A synthetic polypropylene fibre is supplied under the designation MasterFiber 143 for concrete applications in which plastic-shrinkage crack control, plastic-settlement reduction, and improved resistance to fire-induced spalling are required in sprayed concrete and cast-in-place concrete. The material is a hydrophobic monofilament with a density of approximately 0.91 g/cm³ and a melting point commonly reported between 160 °C and 170 °C. In normal-weight concrete, the addition rate is typically specified from 0.9 kg/m³ to 1.8 kg/m³, although fire spalling mitigation in high-strength concrete may justify dosages up to 2.0 kg/m³ when supported by fire test panels. The fibre is classified as a polymer microfibre under EN 14889-2:2006 and may be specified under ASTM C1116/C1116M where project documentation requires an American test framework.

MasterFiber 143 should not be specified as a replacement for structural steel reinforcement or for macro-synthetic fibres in applications requiring post-crack residual flexural strength. The low tensile modulus of polypropylene relative to steel and the small fibre cross-section limit its contribution to hardened-state load transfer. Its primary functions are plastic-state and high-temperature mechanisms: reducing the width of plastic shrinkage cracks in the first 24 h after initial set and creating pressure-relief pathways when the polymer melts during fire exposure. Field batching experience shows that addition sequence controls fibre distribution: the fibre is introduced after aggregate charging and before water to prevent clumping. In pan mixers and twin-shaft compulsory mixers, addition after water can produce fibre balls at shaft corners and inconsistent bag disintegration.

What Restricts MasterFiber 143 Addition Rates in Pumped Wet-Mix Shotcrete?

The addition-rate ceiling in pumped wet-mix shotcrete is set by paste volume, pump line pressure, and nozzle performance rather than fibre thermal stability. Concrete with water-to-binder ratios below 0.40 and slump below 150 mm may show a measurable increase in pump outlet pressure when MasterFiber 143 is added. Because the fibres raise the specific surface area of the granular phase, the plastic viscosity increases even though the fibre itself does not absorb mixing water. Production-scale monitoring on shotcrete lines with 50 mm to 65 mm internal-diameter delivery hoses indicates that dosages above 1.5 kg/m³ can require pump setting adjustments, especially in long vertical rise conditions. Published data for this exact product under every pump type are limited; trial spray panels should include a pressure transducer at the pump outlet to validate the selected dosage.

A relevant processing boundary is wet-mix piston pump intake. Monofilament fibres can bridge across the hopper screen, particularly when material is discharged too quickly into the hopper. Screens of 10 mm to 12 mm aperture are typical; blockages increase if the screen is already partially loaded with coarse aggregate. Maintenance logs from tunnel shotcrete crews indicate that hopper screens require inspection every 6 m³ to 8 m³ when running fibre-containing concrete, while unfibred mixes may run 12 m³ to 15 m³ between inspections. These are equipment-dependent figures, not a universal standard.

In dry-mix shotcrete, the limiting variable is fibre feed uniformity. MasterFiber 143 is pre-blended with cement and aggregate in a batch mixer; if the mix is discharged into a hopper and pneumatically conveyed, the fibre can segregate because of density differences. Fibre balls form when the material passes through a restriction or when the air conveying velocity is too low. The standard control is to maintain the air supply within the manufacturer’s recommended range for the gun and to use a vibrating screen feeder between the hopper and rotor. Nozzle blockage occurs when fibre clusters accumulate at the water ring; this failure mode is more frequent with lengths above 12 mm or when the water pressure is below the air pressure at the nozzle.

Comparative performance between MasterFiber 143 and other reinforcement systems is most clearly separated by modulus, dosage, and post-crack role. Steel fibres conforming to EN 14889-1:2006 have typical tensile strengths from 800 MPa to 1500 MPa and an elastic modulus around 200 GPa; they are selected where residual flexural and shear resistance are required after cracking. Macro-synthetic polymer fibres conforming to EN 14889-2:2006 class II may have tensile strengths from 300 MPa to 600 MPa and moduli from 5 GPa to 10 GPa; they are used where corrosion resistance and post-crack durability are required. MasterFiber 143 is a micro polypropylene fibre and is therefore specifiable only for plastic-state crack control and fire spalling mitigation. The table below establishes this boundary.

