one-way PT slab temperature and shrinkage rebars
one-way PT slab temperature and shrinkage rebars
(OP)
if using 100psi minimum prestressing PT as temperature per ACI318 7.12.3, no need for any mild steel in that direction? What to prevent shrinkage cracks before the stressing? Thank you.






RE: one-way PT slab temperature and shrinkage rebars
In my previous job, we used to have a nominal #4 @ 48" o.c. each way bottom mat, precisely for the reasons you mention.
RE: one-way PT slab temperature and shrinkage rebars
RE: one-way PT slab temperature and shrinkage rebars
The initial prestress (25%) is used to control cracking from early age shrinkage.
RE: one-way PT slab temperature and shrinkage rebars
RE: one-way PT slab temperature and shrinkage rebars
RE: one-way PT slab temperature and shrinkage rebars
You need to get around. Staged stressing for crack control, and then for self weight load balancing is a widely used practice. Some engineers require 25% at one day, 75% at 3 to 4 days, then 100% just before grouting the tendons. And PT slabs do have shrinkage cracking problems, which when they occur typically involve wider cracks at greater spacing than for conventionally reinforced slabs.
RE: one-way PT slab temperature and shrinkage rebars
Ron is correct for unbonded prestressed slabs in the USA. They only do one stress when the strength is up to about 15-20Mpa. Thye do not do the stage stressing as we do in Australia and Asia with bonded PT.
But this lack of stage stressing used to be possible as extra untensioned reinforcemernt was supplied along with the unbonded PT. If there is not untensioned reinforcement in the slab, then stage stressing is necessary. The Australian code also used to allow for the absence of stage stressing, but required a minuimum unstressed reinforcement area of .1% in slabs to provide some ititial crack control in this case.
I also agree with the comments above that the P/A levels of 100psi (about ,7MPa) by itself will not provide good crack control long term. Extra untensioned reinforcement would be needed to provide a reasonable level of crack control.
I also agree with Hokie's comment that P/A does not provide crack control once cracks have formed (does not control width of cracks well), even more so with unbonded prestress but also for bonded PT. untensioned reinforcement at relatively close centres is best for crack control
RE: one-way PT slab temperature and shrinkage rebars