CHRYSO Crackstop F is a fibrillated polypropylene fiber supplied for addition to concrete and cementitious mortars. The raw polymer is stretched into film and slit into net-like fibrils that open during mixing, creating a fine three-dimensional distribution in the fresh paste. The primary performance function is control of plastic shrinkage cracking before initial set, when capillary tension in the drying surface exceeds the low tensile capacity of fresh concrete. The fiber is hydrophobic, has a density of approximately 0.91 g/cm³, is chemically stable in the alkaline cement environment, and does not corrode or contribute to galvanic corrosion. Published polypropylene fiber data place the elastic modulus in the range of 3.5–8.0 GPa, which is far below the modulus of the hardened concrete matrix; the product therefore functions as a micro-reinforcement for crack formation control, not as a structural load-bearing fiber. Under EN 14889-2:2006, polymer microfiber of this type is evaluated for conformity as a non-structural fiber for plastic shrinkage control, and it is not interchangeable with steel macrofibers in structural design. The addition does not require mix water adjustment, but it does require precise batching and dispersion control. The following scenarios are downstream applications where the material appears in production practice. The dosage ranges cited are typical starting points from concrete and shotcrete practice; the manufacturer’s current product data sheet remains the controlling document for formulation.

In industrial slab-on-grade production floors, plastic shrinkage cracking is the dominant early-age surface defect when unfavourable evaporation rates occur during placement and finishing. The fiber is added at a rate of 0.6–0.9 kg/m³, preferably through a silo-fed dosing screw into the aggregate stream before water and cement are added to a twin-shaft compulsory mixer. This sequence prevents fiber balling, which is observed when whole bags are thrown into wet concrete under high mixer speed. ACI 544.3R-08 summarises comparative restrained-evaporation data showing crack area reductions commonly in the range of 60–90% for fibrillated polypropylene fibers at 0.9 kg/m³ compared with plain reference panels. The placement is specified as Type III synthetic fiber-reinforced concrete under ASTM C1116/C1116M-23, and plastic shrinkage crack potential is assessed by ASTM C1579-20. Slump loss associated with the fiber is typically 10–25 mm, depending on water-cement ratio and polycarboxylate ether superplasticizer dosage; water should not be added solely to compensate for fiber-induced slump loss because that increases bleed and shrinkage. Finishing trials on production floors show that the slab must be bull-floated or pan-floated before steel trowel blades are used. If the finisher moves directly to trowel blades at high angle, dry surface fibers can increase blade drag and cause surface tearing. Joint sawing windows remain 6–12 h after final set, and fiber addition does not eliminate contraction joints. The fiber does not improve abrasion resistance under hardened-wheel traffic; wear performance remains dependent on aggregate hardness, water-cement ratio, and curing.

Does Fibrillated Fiber Increase Wet-Mix Shotcrete Pump Pressure?

In wet-mix tunnel linings and slope stabilization, the addition of fibrillated PP fiber modifies fresh-state rheology and rebound behaviour rather than hardened compressive strength. The most common addition is 0.9–1.5 kg/m³ for crack control in temporary and permanent linings; at 2.0 kg/m³ and above, S-valve piston pumps may show increased line pressure and hopper bridging if fiber bags are discharged directly into the hopper. Field records indicate that fiber should be pre-blended with fine aggregate or metered through a silo screw into the charging sequence. In dry-mix shotcrete, fiber added after pre-dampening or at the nozzle creates uneven distribution and excessive rebound; it should be introduced at the pre-dampening stage so the transport air carries a homogeneous aggregate-fiber stream. Rebound of fibrillated PP fiber in dry-mix shotcrete can be 20–50% under non-optimum nozzle angles or air flow, requiring batch adjustment. The shotcrete matrix is tested under EN 14488-2:2006 for compressive strength, while fiber conformity is documented under EN 14889-2:2006. The presence of PP fiber does not replace welded wire mesh or rock bolts; it is a crack-control addition and not a structural bending reinforcement. When tunnel linings are designed for fire exposure, the dosage is typically increased to 1.5–2.0 kg/m³ for spalling reduction. No significant pump blockage is observed below 1.5 kg/m³ when the nozzle is held at 90°±10° to the substrate and the hopper is not allowed to run empty.

Precast Mixing, Vibration, and Demoulding Variables

Precast production lines use the fiber at 0.6–1.2 kg/m³ in architectural cladding, utility boxes, and non-structural elements to reduce shrinkage cracking before curing is effective and to limit chipping during demoulding. In pan mixers and twin-shaft mixers, the fiber is added to dry aggregate before water and admixture; reverse-order addition can produce fiber balls. External vibration at 3,000–6,000 vpm on steel moulds can orient low-modulus fibrillated fibers parallel to the mould face. This tendency reduces visible cracking on the exposed surface but reduces fiber availability through the section and may produce sampling variability in wash-out tests. At 1.0 kg/m³, slump reduction of 10–20 mm is typical; the producer should adjust polycarboxylate ether superplasticizer rather than increase water to maintain workability. The demoulding time is governed by the binder system, and the fiber does not permit earlier stripping than the minimum strength required by EN 13369:2018. In thin architectural panels, the fiber does not substitute for alkali-resistant glass scrim, steel connections, or handling steel because it cannot provide structural flexural capacity. Plant trials should include a mixing time study, because prolonged high-shear mixing above 90 s after fiber addition can damage the fibrillated structure and create re-balled clumps in low-water mixes.

