A post-tensioned slab does not fail on the day it is poured. It fails on the day somebody cannot produce a stressing record, or the day a tendon reads 14% off calculated elongation and nobody on site knows whether that is a problem, or three years later when a tenant fit-out crew puts a core drill through a live strand. Every one of those is a construction-stage control, not a design problem. And every one of them lands on the concrete contractor’s desk first.
After 38 years pouring concrete in this city, we have sat through enough deficiency walks to know which conversations are survivable with paperwork and which are not. Post-tensioning is the clearest example we deal with. The design is somebody else’s work. The evidence that the design was actually delivered is yours.
This is the framework we use on Calgary post-tensioned work: what has to be right before the pour, what gets measured on stressing day, what the numbers mean when they come back wrong, and the one rule that has to outlive your contract.
Post-tensioning moves your critical path off the calendar and onto a cylinder break
On a conventionally reinforced slab, your sequence is governed by access and by stripping. On a post-tensioned slab, three new events sit in the middle of that sequence: tendon installation before the pour, stressing after it, and the release of shoring that cannot happen until stressing is complete and accepted.
The practical effect is that your schedule stops being driven by days and starts being driven by a strength milestone. The engineer of record specifies a minimum concrete compressive strength that must be reached before any tendon is jacked, and that strength is demonstrated by field-cured cylinders stored with the slab under CSA A23.1 conditions — not by lab-cured cylinders sitting in a warm curing tank across town. In Calgary, between late September and May, that distinction is the whole schedule. A slab protected and heated to the spec hits the number. A slab left to a cold night does not, and the stressing crew you booked arrives to a deck they are not allowed to touch.
Build the float in at tender. If you are pricing this scope, the stressing strength milestone belongs in your quote assumptions alongside access and protection — the same discipline we describe in our guide to getting an accurate concrete quote in Calgary.
Six things have to be right before the concrete goes in
Once the slab is poured, every one of these is expensive. Before the pour, they are a walk with a tape and a set of shop drawings.
- Tendon profile. Chair heights at the specified intervals, high points over supports, low points at midspan. The profile is the structure. A tendon that sags 20 mm off profile is not a cosmetic deviation.
- Anchorage edge condition. Live-end and dead-end anchors set square to the slab edge, properly supported, with the back-up bars shown on the drawings actually installed.
- Sheathing integrity. Unbonded monostrand tendons rely on a continuous plastic sheath filled with corrosion-inhibiting grease. Torn sheathing from foot traffic or a dragged rebar bundle is a corrosion path and a friction problem at once.
- Openings and sleeves. Every penetration coordinated and approved before the pour. This is the cheapest hour on the whole job. Every opening you do not coordinate now becomes a scan-and-core exercise later.
- Strand ends and jack clearance. Enough tail beyond the anchor for the jack to grip, and physical room for the jack to sit. Tight edge conditions against a shoring tower are found on stressing day if they are not found before.
- Embeds and conduit. Electrical and mechanical runs that cross tendon zones get resolved on paper, with the engineer, before anyone is standing on a deck holding a conflict.
We treat this as a formal pre-pour walk with the installer and the engineer’s reviewer, documented with photographs. The photographs are the part people skip. They are also the part that settles arguments a year later, the same way a documented pre-pour survey settles ponding arguments on exterior flatwork — see our breakdown of exterior concrete slope tolerance in Calgary for how that plays out on a warranty claim.
Elongation is the measurement that decides whether the slab is accepted
Stressing is not a visual trade. When the jack pulls a tendon to the specified force, the strand stretches a calculated amount, and the crew measures what it actually stretched. The comparison between the two numbers is the acceptance test.
The benchmark is tight. The Post-Tensioning Institute notes that ACI 318 “recognizes that these variable factors can result in differences between the theoretical and measured elongations and has set a tolerance of +/- 7%.” Short tendons are the exception, because seating loss — the slip that happens as the wedges bite when the jack releases — is a fixed length, so on a short tendon it eats a much larger share of the total stretch. PTI Technical Note 16 defines a short tendon using an assumed anchor set distance of 0.25 in. (about 6 mm), which “typically will result in ‘short tendon’ lengths up to 35 ft” (roughly 11 m), and recommends judging those against “an arbitrary length tolerance, such as ±1/4 in.” rather than a percentage.
Two more numbers worth knowing before you argue with anyone:
- Normal seating loss range. PTI puts it at “between 0.125 and 0.50 in.” (about 3 to 13 mm) as “the maximum range reasonably anticipated in normal construction,” with 0.50 in. representing “the maximum seating loss physically possible with commercially available single-strand unbonded wedge anchorage systems.”
- The residential number does not apply to you. The ±10% tolerance you may have seen quoted comes from the PTI Slab-on-Ground Committee and exists “because of the shorter length tendons prevalent in residential slab-on-ground foundations.” It is not the tolerance for a suspended commercial deck.
When the readings come back out of tolerance
Do not let the crew re-pull and move on. The direction of the error tells you where to look, and PTI’s published list is the one to work through.
| Reading | Likely causes to investigate |
|---|---|
| Elongation too low | Excessive friction from damaged sheathing or poor placement; excessive wobble from sloppy tendon layout; excessive seating loss from cement paste in the wedge cavity or worn equipment; higher-than-assumed modulus of elasticity or strand area; improper strand entry into the anchor causing dragging; calculation or documentation error |
| Elongation too high | Lower-than-assumed friction or wobble coefficients; lower modulus of elasticity or strand area than calculated; overstressing beyond calibrated gauge pressure; calculation or documentation error |
Note how many of those are paperwork, not steel. A gauge that has drifted since its last calibration and a calculation built on the wrong strand area produce a deck full of non-conforming readings and no actual defect. That is why the jack and gauge calibration certificates belong in your records before the first pull, not after the engineer asks.
