What a post-tensioning inspection checklist actually covers
A post-tensioning inspection checklist is the field document a structural engineer, special inspector, or quality assurance technician completes while tendon stressing, anchor seating, and grouting operations are underway on a concrete slab, bridge deck, or mat foundation. It is not a marketing brochure and it is not a generic punch list. The purpose is to record, at the moment of construction, the parameters that cannot be re-checked later without destructive testing: jack pressure versus gauge reading, elongation versus calculated elongation, wedge seating, duct placement, and grout bleed at the vent tubes. Industry references such as the Post-Tensioning Institute's Specification for Unbonded Single-Strand Tendons (PTI/ASBI M50.3) and the Manual for Grouting of Post-Tensioned Structures (PTI M55.1) define the variables that must be documented, and most US municipal special-inspection programs (including those written into Chapter 17 of the IBC) require the form to be signed by a qualified special inspector before concrete is placed or tendons are encased.
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The reason the checklist exists is structural: an under-stressed tendon reduces the moment capacity of a slab, a leaking duct admits chlorides that corrode strands, and an improperly vented grout column leaves voids that allow water to migrate along the tendon and attack the anchor. None of these conditions is visible after the slab is closed up. The checklist is therefore the only auditable proof that the designer's assumptions about effective prestress, anchor set, and tendon profile actually exist in the built structure.
Pre-stress inspection items before the jack is connected
Before stressing begins, the inspector verifies tendon profile against the shop drawings, typically within a tolerance of plus or minus 6 mm (1/4 inch) for slabs and 13 mm (1/2 inch) for beams, per ACI 318-19 §25.4. Anchorages are checked for correct manufacturer, correct corrosion-protection cap (where specified), and proper concrete strength at the time of stressing, which is commonly 75 to 80 percent of the specified 28-day compressive strength for bonded systems and 70 percent for many unbonded residential applications. Ducts are checked for damage, couplers are verified for full thread engagement, and the stressing tails are confirmed long enough to seat the wedges and operate the jack without cutting the strands short.
The inspector also records strand size (commonly 12.7 mm or 15.2 mm diameter seven-wire strand), the number of strands per tendon, and the calculated theoretical elongation based on the modulus of elasticity supplied by the strand producer, typically 196 GPa (28,500 ksi). These numbers feed directly into the comparison between measured and calculated elongation that takes place once the jack is pumped.
Stressing operation: the numbers that must be written down
During stressing, the inspector records three quantities per tendon: gauge pressure, jack ram extension, and measured elongation. The ACI 318 tolerance band is plus 7 percent or minus 5 percent of the calculated elongation; many engineers tighten this to plus or minus 5 percent as a matter of contract. If elongation falls outside that band, the inspector flags the tendon and the structural engineer of record is asked to evaluate before the tendon is cut off and sealed.
The checklist should also capture the jack calibration date, the ram area used to convert pressure to force, and any slip at the anchor (commonly 6 mm or 1/4 inch wedge seat set for standard chucks). These numbers are time-sensitive. Once the wedges are seated and the jack is removed, the inspector confirms the tail length remaining, marks the tendon with a tag identifying the tendon number, stressing date, and inspector initials, and photographs the gauge reading if the project specification requires photographic documentation. Where tendons are unbonded, the corrosion-inhibiting grease is verified to have filled the sheath from anchor to anchor before stressing tails are cut.
Grouting inspection items
For bonded (internal) post-tensioning, grouting is a separate inspection event with its own line items: water-cement ratio of the mix (commonly 0.40 to 0.45 by mass), grout temperature at the mixer (typically below 32 °C/90 °F), and the order in which tendons are grouted (typically from low point to high point so air is displaced upward through vents). The inspector observes the grout emerging from each vent tube until a uniform consistency is observed, then caps the vent. Any vents that do not show grout are flagged, and a secondary pumping or vacuum grouting may be required. PTI M55.1 calls for a minimum efflux time from a flow cone of 11 to 30 seconds for most applications.
The checklist should also document the volume of grout pumped versus the theoretical volume of the duct, calculated from duct diameter and tendon length, with an allowance of 10 to 15 percent for waste and over-pump. A short fall of more than 5 percent generally indicates a leak, blockage, or void and is cause to investigate before the tendon is closed up.
