Post-tensioning

Post-tensioning is a construction method used to strengthen concrete by running high-strength steel tendons or strands through ducts in the concrete, which are then tensioned and anchored after the concrete has cured. This process places the concrete under compression, allowing for longer spans, thinner slabs, and greater resistance to cracking, making it especially valuable in dams, bridges, and large-scale structural projects.

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Materials

• Strands (bonded/unbonded): 7-wire, low-relaxation steel
  — Ø 12.7 mm (0.5″), 15.2/15.7 mm (0.6″)
  — fpu ≈ 1,770–1,860 MPa (250–270 ksi); E ≈ 195–205 GPa

• Bars (where used): Ø 26–75 mm, fpu ≈ 1,030–1,080 MPa

• Ducts (bonded multistrand): steel or HDPE corrugated, Ø 50–130 mm typical

• Anchorage hardware: wedges + bearing plates per system; galvanised or stainless options

• Sheathing (unbonded): HDPE with grease corrosion protection

• Grout (bonded): cementitious, non-shrink, low-bleed

Concrete & cover

• Concrete strength before stressing: typically ≥ 25–30 MPa (check design)

• Nominal cover to duct/tendon: interior ≥ 25–40 mm; exterior/exposure ≥ 40–75 mm (environment dependent)

Stressing & acceptance

• Stressing level: typically 0.70–0.80 fpu (do not exceed system limit)

• Elongation tolerance: measured vs theoretical ±7% (common)

• Jack calibration: certified within 30 days/usage, accuracy ±2%

• Seating loss (wedges): allow ~3–6 mm (system specific)

• Stressing sequence: from the most restrained end; alternate/multiple stages to control cracks/deflection

Friction & layout

• Friction coefficient (μ):
  — steel duct ~0.20–0.25; plastic/HDPE ~0.10–0.18

• Wobble factor (k): ~0.001–0.002 per m (system/layout dependent)

• Minimum radius of curvature: typically ≥ 50–100 × duct diameter (follow supplier tables)

• Deviators/blocks: provide bursting/anchor zone reinforcement per design

Grouting (bonded systems)

• w/c ratio: ~0.40–0.45

• Bleed: ≤ 0.3% (low-bleed spec)

• Compressive strength: ≥ 17–25 MPa @ 7 days (project-specific 28-day often ≥ 30 MPa)

• Chlorides: very low—≤ 0.1% by mass of cement (or per spec)

• Temperature (mix/injection): generally 10–32 °C

• Grout verification: density, flow, bleed tests each batch; venting from high points until neat grout

QA/QC & testing

• Tendon ID/tracking per drawing; record jack pressure & elongations

• Lift-off checks: sample tendons (e.g., 1–5%) to verify residual force

• Duct pressure/continuity tests prior to grouting (bonded)

• Protection of unbonded tails: caps + grease; seal all vents/anchorages after grout set

References to align with (South Africa/UK/EU)

• EN 1992 / BS 8110 practices, PTI (Post-Tensioning Institute) manuals, and local SANS 10100/project specs.