STRUX 85/50 is a synthetic macro fiber for concrete secondary reinforcement. The product is supplied as 100% virgin polypropylene filament bundles with a nominal length of 50 mm and an aspect ratio of approximately 85; the 85/50 model designation corresponds to these geometric parameters. Under ASTM C1116/C1116M, the material falls within Type III synthetic fiber reinforcement. CE marking is assessed under EN 14889-2:2006 for polymer fibres for concrete. The fiber density is near 0.91 g/cm³, which is substantially lower than steel fiber density and alters batching, mixing, and pumping response. The primary function of STRUX 85/50 is to provide post-crack residual flexural strength and crack-width control in slabs on grade, shotcrete, precast concrete, and composite metal deck placements. It is not a substitute for primary steel reinforcing bars in structural elements requiring a continuous load path.

In concrete, the fiber is distributed through the full matrix and provides crack control without the need for chairs, laps, or precise elevation associated with light-gauge welded wire mesh. Replacement of mesh is permissible only where project specifications permit alternative reinforcement and where design calculations incorporate the appropriate residual strength values from ASTM C1399/C1399M or ASTM C1609/C1609M. The product is available in pre-weighed bags suitable for direct addition to ready-mix trucks or central batch plants.

What differentiates STRUX 85/50 from steel and micro-synthetic fibers?

Steel hook-end fiber reinforcement commonly uses lengths of 30–60 mm and has a specific gravity of 7.85. The mass dosage required for comparable crack-control performance is often 15–40 kg/m³, and the modulus of elasticity is approximately 200 GPa. STRUX 85/50 is added at mass dosages of 1.8–7.0 kg/m³ and has an elastic modulus below 5 GPa. The lower modulus means that structural response at deflections above first crack is more dependent on fiber count, aspect ratio, and concrete matrix properties than on fiber tensile strength alone. Polypropylene fibers do not corrode and do not produce surface staining when fibers are exposed at the concrete surface. Steel fibers can produce surface rust spots in chloride-rich environments if fibers remain exposed after finishing or abrasion.

Polypropylene microfibers, with lengths typically 6–20 mm and diameters below 0.3 mm, are intended for plastic shrinkage crack control in the first 24 h after placement. They are not designed to provide significant residual post-crack load transfer at crack widths exceeding 0.25 mm. The 50 mm length and aspect ratio of approximately 85 place STRUX 85/50 in the macro fiber class, where the primary test methods evaluate average residual strength at crack openings of 0.5 mm and 3.0 mm. However, macro synthetic fibers are not equivalent to steel fibers for all applications, and design decisions should be based on flexural performance tests with the exact concrete mixture rather than on geometric similarity alone.

Typical published polymer values for polypropylene macro fibers include a specific gravity of 0.90–0.92, a melting point of 160°C, and water absorption below 0.1% when tested by ASTM D570. Alkaline resistance in saturated calcium hydroxide solution is high because the polymer lacks the reactive iron or glass network that can degrade in cementitious pore solution. The fiber is hydrophobic, which limits interference with free-water demand but can increase the apparent plastic viscosity of the concrete. Slump loss from fiber addition is typically small at dosage rates up to 3.0 kg/m³, although dosages above 5.0 kg/m³ may require a mid-range or high-range water-reducing admixture to maintain a placement slump of 100–150 mm. This should be verified by trial batch using ASTM C143/C143M for slump and ASTM C231/C231M or ASTM C173/C173M for air content.

At a dosage of 3.0 kg/m³, a polypropylene macro fiber of 50 mm length and 0.63 mm equivalent diameter yields approximately 200,000 discrete filaments per cubic metre. This estimate follows the mass-per-fiber relationship m = ρ × π × d²/4 × L. In contrast, steel fibers at 20 kg/m³ may provide fewer than 50,000 filaments per cubic metre depending on diameter. The higher filament count contributes to crack intersection and post-crack load transfer at low addition masses, but it does not offset the lower elastic modulus of the polymer in all deflection ranges.

When chloride exposure rules out steel fiber reinforcement

In parking garage slabs, bridge overlays, marine pile jackets, and exterior pavements subject to de-icing salts, the specification may prohibit steel fiber reinforcement because of corrosion risk at exposed fiber surfaces. STRUX 85/50 is non-corrosive and non-conductive. It does not require cover for corrosion protection and does not produce galvanic coupling with reinforcing bars. The performance of the concrete under chloride ingress remains dependent on water-to-cementitious material ratio, curing, and cover to primary reinforcement. Published data for chloride diffusivity of steel-fiber-reinforced versus synthetic macro fiber concrete is limited for direct comparison, but the absence of embedded ferrous fibers removes a source of exposed-surface corrosion.

