MasterFiber F 70 is a fibrillated polypropylene fiber manufactured from 100% virgin homopolymer polypropylene resin. The resin is extruded into an oriented film and mechanically fibrillated to produce an interconnected network of polypropylene filaments with rectangular cross-sections and longitudinal fibrillation. Under ASTM C1116/C1116M-23, the material classifies as a Type III synthetic fiber for fiber-reinforced concrete. The product is commonly supplied as collated fiber bundles with a nominal length of 19 mm. The base resin has a specific gravity of 0.91 when measured according to ASTM D792-20, water absorption below 0.1% after 24 h immersion under ASTM D570-22, and a melting temperature in the range 160–170 °C under ASTM D3418-21. The virgin homopolymer designation excludes recycled polyolefin content, reducing melt-flow variation and pigment-related heterogeneity in the fiber batch. The hydrophobic polymer matrix does not absorb mix water, which separates the product from cellulosic or hydrophilic synthetic fibers that display measurable water uptake.

Fibrillated film morphology creates a high specific surface area relative to round monofilament fibers. The interconnected fibrils separate during mixing and form a three-dimensional network in fresh concrete, but the material is not intended as primary reinforcement and does not replace structural steel or macro synthetic fibers. Its function is limited to secondary crack control in cementitious matrices.

What distinguishes fibrillated virgin homopolymer fibers from monofilament and steel fiber systems?

Fiber geometry and polymer source, rather than fiber count alone, govern dispersion and mechanical behavior in concrete. Fibrillated fibers begin as bundles of interconnected film fibrils and must be mechanically separated during mixing. In contrast, monofilament polypropylene fibers are extruded as discrete round strands and disperse more readily but provide less surface-topographic anchorage. Hooked-end steel fibers provide structural residual flexural capacity because their elastic modulus is approximately 200 GPa and their tensile yield strength is typically 1,100–1,500 MPa. Polypropylene fibers exhibit an elastic modulus of 3.5–4.5 GPa and tensile strength commonly reported at 300–600 MPa, so their structural contribution is confined to early-age crack width control and microcrack arrest rather than replacement of reinforcing bars or welded wire reinforcement. MasterFiber F 70 therefore belongs to the microsynthetic fiber class for concrete, not to the macro-synthetic or steel fiber structural class.

The use of 100% virgin homopolymer resin differentiates MasterFiber F 70 from fibers containing recycled polyolefin feedstocks, which may show greater fluctuation in melt flow rate, tensile response, and pigment distribution. For incoming resin, melt flow rate can be monitored under ISO 1133-1:2022; this test is relevant because fibrillation performance depends on the orientation and molecular weight of the extruded film. Recycled feedstocks can contain mixed polymer species, which create inconsistent fibrillation and lower bundle separation efficiency in low-shear concrete mixers.

Batching sequence affects dispersion of fibrillated fibers more than initial fiber moisture content. On dry-batch plants, the fiber bundle should be added to the fine aggregate stream after the initial aggregate charge but before water introduction. This sequencing allows the dry fiber to spread across the aggregate mass and separates fibrils under subsequent aggregate shear. In truck-mix operations, adding a full bag directly into the water stream or onto wet concrete can create cohesive fiber balls that are not dispersed by normal drum mixing at 12–18 rpm. Production plants use low-rate fiber dispensers to release opened bags over a 20–30 s conveyor period, or add the opened bundle to the central mixer with the aggregate fraction. After water addition, a minimum of 70–100 revolutions of the truck drum at mixing speed is required for uniform distribution at an addition rate of 0.9 kg/m³. Central mixers such as twin-shaft compulsory mixers or planetary mixers disperse the fiber network more efficiently than free-fall drum mixers; 60–90 s of wet mixing after fiber introduction is generally sufficient. If the discharged concrete shows visible fiber clumps, the fleet should be checked for worn blades, insufficient mixing speed, or bag addition rate rather than increasing water content. The fiber is hydrophobic and does not absorb mix water, so observed slump changes are dominated by increased yield stress from the fiber network rather than increased water demand. Slump measured to ASTM C143/C143M-20 should be recorded before and after fiber addition; typical reductions at 0.9 kg/m³ are in the range 10–25 mm for a 0.45 water-to-cementitious ratio mix with crushed limestone aggregate. Workability is restored by adjusting the dosage of a Type A or Type F water-reducing admixture meeting ASTM C494/C494M rather than by adding excess water, which would lower compressive strength. Batching tolerance for the fiber dosage should be controlled within ±0.1 kg/m³ of the specified addition; low-speed auger dispensers generally achieve this more reliably than aggregate weigh-belt systems.

When plastic shrinkage crack control governs specification, how is dosage validated?

