one-way PT slab temperature and shrinkage rebars 1

[Once20036]

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.

 

[slickdeals]

Normally they do an initial stress to prevent shrinkage cracks, before stressing to the design force. I forget if it is 25% or 10%. One of the Aussies here might be able to tell you since that practice is more common there, compared to the US.

In my previous job, we used to have a nominal #4 @ 48" o.c. each way bottom mat, precisely for the reasons you mention.

 

[ron9876]

Not so different from a two way slab. They typically don't have continuous reinforcing steel and they don't have shrinkage crack problems.

 

[asixth]

100psi or 0.7MPa will give a minor degree of crack control, I like to have a minimum of 1.4MPa or tendons at 10 times the slab depth or I will provide reinforcement to provide crack control.

The initial prestress (25%) is used to control cracking from early age shrinkage.

 

[hokie66]

Agree with asixth, and that is for bonded tendons. With unbonded tendons, I don't know...they don't do much for controlling cracks once cracks form. The reinforcement requirement for crack control varies a lot with the degree of restraint.

 

[ron9876]

I have never seen a PT slab with initial prestress. Wait until 2/3 or 3/4 of 28 day strength and then stress slabs. No problems unless the contractor waits to long to stress.

 

[hokie66]

ron,
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.

 

[rapt]

Hokie,

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

 

[hokie66]

Yes, I knew he was talking about US experience. Thus, my gentle dig.