The cost of ignoring it
A post-tensioned slab carries its load through tendons held under high tension. Cutting or coring into one without first locating the tendons, and without an engineered procedure for any that must be cut, can release that force — along the whole length of an unbonded tendon, or over a transfer length either side of the cut in a bonded one — removing capacity well beyond the opening, and creating a serious safety hazard.
How post-tensioned slabs work
In a post-tensioned slab, high-strength steel tendons run through ducts cast into the concrete. Once the concrete has gained strength, the tendons are stressed with hydraulic jacks and locked off at anchorages at the slab edges, placing the slab into compression. The tendons follow a curved profile, high over the supports and low between them, so that the force counteracts the loads the slab carries. This allows thinner, longer-spanning slabs than conventional reinforcement, which is why post-tensioning is common in Australian apartment, office and carpark construction.
The tendons are either bonded, with the duct grouted after stressing so the steel is bonded to the concrete along its length, or unbonded, with greased and sheathed strands that remain free to move within the slab. Which type a building has changes what happens when a tendon is cut, and is one of the first things established.
Tendon truncation and new openings
Alterations to post-tensioned buildings — new service penetrations, stairs, lift shafts, skylights and plant openings — frequently cross tendons. The engineer designs the opening, identifies each tendon affected, and specifies how each is to be treated. The tendons are truncated and re-anchored either side of the opening so that they continue to carry their force, and the slab around the opening is trimmed with carbon fibre laminate or steel where the loss of capacity requires it; see carbon fibre strengthening.
Anchorage and stressing-pocket defects
The anchorages at the slab edge are the most exposed part of a post-tensioned system. On balconies, carpark decks and slab edges exposed to weather, water can enter through cracked or poorly filled stressing pockets and reach the anchorage and the strand. Investigation involves opening the pocket to inspect the anchorage and the strand for corrosion; repair ranges from reinstating the pocket with a suitable repair mortar and protective coating to replacing a corroded tendon.
Diagnostic approach
- Document review — as-built and shop drawings, stressing records and any previous alteration documentation
- Scanning — ground-penetrating radar and cover meter surveys to locate tendons, ducts and reinforcement at the work area
- Exposure — careful hand exposure of a tendon where its position or condition must be confirmed
- Anchorage inspection — opening stressing pockets where corrosion or water ingress is suspected
- Lift-off testing — measuring the force remaining in unbonded tendons where the engineer needs to know it
Standards and responsibility
Post-tensioned concrete is designed under AS 3600:2018, and alterations are designed and certified by a structural engineer. Titan carries out the works to the engineer’s design and procedure, with the scanning, exposure and anchorage records needed for certification.
Typical applications
New penetrations and openings in existing post-tensioned floors, refurbishment and change-of-use works, corrosion at slab-edge anchorages on balconies and carparks, and repair where a tendon has been damaged by coring or cutting carried out without scanning.