Post-Tensioning & Concrete Stressing
Post-tensioned concrete uses high-strength steel strands (tendons) that are stretched with hydraulic jacks after the concrete is poured, squeezing the slab or beam so it can span farther with less material. The strands hold huge energy. A tendon or anchor that fails during stressing, or a tendon cut later, can shoot out with deadly force.
- Post-Tensioning Crews
- Concrete Workers
- Ironworkers
- Concrete Cutters & Drillers
- Renovation Crews
How it works
- Tendons are steel strands, usually coated in grease and encased in plastic sheathing (unbonded) or in ducts that are later grouted (bonded), laid in the formwork before the pour.
- When the concrete reaches the specified strength, crews attach a hydraulic stressing jack at the live end and pull each tendon to a specific force, then lock it with wedges in an anchor.
- The tendon stays permanently under tension, storing great energy in the slab. Ends are trimmed and sealed in pockets.
- Used in parking structures, elevated slabs, bridges, foundations (slabs-on-ground), tanks and residential foundations.
Main hazards
- Tendon or anchor failure during stressing: wedges can slip, strands can break, anchors can fail and jacks can slide, throwing parts and strands out the back of the jack.
- Hydraulic hazards: high-pressure hoses and fittings can burst, and oil injection injuries are serious. See Hydraulic Tools.
- Struck-by flying wedges, strand ends and jack parts, and crushed hands in the jack or between the jack and anchor.
- Drilling, cutting or coring into a post-tensioned slab can sever a tendon, causing it to eject, damaging the structure and killing or injuring nearby workers. See Concrete Cutting.
- Cutting strand ends under tension if done incorrectly.
- Falls at slab edges and openings during stressing. See Guardrail Systems.
- Noise, silica dust and heavy lifting with jacks and coils. See Silica.
Safe stressing
- Only trained and experienced crews should stress tendons, following the engineer's stressing sequence and the equipment maker's instructions. Check that concrete has reached the required strength first.
- Keep everyone out of the line of the tendon and the jack (behind and in front, along the axis of the strand) while stressing. Stand to the side of the jack. Barricade the area at both ends of the tendon and post warning signs. Only the crew operating the jack stays.
- Inspect the jack, pump, gauges, hoses and fittings before use. Use equipment rated for the force and calibrated as required. Don't exceed the rated pressure.
- Make sure the jack seats properly against the anchor and is aligned with the tendon. Use jack safety restraints or guards where provided.
- Check that wedges are the right type and seated evenly, and stop immediately if you see slippage, strand damage or unusual noises.
- Depressurize and secure the jack before releasing it. Handle the jack with proper lifting aids. See Lifting & Ergonomics.
- Wear safety glasses or a face shield, hard hat, gloves, hearing protection and sturdy boots. See Eye & Face Protection.
Drilling, coring and renovation
- Assume every slab might be post-tensioned until drawings or scanning prove otherwise, especially in parking structures, residential slabs and elevated floors.
- Locate tendons with ground-penetrating radar or other scanning before cutting, coring, anchoring or trenching, and mark them on the slab. Get the structural engineer's approval before any penetrations.
- Don't cut or drill into a marked tendon or anchor. Don't cut through a stressed tendon except using a specified procedure from the engineer. Look for slab-edge anchor pockets and tags showing where tendons run.
- Use the engineer's destressing procedure when tendons must be removed, with barricades and trained crews.
- Inform workers of the location and hazards when working near post-tensioned elements. See Demolition.
Pockets, grease and cleanup
- After stressing, tendon ends are cut and pockets sealed as specified. Use proper tools and don't use torches on strands, since heat can weaken or fail the strand.
- Tendon grease and coatings can be slippery and irritating. Wear gloves and clean up spills. See Skin Disease & Dermatitis.
- Coil handling: strand coils are heavy and can spring out when cut. Cut bands carefully and keep clear.
Post-tensioning checklist
- The engineer's stressing plan and the concrete strength test are on file
- Both ends of every tendon are barricaded and nobody stands behind or in line with the jack
- Jack, pump, hoses and gauges are inspected and used within rated limits
- I scanned and marked tendons before cutting, coring or drilling any slab
- I wear eye or face protection and stop work if wedges or strands behave strangely
Standards & references
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