MasterFiber MAC 244 is a macro synthetic reinforcement fiber supplied as collated bundles of blended polyolefin monofilaments. Manufacturer documentation identifies a nominal fiber length of 54 mm and a specific gravity of 0.91. The material is classified under ASTM C1116/C1116M Type III synthetic fiber-reinforced concrete and falls within the macrofiber definition used in EN 14889-2:2006. MasterFiber MAC 244 is specified for secondary reinforcement in cast-in-place slabs, precast concrete, and wet-mix sprayed concrete. Typical addition rates range from 2.0 kg/m³ to 5.5 kg/m³ by mass. The fiber is non-corrosive and contributes negligible water absorption; however, its physical presence in the mixture can reduce slump through mechanical interference, particularly at higher dosage levels.

The product differs from microsynthetic fibers in both geometry and intended function. Microfibers used for plastic shrinkage control are commonly 6–19 mm long with equivalent diameters below 0.3 mm. MasterFiber MAC 244 is a macrofiber with a length suitable for hardened-state crack-width control and post-crack toughness. Unlike steel fibers, the synthetic macrofiber does not corrode, does not produce surface rust staining, and does not abrade pump components at the same rate. However, the lower elastic modulus of polyolefin macrofibers means that a mass-for-mass replacement of steel fiber is not technically equivalent without redesign based on residual strength testing.

What Mixing and Pumping Conditions Are Required for This Fiber?

MasterFiber MAC 244 is introduced at the batch plant or on site. In central mixer operations, the fiber is added after aggregates and before water; in transit mixer operations, collated bags are added after the concrete has been bathed and mixed, then drum speed is increased to 10–14 rpm for a minimum of 70 revolutions. The fiber does not require pre-drying before batching, but condensation on bag surfaces can cause partial clumping of collated bundles before dispersion. Bags stored outdoors should therefore be protected from groundwater and rain.

Production-scale ready-mix experience indicates that introducing bags onto a moving aggregate conveyor reduces fiber balling compared with dropping whole bags into a stationary drum. Fiber balling has been observed when low-slump concrete below 100 mm is mixed at drum speed less than 8 rpm and when more than one bag is added before the preceding bag has disappeared into the concrete. In a twin-shaft compulsory mixer used for precast production, a wet mixing time extension of 15–30 seconds after fiber addition is generally sufficient. In a transit mixer, revolutions should be measured rather than relying on time alone, because drum speed may vary with truck engine condition and drum wear.

Concrete containing MasterFiber MAC 244 can be pumped through standard 100 mm or 125 mm concrete pump lines. Discharge-hose reducers smaller than 65 mm increase the risk of plugging at dosage rates above 4.0 kg/m³. The fiber raises concrete yield stress through mechanical interference, so pump pressure should be monitored during the first placements. Slump adjustment is managed by starting with a base slump 20–30 mm above the target value and using a high-range water-reducing admixture conforming to ASTM C494/C494M Type F within its approved dosage range. Water addition beyond the approved water-cementitious ratio must not be used as a slump adjustment method.

Post-Crack Residual Capacity and Slab Design Classification

The functional difference between MasterFiber MAC 244 and plastic shrinkage microfibers is evaluated through different test protocols. Plastic shrinkage control is often assessed with ASTM C1579, while hardened-state toughness is measured with ASTM C1609/C1609M, ASTM C1399/C1399M, or EN 14651. Under EN 14651, residual flexural tensile strengths are reported at crack mouth opening displacements of 0.5 mm (fR,1) and 3.5 mm (fR,4). These values classify post-crack performance and should be used for specification. Published generic data for macro synthetic fibers in this length class indicate that fR,1 values can range from approximately 1.0 MPa to 2.5 MPa at dosage rates of 3.0–4.5 kg/m³, but product-specific EN 14651 curves for MasterFiber MAC 244 should be requested from the manufacturer. Aggregate type, cement content, fiber orientation, and beam preparation all affect the measured values.