Fire Spalling Mitigation Is Governed by Fibre Melting Point and Permeability Gain

High-strength, low-permeability concretes are vulnerable to explosive spalling during fire because water-vapour pressure develops faster than moisture can escape through the matrix. Polypropylene fiber, including fibrillated types, melts at 160–170°C and leaves continuous channels that raise heated permeability. The critical variable is dosage. Published experimental programmes on concrete with compressive strength between 60 MPa and 100 MPa generally require 1.5–3.0 kg/m³ of polypropylene fiber to reduce spalling, but the minimum threshold changes with aggregate type, initial moisture state, heating rate, and section confinement. Fibrillated fiber may distribute melted channels more uniformly because of its higher surface area than monofilament fiber, but published independent data for this specific CHRYSO fiber configuration is limited. The design fire exposure is defined by ISO 834-1:1999, ASTM E119-22, and EN 1992-1-2:2004; hydrocarbon curves such as RWS used in tunnels impose a more severe heating rate, and standard curve results do not automatically validate hydrocarbon spalling resistance. At 2.0 kg/m³, the fiber reduces workability of low water-cement ratio concrete, usually requiring a higher polycarboxylate ether superplasticizer dosage to maintain a spread flow of 550–650 mm. The fiber must also be protected from sustained exposure above 120°C before placement because softening or distortion before fire reduces its spalling-control function.

For bonded repair mortars and cementitious screeds, polypropylene fibrillated fiber at 0.6–1.0 kg/m³ reduces early-age plastic shrinkage cracking in thin applications where water loss is rapid and the existing substrate restrains free movement. In paddle mixers, the fiber is added gradually into the running mixer because the lower mixing energy of a paddle mixer cannot disperse a full bag added at once; fibre clumping is a common batch failure. The repair product is specified under EN 1504-3 for structural and non-structural repair and under ASTM C928/C928M-20a for packaged dry materials. The fiber functions as plastic shrinkage control and does not provide load-bearing structural strengthening; structural repair capacity must come from the existing concrete section, added reinforcing bar, or approved structural reinforcement. In leveling screeds at 20–50 mm thickness, the fiber may reduce edge-curling cracks during drying but cannot correct substrate instability or excessive water content. Bond strength verification remains a separate requirement, tested under EN 1542 or ASTM C1583/C1583M-20, because published data for this specific fiber’s effect on adhesion is limited.

When Water-Retaining Structures Must Resist Hydrostatic Service Conditions

In water treatment basins, tanks, and marine-adjacent reinforced concrete, the fiber is added at 0.9 kg/m³ to reduce plastic settlement and plastic shrinkage cracks that become preferential water paths before hydration is complete. The fiber is non-corrosive in chloride-laden environments, unlike steel fiber, and is stable in the alkaline pore solution. However, its low elastic modulus means it cannot control long-term crack widths under tensile service loads. Crack control for hydrostatic service conditions must be provided by distributed steel reinforcement designed in accordance with ACI 350-20 or EN 1992-3:2006; the fiber is additional, not a substitute for reinforcement or joint waterstops. In low-permeability concrete with a water-cementitious ratio below 0.45, the addition of 0.9 kg/m³ may reduce visible plastic shrinkage crack counts in the range of 50–80% based on comparative site inspections, but this range is evaporation-rate dependent. The fiber does not reduce chloride diffusion or permeability of sound concrete; chloride resistance is governed by cover depth and binder chemistry. Claims that the fiber alone provides waterproofing are not supported by a recognised test standard.

Concrete pavement and bonded white-topping placements use the fiber at 0.6–1.2 kg/m³ to reduce early-age plastic shrinkage cracking and edge slumping before joint sawing. In slip-form paving, the non-metallic fiber does not interfere with stringline sensors or automatic grade-control equipment. The fiber is metered into the plant mix with the aggregate so that the continuous paver receives a homogeneous mixture; low water content and high-frequency internal vibration can otherwise cause uneven fiber distribution near the surface. The base pavement concrete is specified under ASTM C94/C94M-24 or EN 206:2013+A2:2021, and the fiber is accepted as an addition when its conformity is certified under EN 14889-2:2006. Joint spacing and dowel design are not altered by the fiber; contraction joints are still required at spacings determined by slab geometry and subgrade friction. Published field data for this specific fiber in slip-form pavement is limited, so qualification should compare crack density against an unreinforced control under the same evaporative conditions.

Mass concrete pour protection is limited to surface plastic shrinkage control at 0.6–0.9 kg/m³; the fiber does not influence core temperature rise and is not a thermal cracking control measure.