When Wet-Mix Shotcrete Rebound Exceeds 22% on Site, Fibre Type and Addition Point Become Controlling

In wet-mix shotcrete for underground headings, MasterFiber 343 addition occurs at the batching plant through a calibrated fibre dosing hopper that discharges into the twin-shaft compulsory mixer after coarse aggregate and before water. The macro fibre fraction, commonly 38–54 mm in length with an equivalent diameter of 0.6–0.9 mm, contributes post-crack flexural toughness required for rock support. The micro fibre fraction, typically 10–20 mm in length, reduces early plastic shrinkage and improves material cohesion at the nozzle. A representative dosage window is 3.0–6.0 kg/m³, but the actual addition rate is set by the specified residual flexural strength class under EN 14488-3:2006. For temporary support, an fR1k value of not less than 2.0 MPa is often specified; for permanent linings, fR4k of not less than 1.5 MPa may be required. Delivery through a swing-tube piston pump with a 50 mm internal diameter hose produces line pressures from 18–28 bar at outputs of 12–16 m³/h. Sharp 90° bends and reducer transitions increase the probability of fibre accumulation and should be avoided. Air supply at the nozzle is typically 4–8 m³/min at 0.5–0.7 MPa. At the crown, rebound can exceed 22% when fibre dosage is high and accelerator dosage is insufficient; accelerator dosage is adjusted in increments of 0.2–0.5% by cement mass. Field verification requires trial panels sprayed with the production equipment, then beam specimens sawn and tested under EN 14488-3. Fibre distribution is improved by using a swivel-type nozzle and a well-regulated air ring rather than extended travelling distances. Batch-to-batch variation in fibre count per unit volume should be checked by washout testing of fresh concrete per ACI 506R guidance. If the fibre feed skips during the shift, residual strength in the corresponding advance should be treated as non-compliant until companion panel testing demonstrates otherwise.

For carousel production of precast concrete segments, the dosing sequence for MasterFiber 343 begins with dry mixing for 60–90 s in a twin-shaft compulsory mixer, followed by wet mixing for 90–120 s after water and superplasticiser are introduced. The addition rate is generally specified between 2.5–4.5 kg/m³, with the lower end for segments with dense conventional reinforcement and the upper end for segments requiring improved handling toughness after stripping. The macro fibre fraction acts at demoulding and during lifting by vacuum handling systems; the micro fibre fraction reduces the incidence of edge spalling at mould corners. Steam curing is conducted at 55–80°C for 6–10 h, with ramp rates limited to 15°C/h to avoid thermal shock. Synthetic fibres must retain their geometry through the heating cycle. Published multi-batch field data specific to MasterFiber 343 under segment curing is limited; therefore a plant trial of at least three consecutive batches is required to establish fibre distribution and surface finish stability. Flexural performance is evaluated according to ASTM C1609/C1609M-19a, and residual strength is additionally measured by ASTM C1399/C1399M-10(2015). Compressive strength is verified under EN 12390-3. Visual inspection after demoulding includes checking for clustering at steel mould edges, which is aggravated by over-vibration above 70 Hz. Edge trowelling should be limited because manual working can draw macro fibres to the formed surface. Fibre addition does not replace conventional reinforcement required for bursting, handling, or segment erection loads. If the mix self-compacts on a carousel without external vibration, the slump flow must be controlled to avoid fibre segregation.

Jointless Slab-on-Grade Flooring Demands More Than Crack Width Control

The governing conflict in jointless slab-on-grade construction is the simultaneous requirement for extended joint spacing and acceptable post-crack residual strength under forklift and racking loads. MasterFiber 343 is added at 2.0–5.0 kg/m³. The macro fibre fraction bridges cracks that form at construction joints or saw-cut terminations; the micro fibre fraction controls plastic settlement and surface craze cracking during laser screed finishing. Flexural performance is measured under ASTM C1609/C1609M-19a using 150 x 150 x 500 mm beams loaded at third points. For industrial floors, a performance target of R3 30–50% or residual strength at L/600 of 1.5–2.5 MPa is commonly specified, depending on racking loads. The fibre blend does not eliminate differential drying shrinkage; the water-cement ratio should remain between 0.45–0.50, and wet curing for 7 days is required. If the concrete is pumped through a 36 m boom with a reducer from 125 mm to 100 mm, localised fibre blockages can occur at reducers when the bend radius is less than 1.0 m. Plant dosing must be automated with a tolerance of ±0.5 kg per load to limit variation in flexural toughness across the pour. After strike-off, the use of a laser screed followed by a pan float must be sequenced before the surface crust closes. A skip float applied too late can draw macro fibres to the surface and create visible fuzz. For high-rack warehouses, the test programme also includes ASTM C1581/C1581M-18a restrained shrinkage ring testing to compare crack initiation time. Slab curling is governed by moisture differentials and is not corrected by fibre addition alone.

