What Limits Explosive Spalling Resistance in Dense Precast Tunnel Segments Exposed to Hydrocarbon Fire Curves?

In high-strength precast concrete tunnel segments produced with water/cement ratios below 0.40 and silica fume dosages of 5–8% by mass of cement, the dominant failure mechanism during a hydrocarbon fire is not compressive strength loss but explosive spalling driven by internal pore pressure. The matrix under consideration typically exhibits a capillary porosity of 8–12% after steam curing, and when the RWS or HCM fire curve raises the surface temperature at 200–250 °C/min, free water in pores converts to steam faster than it can migrate. Micro synthetic polypropylene fibers with a melt onset of approximately 160–170 °C create pressure-relief channels after melting, reducing the vapour pressure plateau that otherwise exceeds the tensile strength of the heated cover zone. The conformity of the fiber is assessed under EN 14889-2:2006 for polymer fibers for concrete; the fire design of the segment is governed by EN 1992-1-2:2004, Section 4.5, and the fire test condition is defined by ISO 834-1:1999 or project-specific RWS/HCM hydrocarbon curves. The addition ratio used in production is 1.0–1.5 kg/m³, with the upper bound reserved for matrices with compressive strength class C50/60 or higher and low permeability. In the mixing sequence, a 3.0 m³ twin-shaft compulsory mixer receives coarse and fine aggregate first, then the fiber is added after 80% of aggregate mass is charged, followed by cement, silica fume, water, and high-range water reducer meeting EN 934-2. Mixing continues for 90–120 seconds at high shear; batch-to-batch fiber distribution is checked by washing out 10 L fresh concrete samples over a 0.5 mm sieve. After demolding, segments are steam cured at 60–70 °C for 6–8 hours, below the fiber melting point. Dosages above 1.5 kg/m³ require verification of residual compressive strength under EN 12390-3 because the additional melt-channel porosity may affect the dense matrix. Trial furnace tests are mandatory because published data for this specific product in full-scale RWS-exposed production segments is limited. The terminal product is a reinforced precast concrete tunnel ring segment intended for rail and road tunnels where hydrocarbon fire spalling resistance is specified as a project performance requirement.

Large-area ground-supported concrete slabs placed at thicknesses of 150–200 mm in logistics facilities exhibit plastic shrinkage cracking when the evaporation rate from the fresh surface exceeds the bleed water supply, a condition commonly recorded when wind speed exceeds 4.5 m/s, ambient relative humidity drops below 40%, or concrete temperature exceeds 28 °C. The applicable construction specification for the finished floor class is ACI 302.1R-15, and the plastic shrinkage cracking performance of the fiber-reinforced mixture is evaluated by ASTM C1579-21; the fiber itself is specified as a Type III synthetic fiber under ASTM C1116/C1116M-10a. The formulation addition ratio in the ready-mix truck mixer is 0.6–0.9 kg/m³; at this range, the slump of a 150 mm target mix is typically reduced by less than 10 mm, and if a higher water reduction is required, a polycarboxylate high-range water reducer conforming to EN 934-2 or ASTM C494/C494M may be added without altering fiber dispersion. The production sequence involves charging 80% of the aggregates and 50% of the batch water before the fiber bags are introduced, then mixing at 12–18 rpm for a minimum of 70 revolutions of the drum. Placement uses a laser-guided screed and bull float, followed by power troweling only after bleed water has disappeared from the surface. Curing compound is applied at 0.20 L/m² or the rate specified by the manufacturer; saw cutting of contraction joints follows the schedule in ACI 302.1R-15 based on the concrete mixture and ambient conditions. The micro fiber is not a substitute for jointing or structural reinforcement, and it does not eliminate the need for surface curing in high-evaporation conditions. The terminal product is a hard-troweled industrial floor slab for logistics, distribution, and automated storage facilities where flatness and crack control are specified jointly.

Micro Polypropylene Fiber Dispersion in Wet-Mix Sprayed Concrete for Underground Support

Wet-mix sprayed concrete produced with a robotic manipulator and an accelerating agent injected at the nozzle requires the fiber to remain dispersible through a 50 mm delivery line at volumetric outputs of 8–20 m³/h, a flow condition where the shear rate at the hose wall can exceed 80 s⁻¹. The relevant specification for the sprayed concrete is EN 14487-1:2005, with test methods for flexural and residual strength performed under EN 14488-3 or ASTM C1399/C1399M; the polymer fiber is assessed under EN 14889-2:2006, and the execution guidance follows ACI 506R-16. The addition ratio used for underground support applications is 0.9–1.2 kg/m³; higher dosages increase the viscosity of the pumped paste fraction and may require an additional 0.2–0.5 L/m³ of a polycarboxylate superplasticizer to maintain a fresh concrete rheology compatible with the pump. In the batching sequence, the fiber is added after the coarse aggregate fraction in a truck mixer, and the mix is transported to the shotcrete machine with a maximum drum speed of 10–12 rpm. At the nozzle, compressed air at 7–10 m³/min and a pressure of 0.7–1.0 MPa accelerates the mix onto the substrate, while an alkali-free accelerant is dosed at 4–8% by weight of cement. Rebound during spraying is collected and weighed; published data for the rebound reduction attributable to this specific micro fiber configuration is limited, and the nozzleman must adjust the nozzle angle and distance within 0.8–1.2 m to maintain rebound below 15%. The fiber addition does not replace steel fiber or welded wire mesh where structural ductility is required; it addresses plastic shrinkage cracking in the fresh layer and reduces early-age surface fissures before the sprayed lining gains structural strength. The terminal product is a temporary or permanent primary lining in underground headings, rock slope stabilization, or shaft lining where early crack control and fire spalling resistance are both required.

