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-   -   Making a channel in a post tension slab (http://www.diychatroom.com/f19/making-channel-post-tension-slab-51290/)

firehead67 08-20-2009 08:33 AM

Making a channel in a post tension slab
 
I'm a pretty avid DIY and have worked in the construction trades. I'm trying to remodel my bathroom possibly saw into or chip out a channel in a post tension slab. First can you or should you let the tension out of the cables in the area your working in? Second is there a standard depth I should saw first then chip out the rest to go around the cables if needed? Any suggestions would be helpfull. Thanks

itsreallyconc 08-20-2009 09:47 AM

4" grinder, .110 diamond blade, & 15# bosch chipping gun,,, NEVER EVER let the tension out,,, you cables should be plac'd midpoint vertically,,, that is not to say they were as those guys got paid & they're gone :laughing:

IF you run into the cable(s), remember to restore the sheath when replacing the conc,,, all in all, its not a difficult job - a metal detector would help.

Termite 08-20-2009 09:53 AM

Personally I'd avoid doing this at all costs. The cables are under incredible stress and therefore the concrete itself is under some stress. Don't mess with the cables, period. The problems that screwing with any of this could cause aren't worth it. If such an installation were done during construction the slab would almost certainly be rejected or engineering would be required.

itsreallyconc 08-20-2009 12:21 PM

normally i'd agree w/our forum leader, kc 'mite, as he's knowledgeable, willing, & picks up the bar tab more often than not :thumbup:

that being said, just be careful,,, we've repair'd cables & not had problems,,, yes, our levels of expertise do vary but, as i understand the post, its only a short opening - you're not bisecting the floor,,, point of disclosure - haven't done any residential work - only public bid { usually parking structures }

mpoulton 08-21-2009 11:17 AM

Quote:

Originally Posted by firehead67 (Post 316831)
I'm a pretty avid DIY and have worked in the construction trades. I'm trying to remodel my bathroom possibly saw into or chip out a channel in a post tension slab. First can you or should you let the tension out of the cables in the area your working in? Second is there a standard depth I should saw first then chip out the rest to go around the cables if needed? Any suggestions would be helpful. Thanks

This is a VERY dangerous operation, and I would not recommend it. The amount of energy stored in post-tension cables is tremendous. If you hit one, it might simply go "BANG" and not damage anything. Or it might tear itself up through the floor surface, shredding the concrete and launching chunks everywhere at high speed, flying back like a steel whip and cutting anything in its way. I've seen PT cables rip up 20 feet of concrete and cut a ladder in half just because someone hit it with a hammer drill. Construction workers who've seen one break are scared of them.

You can't tell how deep the cables are. The engineered depth varies throughout the slab, depending on the magnitude and direction of the bending moment in the concrete. The actual installed depth may not even match the engineered plan, since cables are hard to place before a pour. They should be at least an inch into the slab, but I sure wouldn't count on that. There is usually no way to release tension in the cables, since they are grouted into place and the extra cable is cut off during tensioning.

I would never consider cutting or drilling in a PT slab without X-rays of the location first. I don't know if or how a slab on grade can be X-rayed - I've only dealt with high rise PT slabs.

Termite 08-21-2009 11:44 AM

Quote:

Originally Posted by mpoulton (Post 317391)
This is a VERY dangerous operation, and I would not recommend it. The amount of energy stored in post-tension cables is tremendous. If you hit one, it might simply go "BANG" and not damage anything. Or it might tear itself up through the floor surface, shredding the concrete and launching chunks everywhere at high speed, flying back like a steel whip and cutting anything in its way. I've seen PT cables rip up 20 feet of concrete and cut a ladder in half just because someone hit it with a hammer drill. Construction workers who've seen one break are scared of them.

Well said. I've seen the aftermath of a tendon/slab failure during tensioning on an apartment slab project and you wouldn't believe the damage it did. If anyone had been in its way it would've cut them apart.

Definitely a project left for very specialized professional tensioned slab guys. Not a handyman or DIY project under any circumstance.

SNC 08-21-2009 05:03 PM

I have to agree with the above post. You sound like you are WAY out of your leauge on this one. Ever break a rubber band by pulling to hard ? Well think of those cables as giant rubber bands made out of steel.
Your playing with nitro here.

BTW you cannot release the tension because when they pull them tight they cut off all the excess.

ncgrogan 08-22-2009 06:42 AM

You can release the tension in post tension tendons but you need the right tool. That said if its prestress tendons you cannot as the bond between the concrete is what keeps the tendons tight. The tendons will be in the bottom of the section since you said its a slab as I doubt theres room to drape them but you probably don't want to cut into the top of the slab with the tendons still tight because its going to be under tensile stress and will likely crack and propogate cracks. I never liked post tension as it seems its always used as a band-aid for a cross section or span that doesnt work. (my old boss would flip out if he heard me say that as he helped invent prestress design and manufacturing and used in when ever he could.) I like prestress when trying to minimize fatigue though.

