SikaFiber 25 AE is a polypropylene macro-synthetic fibre for concrete reinforcement. The product identifier encodes nominal length as 25 mm and an AE alkali-stable additive package. The fibre is supplied as an embossed monofilament with an equivalent diameter of 0.45 mm, giving an aspect ratio of 55.6. Density is 0.91 g/cm³ when tested to ISO 1183-1:2019. The product carries CE marking under EN 14889-2:2006 as a Class II polymer fibre for structural use in concrete, mortar and grout, and it can be evaluated under ASTM C1116/C1116M Type III for synthetic fibre-reinforced concrete. The functional boundary is post-crack bridging of macro-cracks; SikaFiber 25 AE is not specified as a replacement for primary steel reinforcement in load-bearing members unless the structural design explicitly accounts for the fibre contribution. At the supplier-recommended dosage of 3.0–6.0 kg/m³, the fibre occupies 0.33–0.66% of concrete volume.

How does dosage influence residual flexural strength under notched beam loading?

Post-crack performance is assessed primarily by flexural beam tests. EN 14651:2005+A1:2007 uses a 150 × 150 × 550 mm beam with a 25 mm notch and measures load–CMOD response. ASTM C1609/C1609M-19a uses a 100 × 100 × 350 mm beam under third-point loading. In both procedures, the transition from first-crack behaviour to residual capacity depends on fibre count, anchorage geometry, and matrix-fibre interfacial friction. The embossed surface of SikaFiber 25 AE increases mechanical pullout resistance compared with smooth monofilaments; residual load values for embossed macro-synthetic fibres are commonly reported in the range of 30–45% of first-crack load at 3 kg/m³ and 40–60% at 6 kg/m³ for ordinary 35 MPa concrete, but project-specific verification is required because aggregate type, mixing energy, and curing modify the post-crack envelope. Published independent data for this exact product under all concrete strength classes is limited; supplier-published flexural curves should be confirmed on the project mix.

The dosage-response curve is non-linear. Increasing dosage from 3 kg/m³ to 4 kg/m³ typically raises the equivalent flexural strength ratio f_R,1/f_L by roughly 8–12% in C30/37 concrete; the increment from 5 kg/m³ to 6 kg/m³ may be smaller due to fibre clumping and workability loss. The critical processing constraint is therefore not fibre addition alone but the ability of the mixer to distribute fibres without bundling. A paddle mixer with 28–32 rpm mixing speed and 4 minutes wet mixing after fibre addition typically yields a fibre distribution coefficient of variance below 5%, based on washout tests of 10 L fresh concrete samples. Free-fall mixers show higher variance and occasional agglomerations at ambient temperatures below 10°C.

In full-scale batching operations, the fibre is added to the wet concrete after aggregates, cement, and 80% of the batching water have been combined. Adding SikaFiber 25 AE to dry materials is not recommended because the 0.91 g/cm³ density and electrostatic charge can cause fibres to cling to mixer walls and form ball clusters. In a 500 L compulsory pan mixer, a charged bag is dispersed by wet mixing for 4–5 minutes at 28–32 rpm; in a truck mixer, 5 minutes at 12–15 rpm is used per 3 m³ load. Slump measured to EN 12350-2:2019 decreases by 20–40 mm when dosage exceeds 4 kg/m³; a polycarboxylate ether superplasticizer is normally adjusted in 0.1% increments by mass of cement to restore rheology. The concrete temperature at mixing should be within 5–30°C. Fibre addition does not alter the air-void system appreciably at dosages below 4 kg/m³, but dosages above 5 kg/m³ can increase air content by 0.5–1.0% and should be checked under EN 12350-7:2019. Placement and compaction follow conventional methods; internal vibration should not be applied to a single point for more than 10 seconds to avoid fibre orientation loss near the surface.

When steel fibre is substituted in ground-supported slab design

Substitution logic is based on mass per cubic metre and corrosion resistance. Steel fibres conforming to EN 14889-1:2006 are typically dosed at 20–40 kg/m³, whereas SikaFiber 25 AE is dosed at 3–6 kg/m³. The density differential explains the mass differential: 7.85 g/cm³ for steel versus 0.91 g/cm³ for polypropylene. The lower elastic modulus of polypropylene (4.2 GPa versus 210 GPa) means that at equal fibre volume, steel provides higher post-crack tensile stress at small crack openings; at high deformation, macro-synthetic fibres avoid brittle fracture and do not corrode. For external slabs exposed to de-icing salts, SikaFiber 25 AE eliminates surface rust staining observed with steel fibre. In acid environments with pH below 5.5 or high chloride exposure, steel fibre is more vulnerable to corrosion while polypropylene is inert. The design switch is not mass-for-mass but based on residual flexural strength requirements derived from ASTM C1399/C1399M or EN 14651. For a given slab thickness and subgrade modulus, replacing 25 kg/m³ steel fibre with 5 kg/m³ SikaFiber 25 AE may require an increase in slab thickness of 10–20 mm to maintain equivalent crack-width control, depending on joint spacing and load magnitude. Published comparative data for this exact substitution in industrial floors is limited; it should be verified by trial panels.

Compared with micro-synthetic polypropylene fibres of 6–12 mm length, the 25 mm length and embossed surface shift the function from plastic shrinkage crack reduction to post-crack toughness. Microfibers are added at 0.9 kg/m³ and act mainly during the first 24 hours; SikaFiber 25 AE remains in the hardened matrix and resists crack opening after 7 and 28 days. Compared with glass macro fibres, polypropylene has lower tensile strength and modulus but demonstrates stable performance in high-pH concrete without special sizing selection; glass fibre requires alkali-resistant sizing and can lose strength in wet alkaline environments if incorrectly specified.

Mixer torque, slump retention, and pumpability boundaries

Fibre addition changes mixer energy demand. A 0.33–0.66% volume fraction of flexible fibres raises mixer torque by 5–8% in twin-shaft mixers compared with plain concrete; the torque rise is lower than that of steel fibre because of the lower density and higher aspect ratio but can increase with bundle formation. Pumping trials through a 50 mm diameter steel line have shown no blockage at dosage 4 kg/m³ and slump 120–160 mm; at 6 kg/m³ and slump below 100 mm, line pressures increase and hose pulsation becomes visible. Addition of more than 6 kg/m³ is outside the supplier-recommended range and may cause fibre ball accumulation at the hopper grate. Finishability is affected by surface fibre protrusion; power trowelling should follow initial bleed-water disappearance and blades should be set at low angle. Exposed aggregate or hard-trowelled architectural finishes may require a final 1–2 mm skimming coat of plain mortar if fibre protrusion is unacceptable. Storage limits are 5–35°C in unopened bags; high humidity above 60% does not require pre-drying because the polymer does not absorb water, but wet bags should not be added as intact units because the bag polymer may not disperse and can clog the pump. The product is not intended for use in concrete autoclaved above 160°C or in contact with strong oxidizing acids.