Fibra Nycon RECD15-PVA para reforço de concreto (8 denier)

Nycon RECD15-PVA Fiber for Concrete Reinforcement is supplied as a polyvinyl alcohol monofilament with a nominal filament linear density of 8 denier; the model designation RECD15 distinguishes this material from coarser 15-denier macrofilament grades and from fibrillated polypropylene products. The fiber is produced in discrete cut lengths, typically 6 mm, 8 mm, and 12 mm, and is introduced into conventional hydraulic cement concrete at low to moderate dosage rates to mitigate early-age plastic shrinkage cracking and to provide secondary post-crack bridging. Its primary function is physical reinforcement of the cementitious matrix, not chemical admixture activity. Unlike steel fiber, the PVA filament carries no galvanic corrosion risk, and unlike some lower-cost polyolefin fibers, the hydrophilic surface character of PVA affects wettability, fiber-cement adhesion, and dispersion stability in the fresh paste.

Values in the table are representative of published PVA monofilament fiber data and should be confirmed against lot-specific certificates for project acceptance testing. Published data for this specific configuration is limited in some high-dosage structural applications; therefore, trial batching remains the controlling acceptance method under the relevant concrete standard.

Typical application contexts include slab-on-ground construction, bonded or unbonded overlays, precast concrete panels, repair mortars, and shotcrete exposed to early-age moisture loss. In each case the fiber functions as secondary reinforcement; it does not replace primary load-bearing steel reinforcement or post-tensioning. For a slab with a 100 mm thickness at a dosage of 0.9 kg/m³, the high filament count per unit volume places the material in a microcrack-control regime rather than a structural macro-fiber toughness regime. The distinction is important when comparing RECD15 with hooked-end steel fibers or macro-synthetic fibers specified for load-bearing post-crack performance.

What Limits Dispersion of 8-Denier PVA Filaments in High-Shear Concrete Mixers?

In production-scale batching, dispersion is governed by addition sequence, free-water availability, mixer type, and mixing duration. The fiber is introduced after coarse and fine aggregates have been charged but before the full water and high-range water reducer additions when possible; this sequence allows the rotating aggregate bed to strip individual filaments from the cut bundle and prevents balling at the mixer fins. In a 1.0 m³ twin-shaft mixer operating at 30 rpm, a 0.9 kg/m³ addition is typically distributed within 4 to 5 minutes of wet mixing, but in a 0.25 m³ laboratory pan mixer a longer interval of 6 to 8 minutes may be required because the material transfer rate at the blade surface is lower. The characteristic failure mode in poorly sequenced batching is the formation of fiber-rich clumps containing 10 to 30 filaments bound by cement paste; these clumps are observable after discharge and cannot be fully separated by additional mixing once hydration has progressed. At relative humidity above 65%, opened bags should be resealed because PVA absorbs moisture, altering apparent filament stiffness and increasing clumping tendency. Some precast plants pre-dry opened material at 40 °C for 2 hours before automated dosing to reduce feed variability.

The addition of 0.9 kg/m³ of 8-denier PVA fiber to a 0.40 water/cement ratio concrete with a target slump of 150 mm typically reduces slump by 15 to 30 mm unless water or admixture dosage is adjusted. The surface area introduced by the filament population is significant: at 8 mm cut length, 1 kg of fiber contains on the order of 10⁸ filaments, and the paste requirement for wetting this surface contributes to the observed workability loss. Air content measured under ASTM C231/C231M may shift by 0.5 to 1.0 percentage point, and air-entraining admixture demand can increase because the filaments alter bubble coalescence in the fresh paste. Setting time measured by ASTM C403/C403M is generally not shifted by more than 30 minutes at dosages at or below 1.8 kg/m³, although cement alkali content and high-range water reducer chemistry can produce greater shifts.

When a Conventional Slab-on-Grade Mix Transitions from Plain Concrete to Type III Synthetic Fiber Reinforcement

Under ASTM C1116/C1116M, the material falls within Type III synthetic fiber-reinforced concrete when the dosage and mixing meet the standard requirements. In restrained slab testing under ASTM C1579-20, the primary metric is crack area after exposure to a prescribed evaporation rate; additions of 0.6 kg/m³ to 1.2 kg/m³ of 8-denier PVA fiber have been reported in comparative studies to reduce crack area by 60% to 90% relative to an unreinforced control, although exact reductions depend on slab depth, aggregate grading, bleed rate, and finishing timing. The fiber becomes effective only when the dosage exceeds a geometry-dependent percolation threshold. Below 0.4 kg/m³, little post-crack bridging is available, while above 1.8 kg/m³, workability and finishability can deteriorate without a proportional additional plastic shrinkage benefit.

Plastic shrinkage cracking is initiated when surface evaporation exceeds bleed water supply; in field practice the evaporation rate threshold is often taken as 0.5 kg/m²/h using the ACI 305.1-14 nomograph. The fiber does not reduce bleeding, but it does restrict crack opening and coalescence after the initial crack forms. In a typical comparative panel with 0.9 kg/m³ RECD15, maximum crack width may remain below 0.3 mm under conditions where an unreinforced control exceeds 1.0 mm, though the exact ratio is mix-specific.

