SikaFiber 658 X is a monofilament polypropylene microfibre specified for plastic shrinkage crack control in cementitious systems. In concrete, the reference addition is 0.6 kg/m³; the upper practical addition of 0.9 kg/m³ is reserved for pours with surface wind speed above 5 m/s, concrete temperature above 30 °C, or relative humidity below 40%. Under EN 14889-2:2006 the fibre is classified as Class I because its diameter is below 0.3 mm; under ASTM C1116/C1116M-23 it is a Type III synthetic-fibre reinforcement. The volume fraction at 0.6 kg/m³ is approximately 0.066%, which is too low to alter structural flexural capacity, but sufficient to place a high population of fine filaments across meniscus and bleed-channel discontinuities that form in the first 6 h after casting.

Plastic Shrinkage Crack Narrowing in High-Surface-Area Industrial Floor Pours

Cast-in-place industrial slabs with surface areas above 1,000 m² and thicknesses from 150 mm to 250 mm are routinely placed with laser screeds and power trowels. The plastic shrinkage risk is highest when the evaporation rate exceeds 1.0 kg/m²/h, a condition commonly produced by wind speed above 5 m/s, concrete temperature above 30 °C, or relative humidity below 40%. SikaFiber 658 X is dry-charged at 0.6 kg/m³ together with coarse aggregate before 60% of the batch water is admitted, so that the monofilaments are separated by interparticle shear rather than suspended in already lubricated paste. In truck mixers the batch is mixed at full charging speed for not less than 4 min after the last component is added; shorter mixing creates visible fibre clumping when the truck waits more than 20 min to discharge. The wet-concrete rheology changes by a measurable slump loss of 10–20 mm at the same water-cement ratio, which is compensated by a mid-range water reducer rather than by additional water. Power trowel finishing is started only after the surface bleed sheen disappears; early closure can pull fibres into the burnished layer and produce surface fuzzing under high blade pitch. In comparative plastic cracking tests run under ASTM C1579-21, equivalent polypropylene microfibre additions at 0.6–0.9 kg/m³ have been reported to reduce crack area by 50–80% compared with plain control slabs, with the greater reduction recorded at evaporation rates above 1.0 kg/m²/h. The resulting terminal product is a jointed industrial floor for rack loading, reach trucks, AGV lanes, and high-bay logistics; saw-cut joint spacing and doweled load transfer at construction joints remain governed by ACI 360R-10 and ACI 302.1R-18. The microfibre does not justify widening joint spacing, removing steel at column capitals, or eliminating dowel baskets at planned construction joints.

In precast concrete operations where demoulding occurs between 6 h and 18 h after casting, edge chipping during form stripping and handling accounts for a reproducible share of unit rejects. A twin-shaft compulsory mixer is charged with dry aggregates and SikaFiber 658 X at 0.9 kg/m³; the fibre is blended for 15–20 s before cementitious binder is added, which prevents hydrophobic filaments from floating on the paste surface. The concrete is typically specified at water-cement ratios below 0.45, often with a polycarboxylate superplasticizer, and placed into steel moulds with high-frequency external vibration. Steam curing at 55–70 °C with a heating rate not exceeding 20 K/h is used to reach demoulding strength within 8–16 h; the polypropylene softening range above 150 °C does not interact with this curing window. The presence of 0.9 kg/m³ of microfibre contributes mainly to green-edge integrity and handling toughness rather than to hardened load capacity. Precast elements such as manhole rings, cable troughs, concrete lintels, and stormwater box culverts benefit from reduced edge damage when stacked and transported. Exposed-aggregate and acid-etched finishes require validation because fine fibres can remain visible at the surface after retardation and washing; sandblasting or form-facing can cover this condition. Compliance is demonstrated through EN 13369:2018 for common precast products and EN 14889-2:2006 for fibre conformity, while the product is not substituted for structural reinforcement required by Eurocode design.

What Limits Fiber Dispersion When Microfiber Is Charged After Polycarboxylate Ether Superplasticizer in Central Mix Plants?

For elevated composite floor slabs placed over profiled steel deck, ready-mix concrete is commonly produced in a central mix plant and transported in agitating trucks for 30–60 min. The elevated slab is cast at 120–150 mm thickness over deck crests, and the top surface is exposed to elevated air movement on upper floors; plastic cracking concentrates at the negative moment region over each deck rib before composite action develops. SikaFiber 658 X is specified at 0.6 kg/m³ for standard exposure and 0.9 kg/m³ for high-rise placements with through-floor wind. The critical charging sequence is fibre with dry aggregates first, followed by cementitious materials, then 70% of batch water, and finally a polycarboxylate ether superplasticizer diluted in the remaining water. When the superplasticizer is added before the fibre, the paste is already deflocculated and shear-thinned; the remaining mixing energy is less effective in separating hydrophobic monofilaments, and clumps become visible at the discharge gate and at the pump hopper. Plant experience on equivalent 12 mm monofilament polypropylene microfibre indicates that the post-superplasticizer addition route increases the observed count of fibre agglomerates in the first 2 m³ discharged compared with dry-batch addition, although published data for this specific configuration is limited. Pumping through a deck-mounted boom at 30–40 m³/h applies additional shear to the slurry; line pressure may rise by 5–15% relative to an otherwise identical plain concrete at the same slump, an increase that must be accounted for in pump-capacity checks. The finished composite slab is later swept and power-floated; the microfibre controls early-age crack width until sawing or hand cutting of contraction joints is completed at 12–24 h. Shear stud capacity, deck deflection, and fire insulation are not improved by the fibre, and structural design remains within SDI C-2017 and ACI 318-19.