When MasterFiber 143 is substituted for welded wire mesh in slabs-on-grade or sprayed concrete linings, the construction sequence may be shortened because the fibre is batched into the concrete rather than placed as fixed reinforcement. However, the fibre does not provide crack-width restraint after the plastic phase unless continuity and post-crack stress transfer are supplemented by structural steel or macro-fibre reinforcement. Long-term crack-width control in restrained members therefore requires a design check of the concrete section, degree of restraint, and reinforcement index, not a simple one-to-one substitution of mesh with micro polypropylene.

When a Polypropylene Microfibre Is Specified for Fire Spalling Mitigation in Cast Concrete

When concrete reaches compressive strengths above 60 MPa, the low permeability of the cementitious matrix impedes vapour transport during rapid heating. Fire exposure can produce internal pore pressures that exceed the tensile strength of the concrete, causing explosive spalling and exposing reinforcing steel. MasterFiber 143 is introduced for this scenario because its melting point near 160 °C is below the temperature at which spalling becomes critical. The molten and decomposed polymer creates a network of temporary channels that increases the effective permeability of the heated zone. Typical fire-spalling dosages are 1.5 kg/m³ to 2.0 kg/m³, but the required amount depends on concrete moisture content, silica fume dose, aggregate type, heating rate, and section depth. A reliable dosage determination requires fire test panels under ISO 834-1:1999 or an equivalent standard; published data for this product in every aggregate matrix are limited.

Fibre distribution is the primary field risk for fire spalling applications. The density of polypropylene is lower than that of cement paste, and the fibres can migrate toward the upper surface under prolonged vibration. In slabs of 200 mm to 300 mm thickness, consolidation should be limited to 5 s to 15 s per insertion point; excessive vibration can leave lower sections with insufficient fibre count. Core examination on trial placements may be used to determine the number of fibres per unit area on sawn surfaces. When fibre counts are visibly inconsistent, the fire protection cannot be assumed, even if the batch records show the correct dosage.

Another thermal limitation is that MasterFiber 143 should not be expected to improve post-fire residual strength. Once the fibres have melted and exited the matrix, the increased permeability remains as a system of micro-pathways, but the concrete surface may be weakened. After cooling, damaged cover must be assessed by chisel testing or core compressive strength per EN 13791 before repair decisions are made. The fibre is not a fireproofing agent; it is a spalling-control additive within a larger fire-resistance design.

Material Specification, Standards References, and Batching Tolerances

For conformance control, the fibre should be received with a declared length, diameter, tensile strength, and melting range. The current technical data sheet for MasterFiber 143 should be compared with the project specification before batching. The dosage mass is usually based on concrete volume; the tolerance is commonly held to ±2% of the target fibre mass per batch when automatic dosing is used. Manual whole-bag addition is permitted when the bag mass is matched to the batch volume, but partial-bag addition should be avoided because it introduces dosage variability.

The table below provides a verification matrix for specification compliance.

For sprayed concrete, project specifications typically require that the fibre product be added without changing the water-to-binder ratio. The fibre itself is hydrophobic and does not absorb mixing water, but the increased solid surface area may reduce slump. Any added water or high-range water reducer must be rebalanced through trial mixes. The spray contractor should establish spray panels with the exact fibre dosage, accelerator type, and pump configuration before main placement. Sprayed concrete quality is then assessed by EN 14488-2 for compressive strength and EN 14488-6 for bond strength where relevant; the presence of MasterFiber 143 does not change the required sampling rate.

MasterFiber 143 is specified in cast-in-place applications such as industrial floor slabs, repair mortars, precast panels, and tunnel linings where restraint from adjacent construction or thermal shrinkage is expected. In such placements, the fibre is incorporated centrally at the plant or on site in the truck mixer. The common addition sequence is to charge the fibre with sand before coarse aggregate, add cement and supplementary cementitious materials, then introduce water and admixtures. In truck mixers, mixing speed should be maintained at the manufacturer’s rated mixing speed for at least 5 min after fibre introduction; discharge should not begin until visual inspection of a fresh sample confirms no fibre balls larger than 5 mm. If clumps are present, additional mixing time may be required, but prolonged high-speed mixing can increase air entrainment and should be limited.