Case 3: A Metal Mine Roadway Support Project

Why Tunnel Engineering Needs Steel Fibers?

Tunnel lining structures are subjected to multiple adverse factors such as complex ground stress, surrounding rock deformation, blasting vibration, and groundwater erosion. Although ordinary reinforced concrete has high strength, its toughness is poor, and it is prone to brittle fracture and spalling under surrounding rock deformation.

Handan Chengyuan Technology supplies hooked-end steel fibers, made of high-strength steel wire, tensile strength ≥1100MPa, hooked-end design ensures good bonding with the matrix, significantly improving concrete toughness and impact resistance.

Steel Fiber Application in Various Tunnel Parts

Steel Fiber Type Selection and Dosage Reference

Case 2: A City Metro Shield Tunnel Project

1. Selection and Inspection of Steel Fiber

• Type selection: Tunnel engineering should prioritize hooked-end steel fibers with good bonding performance and significant reinforcement effect. Length 30-50mm, aspect ratio 40-80, tensile strength ≥600MPa (≥1000MPa for critical projects).
• Quality inspection: Each batch of steel fibers should be provided with a factory inspection report, including tensile strength, bending performance, length deviation, diameter deviation and other indicators. Sampling re-inspection should be conducted after arrival, and only qualified products can be used.
• Anti-rust treatment: Steel fibers should be anti-rust treated (copper-plated or galvanized), or stainless steel fibers should be used to ensure long-term durability in concrete. For highly corrosive environments (marine, saline soil), stainless steel fibers should be selected or the protective layer thickness increased.

2. Mix Ratio Design

• Dosage determination: Steel fiber dosage should be determined by testing according to project requirements. Generally 30-60kg/m³ for shotcrete, 20-40kg/m³ for cast-in-place concrete, 40-60kg/m³ for segments. Excessive dosage (>80kg/m³) will affect concrete workability and increase construction difficulty.
• Sand ratio adjustment: The sand ratio of steel fiber concrete should be 5%-10% higher than ordinary concrete, generally 45%-55%, to improve concrete workability and pumpability.
• Water reducer selection: Polycarboxylate superplasticizer should be selected with water reduction rate ≥25% to ensure the workability of steel fiber concrete. If necessary, an appropriate amount of air-entraining agent (air content 3%-5%) can be added to improve workability.
• Mix ratio testing: Mix ratio testing should be conducted before construction, measuring concrete slump, flow spread, compressive strength, flexural toughness, impermeability and other indicators to ensure design requirements are met.

3. Mixing and Transportation

• Feeding sequence: The recommended process is "aggregate + cement + admixture + steel fiber dry mix for 30s → add water + water reducer and mix for 120-180s" to ensure uniform dispersion of steel fibers and avoid clumping.
• Mixing time: The mixing time of steel fiber concrete should be 30-60s longer than ordinary concrete, generally not less than 150s, to ensure uniform dispersion of steel fibers.
• Transportation time: The transportation time of steel fiber concrete should not exceed 1 hour, and should be controlled within 45 minutes in summer construction. During transportation, the mixing drum should be kept rotating at low speed (2-4 rpm) to prevent steel fiber sinking and segregation.

4. Shotcrete Construction

• Spraying equipment: Special steel fiber shotcrete machines should be selected with delivery pipe diameter ≥75mm to avoid steel fiber blockage in the pipe. Wet spraying is superior to dry spraying, with more uniform steel fiber dispersion and lower rebound rate.
• Spraying process: The spraying angle should be as perpendicular to the sprayed surface as possible (75°-90°), with the nozzle 0.8-1.2m from the sprayed surface. The spraying sequence should be from bottom to top, in sections and slices, with each section length not exceeding 6m.
• Single-pass spraying thickness: The single-pass spraying thickness of steel fiber shotcrete can be appropriately increased compared to ordinary concrete, 80-150mm for side walls and 60-100mm for arches. For layered spraying, the subsequent layer should be applied after the previous layer has set.
• Rebound rate control: The rebound rate of steel fiber shotcrete should be controlled below 15% (side walls) and below 25% (arches). When the rebound rate is too high, adjust the mix ratio, spraying pressure or nozzle distance.
• Curing: Water spraying curing should begin within 2 hours after shotcrete final set, with curing time not less than 14 days. Winter construction should take thermal insulation measures to ensure curing temperature ≥5°C.

5. Quality Inspection

• Strength inspection: At least 1 set of standard-cured specimens should be retained per shift to test 28-day compressive strength. Critical projects should add flexural toughness testing to determine toughness index and equivalent flexural strength.
• Thickness inspection: Shotcrete thickness should be inspected by drilling or radar method, checking at least 1 section every 10 linear meters, with each section checking 1 point every 2-3m from the arch crown centerline. The thickness qualification rate should be ≥90%, with the minimum thickness not less than 80% of the design thickness.
• Steel fiber content inspection: Samples should be periodically taken from shotcrete, and steel fiber content determined by ignition or water washing method to ensure dosage meets design requirements. Steel fiber content deviation should not exceed ±10% of the design dosage.
• Appearance inspection: The shotcrete surface should be flat, free of cracks, voids, exposed reinforcement, water leakage and other defects. Problems should be promptly addressed.