For sulfate or acidic process environments, the polymer fiber itself remains inert under normal exposure; however, the surrounding cement matrix may degrade independently. The fiber should not be used as a corrosion-protection strategy for reinforcing steel. It is also not a replacement for coatings, sealers, or low-permeability concrete in aggressive chemical exposure conditions.

Operational storage limits should be observed. Extended warehouse storage above 60°C may cause bundle compaction and creep in polypropylene. The fiber should be kept in sealed bags away from direct flame and strong oxidizing agents. It should not be predispersed in solvent-based admixtures unless compatibility testing is performed. In high-shear horizontal pan mixers operating above 60 rpm, static charge and bundle survival may increase if the fiber is added as a whole bag to the dry blend.

Specification submittals for STRUX 85/50 should include a certified mill test or manufacturer quality certificate showing conformance to EN 14889-2:2006, the product data sheet, and an independent laboratory report for ASTM C1399/C1399M or ASTM C1609/C1609M prepared with the actual concrete mixture. For CE-marked projects, the Declaration of Performance should be checked for the intended concrete strength class and dosage. Batch-to-batch fiber length and aspect ratio tolerances should be declared in the control plan.

ASTM C1399/C1399M reports average residual strength obtained from a cracked beam with a three-point loading configuration and an initial crack width of 0.5 mm. ASTM C1609/C1609M reports flexural toughness and residual strengths at net deflections of L/600 and L/150. These two methods do not produce interchangeable values. A specification that cites only fiber reinforcement without a designated test method is insufficient for quality control.

Batching and mixing require different handling compared with steel fiber. Steel fiber addition often uses magnetic lifting devices and metering conveyors because of the density. Synthetic macro fiber has low density and can become airborne if added too quickly to high-speed pan mixers. The recommended procedure is to add the fiber bundles to the aggregate stream or to the truck mixer at the plant after the coarse aggregate and initial mixing water have produced a cohesive matrix. A minimum of 4–5 minutes at mixing speed in a ready-mix truck, followed by visual inspection at the discharge chute, is typical. Premature addition to dry cement can create fiber/cement balls that survive through mixing and appear at the pump hopper. Air content should be verified after fiber addition using ASTM C231/C231M or ASTM C173/C173M; fiber can alter the air void system and may require adjustment of the air-entraining admixture dosage.

Pumping performance of STRUX 85/50 depends on aggregate gradation, line size, and dosage. In concrete with a maximum aggregate size of 20 mm and a slump maintained between 100 mm and 150 mm, dosages up to 5.0 kg/m³ typically pump through 65 mm minimum line diameter without blockage. Where the specification requires higher dosage, a 100 mm pump line and reduced piston face pressure are advisable. Fiber balling is most commonly associated with high-shear mixing, water-to-cementitious material ratios below 0.40, or slug addition of an entire bag into a stationary drum. The fiber should be introduced as a dispersed stream over the charging hopper, not as a compacted mass.

Shotcrete, Precast, and Slab-on-Grade Processing Parameters

In wet-mix shotcrete, the fiber is added at the batch plant or in the transit mixer before the pump. Dosages in the 3.0–7.0 kg/m³ range are common for ground support and slope protection. The fiber increases the apparent cohesion of the wet-mix but can reduce workability through the nozzle. Rebound of synthetic macro fiber is lower than steel fiber rebound because of lower particle mass; however, rebound of the total aggregate and paste system must still be controlled by air volume and nozzle angle. Published rebound data for specific shotcrete mixes are required for estimating in-place fiber content because fiber count at the nozzle may differ from the batched dosage.

In precast concrete, STRUX 85/50 can be used in thin-walled panels, septic tanks, pipes, and utility vaults where steel fibers are undesirable because of magnetic properties or surface finish. The low modulus of polypropylene requires that demolding and stripping strengths be evaluated separately; fibers do not increase early-age compressive strength. Form release agents and external vibration procedures are generally unchanged, but external vibrator frequencies must be sufficient to consolidate the matrix around the fibers. Field observations on precast lines indicate that vibration times below 15 s per 500 kg batch may leave fiber-rich zones at the form face if the fiber was not fully dispersed.

For slab-on-grade construction, typical specifications require a dosage of 1.8–5.0 kg/m³ and a measured residual strength from ASTM C1399/C1399M or ASTM C1609/C1609M using the project concrete. The fiber is added to the ready-mix truck at the plant or site. A mixing time of 4–5 minutes at full mixing speed after fiber addition is used in many submittals, followed by inspection of the discharged concrete for fiber balls. If fiber balls appear, the addition location should be moved from the top of the drum to the aggregate charging line or the rate of bag addition should be reduced.

Finishing of exterior slabs with macro synthetic fibers requires a slightly delayed troweling pass compared with plain concrete to allow the surface fibers to be worked into the paste. If troweling begins too early, surface fibers can remain visible and may alter the texture of the finished slab. The use of a burlap drag or broom finish provides a conservative surface for exterior flatwork.