Coring through floor slabs to post-tension 1

[a3a]

Does anyone have any knowlege of coring through concrete floor slabs in so that post tensioning cables could be run through them in the event that the original slabs were insufficient (design error). I was curious if there were any articles on this procedure or if it has ever been done. I was especially curioius if problems would be encountered during the grouting because of the smooth walls.

Thanks

 

[Ingenuity]

a3a,

I gather you mean horizontal coring.

Short answer is yes. But they are limited to straight core holes, and you cannot achieve draped holes.

Actually, a large strengthening project is happening as I respond to this thread...4" diameter horizontal thru existing beams at 80 feet core length!!! Laser guided coring technique with 1/2" tolerance!!! Post-tensioning is then placed within the hole and stressed (you may ask why not just use external P-T - it won't fit with the immediate adjacent pipe work etc).

Also, smaller diameter horizontal coring techniques have been used on balcony repairs, whereby external access coring is undertaken to the length required (maybe 20 feet or more etc) then high tensile threaded PT bar is inserted and stressed (dead end is established at remote end via "sausage bag" of epoxy, or debond bar over free length etc).

Check out the Aberdeen Group on a publication about concrete coring where they detail the balcony coring/PT project.

Re the smoothness of the core hole - we have used sand blasting techniques to roughen up the walls, but you will be suprised how much transfer of load you can achieve provided you remove the coring slurry. We usually undertake some bond tests - grout in a few bars of fixed bond length and load test them with a hydraulic center hole jack.

I must say that as a strengthening technique, EXTERNAL P-T is more common than what i describe above, and somewhat well documented, and more cost effective too!!!

Maybe you wish to consider external PT for this project.

Good luck.

HTH

Depending on the % of deficiency, maybe you can consider FRP - but it will not help you with any deflection control.

 

[Qshake]

Ingenuity, WOW that's some core length! Given the laser guidance system it sounds on the up and up but expensive as well.

I've recently engineered some anchors for bridge substructure beams (in the short dimensions) and they'd like you to think I was drawing blood from that subcontractor! The first problem was being able to determine the location of reinforcing in the beam in order to avoid it...not happening! The stirrups don't line up from face to face of the beam and they're having to go at it from both sides (reminds me of the cartoon where two contractors are tunneling from two sides and scratching there heads at the obvious offset at the juncture!).

Good Luck!

 

[a3a]

Thanks Ingenuity

You nailed it. Though I was just touring this project, it was pretty much what you descibed. Coring through a combination of beams, slabs, and columns--except only about 50 feet, a tolerance of 1", and surveyors using very primitive means compared to lazers (piano wire). I am still curious how the lazers were used for the surveying. Were they just used to check periodicly if the holes were on target, or were they actually part of the coring setup that "guided" the bits.

Q-shake,

There was some "head scratching" on this job too. While coring, they were logging the rebar, plates, trash, and whatever else they encountered as the sleeves came out; and they were finding things that no one knew why they were hitting certian structual elements and at those locations.

I'm still at the entry level stage in my career, and so far it was the largest project I've been on--completely overwhelmed by the shear size, amount of effort, and amount of money, that goes into huge, complex projects. Likewise, I was just as overwhelmed by the amount of tension in the people because the project was not going smoothly at all (possible collapse), when the facility should have been in full production already. I guess I was so overwhelmed because I've been on a *small* project that wasn't going smoothly and I can't see how the entire upper echelon hasn't had a heart-attack already. Amazing! Sorry, had to share it with someone that could appreciate it.

 

[dlew]

a3a,
Have you considered placing the post-tensioning cables under the slab, instead of coring.
I have seen this type of exterior prestressing on steel beams and prestressed concrete Tee-beams.
The supporting beams would have to be cored, and the prestressing cable would have to be protected.

AEF