Post tention concrete slabs

[wanttoknow]

Could some one tell me the steps for a post tention concrete slab and how it works for a concrete high rise? In easy terms.

[Kelvin]

Consider that when you load any kind of floor it would tend to deflect downward. In so doing, the floor -- and in this case -- the concrete slab would develop compression at the top surface, but tension in the bottom face. This condition is especially dangerous for concrete floors, because it is much weaker in tension and will lead to cracking.

To prevent a dangerous condition, we lay steel bars near the bottom face so that if cracks do develop (from excessive loading), a tensile-capable material is present to bridge the cracks and carry tensile forces beyond the cracks.

Alternatively - a prestressed or post-tensioned concrete system not only adds steel to the concrete where it is needed most, but it is pulled very taught so as to induce a compressive stress field into the concrete. If the magnitude of the compressive stress applied into concrete is larger than what would be developed in tension (as when our floor gets loaded), then the concrete never cracks and never becomes dangerous.

The difference between Pre-stress and Post-tension is merely when the steel is tensioned w.r.t. when the concrete is poured (i.e. before or after).

If the degree of post-tensioning is large, then it is also possible to make slabs thinner. It is this fact that helps make economical in highrises, where transporting concrete upward is done at a premium.

Tempered glass works on the same principle.

[wanttoknow]

How is the Post-tension done after the concrete slab is poured.

[harryc]

Very good question, I haven't been able to catch the actual tighting process (yet).

Here are a few shots of the layout.
Aqua
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2


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And after
161 W Kinzie


This series 400 N Dearborn Rising shows the layout on the standard floor plate well.
In this frame you can see the coiled up sleeves in the upper frame, and stretched out on the lower part.

[slide_rule]

i remember a vivid example of post-tensioning gone wrong. a midrise built with post-tensioned cables began to have its cables snap. the evidence cited improperly sealed grout caps. moisture seeped in, and the steel cables began to rust and lose their strength. lawyers were hired and threatened lawsuits, various contractors eventually settled. prestressed concrete could have been built under climate-control, and could have more easily avoided these problems.

there's nothing wrong with post-tensioning. given proper oversight, it can result in much more delicate structures. but negligence has a way of messing all shorts of things up.

[Kelvin]

Originally Posted by wanttoknow

How is the Post-tension done after the concrete slab is poured.

Refer also to HarryC's excellent montage. In the case of PT, flexible ducts are laid with the rebar (epoxy coated green rebar in this case). These ducts can also be ribbed to develop anchorage with the concrete. The ducts may already have the steel tendons inset prior to the pour or they can be pushed in after the pour.

Once the concrete is hardened and developed sufficient strength, then the anchors/wedges are set at each end and the tendons are jacked (against the edge of the slab). The wedges have little teeth on them so as to prevent the tendon from slipping back once the jacks are removed.

The final process is to then grout the interior of the tube, so as to fill the space left inside the duct and the steel tendon. As Slide-Rule indicated, improper grouting can be a disaster waiting to happen if not done correctly (and it's a mess to fix). An alternative to grouting is "greasing" or using a thick grease pumped into the cavity which will hopefully remove air and water. Greased PT conduit has the advantage of allowing tendons to be removed in the future if problems occur. However a properly laid duct with properly positioned vents and properly "cleaned" prior to grouting will almost always be a problem-free installation.

[scalziand]

The jacking for the post tensioning of slabs looks to be quite similar to that used in cable-stay bridges.

[harryc]

The slabs over the garage entrances for Daley plaza are post tension granite.



the covers

[Kelvin]

OK - I'll bite! Why are the granite slabs over the parking garage entries at Daley Plaza post-tensioned?

[1ajs]

because granite as hard as it is does crack esay

[Kelvin]

Well perhaps it is still a "brittle" material, but the tensile strength of granite and similar rock is in the range of 4 - 8 MPa (as compared to concrete that would be 1 - 3 MPa). For the application shown it still appears to be gratuitous overkill given the thickness of the slab used, the support arrangement (e.g. pinned on 3 edges, fixed on one), and the fact that there appears to be no additional or superimposed load acting on the surface.

Even without the PT, the FOS would be ~3 as a conservative estimate (from my basic calcs) against a service load crack.

[1ajs]

even at that thicknes that span needs the extra suport

[vexxed82]

Next obvious question for the uninformed...How exactly does one go about post-tensioning granite?

[1ajs]

u bore holes through it

[Kelvin]

Originally Posted by 1ajs

even at that thicknes that span needs the extra suport

That's my point - it doesn't appear to require the assistance of PT. The tensile capacity of the material is more than adequate and even if you wanted to guard against a freak failure, why not just slide a few channel sections in under the slab hidden from view and doesn't involve the additional expense.

Span to depth is pretty modest (about 12 or 15 to 1), so it still defies logic. An FOS of 3 (or even more) is superb, so again PT appears to be an expensive solution with little reason to substantiate it. So the question remains - why PT it?

To expand on Vexx's question - a series of holes are drilled through the slab but each would also require a counter bore at each end in which to accept the strand anchors. The photo shows each strand pocket capped with an epoxy sealant to keep it air tight, so it (the strand) may not need to be grouted. However, it may not be prudent to rely on said sealant, so grouting the hole seems warranted. To do that though, there would have to be a second grout tube and vent drilled alongside each primary hole - which probably explains the oblong shape of the cap.

[vexxed82]

Originally Posted by Kelvin

That's my point - it doesn't appear to require the assistance of PT. The tensile capacity of the material is more than adequate and even if you wanted to guard against a freak failure, why not just slide a few channel sections in under the slab hidden from view and doesn't involve the additional expense.

Span to depth is pretty modest (about 12 or 15 to 1), so it still defies logic. An FOS of 3 (or even more) is superb, so again PT appears to be an expensive solution with little reason to substantiate it. So the question remains - why PT it?

To expand on Vexx's question - a series of holes are drilled through the slab but each would also require a counter bore at each end in which to accept the strand anchors. The photo shows each strand pocket capped with an epoxy sealant to keep it air tight, so it (the strand) may not need to be grouted. However, it may not be prudent to rely on said sealant, so grouting the hole seems warranted. To do that though, there would have to be a second grout tube and vent drilled alongside each primary hole - which probably explains the oblong shape of the cap.

Ahh, makes sense now. I'm an idiot...I didn't realize those were caps on the end of the granite. The first time I looked the photo it appeared as though something had been punched through, similar to a large hypodermic needle...I guess the simplest explanation is usually the best.

[1ajs]

who knows but i'm sure according to some chart out there it says to post tension it so thats what they did

[harryc]

A better shot of some cables from 161 W Kinzie

[wushu18t]

it's been a while since i've been on this forum, especially the engineering threads. now that i graduate in may with my bachelors in civil engineering, i understand these threads so much more. but, i think i'm headed towards water now instead of structural.

[harryc]

From 200 W Lake, Chicago Ill.
s the green where the cables were before tensioning ?


More post tension lines waiting their turn.