How Does MasterFiber 320 Modify Plastic Shrinkage Cracking in Ground-Supported Slabs?

In ready-mixed concrete supplied for ground-supported slabs, MasterFiber 320 is treated as a microsynthetic Class II addition at 0.6–1.2 kg/m³, with most industrial floor mixes standardized at 0.9 kg/m³. The fibre is charged at the batching plant after coarse and fine aggregates have passed through the weigh batcher, either through an automatic fibre dispenser mounted above the truck mixer charging chute or directly into the central twin-shaft mixer. Batch records from central mixers with twin horizontal shafts require a wet mixing interval of 4–5 minutes after fibre addition at ≥12 rpm to disperse the monofilaments through a 20–40 mm slump mix without forming fibre balls. The product falls under EN 14889-2:2006 Class II and ASTM C1116/C1116M-10a Type III, and the CE marking is issued under Construction Products Regulation EU 305/2011. On-site slump is measured to EN 12350-2:2019, while plastic shrinkage cracking can be evaluated by ASTM C1579-21 on restrained ring specimens. Downstream, the concrete is placed by laser-guided screed or conventional wet screed, then power-trowelled; careful timing of trowel passes is required because the fibre can hold bleed water at the surface slightly longer than an unmodified mix. Finished product types include warehouse floor slabs, external freight aprons, parking decks, machine foundations, and interior slabs on grade. The fibre is not a substitute for structural steel reinforcement, and its addition does not alter the slab thickness design carried out under ACI 360R-10; it functions only as a plastic cracking control component.

Because MasterFiber 320 is hydrophobic and does not absorb mixing water, no pre-drying is required even at aggregate moisture contents above 6 %. The main processing bottleneck on automated batching lines is fibre bridging in screw conveyors if the hopper outlet is narrower than 150 mm; low-speed augers with knurled flighting or belt feeders are preferred over pneumatic blowlines with sharp bends, which can generate static and cause agglomeration at the delivery point. The fibre is exempt from registration as a polymer under REACH Regulation (EC) No 1907/2006, Article 2(9); the propylene monomer is registered and handled according to the manufacturer’s safety data sheet. In truck-mixer use, the batch should not be discharged before 70–100 revolutions after fibre addition, unless plant trials confirm a shorter mixing interval. A visual check at the hopper is required to detect early fibre balls, and any clumps found before discharge must be removed by passing the concrete through a 50 mm bar screen at the pump hopper.

Dosage Thresholds During Demolding of Thin-Wall Precast Panels

Precast plants operating zero-slump or 10–30 mm slump mixes for thin-wall architectural panels and utility vaults use MasterFiber 320 at 0.9–1.8 kg/m³ to reduce arris spalling and microcracking during mold stripping and stacking. The fibre is added in a planetary pan mixer after dry blending of cement and aggregate for 30–60 seconds; because polypropylene has a melting point above 160 °C, standard steam-curing cycles at 50–65 °C do not induce visual filament degradation. Compliance for the fibre remains EN 14889-2:2006 Class II and ASTM C1116/C1116M-10a Type III; the precast element itself is subject to EN 13369:2018 and the relevant product standard. Demolding is scheduled after the plant’s minimum stripping strength is confirmed on companion cubes tested to EN 12390-3:2019, not on calendar time alone. Downstream production uses external vibrators or vibrating tables until air discharge ceases, followed by stack curing or accelerated steam curing. Finished product types include thin-wall cladding panels, boundary wall units, drainage channels, transformer vaults, and small modular retaining-wall elements. A key operational boundary is that MasterFiber 320 is not intended to provide post-crack residual strength in precast concrete; if a project specifies flexural toughness for a load-bearing element, a macro-synthetic Class I fibre or steel fibre should be evaluated separately.

