SINTA F is specified as a collated fibrillated polypropylene fiber for secondary reinforcement of Portland cement concrete, mortar, and wet-process shotcrete. The product is manufactured from oriented polypropylene film that is slit, fibrillated, and collated into water-dispersible bundles. The collated bundle format permits addition through automated dosing equipment without the static clumping and feed-line blockage observed with loose monofilament or unbundled fibrillated product. Manufacturer-published nominal values place base polymer density at 0.91 g/cm³, tensile strength at 400 MPa to 650 MPa under ASTM D2256, elastic modulus at 3.5 GPa to 4.8 GPa under ASTM D2101, and melting point at 160 °C to 170 °C under ASTM D3418. Standard cut length is 19 mm, with 12 mm and 24 mm lengths available for applications where finishing and aggregate gradation require a shorter or longer fiber. The normal dosage is 0.6 kg/m³ to 1.8 kg/m³ for restrained plastic shrinkage control and 3.0 kg/m³ to 4.5 kg/m³ when project specifications require post-crack residual strength evaluation under ASTM C1399/C1399M or ASTM C1550. Because the polymer is hydrophobic, water absorption is less than 0.1% by mass under ASTM D570, and the fiber does not alter free-water demand at the standard dosage.

What distinguishes collated fibrillated polypropylene from monofilament and steel fiber?

The fibrillated network opens under mixer shear and forms a two-dimensional grid that mechanically interlocks with cement paste and fine aggregate. Monofilament fiber develops bond primarily through surface friction along a circular cross-section and may require a higher dose to reach the same plastic shrinkage crack reduction observed under ASTM C1579. Steel fiber has a modulus of approximately 210 GPa and can participate in structural moment resistance; the polypropylene modulus of 3.5 GPa to 4.8 GPa places SINTA F in the secondary-reinforcement category. In a comparative washout test using a 0.1 m³ laboratory pan mixer at 24 rpm, a 0.9 kg/m³ dose of collated fibrillated bundles dispersed to individual fibrillated networks within 4 min to 5 min, whereas loose unbundled fiber required 8 min and still exhibited visible clumps at paddle tips. The collated construction specifically addresses batching and dispersion problems; it does not convert the material into a macro-synthetic structural fiber.

Collation does not function as a water-binding agent in the mix.

Collation is a mechanical or water-rupturable bundling step; it is not a superabsorbent polymer treatment. The bundle separates when wetted and sheared, but it does not bind free water or change the effective water-cement ratio. At 0.6 kg/m³ to 0.9 kg/m³, no batch-water correction is necessary. At dosages above 3.0 kg/m³, the increased solid surface area can reduce slump by 20 mm to 40 mm in truck-mixed concrete, which is a rheological effect rather than water absorption. The preferred addition sequence is after dry mixing of aggregate and cement but before the full water and admixture charge. In a 2.0 m³ twin-shaft compulsory mixer operating at 18 rpm to 24 rpm, fiber is introduced over 30 s to 60 s, followed by remaining water and polycarboxylate ether superplasticizer meeting ASTM C494/C494M. Minimum wet mixing time after fiber addition is 4 min to 5 min. In truck mixers, the drum should operate at 12 rpm to 18 rpm for 5 min to 7 min; drum speeds below 8 rpm have been observed to leave unopened bundles at the back of the drum.

Rheometer measurements on a 0.40 w/c mortar fraction in a coaxial-cylinder concrete viscometer indicate that 1.8 kg/m³ of fibrillated polypropylene increases plastic viscosity by 15% to 25% relative to the plain reference at the same superplasticizer dosage, while yield stress remains within ±5%. Above 3.0 kg/m³, yield stress may begin to increase, particularly in mixes with cementitious content below 300 kg/m³. Adding fiber after a fully air-entrained mix has been mixed can reduce air content by 1% to 2% by volume in truck mixers due to prolonged mixing and fiber interaction with the air-void system. Therefore, the fiber should be introduced before the air-entraining admixture or immediately after the initial aggregate-cement dry mix to avoid air-loss variability. These values are indicative field and laboratory observations and vary with aggregate shape, cement fineness, and ambient temperature.

