Post Tensioning Advice

I think he may be referring to the practice of embedding cables in concrete slabs that are tightened, or tensioned, after the concrete has cured. It helps to keep the slab from cracking.

"Being a mech engineer, I obviously didn't." You know the theory involved, just not the practice.

It probably lets builders pour slabs on less stable ground, but I no expert in building. _{Come to think of it, I no expert in anything.}

Changed my mind about this after some research.....

Ask him where he heard it.

other than that it would be centered around structural engineering, some people think being a engineering you can ask them very broad questions, but atleast its a start.

Sounds weird, I've heard of pre-stressed concrete, but post stressed makes no sense to me. How do you garantee even stress in the cables/rods whatever? You could easilly just pull the ends off, surely you must stress it while the concrete is wet?
Heck why not just use some bags of shingle, clamp 'em up hard enough, they'll be near enough to solid?
I think maybe the clue is in that word 'manager' .
Del
(I'm happy to be completely wrong on this...)

Yeah, thanks for that. Actually I kind of absorbed some content from the internet about what it is and how it would help bear greater loads then what's apparent. I just wanted to know the practical implications it would have, possibly if someone already has an experience with it.

Post-tensioning is also a pre-stressing process. You do not stress wet concrete. You apply the stress to the steel either prior to placement or after placement of concrete, then let it set to develop resisting strength. You do not transfer the stress loads from the tensioning steel onto the concrete until it has achieved sufficient set strength. Pre-stressing actually relates to stressing the concrete before it is placed into operation, or loaded with operating loads. Post-tensioning is a process of stressing the tensioning cables after concrete placement. You replace some of the reinforcing steel with steel cables that are encased in a sleeve. After the concrete has set and achieved a minimum set strength to support the tension, you apply a jacking force against the cables to stress them, then clamp them against the concrete to transfer the stress from the tensioned cables into the concrete as compression against the concrete. It utilized the strength of the steel agaisnt tension and the tranferring some tension stresses in the concrete into the cables and loading the tension onto the concrete as compression loads (concrete really only effectively pocesses substantial strength under compression, so you redirect the loading into more compression on the concrete, and more tension gets applied to the steel, whuich is substantially stronger than concrete). Post-tensioning in slabs tends to be more expensive, but develops much stronger slabs that can resist differential settlements and other subgrade reactions much better. It requires special inspections and personnel to install properly. It limits the bends and radius that such reinforcement can be aligned (non-tensioned reinforcing steel may be used elsewhere as needed). It substantially stiffens the concrete against bending forces. It is not amenable to future modifications (it is dangerous to cut post-tensions cables, and once cut the benefit is lost as the cable itself is not bonded to any concrete). there is a ton of information available on post-tensioning. It is more common in beams, and slabs on expansive or wet clay foundations or in liquifiable shallow groundwater areas where substantial differential settlements can occur.

Hi, It is the pretension added in the opposite direction . its concept is exactly similar to the leaf spring used in automobiles. Here the beams used are pre stressed. For example a post tension concrete slab of 400mm can carry load equivalent to the general concrete slab of 1000mm. It also depends on the rebars used to make the slab. being a mechanical engineer i have only practical idea and not sure about the theory behind it. -RK

post stress, what if there is no stress put on the concrete to react with the post stress.....i'm sure thats figured in

In the event that later you want to core drill any holes through the concrete, it would be good to know where the post-tension cables are, in order to avoid them.

I will begin by saying that I am a civil engineer, but concrete design is not my specialty. I will also say my experience with post tensioned concrete is nil. I do however work close to contractors that build it. Here is my understanding of how it works and the contractors take on it.

The post tensioning wires are placed similar to rebar except they are put under a constant tension. The concrete is poured and after curing the wires are cut and thus putting the stress onto the concrete (in theory anyhow). Like a stretched spring trying to return to its normal state.

The contractors I have dealt with believe this method to work fine in the short run, but don't expect it to last as long as a standard reinforced concrete design would. They have said that is the number one reason for repair and rework jobs.

I believe the pro is that you can lower the cost of the project by minimizing how much steel is needed. The con is that you can expect about a 10 year life from it.

Again I don't do these designs and have no personal experience with post tensioning, so if I am wrong please feel free to educate me!!

thanks for those thoughts, seems logical....

This link gives a brief description of the method. This link has some interesting comments and pictures.

I have used post tensioning on several medium or high rise buildings. The advantage is that long spans can be achieved with shallow slabs. This saves expensive transfer structures when a residential or commercial building lies above an underground parking facility. Many of the high rise buildings in the U.S.A. and Canada have multiple floors of post tensioned concrete.

The reason the slabs can be so shallow is that the tendons are draped from end to end in two orthogonal directions. At columns they are high and at midspan they are low, so that when the tendons are stressed, they balance some or all of the dead load of the slab.

The main advantage of post tensioning is the saving in material, weight and money.

There have been some problems with post tensioning. One is the accidental cutting of one or more cables which has already been mentioned. No drilling should be permitted in a post tensioned floor without first determining the location of the tendons. If an unbonded tendon is cut, it loses its tension for the entire length or width of the floor, so it affects many spans. If the tendon is bonded, i.e. grouted after stressing, the cutting of one cable affects only the span where it was cut.

Also, there have been some serious problems with corrosion of unbonded tendons. If the end anchorages are not watertight, water can enter the plastic duct and sit in the low point at midspan causing corrosion over time. If the tendons are grouted, this is not a problem.