Bottom line, call the EOR and get them involved.

SNC 08-22-2009 07:17 AM

Quote:

Originally Posted by ncgrogan (Post 317720)
You can release the tension in post tension tendons but you need the right tool. That said if its prestress tendons you cannot as the bond between the concrete is what keeps the tendons tight.

I cant imajin how you can release them, after pulling they are cut off down to just a couple inches. Enough to grab onto with the jack but to release, the cable has contract back into the concrete, and as Ive seen these pulled as much as a couple feet, a couple inches is hardley going to relive much tension.

firehead67 08-22-2009 10:23 AM

Thanks for all the input on post tension
 
Thanks for all the help, I too was very sceptical when I was told you could relieve the tension, it just didn't make sense. I was also informed by a remodeler that he had gone around the cables to install pluming, like I was thinking of doing. With that said I know how much tension these cables are under just wanted to hear some thoughts on expierence with residential systems and even attempting it. Thanks :)

Daniel Holzman 08-22-2009 10:48 AM

I think there is some confusion on this topic, due to the complexity of modern engineering concrete stressing practice. Post tensioned steel refers to the fact that the steel tendons are tensioned AFTER the concrete has been placed, but BEFORE the service loads have been applied. Pre-tensioning refers to the practice of tensioning the steel BEFORE the concrete has been placed.

Pre-tensioning must be done at the factory, using special jigs, and is typically used to produce standard shapes such as T beams. Regardless of whether the process is pre-tensioning or post-tensioning, the end result is a concrete slab with steel tendons that are stressed using a jack, and retain their tension after the slab has cured.

See the website http://www.concretenetwork.com/post-...n/what-is.html for an interesting discussion about post-tensioned and pre-tensioned concrete. Both pre-tensioning and post-tensioning are forms of prestressing, meaning that the steel is stressed before service loads are applied. A different form of pre-tensioning is to stress the steel, but instead of installing the steel in a plastic sheath, the concrete cures around the steel. This is known as prestressed concrete, which is a bit of a misnomer, since pre-tensioned and post-tensioned concrete are also prestressed concrete, but typically if you hear the term prestressed, it means that the steel was stressed, and the concrete cured around the steel, so the bond between the concrete and steel holds the stress in the concrete. In the case of pre-tensioned and post-tensioned concrete, the stress is maintained by a large nut on the end of the steel strands, whichi is embedded in the concrete, and forms a permanent connection.

Releasing tension on a prestressed concrete slab is a specialized practice, typically handled by a professional company with lots of experience. Definitely NOT A DIY project in my opinion. Similarly, cutting into a prestressed concrete slab is a specialty project, typically undertaken by a specialty company with lots of experience. I used to design wireless systems, and we would occasionally cut into a prestressed slab to install wires. This work was always done by a professional firm with experience, insurance, and very carefully, usually after X-rays had located with position of the strands. Definitely not a DIY project in my opinion. Far too dangerous for an amateur.

ncgrogan 08-22-2009 01:38 PM

Quote:

Originally Posted by Daniel Holzman (Post 317809)
I think there is some confusion on this topic, due to the complexity of modern engineering concrete stressing practice. Post tensioned steel refers to the fact that the steel tendons are tensioned AFTER the concrete has been placed, but BEFORE the service loads have been applied. Pre-tensioning refers to the practice of tensioning the steel BEFORE the concrete has been placed.

Pre-tensioning must be done at the factory, using special jigs, and is typically used to produce standard shapes such as T beams. Regardless of whether the process is pre-tensioning or post-tensioning, the end result is a concrete slab with steel tendons that are stressed using a jack, and retain their tension after the slab has cured.

See the website http://www.concretenetwork.com/post-...n/what-is.html for an interesting discussion about post-tensioned and pre-tensioned concrete. Both pre-tensioning and post-tensioning are forms of prestressing, meaning that the steel is stressed before service loads are applied. A different form of pre-tensioning is to stress the steel, but instead of installing the steel in a plastic sheath, the concrete cures around the steel. This is known as prestressed concrete, which is a bit of a misnomer, since pre-tensioned and post-tensioned concrete are also prestressed concrete, but typically if you hear the term prestressed, it means that the steel was stressed, and the concrete cured around the steel, so the bond between the concrete and steel holds the stress in the concrete. In the case of pre-tensioned and post-tensioned concrete, the stress is maintained by a large nut on the end of the steel strands, whichi is embedded in the concrete, and forms a permanent connection.

Agreed except I've always used the term prestressed for pretensioning to not confuse people When you say precast prestress pretensioned concrete, people look at you funny. Nuclear power containments were mostly done with post tensioning in the 70s and the NRC requires restressing/checking the stress in the tendons as part of the license extension process, so I know it can be done.


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