Prestressed Corrugated Metal Ducts ‌Installation Guide

Prestressed Corrugated Metal Ducts ‌Installation Guide (PCMD)

(For Post-Tensioning Applications per ASTM A929/A929M & EN 523)


1. Functions & Advantages

Prestressed corrugated metal ducts are fabricated from galvanized/non-galvanized steel strips through spiral folding and lock-seam forming. Key benefits include:

  • Structural Performance‌: Provides secure conduits for tendons, preventing corrosion from concrete bleed water (pH≥12.5) and atmospheric moisture (RH≤75%).
  • Construction Efficiency‌: Lightweight (0.8-1.2kg/m for Ø50mm) with bend radii ≥30×D without kinking.
  • Bond Enhancement‌: Helical grooves (pitch: 6-12mm) improve 28-day grout bond strength by 40% vs. smooth ducts.

2. Pre-Installation Preparations

2.1 Material Inspection

  • Visual Checks‌: Reject ducts with:
    • Dents >3% of diameter
    • Zinc coating defects (<100g/m² per ISO 1461)
    • Seam gaps >0.5mm
  • Dimensional Tolerance‌:
    ParameterAllowance
    Diameter±1.5mm
    Wall thickness±0.1mm

Prestressed Corrugated Metal Ducts ‌Installation Guide
Prestressed Corrugated Metal Ducts ‌Installation Guide

2.2 Site Preparation

  • Clearance‌: Maintain 500mm working space around duct routes.
  • Tool Checklist‌:
    • Torque wrench (10-50Nm range)
    • Seam crimper (for field joints)
    • Leak tester (0.3MPa air pressure)

2.3 Layout & Positioning

  • Tolerance Limits‌:
    • Vertical: ±10mm (per EN 1992-1-1)
    • Horizontal: ±20mm
  • Marking‌: Use laser levels for curves (min. radius=4m).

3. Installation Procedures

3.1 Jointing Methods

A. Sleeve Connections

  1. Select sleeves 10mm larger than duct Ø (e.g., Ø60mm sleeve for Ø50mm duct).
  2. Insert ducts 150mm minimum into sleeve.
  3. Seal with:
    • Butyl tape (3mm thick, 50% overlap)
    • Stainless steel hose clamps (2 clamps/joint, torque to 8Nm).

B. Spigot-and-Socket Joints

  1. Chamfer spigot end at 15°.
  2. Apply polysulfide sealant (ASTM C920 Type M) to socket.
  3. Engage with ¼ turn until alignment marks match.

3.2 Fixing to Reinforcement

  • Spacing‌:
    Segment TypeInterval
    Straight800mm max.
    Curved400mm max.
  • Binding‌: Use 1.2mm annealed wire (2 turns per strap).

3.3 Anchorage Connection

  1. Insert duct 50mm into trumpet.
  2. Weld/seal per:
    • Option 1‌: 3 tack welds (max. 150°C to avoid zinc vaporization)
    • Option 2‌: Epoxy putty (24h cure @20°C)

3.4 Leak Prevention

  • Critical Areas‌:
    • Penetrations: Wrap with hydrophilic waterstop
    • Couplers: Pressure-test at 0.2MPa for 5min

4. Key Operational Controls

4.1 Concrete Placement

  • Pouring‌: Maintain 300mm free-fall distance from ducts.
  • Vibration‌:
    • Minimum offset: 300mm from duct surface
    • Frequency: ≤10,000 rpm to avoid seam resonance

4.2 Pre-Stressing Checks

  • Mandrel Test‌: Pass Ø90% tendon diameter mandrel through entire run.
  • Airflow Test‌: ≤3% pressure drop in 10min @0.15MPa.

4.3 Grouting Protocol

  1. Flushing‌: Pump pH 7-8 water at 0.5m/s velocity.
  2. Mixing‌:
    • W/C ratio: 0.40±0.02
    • Marsh cone flow: 25-35 seconds
  3. Injection‌:
    • Pressure: 0.5-1.0MPa
    • Venting: Continue until effluent density matches inlet (±2%).

5. Troubleshooting Guide

IssueRoot CauseSolution
Seam leakageInsufficient crimp forceRe-crimp at 15% higher pressure
Grout voidsRapid setting (<30min)Use retarding admixture (0.1% by weight)
Tendon seizureDuct ovalization (>5%)Install internal stiffener sleeve

6. Maintenance

  • Exposed Sections‌: Apply zinc-rich paint (DFT≥80μm) annually.
  • Structural Monitoring‌: Use ultrasonic tomography if cracks >0.3mm appear.

This guide complies with PTI M50.3-20 and FIB Bulletin 33 standards. For seismic zone adaptations (Z≥3), consult Section 8.4 of EN 1998-1.

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