When specified for plastic shrinkage mitigation in slab-on-grade placements, the addition rate is generally 0.9 kg/m³ according to the manufacturer’s technical documentation. Rates from 0.6 kg/m³ to 1.8 kg/m³ are used for higher-risk placements such as large exposed panels, wet-mix shotcrete, and slip-formed barriers. The 0.9 kg/m³ addition provides sufficient fiber count to bridge plastic capillary pores while maintaining pumpability. Validation of crack reduction is performed using ASTM C1579-21, in which a restrained rectangular panel is subjected to controlled evaporative loss and the crack width reduction is compared with an unreinforced control. A crack-width reduction of at least 50% at the specified dosage is a commonly referenced acceptance threshold, although project specifications may require higher reduction at 60% relative humidity and 30 °C surface temperature. For each mix revision—especially changes in supplementary cementitious material content, fine aggregate angularity, or water-to-cementitious ratio—the plastic shrinkage test should be repeated because fiber effectiveness is influenced by matrix pore pressure, bleeding rate, and early-age tensile stress development. The product does not reduce drying shrinkage occurring after final set; hardened drying shrinkage is evaluated by ASTM C157/C157M and is not the acceptance method for this fiber.

Under hot-weather placement, the concrete bleeding rate and set time govern plastic shrinkage. The fiber must be present in the near-surface mortar fraction to intercept capillary stresses. Contractor experience on slab placements with laser screeds indicates that fiber ball accumulations can be reduced by maintaining drum speed at 14 rpm and adding the fiber before the last third of the batch water. Because the fiber network reduces bleed water transport to the surface, finishing operations may begin earlier than on plain concrete; bleed rate can be measured by ASTM C232/C232M-20. Surface texture should be checked with a straightedge because fiber-reinforced concrete may produce a tighter surface with less available bleed water.

On slab-on-grade construction, MasterFiber F 70 is typically introduced through the aggregate weigh hopper or by a fiber dispenser mounted over the coarse aggregate conveyor. The fiber network distributes through the low-slump concrete without changing the water-to-cementitious ratio. In precast operations using self-compacting concrete, the addition rate should be limited to the lower range, usually 0.6–0.9 kg/m³, because higher fiber volumes can reduce passing ability through narrow form sections; J-ring tests to ASTM C1621/C1621M-17 should be performed on the self-compacting concrete after fiber addition. In wet-mix shotcrete, the fiber can be added during batching; nozzle clogging is not generally caused by the fiber itself but by balled bundles if mixing energy is inadequate. The use of pre-dampened aggregates is allowed, but fiber bags must remain dry until addition. The material is stable in the alkaline cement matrix and does not undergo the corrosion mechanisms associated with steel fibers. However, because polypropylene melts between 160–170 °C, the fiber should not be considered a fire-resistance reinforcement; its contribution diminishes as the matrix temperature exceeds the polymer melt temperature.

At addition rates above 1.8 kg/m³, pump line pressure can increase due to higher plastic viscosity. For long pump lines over 150 m, a pumping trial is recommended using the actual boom pump configuration and a pressure transducer recording line pressure. Fibrillated fiber addition does not alter cement hydration kinetics, because the polymer is inert and hydrophobic; isothermal calorimetry shows no measurable acceleration or retardation of silicate phase hydration attributable to the fiber. However, the three-dimensional fiber network can physically interact with entrained air void stability, so air content should be measured before and after addition with a pressure meter meeting ASTM C231/C231M-17a.

Material identity, storage, and batch-to-batch quality verification

Incoming lot inspection begins with identification of fiber type, nominal length, and fibril bundle integrity. Because the product is manufactured from virgin homopolymer resin, the melt flow rate and ash content are generally more stable than recycled blended fibers; nevertheless, each shipment should be checked against supplier certificates and retained samples subjected to oven-dry mass loss at 105 °C for 1 h. Storage below 50 °C and away from direct sunlight prevents softening and bundle compression from causing fiber agglomeration. Opened bags should be resealed to keep the fibers dry and free of cement dust; contamination with fine cement powder creates localized clumps that reduce dispersion. A standard lot check includes specific gravity by ASTM D792-20, water absorption by ASTM D570-22, and melt temperature by ASTM D3418-21. Fiber length distribution can be verified from the manufacturer’s certificate against ASTM C1116/C1116M-23. For full compliance, the concrete producer maintains batch records of fiber addition per cubic meter, mixing time, slump before and after fiber, and plastic shrinkage test results where specified. In jurisdictions requiring CE marking for fibers used in concrete, the applicable harmonized standard is EN 14889-2:2006; product properties must be declared on the basis of type testing and factory production control.

Because polypropylene has a melt temperature near 165 °C, the fiber loses tensile capacity in fire-exposed concrete; therefore, fire-resistance rating is not improved by this product. MasterFiber F 70 is not intended to replace steel reinforcing bars, welded wire reinforcement, or structural macrofibers. It is incompatible with applications requiring post-crack residual flexural strength measured to ASTM C1609/C1609M-19, because the fiber length and modulus are too low to generate significant residual load after crack opening. In flooring systems where high point loads or jointless structural slabs are specified, the design should be performed with steel reinforcing or macro synthetic fibers, and MasterFiber F 70 should be limited to secondary crack control. Do not store in a moist environment above 60% relative humidity if bags are not sealed; although polypropylene absorbs minimal water, surface moisture on bundles promotes clumping. Published product-specific data for high-temperature performance and long-term creep of the fiber in cracked concrete is limited.