SikaFiber 618 PPM is a monofilament polypropylene microsynthetic fiber supplied for secondary reinforcement of concrete and cementitious mortars. The product is based on a polypropylene polymer system and is introduced into the mix to contribute to control of plastic shrinkage cracking, reduction of early-age crack formation, and improved homogenous distribution of microcracking under restrained drying conditions. The numerical designation is consistent with supplier nomenclature for a fiber length of approximately 6 mm and an equivalent diameter near 18 µm, producing a high surface-area-to-volume geometry relative to fibrillated polypropylene fibers. The material is non-corrosive, hydrophobic, and chemically inert in normal cementitious environments, does not absorb mix water, and does not participate in cement hydration chemistry. These characteristics make it suitable for direct addition to ready-mix concrete, precast concrete, slabs-on-grade, screeds, mortars, and wet-process sprayed concrete where early-age crack control is specified as a durability measure under relevant construction standards.

SikaFiber 618 PPM Physical and Chemical Specification

The table below consolidates commonly reported physical and mechanical parameters for polypropylene monofilament microsynthetic fiber in the class represented by SikaFiber 618 PPM. Values are typical bench-level characterizations for the polymer class and should be verified against the manufacturer certificate of analysis, because batch-specific melt extrusion and draw conditions can shift tensile response by up to 10 %. Final compliance is determined by the declared values in the applicable Declaration of Performance under EN 14889-2:2006 and by classification under ASTM C1116/C1116M.

The monofilament form is produced by extrusion and drawing, which aligns polymer chains along the fiber axis. That alignment increases tensile strength relative to unoriented polypropylene film and provides consistent cross-sectional geometry for dispersion calculations. Because the fiber does not absorb water, it does not alter free-water demand by absorption, although it can influence the apparent yield stress and slump response of the mixture through particle interference and increased total surface area. The degree of slump modification is not linear with dosage and is affected by aggregate shape, paste volume, and admixture chemistry.

In ready-mix production, batch-to-batch dispersion variability is the primary processing risk. SikaFiber 618 PPM should be added to the coarse or fine aggregate stream before water addition or introduced into a mixer operating at high-shear speed. Addition of the loose fiber directly onto dry cement or into an idle mixer can create agglomerates that are difficult to separate once wetted. Pan mixers, planetary mixers, and twin-shaft mixers typically require 30–60 s additional mixing after fiber introduction for visual uniformity. Ready-mix truck mixers at low drum rotation may require 5–8 min of full-speed mixing after fiber dosing, depending on batch size and blade condition. The effect on slump can be evaluated under ASTM C143/C143M, with comparative measurements before and after fiber addition. Reported slump reductions for similar monofilament microsynthetics at 0.9 kg/m³ range from 10 mm to 30 mm; published data for this specific configuration is limited, and full-scale plant trials are required to establish dosage-specific adjustment ranges for high-range water-reducing admixtures.

Air content should be monitored under ASTM C231/C231M or ASTM C173/C173M, because fiber addition can alter air-void stabilization in air-entrained mixtures. The change is generally less than 1.0 %, but interaction with synthetic air-entraining admixtures may require re-verification of dosage. Placement and finishing should follow standard practice for fiber-reinforced concrete. Hard troweling of slabs with exposed monofilament fibers can produce surface fuzzing if the concrete is finished too early or too aggressively. Delaying final troweling until bleed water has dissipated and using a low-angled steel blade generally minimizes fiber protrusion.

What Limits Crack Control for Plastic Shrinkage in Monofilament Microsynthetics?

The function of SikaFiber 618 PPM in plastic shrinkage crack control is primarily geometric: the high count of fine monofilaments per unit volume distributes local tensile strains that develop during surface drying before final set. A dosage of 0.9 kg/m³ for the 6 mm fiber geometry provides on the order of several hundred million fibers per cubic meter, depending on the measured diameter and density. That fiber population reduces the coalescence of shrinkage microcracks into visible cracks under restrained evaporative conditions. The relevant test method is ASTM C1579-21, which uses a restrained slab panel exposed to forced airflow to evaluate plastic shrinkage cracking. In such tests, monofilament polypropylene microsynthetics are typically compared at dosages between 0.6 kg/m³ and 1.8 kg/m³. Crack width and aggregate crack length can be reduced substantially, but the response is sensitive to paste content, slag or silica fume content, air content, curing initiation, and ambient evaporation rate.

The product is not a structural reinforcement substitute. It does not provide post-cracking flexural capacity in the manner of steel fibers or macro-synthetic fibers with a diameter greater than 0.3 mm. Load-carrying capacity after cracking remains dependent on conventional reinforcing steel, welded-wire reinforcement, or structural macrofibers classified under EN 14889-2:2006 Class II or ASTM C1116/C1116M Type III macrofiber. Design calculations for flexural strength may not include SikaFiber 618 PPM unless a specific verified performance model exists in the project specification. Elevated dosages above 1.5 kg/m³ may improve crack control but can cause reductions in workability that require mix redesign and may not be economically justifiable without documented test data.

Plastic shrinkage cracking is strongly influenced by ambient conditions. The American Concrete Institute hot-weather guidance recommends limiting evaporation rate to below 1.0 kg/m²/h for conventional concrete. Monofilament fiber is an auxiliary measure and does not remove the need for fogging, evaporation retarders, damp subgrade preparation, and early curing, especially when air temperature exceeds 30 °C or wind speed exceeds 15 km/h. Without curing, plastic fiber cannot prevent cracking driven by surface tension gradients.

