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Post tension cables
2

Post tension cables

Post tension cables

(OP)
My apartment complex has an above grade concrete slab with PST construction.It is estimated to have about 1200 cables.
4 of the cables are known to have broken. What percentage of the cables are permitted to be broken under Canadian (B.C.) building codes?

RE: Post tension cables

2
Westhampton,

Your apartment complex is constructed with UN-BONDED post-tensioning. What that means to the lay person is that the steel strands are not bonded to the surrounding concrete, and if the steel strand is severed/cut for what ever reason(corrosion, coring, alterations etc) then the force in that tendon is fully lost. The steel strand is structural reinforcement, so loss of force also means loss of structural reinforcement. These structures are typically built with very small quantities of bonded mild steel reinforcement.

Unfortunately, a statistical approach does not work on the complete tendon population, and certainly not based upon exterior visual observations. It needs to be assessed on a floor-by-floor basis, element-by-element or varying exposure condition, or similar. And localised chipping of concrete needs to take place to determine the probable extent of the problem.
 
The American Concrete Institute (ACI) code 318 has an allowance of 2% of broken strands, but that is usually taken across an slab panel or beam element, and certainly NOT applied to the total tendon population of the complete building.

I presume the 4 cables that have broken are the result of corrosion - maybe the strand or maybe an anchorage. There may be many more that have corrosion too that have NOT manifested themselves visually to the concrete exterior. A parking structure where de-icing salts are prevalent with lots of freeze/thaw cycles increases the susceptibility to corrosion. The tendons population for the parking elements needs to be separated from other exposure conditions. The grout pocket at the tendon anchorages are also another source of water ingress. Construction joints where intermediate stressing occurred are also locations of potential corrosion - mainly in exposed conditions.

It is on rare occasions that broken tendons are evident from the concrete exterior - strand tails or loop failures - so you could have on-going corrosion and not really know about it until you will have to spend big bucks. Strands flying out of buildings are really somewhat of an urban myth (although I do have a photo of one that did exit 25 feet in Jan 2002 - but very rare).

You need to commission the services of a consulting engineer experienced in the evaluation of un-bonded P-T structures. They will undertake an evaluation and report on it probable condition and possibly report on repairs required.  There has been several unbonded P-T problems in Canada and the USA with the older systems. I do not want to scare you but Burnaby City, BC has issued a notice for un-bonded P-t buildings. See:

http://www.city.burnaby.bc.ca/building/concrete.html

Hope this helps.

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