Adfil Strux 75/32 (STRUX 75/32) is a synthetic macro fiber manufactured from a polypropylene/polyethylene blend. It is supplied as a discrete secondary reinforcement for integration into fresh concrete and wet-mix sprayed concrete. The fiber is intended to resist crack opening after first crack and to improve post-crack residual load capacity in slabs, pavements, precast elements, and sprayed linings. Under EN 14889-2:2006, polymer fibres for concrete are divided into Class I microfibres with an equivalent diameter below 0.3 mm and Class II macrofibres with an equivalent diameter of 0.3 mm or greater; STRUX 75/32 is classified as Class II. In North American practice, the fibre is considered a Type III synthetic fibre-reinforced concrete component under ASTM C1116/C1116M. The designation 75/32 identifies a nominal length of 75 mm and a grade indicator 32. The manufacturer’s technical data sheet is the controlling document for tensile strength, elastic modulus, cross-sectional geometry, and permissible dosage range. Published data for the exact properties of this grade are limited; the technical description below therefore refers to the product class and to established test methods governing specification, batching, and performance verification.

How Is Adfil Strux 75/32 Classified Within Synthetic Fibre Standards?

The European harmonized standard EN 14889-2:2006 establishes requirements for polymer fibres for concrete. Class II macrofibres are defined by an equivalent diameter of at least 0.3 mm; Class I microfibres fall below this threshold. The distinction controls whether a fibre is specified for plastic shrinkage suppression only or for post-crack resistance in semi-structural applications. STRUX 75/32, because of its length and diameter class, is specified where a macrofibre is required rather than a microfibre. Under ASTM C1116/C1116M, synthetic fibre-reinforced concrete is classified as Type III. The standard describes requirements for fibre dimensions, dosage, and mixing. For CE marking, factory production control and initial type testing must follow EN 14889-2:2006. Table 1 lists the compliance matrix and test methods.

The macrofibre classification does not imply a fixed residual strength class under EN 14889-2:2006; the manufacturer declares the effect on concrete in terms of residual flexural strength class. Because published data for the 32 grade are limited, a trial mix is required to establish the actual class. The PP/PE matrix is not electrochemically active, so the product has no corrosion mechanism in chloride-containing concrete. This property is one of the main differences from steel fibre and welded wire mesh in environments where chlorides penetrate to the reinforcement.

The polypropylene/polyethylene matrix places the fibre density between 0.90 g/cm³ and 0.94 g/cm³, compared with approximately 7.85 g/cm³ for low-carbon steel. At equal volume fraction, therefore, STRUX 75/32 contributes approximately 12 % of the mass of steel fibre. Commercial polyolefin macrofibres of comparable length are typically specified with tensile strength between 400 MPa and 650 MPa and elastic modulus between 3 GPa and 10 GPa. Steel fibre is typically specified with tensile strength of 1,000 MPa to 2,200 MPa and elastic modulus near 200 GPa. This stiffness difference means that post-crack capacity in concrete containing PP/PE macrofibres is developed through fibre pullout and crack-bridging rather than through direct elastic modulus equivalence to steel. Direct mass-for-mass substitution of steel fibre by STRUX 75/32 is therefore not valid. The dosage must be re-established by flexural performance testing or project-specific design calculations.

The PP/PE backbone is stable in the alkaline pore solution of portland cement concrete, where pH normally exceeds 12.5. It is not subject to chloride-induced corrosion. The polymer is not chemically resistant to strong oxidizing acids or aromatic hydrocarbon solvents, and prolonged storage should avoid direct sunlight and temperatures above the manufacturer’s stated limit to prevent fibre distortion. These limitations are usually not controlling in building and civil engineering concrete; in chemical plant floors exposed to organic solvents, fibre supplier guidance should be obtained.

Dose-Dependent Flexural Response and Residual Strength Characteristics

Flexural performance is evaluated by four-point bending in accordance with ASTM C1609/C1609M. The test reports first-peak flexural strength and residual strengths at net deflections of L/600 and L/150. For a span of 450 mm, these deflections correspond to 0.75 mm and 3.0 mm. The ratio of residual strength to first-peak strength, often designated R₃/R₁, indicates whether the section is deflection-softening or deflection-hardening. At macrofibre dosages below 3.0 kg/m³, the fibre primarily controls crack width and may not produce significant post-crack flexural capacity in suspended slabs. Dosages between 4.0 kg/m³ and 7.0 kg/m³ are commonly evaluated for ground-supported slabs and pile-supported floors. The required dosage for STRUX 75/32 must be determined from load-deflection curves supplied by the manufacturer and verified by trial mixing and ASTM C1609/C1609M testing.

Round panel energy absorption testing under ASTM C1550/C1550M is used for sprayed concrete and some slab applications. The panel is 800 mm in diameter and 75 mm thick; energy absorption is measured up to 40 mm central deflection. This test captures fibre-matrix interaction over large crack rotations and is sensitive to fibre length, aspect ratio, and dosage. Published energy absorption values for the 32 grade are limited; therefore, prequalification by an independent laboratory or notifying body is necessary for performance specifications.