The production constraint in thin-wall precast is early strength gain because panels are often lifted from the mold at 10–15 MPa cube strength; the fibre does not participate in strength gain, but it reduces brittle edge damage at the same demolding strength. Steam-curing regimes should not exceed 65 °C if the fibre is stored beside the kiln or if hot water is used, not because of filament melting but because any accidental contact with surfaces above 160 °C will soften the polypropylene and create deformed filaments that no longer disperse properly in a later batch. Plant quality control records batch-to-batch fibre length and tensile strength under EN 14889-2:2006 Annex A; deviations in fibre cut length greater than ±5 % from the certified value will change the number of filaments per kilogram and should trigger a supplier corrective action.

Wet-mix shotcrete supplied to NATM headings and decline development is batched with MasterFiber 320 at 1.0–2.0 kg/m³, primarily as a cohesiveness and plastic cracking control component rather than as a rock-support reinforcement fibre. The fibre is incorporated during aggregate charging at the batching plant, and the wet-mix is transported in truck mixers before being pumped through a piston-type shotcrete pump with accelerator dosing at the nozzle. Spraying in layers of 50–150 mm per pass is performed on scaled and water-jetted rock surfaces, and the applied shell is specified under EN 14487-1:2005; fibre conformity is controlled under EN 14889-2:2006 Class II. Contractors record rebound as a percentage of nozzle output during trial sprays, and experience on site indicates that MasterFiber 320 improves cohesion and reduces sag in overhead zones, though published data for this specific configuration is limited. Finished product types include temporary rock support shells, portal stabilization linings, underground chamber skin walls, and slope stabilization facings. The fibre shall not be used as a direct replacement for steel fibre or macro-synthetic fibre where shotcrete energy absorption classes are specified; for those systems, the lining design must be verified by panel and beam testing to EN 14488-5:2006 or the relevant project-specific method.

The main field failure mode is not fibre degradation but nozzle blockage when fibres are added to a mix that already contains steel fibres at high dosage. MasterFiber 320 should be added only after the steel fibre has been fully dispersed, because the mixed fibre types can form interlocking agglomerates in the hopper screen and at the S-tube transfer point on piston shotcrete pumps. Spray nozzle operators report that the material remains pumpable at slumps down to 120 mm, but below that value the pressure at the piston head increases more rapidly in lines longer than 60 m; this is a practical boundary for a site batching setup. The polypropylene filaments pass through the nozzle and do not obstruct the accelerator ring, provided the accelerator is injected downstream of the air ring. Quality control of the hardened shell uses core specimens according to EN 12504-1:2019 and fibre distribution checks on sawed panels.

When Polypropylene Microfibers Are Dry-Blended into Binder Systems

When MasterFiber 320 is supplied to producers of cementitious self-leveling underlayments and bagged screed compounds, the fibre is metered into the dry blend at 0.1–0.3 % by total dry weight, which corresponds to approximately 0.6–1.5 kg/m³ after mixing at the specified water-to-powder ratio. Ribbon mixers with paddle-tip clearance of 5–10 mm are used for 120–240 seconds to break electrostatic clumps and distribute the hydrophobic polypropylene filaments across the binder, silica sand, calcium carbonate, and redispersible polymer powder. The dry mortar is then bagged or silo-delivered, and on site it is mixed with a continuous or batch mixer, pumped through a rotor/stator pump, and adjusted to a thickness of 3–30 mm with a gauge rake and spike roller. Fibre conformity is assessed under EN 14889-2:2006 Class II, while the screed mortar itself is specified under EN 13813:2002 with the declared wear resistance, strength, and drying profile determined by the finished-floor system. Finished product types include self-leveling underlayments under LVT and sheet flooring, renovation screeds over embedded services, heated floor encapsulations, and acoustic isolating layers. Processing limitations include the need to avoid extended high-shear mixing after hydration begins, because the fibre can trap air if the mortar is over-mixed and air-release admixtures are not matched to the modified rheology.