I understand that in Australia, unbonded systems are not permitted by code. Perhaps someone in the forum can confirm that. If I were to design any more post tensioned slabs, I would consider only grouted tendons.

Many thanks for the info, this should surely help....i have already been through the PTI website.....but the other pdf was very useful......

Post tensioning is the use of steel tendons that are strung through special conduit in the slab after it is poured. This is used extensively on concrete bridges especially for curved concrete. Pre-cast prestressed concrete the steel tendons are stretched inside the forms first and the concrete is poured around the tendons. When the concrete reaches a specified strength the tendons are cut, and the concrete is then pulled into compression which produces a camber in the member. In post tensioning the concrete is poured with the conduit for the tendons in place. The concrete can be pre-cast pieces as they are for some curved bridge supports, or they can be cast in place as when they are used in the floors of parking structures or buildings. Either way, after the concrete has reached a specific strength, the tendons are mechanically tensioned, and cone shaped fittings are placed around the tendons that cam into place and hold he tendons as they try and pull themselves back into a relaxed condition. This brings the concrete into compression and camber is the result of this process. When camber is built into a concrete structure such as a beam or slab, the allowable live load can be increased since the tension in the bottom fiber of the concrete is calculated out and the entire concrete member is now in compression. This is pretty much a simplified explanation, but you should be able to get the gist of it. Pre-stressed means tendons stressed first then concrete poured and cured around it. Post tensioned means the concrete is cured first and the tendons placed inside conduits inside concrete and tensioned later. Both accomplish the same thing: to produce camber in a member, allowing the member to be totally under compression under its loading. This is definitely a project that requires a structural engineer who knows post tensioning. I have done a lot of prestressed pre-cast stuff, but never had the chance to work with the guy in our office that did post tensioning projects. It has some great advantages, but of course has some disadvantages as well. An engineer well versed in post tensioning can explain the pros and cons. I hope this helps.

Cheers,

John

thank you Idrive....cleared any minor ambiguity I might have had.....

I don't know anything about this sort of thing so forgive me for this.

I wonder if you could achieve the advantages of both pre-and post stressed by including a small heating wire up the centre of the steel cables.

Apply power to the heating wires so that the steel gets to about 95°C; hang the cables in their natural catenary shape; pour the concrete; allow to set bonded to the cables, and finally, remove the power from the heating wires.

I suspect that the main problems would be:-

1.) Keeping the cables at the pre-heat temperature accurately, whilst the concrete is being poured.

2.) The concrete in contact with the cables would be at an elevated temperature as it set. When the whole "assembly" cooled the concrete close to the cables would shrink relative to the surrounding concrete, and, become weak/crumble.

If you read up, you will quickly note that the steel cables have to be "free" not in any way connected to the concrete. You can even thread them through a hole left in the concrete to string many together for certain situations.....

You seem to be confusing the method of "Post Tensioning" with "Pre- stressed" concrete....that is another, different "can of worms"!

You seem to be confusing the method of "Post Tensioning" with "Pre- stressed" concrete....

That was the idea: reading some of the other posts seemed to suggest that the problem with pre-stressed is that the steel has to be straight, and, one of the problems with post tensioning is that the steel is not fixed in the concrete throughout its whole length. I was just trying to combine some of the advantages of both methods.

As you implied contact between the wires and freshly poured concrete, you were not talking about Post tensioning.....

ergo. Pre-stressed....

Understood?

I have come across Pretensioned RCC poles being used for power transmission in rural areas. High carbon steel wires are pretentioned before pouring of concrete mix in the mould. We had supplied a Chain Electric Hoist with a stand and guide pulley so wires were pulled to length of 8". It was pretensioned wire not post tensioned.

Pros: Post Tension can significantly increase the ability to withstand moments and reduces deflection.

Cons: IT IS MORE EXPENSIVE THAN PRESTRESSED!! You need to bring in the post tension mechine and this limits the amount of work you can do as you can on load one element at a time.

If you are using typicall shapes it is best to go with prestressed as they are build offsite allowed to cure and once they get to the site, off the trailer and into the frame.

Post tension is used when there are HIGH negitive moments, the cable location is set to provide compression on both the top and bottom planes.

This practice came into vogue for the foundations of single family dwellings here in the US about 30-40 years ago. The foundations are much more flexible than standard reinforced concrete foundations (because they are thinner) and when they are built on unstable soil (clay most often.) they usually fail. Sometimes spectacularly (brickwork falling off walls, chimneys collapsing, huge cracks in walls, doors that won't close, that sort of thing.). Foundation repair companies are making money hand over fist fixing these failed foundations. Since the foundation is flexible and "floating" on top of the soil which is moving, the structure built on top of the foundation is being flexed too. this causes structural failure more often than not. the whole point of a foundation is to be a rigid member for the structure to be built on so making it intentionally flexible seems stupid to me.

My advice (coming from someone who has no specific experience other than seeing a LOT of them being repaired around here) would be to contact a qualified civil engineer with experience in this area and i would further tend to lean to the conservative side and build that foundation using lots of conventional rebar and make it rigid enough that if the soil shifts, it doesn't cause the foundation to shift.

informative.....I see that you have put it in a bit more elaborate yet precise manner (all the while I was thinking pre-stressed and pre-tensioned were the same!).....I am not sure what the construction guy was planning, bonded or unbonded....I will definitely pass on the info....

also, as per the comments, I think we will have to look at the possible loads in more detail once....