Post-tensioning: tendons, stressing and grouting

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

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

Post-tensioning: tendons, stressing and grouting — concept diagram — Concept diagram (TheBridgeEng)

How it is done

Install the ducts (corrugated plastic for enhanced protection, or steel) to the profile on supports at about 0.6–1.0 m, with sealed couplers, vents at high points, drains at low points and anchorages fixed to the forms.
Pressure-test the duct system before concreting where specified, and keep the ducts clear during the pour with mandrels or swabbing.
Thread the strands, pushed or pulled, once the concrete has reached stressing strength, and fit the anchor heads and wedges.
Stress with calibrated jacks in the specified sequence, recording force, elongation and wedge draw-in; investigate any tendon outside the elongation tolerance before cutting the tails.
Grout promptly with a prebagged, low-bleed thixotropic grout, pumping from the low point until clean grout flows from each vent in turn, then hold pressure and close.
Inspect after grouting (vent probing or endoscope), fill any voids by vacuum grouting, and protect the anchorages with caps and sealed pour-backs.
Hand over the stressing and grouting records; for external tendons meant to be replaceable, use flexible fillers (wax or grease) or detail them for removal.

Key numbers

Strand15.2 mm (0.6 in) seven-wire, fpu 1,860 MPa, area 140 mm2
Tendonscommonly 7–31 strands
Jackingup to about 0.80 fpu; wedge draw-in about 6 mm
Elongationwithin about ±5–7 % of the calculated value
Duct friction coefficientabout 0.15–0.25
Groutingcommonly within 7 days of stressing
CostUSD 6–12 per kg of installed tendon

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.

Choosing it

Choose internal bonded tendons for crack control and ultimate capacity in most decks; choose external tendons when inspection, monitoring and replacement matter, as in segmental boxes.
Prefer flexible fillers (wax or grease) over cement grout for external tendons that must stay replaceable, at higher cost.
Choose bars over strands for short, straight tendons (anchorage zones, temporary PT, pier caps).

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.

Origins

Freyssinet patented prestressing in 1928 and introduced his wedge anchorage and double-acting jack in the late 1930s; after grout failures such as Ynys-y-Gwas (1985), the UK suspended new grouted post-tensioned bridges from 1992 to 1996, and today's practice of plastic ducts, prebagged grouts and pressure testing grew out of that period.

Examples

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

Case studies

Ynys-y-Gwas Bridge, WalesUK · 1985A 1953 segmental post-tensioned span collapsed without warning after tendons corroded at its joints, leading to the UK moratorium on grouted post-tensioning.

Mid-Bay Bridge, Destin, FloridaUSA · 2000An external tendon was found failed and others badly corroded where grout voids and bleed water had collected, prompting changes to Florida's grouting practice.

Hammersmith Flyover, LondonUK · 2011The 1961 post-tensioned viaduct was closed when corrosion of its prestressing strands was found, and was strengthened with new post-tensioning.