Dry-cast pipe manufacturing operates with zero-slump concrete compacted by vibratory action at 40–60 Hz. MasterFiber 343 is added at 2.0–4.0 kg/m³. The micro fibre fraction is more relevant for surface tearing at immediate demoulding; the macro fibre fraction improves handling resistance after the pipe is stripped and moved to curing racks. Fibre balling in dry mixes is a known process failure when water is added before fibres or when mixer pan liners are excessively worn. The standard governing polymer fibres for concrete is EN 14889-2:2006; it requires geometric consistency and defines compliance for relevant material properties. No fully published stress-strain data for MasterFiber 343 in dry-cast zero-slump concrete appears in open literature; validation therefore requires a full-scale trial pipe. Three-edge bearing strength is evaluated according to EN 1916 or ASTM C76. Visual acceptance after demoulding includes checking for fibre clusters at the spigot and bell ends. If clusters are present, the addition sequence must be changed or the mixing time extended. Conventional reinforcement remains mandatory for large-diameter classes where specified. Surface finish on the inner shell is verified immediately after demoulding under task lighting. The fibre blend does not act as a replacement for structural steel cages in high-load pipe classes; its primary function is crack control and handling toughness.

Does Synthetic Macro Fibre Survive the Steam Curing Cycle in Precast Facade Elements?

When MasterFiber 343 is specified in architectural wall panels produced on long-line steel tables, the primary thermal concern is not fibre degradation but fibre read-through and surface blemish after steam curing. The addition rate is typically 1.5–3.0 kg/m³. Synthetic macro fibres have a specific gravity near 0.90–0.96; in wet mixes with a water-cement ratio above 0.45, fibre migration to the top surface becomes likely during vibration and trowelling. The macro fibre fraction improves handling strength after demoulding; the micro fibre fraction reduces plastic cracking at edges and returns. Curing is controlled with a ramp rate of 10–15°C/h and a maximum holding temperature of 80°C. Compliance with fibre specification is assessed under EN 14889-2:2006 and ASTM C1116/C1116M-23. Flexural performance is measured on companion specimens under ASTM C1609/C1609M-19a. Visual acceptance follows the project-specific architectural precast tolerance, and a minimum cover of 15–20 mm over the fibre-reinforced zone is maintained to prevent exposed fibre at the mould surface. Surface retarders and acid etching cannot remove exposed macro fibres after curing. The fibre blend does not replace steel connection reinforcement, edge trims, or structural wythe connectors in insulated sandwich panels. If polishing or sandblasting is specified, a full-scale mockup panel is required to evaluate fibre visibility before production. Batch-to-batch variability in fibre length distribution should be checked against the supplier certificate.

Under slipform paving conditions, the specified addition rate for MasterFiber 343 in rigid pavement and bonded white topping ranges from 1.8–4.0 kg/m³. Total air content is maintained at 5.5–7.5% by volume under ASTM C231 because synthetic fibres do not provide freeze-thaw resistance. The macro fibre fraction reduces crack opening under traffic-induced flexural stress; the micro fibre fraction reduces plastic shrinkage cracking during hot, windy placement. Flexural performance is evaluated by ASTM C1609/C1609M-19a; for white topping, residual strength at L/600 of 1.0–2.0 MPa is often specified. Joint layout is still governed by pavement thickness, support conditions, and curling stress analysis. Fibre addition may allow modest increases in joint spacing only if supported by structural analysis and a site trial panel. Texturing operations with burlap drag or tining must be completed before the surface crust closes; late tining can expose fibres at the pavement surface. If a slipform paver with slab width 7.5–9.0 m is used, fibre clumps at the auger can cause longitudinal streaks; paver speed and auger speed must be tuned during trial placement. The use of an internal vibrator array with 6,000–10,000 vpm may locally redistribute fibres; spacing of vibrators must not exceed 1.5 times the radius of influence. For airfield pavement, the more stringent flexural strength and surface friction standards apply, and fibre addition does not replace dowels or tie bars at joints.

Bond Line Compatibility in Fibre-Reinforced Concrete Overlays

Bonded concrete overlay placement begins with surface preparation to ICRI 310.2R CSP 6–9. MasterFiber 343 is added at 1.5–2.5 kg/m³ for crack control in cementitious overlays. The macro fibre cannot compensate for a contaminated bond line; laitance, oil, and dust must be removed before placement. The micro fibre fraction reduces plastic cracking at the surface during hot weather and high evaporation conditions. Bond strength is tested by ASTM C1583/C1583M-13 and must exceed 1.0 MPa at 28 days. Shrinkage is measured under ASTM C157/C157M-17; the fibre blend reduces visible crack pattern but does not alter chemical drying shrinkage of the cement paste. Application thicknesses below 25 mm may expose macro fibres after trowelling; therefore the maximum fibre length should be less than one-third of the overlay thickness. Pumping through small-line pumps with 25 mm or 32 mm hoses is possible but requires pre-wetted fibres and a mixer uniformity check. The overlay system must comply with ASTM C928/C928M-20 or the specified repair material standard. The presence of synthetic fibres does not reduce the need for a saturated surface-dry substrate and an appropriate bonding agent. If the overlay is placed on a substrate with high suction, a scratch coat or pre-wetting is required to prevent rapid water loss at the bond line.