In self-compacting concrete mixes for thin-walled architectural precast panels with exposed aggregate finishes, the addition of micro polypropylene fibers alters the paste volume and water demand differently from steel fibers because the high specific surface area of the polymer fiber increases the yield stress of the paste fraction; a slump flow reduction of 30–50 mm is commonly observed at a constant water/cement ratio of 0.35. The governing specification for the precast element is EN 13369:2018 in combination with PCI MNL-116 for architectural precast quality control; the fiber-reinforced concrete is specified under ASTM C1116/C1116M-10a, and the self-compacting flow retention is measured by ASTM C1611/C1611M-21. The addition ratio is 0.6 kg/m³, selected to control plastic shrinkage cracking in thin sections without producing visible fiber bundles at the exposed surface after retardation and aggregate exposure. In production, the fiber is introduced into the compulsory mixer after the sand and coarse aggregate are charged but before the cementitious paste is fully dispersed; the mix is then held at a slump flow of 650–700 mm and discharged into a bucket for transfer to the casting bed. Formwork is filled without external vibration; only light tapping with a rubber mallet is permitted along ribs and returns. The panel is demolded after 16–24 hours, and surface retarding agents are removed by low-pressure washing to expose the coarse aggregate to a depth of 3 mm. If the specified aggregate exposure depth exceeds 3 mm, compatibility testing of the surface retarder is required because the micro fibers can become visible at the interface. The terminal product is an architectural precast cladding panel with a textured aggregate finish for commercial and institutional facades.

When Machine-Applied Repair Mortars Are Pumped Through 25 mm Hoses at High Shear

Machine-applied cementitious repair mortars used for vertical and overhead concrete repair are pumped through 25 mm internal diameter hoses at delivery pressures of 1.0–2.0 MPa, a process that imposes high shear and can mechanically bundle polymer fibers if the fibers are added before the cementitious binder is fully wetted. The compliance framework for the repair mortar is EN 1504-3:2005, with performance class R4 for structural repair; in North American practice the material is specified under ASTM C928/C928M. The fiber addition ratio is 0.6–1.0 kg/m³ of dry mortar, depending on the section thickness and the exposure class. The production sequence in a continuous mixing and pumping machine involves adding the dry mortar to the hopper first, wetting the material in the mixing chamber for 30–45 seconds, and then introducing the fiber into the wet paste so that the fiber is distributed before the mix reaches the worm pump. At the spray nozzle, air at 0.4–0.6 MPa is introduced only when the mortar is sprayed, not when it is poured into formwork. The substrate is prepared by removing loose concrete and cutting the repair edge at a minimum 10 mm depth, then pre-wetting to a saturated surface-dry condition before the mortar is applied in layers; successive layers are applied only after the previous layer has reached a firm set. The micro fiber controls plastic shrinkage cracking during the first 6–8 hours, when wind and low humidity can create surface fissures in repair patches thicker than 20 mm. It does not replace reinforcing steel, shear connectors, or structural strengthening systems specified by the responsible engineer. The terminal product is a repaired concrete element, typically a parking garage soffit, bridge pier cap, balcony edge, or column face where the original cross-section is restored and early-age surface cracking is controlled.

Forced-Air Heated Curing Induces Differential Moisture Gradients in Flowable Screeds

Flowable cementitious screeds installed at thicknesses of 30–50 mm and subjected to forced-air heating at 35–40 °C after 24 hours develop differential moisture gradients between the top surface and the substrate that can initiate superficial map cracking before the screed reaches sufficient tensile strength. The material requirement is defined by EN 13813:2002 and BS 8204-1:2003+A1:2009 for in situ cementitious levelling screeds; the restrained shrinkage performance of the fiber-modified screed mortar can be compared with the control using ASTM C1581-04. The addition ratio is 0.6–0.9 kg/m³ of screed mix, and the fiber is introduced into the mixing drum after the fine aggregate and 70% of the mixing water are combined to prevent dry clumping at the paddle shaft. The production process uses a truck-mounted pump with a 25–50 mm delivery hose, a dappling bar or spike roller to remove entrapped air, and a finishing pass with a steel trowel; forced-air heaters are positioned no closer than 1.0 m to the surface and are not operated until final set has been verified by a penetrometer reading. Polyethylene sheet curing is maintained for 7 days at the surface; the micro fiber does not substitute for curing, but it reduces the width of any early-age cracks that form before the screed is covered. The terminal product is a bonded or unbonded cementitious floor screed beneath vinyl, ceramic tile, or resin flooring in residential and commercial buildings where forced drying schedules are imposed by the construction programme.