During the initial 6 h after screeding of a slab-on-grade placement, plastic shrinkage cracking is controlled by capillary pressure development in the top 20 mm of fresh concrete, not by long-term drying shrinkage. Adfil Syntec 12, a 12 mm ultra-fine monofilament polypropylene microfiber with a specific gravity near 0.91, is charged in ready-mix operations at 0.6 kg/m³ to 1.8 kg/m³ to provide a multidirectional filament network capable of restraining microplastic cracking before the cementitious matrix reaches final set. At a central mix plant, the fibre is introduced into the skip or aggregate conveyor before the cement and aggregate charge enters the drum mixer; mixing is continued for 5 min to 7 min after final water addition at drum rotation rates of 12 rpm to 18 rpm to break interfilament electrostatic clumps without inducing excessive air entrainment. The resulting concrete falls under ASTM C1116/C1116M-10a Type III for synthetic fibre-reinforced concrete, and plastic shrinkage crack reduction is evaluated using ASTM C1579-21 restrained panel testing under an evaporative air-flow channel that simulates a 1.0 kg/m²/h evaporation rate. Slump measured by ASTM C143/C143M-20 is normally limited to a loss of 15 mm or less at the 0.9 kg/m³ midpoint for a 150 mm target slump mix with a water-cement ratio of 0.45 to 0.55; higher fibre additions up to 1.8 kg/m³ are reserved for placements where wind speed exceeds 2.5 m/s or where the ACI 305R-10 evaporation threshold of 1.0 kg/m²/h is expected within the first 4 h. Terminal components include power-troweled industrial floors, exterior hardstands, and freezer slab installations where joint spacing is retained according to the saw-cut schedule and the fibre is not a substitute for structural steel reinforcement.

Why Does Wet-Mix Shotcrete Rebound Drop at Specific Polypropylene Addition Rates?

For underground cavern support and slope stabilization, wet-mix sprayed concrete is conveyed as a premixed paste-aggregate stream and accelerated at the nozzle with compressed air; Adfil Syntec 12 is charged at the concrete plant or during transit to avoid segregation in the pump hopper. The 12 mm fibre length is sufficiently short to pass through 50 mm delivery hose and nozzle manifold components without accumulating behind the air ring, provided the fibre count remains below the threshold that creates fibre-fibre mechanical interlocking in the S-valve. Addition rates of 0.9 kg/m³ to 1.5 kg/m³ are used because the three-dimensional monofilament network reduces aggregate rebound during high-velocity impact; this fresh-state cohesion effect is not a mechanism of hardened flexural strengthening and must not be substituted for steel wire or macrofibres where structural ductility is required. Sprayed concrete requirements are defined in EN 14487-1, fibre content in the hardened shell is checked by EN 14488-7, and sampling of fresh shotcrete for consistency follows ASTM C1436/C1436M-17. On piston pumps with 50 mm delivery line, fibre additions beyond 1.5 kg/m³ produce a measurable increase in the yield stress of the fine mortar phase, appearing as manifold pressure rise rather than as hopper segregation; published pressure loss curves for this specific product in wet-mix shotcrete are limited, and plant trials using a 10 m U-loop circuit are required before production. Terminal products include primary and secondary tunnel linings, rock slope stabilization shells, and shaft wall encasements where early re-entry and rebound minimization within the spray cycle dictate the fibre addition rate.

Self-Leveling Underlayment Rheology and Microfiber Settlement Thresholds

Where sheet vinyl or LVT flooring is specified, anhydrite- and CSA-modified self-leveling underlayments are dosed with ultra-fine monofilament polypropylene fibres at 0.05 wt% to 0.15 wt% of dry mortar mass, with the lower bound selected for high-flow systems where visible fibre protrusion would impair vinyl sheet or LVT adhesion and the upper bound selected for rapid-drying patch screeds. The fibre is dry-dispersed in a pin mixer or ploughshare mixer with a tip speed of 20 m/s to 30 m/s for 90 s to 120 s; longer mixing at high tip speed can fracture the 12 mm monofilament and generate short fibre fines that raise the yield stress of the slurry without contributing to crack control. Flow characteristics are evaluated with the EN 12706 flow cone method, and the dosage is normally selected so that the 20-minute ring spread is not reduced by more than 5 mm relative to the unmodified control at a target initial spread of 150 mm. The finished screed material is classified under EN 13813:2002 for both compressive strength class and water permeability; the fibre content is not used to modify the bearing strength class of the screed but to reduce surface microcracking and edge curl during the first 24 h of hydration. Terminal products include self-leveling underlayments for sheet vinyl and LVT flooring, heated floor screeds, and rapid repair screeds in hospital and retail environments where restrained perimeter detailing can otherwise produce shrinkage cracks at door openings and thermal expansion joints.

