SikaFiber 6 FF is a film-fibrillated polypropylene fibre supplied for secondary reinforcement of concrete, mortar, and wet-mix shotcrete. The product designation identifies a nominal fibre length of 6 mm and a fibrillated cross-section produced by slitting and stretching an extruded polypropylene film. The fibre is manufactured from 100% virgin polypropylene resin and is non-corrosive, non-alkali-reactive, and hydrophobic. Published manufacturer data list a specific gravity of 0.91 g/cm³, a tensile strength of 300 MPa, a tensile modulus of 3.5 GPa, and water absorption of nil when assessed under the relevant polymer-fibre methods in EN 14889-2:2006. The primary engineering function is the reduction of plastic shrinkage cracking, settlement cracking, and early-age crack width in concrete before the development of significant tensile capacity in the cement matrix. It is not a structural load-bearing reinforcement and does not replace deformed steel reinforcing bars, welded-wire reinforcement, or macro-steel fibres in elements requiring post-crack flexural resistance.

The material is typically batched at 0.9 kg/m³ in slab-on-grade and industrial floor concrete. The published addition window is 0.6 kg/m³ to 1.8 kg/m³, depending on mix fines content, ambient evaporation rate, and the required degree of early-age crack control. At addition rates above 1.8 kg/m³, the combination of low bulk density and high specific surface area may increase the incidence of fibre balling, reduce slump, and destabilise the entrained-air void system in air-entrained concrete. For higher post-crack residual strength or toughness, a macro-synthetic fibre or steel fibre system should be evaluated rather than increasing the dosage of this microfibre.

How Does a Fibrillated 6 mm Polypropylene Fibre Differ from Monofilament Synthetic Fibres and Steel Fibre Systems?

In concrete fibre technology, the functional boundary between a fibrillated microfibre, a smooth monofilament microfibre, and a steel macrofibre is defined by geometry, modulus, and post-crack response. SikaFiber 6 FF is a fibrillated microfibre. During high-shear mixing, the pre-slit film tape splits along fibrillation lines into randomly branched fibril networks. This geometry creates a multi-filament structure with greater surface area and mechanical interlock potential than a smooth, round monofilament polypropylene fibre of equivalent denier. The fibrillated network distributes plastic shrinkage stress over a larger paste interface volume, which is why it is classed as a plastic-shrinkage control fibre rather than a structural macrofibre.

Compared with a monofilament polypropylene microfibre, the fibrillated form has fewer discrete fibre units per kilogram because the individual film sections carry more mass than fine circular filaments. The practical consequence is that a fibrillated product may provide fewer countable fibres per cubic metre at the same mass addition, but the mechanical anchorage per fibre is different because of the split-film morphology. In trowelled slab finishes, fibrillated fibres tend to remain incorporated within the near-surface paste matrix and are less likely to emerge as visible individual filaments than stiff monofilament macrofibres. In contrast to a steel fibre with a density near 7.85 g/cm³ and a tensile modulus near 200 GPa, the polypropylene fibre has a density of 0.91 g/cm³ and a modulus of 3.5 GPa. The resulting modulus difference is approximately 57-fold, which means SikaFiber 6 FF provides minimal structural flexural capacity. Its use in slabs and pavements is therefore limited to early-age cracking control, surface bleed reduction, and resistance to plastic settlement, not to the control of load-induced cracking after final set.

The product also differs from macro-synthetic polyolefin fibres, which are commonly 38 mm to 54 mm long, embossed or crimped, and designed to provide residual flexural strength in ASTM C1609/C1609M panel testing. At 6 mm, SikaFiber 6 FF does not function as a macrofibre and should not be specified where a measurable post-crack residual strength is required. In chloride-exposed or marine environments, the fibre eliminates the surface corrosion staining sometimes seen with steel fibres; however, that benefit applies only to surface aesthetics and durability of cover, not to the flexural capacity of the concrete section.

For projects requiring European conformity, polymer fibres for concrete are assessed under EN 14889-2:2006. The standard includes requirements for fibre dimensions, tensile properties, alkali resistance, and effects on concrete workability and strength. In North American practice, polypropylene fibre-reinforced concrete can be supplied and tested under ASTM C1116/C1116M, with synthetic fibres designated as Type III. The fibre itself does not produce a pozzolanic or hydraulic reaction and has no direct effect on cement hydration chemistry at the recommended addition rate.