Comparison of the two main inspection regimes
The two principal codes a US inspector works against are ACI 318 (Building Code Requirements for Structural Concrete) and the PTI specifications. They differ in scope and philosophy.
| Feature | ACI 318-19 §25 | PTI/ASBI M50.3 / PTI M55.1 |
|---|---|---|
| Elongation tolerance | +7% / -5% of calculated | Often tighter at +/-5%; project-specific |
| Concrete strength at stressing | Engineer's discretion; 75% common | Explicit by tendon type, 70-80% |
| Grouting requirements | Brief; references PTI | Detailed mix, equipment, sequence rules |
| Inspector qualification | Per IBC Chapter 17 special inspector | PTI-certified bonded PT inspector or equivalent |
| Tendon profile tolerance | +/- 6 mm slabs, +/- 13 mm beams | Similar, with photographic record often required |
Common mistakes the checklist must defend against
The single most frequent error recorded on real projects is mis-recording the jack ram area when a different jack is substituted on site. If the ram area changes from 250 in² to 300 in², the gauge pressure required to deliver the same force drops by roughly 17 percent, and elongation will appear overcalculated. A second common error is failing to subtract the elastic shortening of the concrete itself from the measured elongation, which for a long, heavily stressed tendon in a short column can be 5 to 10 mm and can flip an otherwise acceptable tendon into a flagged condition. Third, inspectors sometimes fail to record tendon profile at the low and high points; if a tendon has been pushed up during a concrete pour and is no longer in the design profile, the moment capacity at midspan drops silently.
A fourth error is treating grouting as a secondary operation. The 2020 collapse of the Morandi Bridge in Genoa, while a cable-stay rather than post-tensioned structure, revived attention to corrosion of prestressing steel across Europe, and several national authorities including the Italian Ministry of Infrastructure have since tightened grouting records. In the United States, the Federal Highway Administration has funded research through TFHRC (e.g., Post-Tensioned Box Girder Bridges: Inspection and Maintenance, 2022) showing that chlorides ingress through poorly grouted ducts in as little as 4 to 7 years in marine exposure.
When to act and how the form is signed off
The checklist is a live document, not a final report. Tendons are typically signed in groups (a "pour strip" or "bay") once all tendons within that area have been stressed, profiled, and grouted. The special inspector signs and dates the form, the contractor's superintendent countersigns to acknowledge observations, and the structural engineer of record signs after a review of any flagged tendons. Where elongation is outside tolerance, a non-conformance report (NCR) is opened and the engineer of record either accepts the tendon as-is with a calculated revised effective prestress, or orders supplemental stressing, restressing, or, in rare cases, addition of an external tendon.
Most building departments require the completed checklist to be submitted before the slab is loaded with significant construction live load (commonly more than 1.5 kPa or 30 psf of stacked material). For parking structures and decks that will receive a waterproofing membrane, the form must be closed out before the membrane is placed, because subsequent repair to a corroding tendon under the membrane is roughly 8 to 12 times the cost of the same repair done while the duct is still accessible.
Practical steps for setting up a usable checklist
A workable checklist for a US unbonded slab-on-ground project can be assembled in two pages. The first page covers pre-stress verification: drawing revision, tendon profile, anchor hardware, concrete strength test results, and jack calibration certificate. The second page is the stressing log, with one row per tendon and columns for date, tendon ID, gauge pressure, ram extension, measured elongation, calculated elongation, percentage deviation, anchor set, and inspector initials. A third half-page captures the vent-by-vent grout record. Digitally, the same form is now routinely filled on a tablet using Procore, BIM 360, or a custom iPad form, and the PDF is filed against the tendon drawing in the project record. AI-assisted platforms such as the one this site is built on can pre-populate the calculated elongation column from the stressing schedule, flagging tendons whose measured values exceed the tolerance band in near real-time, which removes roughly 30 to 40 minutes of clerical work per pour.
The point of any checklist, paper or digital, is that it must survive an audit by a third-party reviewer, a building official, or a future structural engineer pulling files during a renovation. If the form does not produce an unambiguous answer to did this tendon carry the force the designer assumed, it has failed its purpose.
Cost, frequency, and who signs
Special inspection fees in the United States for post-tensioning typically run between $0.20 and $0.45 per square foot of slab area, or roughly $1,500 to $6,000 per typical 30,000-square-foot pour for unbonded residential or commercial slabs. Bonded bridge work is higher, often $4,000 to $12,000 per segment, because of the additional grouting observation. The special inspector is required to hold a registration as a professional engineer, or be a trained technician working under a registered engineer, depending on the jurisdiction. PTI also administers a Bonded Post-Tensioning Inspector certification which several state DOTs now require for bridge work.
The checklist is therefore not free in time or money, but it is cheap compared with the consequences of a tendon that does not perform as designed. A single unbonded tendon that is incorrectly stressed and not flagged can reduce the effective prestress of an entire bay by 1 to 2 percent, and over a 50-bay slab that compounds into a measurable long-term deflection issue that no amount of cosmetic repair will hide.