Bonded Post tension slab Cracking due to restraint of Stiff Vertical Elements

[MAB70]

I am new in PT design and a bit confused on the reasons of cracks due to restraint of stiff vertical elements in post-tension, My questions are:

1. Are the cracks a result of loss of pre-compression in the slab and diverted back to the vertical elements?or is just restraint due to drying shrinkage? or what is the difference between restraint and shrinkage cracks in PT?

2. Why does PT restraint cracks usually full depth not like normal shrinkage cracks on the surface?

3. If that is true that pre-compression is diverted back to the stiff element, then this becomes a strength issue and cannot be ignored or repaired later like shrinkage cracks.

4.In Adapt TN451 - 2 defines the restraint cracks as '' In theory, if the supports prevent any shortening (part b of the figure), the entire post-tensioning force will be diverted to the supports, leaving the member with no precompression'' if that is the case and all forces are diverted back to the supports, why would such restraint cracks happen? In my opinion the slab can fail under flexure with typical flexural cracks not as image shown below:

5. Is this statement correct: ''Force in tendon does not reduce by support restraint. In fact, it is less loss due to shrinkage of concrete. Strength is not a problem for support restraint. The problem is crack mitigation due to shrinkage of concrete''

 

[hokie66]

1. Restraint is not due to drying shrinkage, but cracks are. Shrinkage cracks are always due to restraint. If the concrete is unrestrained, it is free to shorten without cracking.

2. Drying shrinkage cracks occur in restrained concrete sections, whether PT or conventional reinforcement. And they are generally full depth. As to cracks which are not full depth, perhaps you are thinking of plastic shrinkage crack, plastic settlement cracks, or similar.

3. There can be strength issues, but this type crack can be repaired.

4. Not familiar with Adapt, but the cracks shown in (ii) are typical drying shrinkage cracks initiated because of restraint.

 

[r13]

I read somewhere that states the cracks are likely formed during curing, prior to post tensioning, due to shrinkage, and the lack of mild steel to counter the movement. Advice has been given to contractors to perform limit tensioning during curing, but very few followed. Once full tensioning is performed, the cracks should have minimal effect on the performance of the structure. But, I would be worried if the cracks occurred after post-tensioning, as post-tensioning shall be in general "counteracting shrinkage and flexural cracks" (see excerpt below), the cracks developed after tensioning, therefore, means some other mechanism has failed to work properly.

The excerpt from PTI (Post Tensioning Institute):

Crack Control-Watertightness: Post-tensioned structural systems eliminate closely spaced joints and help ensure water-tightness by placing the floor in bi-axial compression, thereby controlling and counteracting shrinkage and flexural cracks.

 

[hokie66]

retired13,

Prestressing concrete certainly helps in controlling cracks due to shrinkage. But in post-tensioned construction, if all or most of the prestress never gets into the slab due to restraint at the ends, then it is not effective. Therefore, detailing of slab to vertical elements must take this into account, allowing the slab to shorten due to shrinkage and prestress. This is true of both post-tensioned and conventionally reinforced slabs.

 

[r13]

hokie66,

Make sense to me. PTI shall have papers on the effect you have just mentioned.

[Added]

That will be a nightmare, if the post tensioning force couldn't get through, renders the slab unusable, onsider it does not have conventional reinforcing.

 

[hokie66]

I don't know about PTI, as I am in Australia. I think the PTI is mostly in North America, and it promotes unbonded PT, but the restraint issue would apply equally.

There are well accepted detailing practices to overcome the restraint problem. Delay pour strips, temporary slip connections, etc.

 

[r13]

hokie66,

Yes. PTI is in the USA. And, agreed, at the end, support details are very important to the performance of most, if not all, precast/prestressed structural elements.

 

[hokie66]

I wouldn't argue with that, but what we are talking about here is cast in place concrete slabs, post-tensioned or not.

 

[Ingenuity]

3. If that is true that pre-compression is diverted back to the stiff element, then this becomes a strength issue and cannot be ignored or repaired later like shrinkage cracks.

Whilst restraint-to-shrinkage of PT slabs may (will?) cause cracking to slabs, there is also increased potential for cracking to the vertically stiff elements that are causing the restraint. However, the PT tendon force is still present (after allowances for losses), and the draped tendon effects (eccentricity, uplift etc) are still present.

With reduced P/A within the PT slab, the flexural tensile stresses will be increased ==> cracking etc, but this is largely a serviceability issue.

Ultimate flexural strength of PT slab sections will be largely the same as an unrestrained slab - same tendon force, same eccentricity etc.

 

[Tomfh]

Whilst restraint-to-shrinkage of PT slabs may (will?) cause cracking to slabs, there is also increased potential for cracking to the vertically stiff elements that are causing the restraint.

Yes in our experience many walls crack due to the restraints. The release mechanisms often don't work very well, and the walls are often easier to crack than the slabs. It's a tug of war. Can go either way.