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RE:Elevated post-tensioned slab cracks

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Oshin,

Seems like the perimeter walls have certainly provided considerable
restraint and caused/assisted the cracking - shame the engineer never
detailed it sufficiently well to reduce the effects.

Are you sure there was no intermediate pour strip - for subterranean slabs
perimeter stressing is unusual, so most often a closure or pour strip is
placed. If you can get hold of the PT shop drawings that would tell you
more, but often PT shop drawings are not readily available for older
structures. As-built drawings often do not address issues/changes that the
PT shop drawings detailed.

The partial step you refer to (although I am not certain what exactly that
is - does the soffit step?, the top surface step?), where these banded
tendons are pulling in opposite direction (and also the location of the 1/4"
crack) that may be the location where internal stressing took place.
Regardless, tendons that are not continuous and are pulling in opposite
directions are going to cause excessive movement unless an adequate quantity
of rebar (and it will be quite large) was designed and installed. Seems that
the structure is wanting a control joint at this location, since it formed a
1/4" crack here. You really need to investigate this location before you can
give any real assessment.

Are these banded tendons that are pulling in opposite directions in a much
shorter span compared to the typical spans? Are they discontinuous at
approximate mid-length of the total slab length? Was structural continuity
expected across this 1/4" crack?

Re structural performance - it really comes down to serviceability and
looking at the effects of strength at the 1/4" crack location. Deflections
are possibly okay (the uplift component of the PT always exists even if 100%
restraint actions), although you are now dealing with a cracked section.
Cracked sections can result in long-term deflections that are more 2 times
that of uncracked sections. Hair line cracks are not uncommon in these type
of structures, but at the 1/4" crack locations crack control is not good,
and durability will be a concern - are the cracks leaking?. I am pretty sure
a 1/4" crack fails most engineers (and certainly all owners) definition of
"serviceable".

If restraint has indeed caused partial or full loss of precompression then
punching shear capacity will be reduced - the vertical component of the
prestress will not change but the angle of a tendon in a flat plate is
negligible and seldom used in capacity checks. But if no "spider-type"
cracks are evident at column locations then punching shear is probably okay.

Regarding flexural strength, it is probably okay and the live load is only
50 psf, the cracking has obviously redistributed moments and the slab will
now behave essentially like it is reinforced - more one-way action and peak
moments over the columns in the banded direction. Very difficult to justify
a ACI 318 2-way average moment analysis where the total panel moments are
averaged over the full panel width and behavior is assumed to be UNcracked -
you now have a cracked section and the area between the banded tendons
(middle strip - where there is probably no rebar or PT) have no flexural
capacity, so 100% of the moments are now taken over the banded tendons in
the column lines.

Unbonded PT has little ability to control cracking once P/A is consumed by
restraint, and other time dependent effects, and
since a crack HAS formed, the crack width will basically increase somewhat
uncontrolled due to future loading and future/remaining restraint effects.

It is maybe worth spending a little more time trying to understand what is
happening at the 1/4" crack at the partial step and the vertical offset you
mentioned - epoxy injection may not be the best method of repair if the
crack is active.


HTH

Mark Geoghegan
Honolulu, HI


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