The deliverable at the end of stressing is a tendon-by-tendon record: identification, calculated elongation, measured elongation, variance, date, operator, and the gauge used. Keep it with the field-cured cylinder breaks. Together they are the proof that the slab was stressed at the right strength to the right force, and they are the documents that decide how a dispute ends — the same reason we are blunt about what a concrete contractor’s warranty actually covers.
Shoring stays until stressing is accepted, and the slab has to be allowed to shorten
Two shoring rules get broken on post-tensioned jobs in Calgary, both for the same reason: people treat the slab as finished when the concrete is hard.
The first is simple. Shoring and reshoring come out on the engineer’s instruction after stressing is complete and the elongations are accepted — not when the deck looks ready and the next trade wants the space.
The second is less obvious. A post-tensioned slab shortens when it is stressed. That movement is designed for, and it has to be able to happen. Shoring clamped hard against stiff elements, or infill walls and stair cores built early at the slab edge, restrain the shortening and attract cracking into exactly the places nobody wants it. If your sequence puts rigid construction against a slab before it is stressed, raise it before you build it.
In Alberta, somebody has committed in writing to review this
Post-tensioning is structural work with professional involvement attached. Under the National Building Code — 2023 Alberta Edition, the registered professional of record for the structural components signs a Schedule B, committing that they “shall conduct or coordinate field reviews during construction of the above referenced components to verify the final design of the components (including approved changes), as constructed, substantially complies with the NBC(AE).” That schedule is submitted to the coordinating registered professional before the building permit application goes in.
For you, that has a practical meaning. There is a named professional whose field review covers tendon placement and stressing, and your job is to give them something to review: notice before the pre-pour walk, notice before stressing, and records afterward. Projects that run into trouble are almost never the ones where the engineer was called too often.
Nobody cores a post-tensioned slab blind — including, five years from now, somebody who is not you
This is the part of the job that outlasts the contract, and it is worth saying plainly to your owner.
Tendons in a finished slab are live. They are under full force for the life of the structure, and they are invisible. PTI’s position is direct: “Mechanical damage to post-tensioning tendons generally results from careless coring or drilling through a concrete slab (and subsequently through post-tensioning tendons),” and “a small investment in structural scanning services prior to creating any openings in a slab will significantly reduce the risk of damaging tendons and the associated repair costs.”
So the rule for every future penetration — a tenant fit-out anchor, a plumbing core, a handrail bolt — is: locate first, then cut. Scan the area, mark the tendons, confirm against the as-built tendon layout, and get the engineer’s approval for the opening. Hand the owner a labelled set of as-built tendon drawings at closeout and tell them, in writing, what the slab is. That document is the difference between a routine core and an emergency repair. When an opening is the right answer, it is a planned, scanned, engineered operation — which is how our concrete cutting and coring crews in Calgary approach any slab whose reinforcement is not fully known.
FAQ
Does a post-tensioned slab need less concrete than a conventional slab? Generally it allows a thinner slab and longer spans with fewer columns, which is usually why it was chosen. That saving is a design outcome, not a site outcome — your pour volume comes from the drawings.
What happens if a tendon is damaged during construction? Stop work in that area and report it to the engineer of record immediately. Repair options exist for sheathing damage and for damaged strand, but they are engineered repairs. A damaged tendon that gets buried and poured over is the single most expensive thing that can happen on this scope.
Can we strip formwork before stressing? No, unless the engineer has specifically allowed a partial stripping sequence in writing. Until the tendons are stressed, the slab does not have the structural system it was designed around.
Who pays when elongations come back out of tolerance? It depends entirely on cause, which is why the investigation matters more than the argument. A worn jack or an uncalibrated gauge, torn sheathing from site traffic, and a calculation based on the wrong strand properties each point at a different party. Records decide it.
How do I know whether an existing Calgary building has post-tensioned slabs? Check the structural drawings first, and if they are unavailable or unclear, scan before you touch anything. Treat an unknown suspended slab as post-tensioned until proven otherwise.
Talk to us before the tendon installer books stressing day
The cheapest hour on a post-tensioned job is the pre-pour walk, and the second cheapest is the conversation about strength at stressing and winter protection before the pour date is locked. Both happen weeks before anyone pulls a jack.
Reach our commercial concrete contractors in Calgary to set up that review, and bring the structural drawings and the tendon shop drawings with you.
Sources
- Post-Tensioning Institute, FAQ No. 6 — Why are discrepancies in field elongation measurements so difficult to resolve? — https://www.post-tensioning.org/Portals/13/Files/pdfs/Education/FAQ%20no.%206.pdf
- Post-Tensioning Institute, Technical Note No. 16 — short tendons, seating loss and elongation tolerance — https://www.post-tensioning.org/Portals/13/Files/Pdfs/Education/Technical%20Note%2016.pdf
- Fulton and Trepanier, Dispelling the Myth About Unbonded Post-Tensioned Buildings, PTI Journal — https://www.post-tensioning.org/Portals/13/Files/PDFs/Publications/Reprints/2015%20August/Fulton-Trepanier-Dispelling%20the%20Myth%20About%20Unbonded%20PT%20Buildings.pdf
- Government of Alberta, Schedule B — Assurance of Professional Field Review and Compliance, National Building Code 2023 Alberta Edition — https://www.alberta.ca/system/files/ma-national-building-code-2023-alberta-edition-schedule-b-fillable.pdf