Mixing sequence controls fibre dispersion and early-age workability. In a twin-shaft compulsory mixer with rated batch capacity of 3.0 m³, STRUX 75/32 is typically introduced after the aggregates and cementitious materials have been blended with part of the mixing water. The fibre should be discharged as a free-flowing stream over the aggregate surface rather than into the water alone or into the fines first; this reduces the tendency to form fibre balls. After fibre addition, mixing is extended for 30 s to 60 s, and the slump is checked according to ASTM C143/C143M or EN 12350-2. If the slump is below specification, water adjustment should be made only within the mix design limits and with the approval of the responsible engineer, because water added after fibre inclusion changes the water-cementitious materials ratio and may reduce compressive strength. In truck mixers, high-speed mixing should be limited to the manufacturer’s recommended interval to avoid excessive air entrainment and workability loss.

Batch-to-batch uniformity is verified by washing out a known fresh concrete sample and weighing and counting the fibres. The supplier’s reported fibre count per kilogram can be used for this on-site quality assurance check. The fresh concrete should be sampled in accordance with ASTM C172/C172M. If the recovered fibre count differs from the expected count by more than the project tolerance, the batching sequence or mixing time should be corrected. Localised fibre clumps visible in hardened concrete saw-cut sections indicate insufficient dispersion and can reduce the intended post-crack performance.

When STRUX 75/32 Replaces Steel Fibres or Welded Wire Mesh in Industrial Flooring

Steel fibres and welded wire mesh provide post-crack strength through high elastic modulus and ductile steel deformation. Synthetic macrofibres provide post-crack strength through a larger number of lower-stiffness fibres bridging the crack. In ground-supported slabs, steel fibre dosages of 15 kg/m³ to 40 kg/m³ are common, whereas PP/PE macrofibres are specified at 3 kg/m³ to 9 kg/m³ because of lower density and different tensile properties. The lower mass dosage corresponds to a much higher fibre count per unit volume. For a 75 mm macrofibre of equivalent diameter near 0.8 mm, the number of fibres per kilogram may exceed 20,000; comparable steel fibre counts are lower at equal mass dosage. This high fibre count improves crack spacing control but does not automatically replicate steel fibre toughness. Design guides such as Concrete Society TR34 and ACI 544.4R provide methods for using fibre-reinforced concrete in slabs-on-ground; the residual strength input must come from actual ASTM C1609/C1609M test results.

When steel corrosion is the governing performance requirement, the PP/PE macrofibre removes the need for galvanizing or stainless steel fibre. It also reduces stray current and electromagnetic effects, which is relevant in floors supporting automated guided vehicles, MRI equipment zones, and induction heating machinery. The lower elastic modulus of the synthetic macrofibre, however, means that long-term creep under sustained loading may be greater than in steel fibre-reinforced concrete. Published creep data for this specific grade are limited; if sustained loads are high, a project-specific creep assessment based on fibre pullout creep and concrete matrix creep should be performed.

Wet-mix sprayed concrete containing STRUX 75/32 is batched before transport; the fibre should not be introduced at the gun or nozzle. The delivery hose inner diameter should be not less than 50 mm; the sprayed layer thickness should be at least 2.5 times the fibre length, or approximately 190 mm for this 75 mm product, to limit rebound and avoid unfavourable fibre orientation. Rebound of synthetic macrofibres can be lower than that of steel fibre because of the lower density and elongated shape, but rebound losses still require a trial spray evaluation. The material is qualified for tunnel linings by ASTM C1550/C1550M round panel testing or by European sprayed concrete standards EN 14487-1 and EN 14488-5. For rock support, the fibre-reinforced shotcrete is specified by energy absorption class, not solely by compressive strength. Published data for STRUX 75/32 in this application are limited; the mix design must be validated with the specific cement, accelerator, and water-reducing admixture combination proposed for the project.

Admixture compatibility should be verified through trial mixes. Polycarboxylate-based high-range water reducers are commonly used with macrofibre concrete; the compatibility risk is not with the polymer fibre itself but with the early stiffening effects of some accelerators and the fibre’s influence on slump retention. In sprayed concrete, alkali-free accelerators are generally preferred over alkaline aluminates when macrofibres are present, because rapid stiffening and high early pH can increase rebound and reduce fibre encapsulation. Trial mixes should be prepared with the exact cement type, supplementary cementitious materials, accelerator, and fibre dosage, and tested at the specified slump and pump discharge temperature.

Thermal exposure alters the modulus of polyolefin fibres. The coefficient of thermal expansion of PP/PE is higher than that of surrounding concrete; at normal ambient temperature ranges this mismatch is accommodated by the fibre-matrix interface. At elevated temperatures approaching the softening point of polypropylene, the fibre modulus and load-transfer capacity decrease. In fire-resistant concrete design, PP microfibres are commonly used to reduce explosive spalling by creating water-vapour release channels, but STRUX 75/32 is a macrofibre and should not be used as the sole fire-spalling protection. Fire resistance should be specified by EN 1992-1-2 or ACI 216.1 without relying on the macrofibre for structural load capacity. Published data for the thermal softening behaviour of the 32 grade are limited.

The finished surface of a slab containing synthetic macrofibres should be power-troweled only after the initial bleed water has disappeared and the fibres have been covered by paste. Early troweling can drag fibres to the surface and create protruding surface fibres. If surface aesthetics are specified, light blast cleaning or diamond grinding may be required. The product does not rust, so surface staining from fibre corrosion is not a concern. For abrasion resistance, the concrete matrix should be specified according to ASTM C779/C779M or EN 13813 as applicable; the macrofibre does not replace hardeners or dry-shake surface treatments where those are separately specified.