When Monofilament Geometry Is Preferred Over Fibrillated Polypropylene in Concrete

Polypropylene fibers for concrete are available in monofilament and fibrillated forms. Fibrillated fibers form a cross-linked net-like structure during mixing, whereas monofilaments remain as discrete cylindrical filaments. The monofilament form in SikaFiber 618 PPM is selected when surface finish, pumpability, and fine dispersion are more critical than post-crack pullout resistance. Compared with fibrillated polypropylene fibers of similar denier, monofilaments tend to disperse more readily in low-viscosity mortars and self-consolidating concrete, and are less prone to balling at moderate addition rates. The smooth geometric form also reduces visible fiber clustering in troweled surfaces.

The main technical trade-off is reduced anchorage. A single monofilament has no branching to develop mechanical engagement across a crack. Pullout resistance is governed by interfacial friction between the polypropylene surface and cementitious matrix. For this reason, monofilament microsynthetics are not considered for structural crack-widening control in hardened concrete. The product is differentiated from macro-synthetic fibers by diameter and length. Macro-synthetic fibers typically have diameters of 0.3–1.0 mm and lengths of 30–60 mm, and are used at dosages of 3–9 kg/m³ to produce measurable residual flexural strength. SikaFiber 618 PPM is used at lower dosage for early-age behavior and is classified as microsynthetic through the 18 µm diameter value.

The choice among these fibers is governed by specification language. If the specification requires a minimum residual flexural strength value under EN 14651 or ASTM C1609/C1609M, a microsynthetic monofilament such as SikaFiber 618 PPM is not suitable. If the specification requires crack-width reduction in the first 24 h after casting under ASTM C1579-21, the monofilament class is appropriate. Mixed fiber systems combining microsynthetic and macro-synthetic products may be used, but compatibility must be established through trial batching, because total fiber surface area and water demand can interact unpredictably with air entrainment and high-range water reducer.

For mortars, screeds, and repair materials, the fiber can be added to dry mix formulations or site-mixed products. In dry-mix mortars, pre-blending in a ribbon blender or similar low-shear mixer is required to avoid fiber segregation. Bulk density of the loose fiber is typically low, and handling systems should be designed to avoid entrained air pockets and static cling. Automated dosing equipment with pneumatic transfer may require static grounding and sufficient agitation to maintain free flow. In wet-mix shotcrete, SikaFiber 618 PPM may be introduced at the batch plant rather than at the nozzle, because its fine monofilament form is generally not compatible with nozzle-injection of high concentrations due to clumping risk. The plastic shrinkage benefit in sprayed concrete is secondary to the requirements for early-age curing and surface protection.

Use in concrete containing lithium nitrate admixtures, calcium nitrite corrosion inhibitors, or silica fume slurry does not inherently degrade polypropylene fiber performance, but mix stability must be verified because these admixtures can change plastic viscosity and alter fiber distribution. The fiber itself does not induce corrosion, does not support galvanic cells, and does not contribute to alkali-silica reaction. It is resistant to the alkaline pore solution of concrete, which typically has a pH above 12.5 during early hydration. Long-term chemical degradation of polypropylene in concrete is negligible under normal temperature and humidity conditions, but service temperatures above 80 °C may soften the polymer and reduce mechanical effectiveness.

Declared Performance Under EN 14889-2 and ASTM C1116/C1116M

SikaFiber 618 PPM falls under the scope of EN 14889-2:2006, “Fibres for concrete — Part 2: Polymer fibres — Definitions, specifications and conformity.” Because the equivalent diameter is less than 0.3 mm, the classification is Class I microsynthetic fiber. The standard requires the manufacturer to declare fiber length, diameter, tensile strength, modulus of elasticity, density, and alkali resistance, among other properties. The product must be accompanied by a Declaration of Performance under the Construction Products Regulation to be CE marked for use in concrete. The exact declared values for each batch may vary within the manufacturer’s acceptable tolerance and must be verified against the actual certificate.

Under ASTM C1116/C1116M, the product is classified as a Type III synthetic fiber-reinforced concrete fiber. Type III covers monofilament and fibrillated polypropylene and polyethylene materials that do not meet the strength and stiffness requirements of Type I steel or Type II glass fibers. This classification signals that the fiber may provide crack-control benefits but is not inherently identical to steel or glass in strength or modulus. Testing to ASTM C1579-21 or ASTM C1581/C1581M may be specified by the engineer for plastic shrinkage cracking and restrained shrinkage cracking, respectively.

The product does not require pre-soaking, and moisture content of the fiber is negligible under normal storage. At high relative humidity above 60 %, packaging should remain sealed to prevent condensation on the fiber surface, which can cause flow blockages in automated dispensers. The fiber is combustible; storage near open flame or ignition sources is prohibited. During concrete placement, the polymer fiber does not present the same hazard profile as steel fibers and does not damage pump hoses in the same manner, although static accumulation in dry feeding systems should be controlled.

No attempt should be made to use SikaFiber 618 PPM as a substitute for jointing, air entrainment, structural steel, or proper curing. Incompatible conditions include use in concrete intended for high-temperature refractory service above 80 °C, use in contact with strong oxidizing agents or hydrocarbon solvents, and use as the sole reinforcement in structural elements requiring a minimum post-crack tensile capacity. When applied in slabs with power trowels, the risk of surface fuzzing increases if the blade angle is too high or if finishing occurs while bleed water remains. The product is also not intended for external post-tensioning, anchorage, or stress-transfer applications. Where published test data cannot be located for a particular mix design, a preconstruction mock-up panel under site conditions is required before production concrete is placed.