Construction methods · Precast concrete decks

Post-tensioning: tendons, stressing and grouting

Steel strands in ducts tensioned after the concrete hardens, anchored at the ends and usually grouted for bond and protection. Internal tendons live inside the concrete; external tendons run inside the box and can be inspected and replaced. Grout quality decides whether the bridge lasts a century or corrodes in twenty years.

How bridges get built · Precast concrete decks · Post-tensioning: tendons, stressing and grouting

Also called: PT, internal tendons, external tendons, grouting of ducts

How it is done

Install ducts (corrugated steel or plastic) to the profile with supports every metre and vents at high points; anchorages fixed to the forms.
After concrete strength is reached, thread strands, stress with calibrated jacks to the design force, record force and elongation, and compare with calculated (within 5-7 %).
Grout with a low-bleed, thixotropic grout from the low point until it emerges clean at every vent; vacuum-assisted grouting where required; seal anchorages with caps and protective concrete.

Where it fits

All post-tensioned decks, pier caps, pylon anchorage zones, transverse deck PT.

Where it does not

Grouting in freezing weather, grout with chloride or excessive bleed, leaving ducts ungrouted for weeks.

Plant, pace and money

PlantStrand pushers, jacks and pumps, grout mixers and pumps, vacuum unit.

ProductivityA tendon of 20 strands stressed in an hour; grouting a span in a day.

CostUSD 6-12 per kg of installed tendon.

Risks and controls

What goes wrong

Grout voids (the Ynys-y-Gwas and Florida failures), strand corrosion at anchorages, duct damage during concreting, strand slip at stressing.

Quality assurance

Stressing records for every tendon, grout tests (fluidity, bleed, strength), vent inspection, anchorage sealing inspection; modern practice adds duct pressure tests before grouting.

Examples

All the game's PSC decks assume proper PT; the upkeep score penalises nothing here only because the game cannot see grout.