Because the fibre is dry, stored moisture below 0.5 % is not required due to the polymer’s hydrophobicity, but bags should be kept closed to prevent extraneous paper fragments from entering the batch. In continuous slurries, the addition rate must be recalculated when the water-to-powder ratio shifts from 0.19 to 0.24, because fibre content per volume changes with pumpable viscosity. Production-scale trials on 25 kg bagging lines show that pre-blending order has a measurable effect: adding the fibre to the sand before the binder reduces fibre float during mixing and improves visual distribution in the hardened screed. The material does not require pre-drying at relative humidity above 60 %, and no corrosion protection is required because polypropylene is inert in alkaline cement paste.

For hand-applied and machine-sprayed structural repair mortars specified under EN 1504-3:2006, MasterFiber 320 is pre-blended in the dry mortar at 0.5–1.2 kg/m³ to control plastic shrinkage cracking on exposed vertical and overhead patches. The repair process begins with substrate preparation by hydrodemolition, grit blasting, or mechanical grinding to remove laitance and friable concrete, followed by saturation to a surface-dry condition. The mortar is mixed in a low-speed paddle mixer or continuous mortar mixer, and the fibre remains dimensionally stable through the mixing cycle, with no measurable effect on the open time or pot life of the hydraulic binder. Compressive strength of the repair mortar is assessed by EN 12190:1998, adhesion by pull-off testing to EN 1542:1999, and the fibre itself is certified under EN 14889-2:2006 Class II. Finished product types include edge reinstatements, soffit repair patches, concrete repair overlays from 5–40 mm, and corrosion-protection render coats after reinforcement priming. The fibre does not increase the structural capacity of a repaired section, and any load-bearing repair must include the original reinforcement or supplementary steel according to the structural design.

The operational boundary for repair mortar is ambient temperature; application below 5 °C is not recommended unless heated enclosures maintain the substrate above 5 °C for the first 24 hours, but this is a cement hydration limitation rather than a fibre limitation. The fibre does not influence adhesion if surface preparation and repair profiling are carried out correctly; adhesion failures recorded in pull-off tests are typically substrate failures in unprepared or saturated concrete, not fibre-related. In machine-sprayed repair, nozzle size should be no smaller than 8 mm when MasterFiber 320 is used at the upper dosage, because smaller orifices increase wear and can create fibre roping if high pressure is applied.

Precast drainage products and non-pressure pipe segments manufactured on automated carousel lines incorporate MasterFiber 320 at 0.6–1.5 kg/m³ to reduce reject rates linked to corner spalling and hairline cracking during steam curing, demolding, and yard stacking. The concrete is produced in a counter-current pan mixer or twin-shaft mixer with a total mixing time of 120–180 seconds, and the fibre is added after the dry constituents but before or with the first portion of mixing water to avoid late wet-stage clumping. The pipe or box culvert elements are compacted by vibration, steam-cured at temperatures not exceeding 65 °C, and then moved with vacuum lifters or sling systems; the microfibre addition is specified only for crack control and does not modify the structural design. Fibre conformity under EN 14889-2:2006 Class II and ASTM C1116/C1116M-10a Type III is maintained in the plant’s factory production control; the final precast product is covered by EN 13369:2018 together with EN 1916:2002 for pipes or EN 14844:2006 for box culverts where applicable. Finished product types include reinforced and unreinforced drainage pipes, box culvert sections, cable troughs, and stormwater channel units. Operators should not exceed the upper dosage without a plant trial, because elevated fibre volume can reduce the visual surface consistency of trowelled or slipformed surfaces in high-alkalinity mixes.

The critical threshold in dry-cast and wet-cast drainage product production is vibration energy. If the paste fraction is too low or vibration is applied for too long, polypropylene fibres can migrate toward the upper surface and become visible as a fine hair on the interior pipe surface; this does not affect hydraulic performance but may be a visual defect in products with strict surface appearance requirements. Plant trials should therefore fix mixing time, fibre addition point, and high-frequency vibration duration before serial production. Because the fibre is chemically inert, it does not contribute to sulfate attack resistance or reduce permeability directly; the concrete mix design remains the primary control for aggressive effluent exposure.