mechanical separation. If surface moisture has caused agglomeration, the material should be dried below 60°C or replaced. Continuous service in cementitious matrices is generally below 90°C, which is safely below the polymer melt transition; the product is not recommended for autoclave curing regimes above 150°C because filament softening and shrinkage may occur. Solvent-based curing compounds applied to surface-exposed fibres should be checked for compatibility because polypropylene is soluble or swelling in some aromatic and chlorinated solvents.

Bags should be stored off the ground in dry conditions below 40°C, protected from direct sunlight. Loose fibres exposed to ultraviolet light for extended periods may undergo surface embrittlement before incorporation; once embedded, the cementitious matrix blocks UV radiation. The product has negligible electrical conductivity and is not intended for use in conductive floor toppings or where the concrete is required to conduct current. No reactive incompatibility with ordinary portland cement, ground granulated blast-furnace slag, or fly ash is known, but combinations with high dosages of air-entraining admixtures can shift the air-void system and require updated trial batches under EN 12350-7 or equivalent.