When ASTM C1609/C1609M is used, residual loads at deflections of L/600 and L/150 provide a comparison of fiber types. Because the elastic modulus of synthetic macrofiber is lower than steel, the load drop after cracking is more pronounced. A single tensile strength value for the fiber should not be used as a design parameter. The performance class obtained from beam tests or slab tests should be the specified value, and the test method must be consistent between qualification and production control.

When Macro Synthetic Fiber Substitutes Steel Fiber in Ground-Supported Slabs

The substitution of steel fiber by MasterFiber MAC 244 is permitted only where the design objective is crack-width control, toughness, and impact resistance. Steel fiber has a specific gravity of approximately 7.85 and an elastic modulus of approximately 210 GPa. Macro synthetic fibers in this class are commonly reported with elastic modulus values between 3 GPa and 10 GPa. At an equivalent fiber volume, the synthetic fiber will generally produce lower residual flexural strength at small crack openings. The replacement must therefore be engineered using EN 14651 residual strengths or ASTM C1609/C1609M residual loads, not by mass substitution ratios.

Operational differences are measurable. Steel fibers are magnetic, can cause rust staining if fibers become exposed at the slab surface, and can accelerate wear in pump hoses and screw conveyors. MasterFiber MAC 244 is non-magnetic, non-corroding, and lower in density. A dosage of 2.5 kg/m³ occupies a larger volume than the same mass of steel fiber but does not require cut-resistant gloves or magnetic dosing equipment. For slabs exposed to deicing salts or marine aerosol, the absence of steel fiber avoids surface oxidation. In batching systems, steel fibers typically require vibratory or weighing hopper dosing, while MasterFiber MAC 244 can be added directly from collated bags or paper bundles by mass.

The Product Must Not Be Used as Primary Flexural Reinforcement

MasterFiber MAC 244 is not a substitute for deformed reinforcing bars, welded wire reinforcement, or prestressing strand. Its contribution to ultimate flexural capacity is limited by the low fiber modulus and pull-out resistance. In suspended slabs, beams, columns, and other elements requiring primary moment reinforcement, reinforcing steel conforming to ASTM A615/A615M or EN 10080 must remain in place. The fiber is also unsuitable for sustained service temperatures above approximately 160 °C, because polyolefin softens and loses load-transfer capacity. In fire-exposed concrete, macro synthetic fibers may provide pressure relief channels after melting, but this is a fire response function and not a structural load-bearing function.

The fiber should not be exposed to aromatic hydrocarbon solvents or concentrated mineral spirits before batching. Compatibility with form release agents, curing compounds, air-entraining admixtures conforming to ASTM C260/C260M, and high-range water reducers conforming to ASTM C494/C494M should be confirmed by trial batch. Published data for MasterFiber MAC 244 in contact with solvent-based curing compounds is limited; concentrated spills should therefore be avoided.

Conformance documentation for MasterFiber MAC 244 is normally provided through a Declaration of Performance under EN 14889-2:2006. The verification standards used for production and application testing are summarized below. Shipments should be checked for intact bags, correct product identification, and absence of visible contamination.

For precast factory production, MasterFiber MAC 244 is dosed by mass into the aggregate weigh hopper or onto the aggregate conveyor. Weigh hopper accuracy should be at least ±1% of the design dosage. Small bag sizes allow low-dosage qualification trials. For wet-mix shotcrete, the fiber is added at the batch plant or through a pre-dosing hopper at the pump. A test panel under the intended site conditions should be sprayed and evaluated for fiber distribution and toughness class before production is accepted.

No single mix design can be transferred from laboratory trials to production without adjustment for aggregate gradation, air content, and mixing equipment. Product-specific residual strength values depend on fiber orientation, edge effects, and the number of fibers bridging a crack. Therefore, production qualification must include trial batches with the actual cement, supplementary cementitious materials, and admixtures intended for the project.