Where wet-process shotcrete is applied as a temporary slope protection lining or as an erosion-resistant drainage channel lining, the microfibre is incorporated at 0.9–1.8 kg/m³ into a pumpable concrete with a maximum aggregate size of 8 mm. Wet-mix shotcrete is delivered by a positive-displacement rotor/stator pump with an air supply introduced at the nozzle; the concrete slump immediately before the pump is maintained at 180–220 mm to avoid plugging, and an alkali-free set accelerator is dosed at the nozzle at 3–6% by mass of binder. SikaFiber 658 X is dry-charged at the batching plant with fine aggregate, not through the nozzle, because post-nozzle fibre addition lacks the mixing residence time needed to separate hydrophobically charged filaments. The addition changes pump pressure minimally in the first 200 m of line, but pumping distances above 150 m require confirmation of the pump capacity because the fibre increases paste yield stress in low-paste shotcrete mixes. Rebound measured at vertical test panels under EN 14487-1 and EFNARC guidance is reduced by the fibre only in the fine-fraction rebound; the reduction is commonly 5–10% in field trials, but it is insufficient to correct an improperly adjusted air stream or accelerator dose. The cured shotcrete layer is specified for erosion control and temporary face treatment, not for permanent structural ground support; where post-crack flexural performance is required, a steel or polymer macrofibre conforming to EN 14889-2 Class II and energy absorption testing under EN 14488-5 must be substituted or combined with the microfibre. The terminal product is a thin shotcrete shell, typically 50–120 mm thick, applied to cohesionless slope faces or drainage ditches.

Across the wet-cast and sprayed routes, the addition rate is not interchangeable because mixing energy, pump shear, and bleed conditions differ. The following compliance matrix identifies the governing test standards and the operational boundaries that remain in force.

When Dry-Mix Repair Mortars Are Processed Through Horizontal Ploughshare Blenders

For bagged cementitious repair mortars and self-leveling underlayments, SikaFiber 658 X is used as a dry-component additive at 0.5–1.5 kg per tonne of dry blend. The addition is made during the dry cycle of a horizontal ploughshare blender or a ribbon mixer, before liquid polymer dispersions or redispersible polymer powders are introduced, because the fibre must be distributed by mechanical shear against the sand particles. The mixed sand and fibre are conditioned to a moisture content below 0.3% and the blender is grounded to dissipate static charge; otherwise, hydrophobic polypropylene filaments may cling to the blender walls and appear later as fibre-rich lumps in the bag. Mixed materials are packed in moisture-resistant bags and later site-mixed with a prescribed water content, typically 4.0–5.5 L per 25 kg bag for trowel-applied repair mortars. In thin overlays from 5 mm to 30 mm, the microfibre reduces plastic cracking at edges, around embedded steel, and over substrate joints. The mortar is specified under EN 1504-3 as a structural or non-structural repair product depending on its compressive strength class; the addition of microfibre does not change the R3/R4 classification because these classes are defined by mechanical properties rather than fibre content. Repair products used in contact with drinking water or in aggressive chemical environments must be separately assessed; the microfibre itself is inert, but the matrix permeability and surface defect rate determine the final durability. The terminal product is a trowel-applied patch mortar, a floor-levelling compound, or a waterproofing slurry applied to balconies, concrete repair areas, and internal screed edges.

Concrete Pavement and External Slab Jointing Under Restrained Temperature Gradients

External concrete pavements, container yards, and airport service roads are placed at slab thicknesses from 200 mm to 350 mm, often with fixed-form or slipform pavers. In these placements the microfibre is added at 0.9 kg/m³ to control plastic shrinkage and early-age thermal microcracking before the first contraction joint is sawed. The concrete is produced to a water-cement ratio of 0.40–0.45 and is consolidated by vibration; slipform paving requires a slump of 20–40 mm, so the fibre-induced workability reduction is compensated by the mix design rather than by adding water. After paving, the surface is immediately sprayed with a resin-based curing compound meeting ASTM C309, and the joints are sawed to a depth of one-third of the slab thickness at an early age, typically 6–24 h, before the sawed joint opens due to restrained thermal contraction. The microfibre narrows the plastic cracks that may occur at reentrant corners around inlet boxes and light-pole bases; it does not control the later thermal expansion and contraction of hardened concrete. Dowel bars, tie-bars, joint sealing, and base preparation remain mandatory because the fibre provides no load transfer across contraction joints. For durable exterior concrete, the mix is also tested for freeze-thaw resistance under ASTM C666/C666M; the microfibre does not entrain air, so air-entraining admixture dosing remains the primary protection against scaling. The terminal product is a jointed concrete pavement or container terminal slab with unloaded crack widths controlled during the first day of placement but with conventional joint patterns retained for mature thermal behaviour.