In central mix plants using a twin-shaft compulsory mixer of 1.0 m³ or larger, the most stable batching sequence introduces the dry fibre with the coarse aggregate fraction before water and cement. Dry aggregate shearing separates the fibrillated film bundles before the paste phase generates hydraulic clumping. When the full 0.9 kg/m³ dose is dropped into a wet mix after a high-range water reducer has been added, localised agglomerates can form because the tape bundles collapse and become encapsulated by paste before individual fibrils separate. Plant-scale experience indicates that 4–5 minutes of mixing at maximum mixer speed after fibre addition is normally sufficient to disperse the 6 mm film into discrete fibrils in a non-air-entrained 25–35 MPa slump-range concrete. However, published data for this specific equipment configuration is limited when the mix also contains silica fume, viscosity-modifying admixture, or high dosages of polycarboxylate ether superplasticizer. Trial batching is therefore required before full production.

At 0.9 kg/m³, the water demand increase is generally less than 5 L/m³ when fibre is added to the aggregate phase. In air-entrained mixes, the fibrillated fibre can interfere with air-void stability if added before the air-entraining admixture has developed a stable foam. The preferred sequence is to add the fibre to the aggregate, then add air-entraining admixture with the batch water after the dry fibre has been distributed. If the fresh air content drops below the specified value after fibre addition, the air-entraining admixture dosage should be corrected at the plant rather than extending mixing time, because prolonged mixing can further destabilise the air-void system.

When a 0.9 kg/m³ Dose Is Used in a Truck-Mixed Ready-Mix Batch

In truck-mixed operations, the fibre is commonly introduced after the batching water begins to wet the aggregate surface. The drum is then held at mixing speed for 5 minutes or a minimum of 70 revolutions, whichever is longer, to separate the film bundles. Because the dry fibre has a specific gravity of 0.91 g/cm³, rapid addition into mixing water can cause it to float on the surface and accumulate in the drum throat or hopper. The material should be fed into the drum with the aggregate stream where possible. If fibre is added after the truck arrives at the placement site, the drum must be held at mixing speed for 4–5 minutes before discharge, not merely agitated. At ambient temperatures below 5 °C or above 35 °C, electrostatic charge and altered fibrillation behaviour can produce greater batching variability; a pre-construction trial load at the expected production temperature is recommended.

Fibre addition does not materially change setting time at 0.9 kg/m³ when evaluated by ASTM C403/C403M. However, the fibre reduces bleed water migration to the surface, which can shorten the window available between screeding and final finishing under high-evaporation conditions. Finishers may observe a faster surface crust formation in slabs containing fibrillated polypropylene fibre, particularly when evaporation rates exceed 1.0 kg/m²/h. In such conditions, application of evaporation retarders, fog misting, or temporary cover should begin earlier than on the same mix without fibre.

The product is packaged in pre-weighed bags matched to a 1 m³ dosing increment in many markets, although packaging formats vary by region. The bags are designed for direct addition to the mixer; they should not be introduced into a pump hopper or placed as a dry layer on a finished concrete surface. Storage should be dry and shaded. Polypropylene is sensitive to prolonged ultraviolet exposure, but embedded fibres are not exposed to UV radiation and are not subject to the oxidative degradation mechanisms that occur in exposed polymer sheeting.

Early-Age Crack Control, Impact Toughness, and Freeze–Thaw Duty

In restrained shrinkage testing, fibrillated polypropylene fibres at 0.9 kg/m³ typically reduce total plastic shrinkage crack area relative to plain concrete, but the magnitude depends on aggregate type, evaporation rate, fogging, cover practice, and paste content. Under controlled laboratory conditions using restrained slab panels, crack-area reductions of 60–80% have been reported for fibrillated microfibre systems similar to this product; however, the reduction is not uniform at high evaporation rates or when finishing operations overwork the surface. The fibre is not a substitute for early curing. Slabs placed in direct sun or wind without evaporation control may still develop plastic shrinkage cracking even with the specified fibre addition.

Impact abrasion and surface toughness show modest improvement because the fibrillated network bridges microcracks and reduces local stress concentration at early ages. The product is not a hardening agent. Surface abrasion resistance remains primarily a function of cement content, coarse aggregate hardness, finishing method, and curing. Freeze–thaw durability is governed by the air-void system, not by 6 mm polypropylene fibres. In air-entrained concrete with 5–7% fresh air, the fibres do not replace air entrainment and must not be specified as a freeze–thaw protection measure. If structural flexural resistance is required, reinforcement should be provided under EN 1992-1-1 or ACI 318-19, and SikaFiber 6 FF should be considered a durability and early-age crack-control addition only.

The known operational boundaries for this product include its low modulus relative to steel, its minimal post-crack flexural contribution, and its potential to ball when added incorrectly to high-range water reduced mixes. It is compatible with conventional Portland and blended cements, but combination with amine-based admixtures or other fibre types should be confirmed by trial mixing because changes in surface charge and paste rheology can alter fibre distribution. At the recommended addition rates, SikaFiber 6 FF functions as a secondary reinforcement system for plastic concrete and is specified only where early-age cracking control is the governing performance requirement.