When restrained shrinkage governs early-age crack width

Concrete elements with a surface-area-to-volume ratio above 3 m⁻¹ and an evaporation rate above 1.0 kg/m²/h are susceptible to plastic shrinkage cracking during the first 6 h to 12 h after placement. Comparative testing under ASTM C1579 has shown that a 0.9 kg/m³ dose of SINTA F in a 0.45 w/c concrete can maintain average maximum crack width below 0.10 mm, while a plain reference under the same conditions may exceed 0.50 mm. The fiber does not reduce total drying shrinkage of the matrix; it redistributes tensile stress and arrests coalescence of microcracks. On exterior slabs exposed to wind speeds above 5 m/s, the dosage is commonly increased to 1.2 kg/m³ to 1.5 kg/m³, but an evaporation retardant remains necessary when bleed rate is less than evaporation rate. The results depend on cement type, supplementary cementitious materials, finishing timing, and the use of curing compounds.

Wet-process shotcrete applications batch the collated fiber at 3.0 kg/m³ to 4.5 kg/m³ before pumping. Field panel comparisons at 0.5 m nozzle distance and 0.5 m³/h output have recorded rebound mass reductions of 10% to 15% relative to plain shotcrete. Pumping pressure through a 50 mm line typically increases by 0.3 MPa to 0.7 MPa at these dosages. In precast concrete, addition at 1.5 kg/m³ to 2.0 kg/m³ can reduce edge cracking during demolding and turning when the element is mixed in a central mixer with at least 4 min wet mixing time. Published data for this specific configuration is limited to equipment-specific field reports, and a plant trial panel should be cast before production.

Maintain batching rate below 1.0 kg/min in compulsory mixers

In a 2.0 m³ twin-shaft compulsory mixer, discharging a full 3.0 kg/m³ dose in less than 60 s can overwhelm the wetting front and produce persistent fiber balls. The recommended batching rate is 0.5 kg/min to 1.0 kg/min when the mixer is running at 18 rpm to 24 rpm. Batch-to-batch fiber length variation is controlled to ±1.5 mm; a washed-out 10 L sample at 5 min of mixing should show no more than 1 unopened bundle per 10 L. If unopened bundles persist, mixing time is extended in 60 s increments rather than increasing mixer speed, because higher shear can tear outer fibrils from the bundle before inner strands are wetted. On high-speed counter-current mixers above 30 rpm, this tearing mechanism has been associated with fiber wrapping on paddle shafts and requires slower addition or the use of a vibrating screen at the charging point.

Fibre orientation in the hardened element is governed by placement method. In slab construction, vibration and finishing tend to align the fibrillated network in the horizontal plane, which is favourable for resisting plastic shrinkage cracks but less effective for vertical shear resistance. In walls and columns, pump placement and external form vibration can produce variable orientation; post-crack performance under ASTM C1399/C1399M should not be extrapolated from slab panels to congested vertical elements without a plant trial. These field observations are consistent with published fiber-reinforced concrete placement studies, but project-specific measurements are recommended when residual strength is a specification requirement.

Manufacturer-published nominal specifications

Under ASTM C1116/C1116M, synthetic fiber-reinforced concrete must meet material requirements for Type III fiber. Alkali resistance is typically evaluated by immersion in saturated calcium hydroxide solution at 20 °C to 25 °C for 28 days; polypropylene does not contain ester or amide groups and is not attacked by portland cement alkali. For European Economic Area projects, the product is placed under EN 14889-1:2006, which requires declared performance values for tensile strength, Young’s modulus, length, and alkali resistance. The collated format does not alter the base polymer compliance; it is a physical bundling method for batching and dispersion. Users should request the manufacturer’s current Declaration of Performance and verify that the notified body certificate covers the specific cut length and collated format.

To verify compliance, review the declaration of performance

Operational boundaries are set by temperature and chemical exposure. Polypropylene fiber does not corrode in chloride-bearing concrete and is not attacked by portland cement alkalis, but it is not recommended for immersion in strong oxidizing acids or aromatic hydrocarbon solvents that soften the polymer. The product is not a replacement for structural steel reinforcement in load-transfer applications. Storage should be in sealed bags below 50 °C, and direct ultraviolet exposure beyond 12 months may embrittle the polymer before batching. Steam curing at concrete temperatures above 60 °C may soften the fiber and reduce mechanical anchorage; published data for this specific configuration is limited. Because the fiber does not absorb water, no increase in mix water is required at 0.6 kg/m³ to 0.9 kg/m³, but at dosages above 3.0 kg/m³, the increased surface area can reduce slump by 20 mm to 40 mm, requiring adjustment with a high-range water reducer meeting ASTM C494/C494M.