Evaluating Plastic Shrinkage Crack Suppression under Restrained Slab Protocols

In ASTM C1579-20 testing, a restrained slab is placed over a steel insert to create a stress riser, and crack width is measured at intervals after initiation. The hydrophilic surface of PVA contributes to higher fiber-cement bond than untreated polypropylene, which is reflected in reduced crack width per unit dosage in restrained shrinkage tests. That bond also increases load-transfer capacity across a microcrack at early age, delaying the transition from a single planar crack to a network of additional cracks. The relevant measured property is not tensile strength of the filament alone but the critical fiber volume fraction and the fiber aspect ratio. With an equivalent diameter near 29 µm and cut length of 8 mm, the aspect ratio is approximately 276, placing the product in a high aspect-ratio class where dispersion quality is the controlling variable. Under field conditions, fiber-reinforced concrete may still exhibit visible cracking if substrate restraint, wind speed, or ambient temperature exceed the limits assumed in the concrete mixture design.

Post-crack flexural response is characterized under ASTM C1609/C1609M in a 150 mm × 150 mm × 500 mm beam tested with third-point loading. The residual strength at net deflection of 1/150 of span is reported as the load capacity after a crack has formed. For 8-denier PVA at 0.9 kg/m³, residual flexural strength is substantially lower than a hooked-end steel fiber at 20 kg/m³ to 40 kg/m³; the filament population is intended for crack-width control rather than primary flexural load resistance. Where a floor slab is designed with welded wire reinforcement, the fiber can reduce visible crack count but does not replace the continuous steel reinforcement required by load calculations.

Across the Post-Crack Regime: RECD15 Versus Steel and Macro-Synthetic Fibers

Compared with hooked-end steel fiber, the 8-denier PVA filament has a lower tensile modulus (37 GPa versus approximately 200 GPa for carbon steel), a lower specific gravity (1.30 versus 7.85), and a much higher filament count per unit mass. Steel fibers typically require dosages of 20 kg/m³ to 60 kg/m³ to achieve structural post-crack toughness, while RECD15 is used at 0.6 kg/m³ to 1.8 kg/m³ for early-age shrinkage and microcrack control. The lower mass dosage is possible because of the smaller diameter and higher count: an 8 mm PVA filament at 8 denier has an aspect ratio near 276, whereas a hooked-end steel fiber with 0.75 mm diameter and 60 mm length has an aspect ratio near 80. However, the steel fiber carries tensile stress at a much higher absolute value, so post-crack residual strengths under ASTM C1609/C1609M are not equivalent at those mass dosages.

Macro-synthetic fibers of 0.2 mm to 0.5 mm equivalent diameter and 30 mm to 50 mm length are intermediate in post-crack toughness but may be more visible at the concrete surface after troweling than 8-denier PVA. Fibrillated polypropylene filaments are hydrophobic and tend to bond mechanically by unraveling, whereas PVA forms a stronger adhesive bond with hydrated cement paste because of hydroxyl groups on the filament surface. This bond difference is one reason lower dosages of PVA can produce narrower plastic shrinkage cracks than equivalent dosages of untreated polypropylene. However, PVA is not a direct substitute for macro-synthetic fiber in applications where structural designers require large crack openings to be resisted by high-energy fiber pullout rather than by microcrack distribution.

Compliance Benchmarks for Type III Synthetic Fiber-Reinforced Concrete

The product is supplied with documentation intended to support acceptance under ASTM C1116/C1116M for Type III synthetic fiber reinforcement. Individual filament tensile properties are reported according to ASTM C1557-20. When concrete containing the fiber is tested for flexural performance, the applicable methods are ASTM C1609/C1609M or ASTM C1399/C1399M for residual strength and ASTM C78/C78M for modulus of rupture. For restrained plastic shrinkage, ASTM C1579-20 is the established comparative protocol. The fiber does not require changes to aggregate grading under ASTM C33/C33M beyond normal mix adjustments, but the batch water and air-entraining admixture demand may require correction.

Operational boundaries are primarily defined by mixing time, dosage, and concrete finishability. At dosages above 1.8 kg/m³, the fiber population can create a matt-like effect at the troweled surface, and the slip resistance of a hard-troweled floor may change as filaments bridge the immediate paste layer. The material is not intended for use as the sole flexural reinforcement in suspended structural members; structural load resistance must be provided by reinforcing steel, post-tensioning, or properly designed macro-fiber reinforcement in accordance with the applicable building code. PVA fiber is resistant to alkaline cement pore solution and does not corrode, but prolonged exposure to acidic conditions below approximately pH 4 can hydrolyze the polymer. Opened fiber bags should not be stored in ambient relative humidity above 65% for more than one shift without resealing; absorbed moisture increases clumping and can reduce dispersion efficiency in automated dosing equipment. The product should not be introduced into a dry concrete mix before water because electrostatic and pneumatic conveying effects may cause filament bloom at the mixer charging point. When a mid-range or high-range water reducer is used, the admixture dose is preferably optimized after the fiber has been added to avoid exceeding the target slump. Avoid combining the fiber with unconfirmed amine-based admixtures in steam-cured precast concrete unless trial batching verifies that the admixture chemistry does not alter fiber-matrix bond.