Application	Typical addition range	Key standard or method	Process control threshold
Ready-mix slab-on-grade	0.6–1.8 kg/m³	ASTM C1116/C1116M-10a Type III; ASTM C1579-21	slump loss ≤ 15 mm at 0.9 kg/m³
Wet-mix shotcrete	0.9–1.5 kg/m³	EN 14487-1; EN 14488-7; ASTM C1436/C1436M-17	fibre addition capped at 1.5 kg/m³ to limit line pressure rise
Self-leveling underlayment	0.05–0.15 wt% dry mass	EN 13813:2002; EN 12706	20-minute flow spread loss ≤ 5 mm
Architectural precast panels	0.6–1.2 kg/m³	ASTM C157/C157M-17; ASTM C666/C666M-15	pan mixer 4–6 min after water; demolding within 24 h
Dry-mix repair mortar	0.05–0.20 wt% dry mass	EN 1504-3:2005; EN 1504-10:2017	residual moisture < 0.2 %; ribbon speed 20–40 rpm
Wet-cast utility vault/pipe	0.6–1.2 kg/m³	ASTM C39/C39M-21; ASTM C497-20; ASTM C76-22a	not for dry-cast; discharge screen check every 20 cycles

Thin-gauge architectural precast concrete panels with exposed aggregate finishes develop edge spalling during demolding when the surface zone has not reached sufficient tensile strength to resist mould suction and demolding stress. In precast mixes with 30 MPa to 50 MPa 28-day compressive strength and 10 mm to 20 mm coarse aggregate, Adfil Syntec 12 is added at 0.6 kg/m³ to 1.2 kg/m³ to restrain brittle microcracks at the perimeter during the first 24 h after casting. The fibre is introduced into a pan mixer with counter-rotating blades after the aggregate and cement have been dry-mixed for 30 s; mixing continues for 4 min to 6 min after water addition to achieve homogeneous distribution without visible fibre bundles on the exposed aggregate face. Dimensional stability of the hardened element is monitored by ASTM C157/C157M-17 length-change bars, and freeze-thaw resistance for exterior cladding is evaluated with ASTM C666/C666M-15 where the panel is exposed to cyclic moisture. The fibre is not intended to replace conventional welded wire reinforcement or lifting inserts; its function is limited to early-age microcracking and handling damage reduction from demolding to erection. Terminal products include architectural spandrel panels, sound barrier walls, and large-format cladding elements where demolding cycles shorter than 18 h create high surface stress at the form return.

When Dry-Mix Repair Mortars Are Charged in Vertical Ribbon Blenders

When charging dry-mix repair mortar ingredients into a vertical ribbon blender, fibre agglomeration occurs if the ultra-fine monofilament fibre is loaded before densified silica fume, producing fibre balls larger than 5 mm at the bagging hopper and creating dead zones within the mixer shell. The charge order for a 1,000 kg batch is mineral binders, then 0.05 wt% to 0.20 wt% of the 12 mm ultra-fine monofilament fibre, then coarse aggregate, and finally low-bulk-density fillers; the ribbon speed is held at 20 rpm to 40 rpm for 5 min to 8 min with side choppers engaged to prevent electrostatic clumping. Because polypropylene is hydrophobic, the residual moisture content of the dry mix must remain below 0.2 % to avoid water-bridge agglomeration on the fibre surfaces; storage in sealed 25 kg bags below 80 % relative humidity is specified. The hardened repair mortar is classified under EN 1504-3:2005 as R3 or R4 according to compressive strength and adhesion, and the selection of the product within a repair system follows EN 1504-10:2017; the synthetic microfiber does not replace structural steel reinforcement and is not considered a strengthening component for crack bridging in structural repair design. Terminal products include 25 kg bagged polymer-modified repair mortars for concrete patch repair, formed surfaces, and overhead applications where batch-to-batch consistency of fibre dispersion controls pumpability and surface finish.

Precast Utility Vault and Pipe Concrete Surface Crack Control in Wet-Cast Applications

In wet-cast precast production of utility vaults and manhole bases, synthetic microfiber at 0.6 kg/m³ to 1.2 kg/m³ is used primarily to suppress plastic settlement cracking around embedded structural inserts, knockouts, and lifting sockets. The fibre is dispersed in a twin-shaft horizontal mixer at 30 rpm to 40 rpm before placement into oiled steel forms; external form vibrators are operated at a frequency compatible with 10 mm to 20 mm aggregate consolidation to prevent fibre layering and ensure uniform fibre distribution in vertical walls. Compressive strength of the finished concrete is tested by ASTM C39/C39M-21, absorption by ASTM C497-20, and the structural conduit is specified under ASTM C76-22a for reinforced concrete culvert, storm drain, and sewer pipe; the 12 mm synthetic microfiber is not a substitute for the steel cage or steel reinforcement required for load-bearing pipe classes. The use of this fibre is incompatible with dry-cast pipe manufacturing, where very low water-cement ratios and immediate form stripping lack sufficient shear to uniformly disperse hydrophobic monofilaments and can cause fibre ball formation in the discharge gate; published data for this specific product in dry-cast machinery is limited, and site trials should evaluate the discharge hopper screen after each 20 casting cycles. Terminal products include wet-cast utility vaults, manhole bases, and box culverts where restrained shrinkage around blockouts and curved sections is a watertightness concern.