Ideas solicited on how to shorten an anchor rod in concrete

cwilcox · Jan 24, 2019

Hi all, I'm requesting your experience or ideas in practically shortening some high tensile anchor rods in concrete.

Anchor Rod Specs: 1-1/4"-7 tpi, High Tensile, Galvanized.

Situation is that about 80 to 100 anchor rods have been damaged and we need to cut them off flush with the concrete, core drill down around the anchor maybe 6 inches deep with an approximate 4" diameter hole, break out the concrete and then install a coupling nut on the good threads below top of concrete (TOC). Some of the rods are damaged a bit below TOC so they need to be cut off in the hole.

Once all rods are cut to a suitable length exposing undamaged anchor rod we need to chase the threads to remove bits of leftover concrete, a plan is in place for that.

Ideas include a mag drill with a center drilling type endmill but I'm not sure the mag drill is suitable for driving endmills. (I know concrete isn't magnetic, we will worry about mounting it)
What other tools or technologies can get in the hole and create a finished end on the top of the anchor rod?
Maybe there is a company that specializes in this repair?
I've seen the AISC field repair manuals etc so no need to post them.

Thanks, Chris

Milland · Jan 24, 2019

If you can set up safety shielding around each stud as you work on it, try adapting cut-off wheels (like for smaller abrasive chop saws) to 7" angle grinders. As long as you're careful to avoid twisting the grinder, you should be able to slide the saw into each stud pretty quickly.

cwilcox · Jan 24, 2019

This image is something taken from Google. I drew the red lines as an idea of how we need to cut the anchors below the concrete. I don't think a 7" grinder will fit in a 4" diameter hole but it is great to cut off the tops to the concrete level I agree.

dazz · Jan 24, 2019

Hi
Have you considered a bolt chamfer tool to debur the end of the anchor rods?

Dazz

magneticanomaly · Jan 24, 2019

What is the nature of the damage? Seems a LOT of trouble could be saved if the existing ends could be salvaged

magneticanomaly · Jan 24, 2019

I've seen the photo now. Bent. I would talk to the engineer about the possibility of bending 'em back. Yes, they will be weaker, but perhaps not by so much as to get past safety factor. Certainly there is some degree of bend that CAN be safely straightened, and that may save you drilling quite a few.

I think it would also be cheaper to drill right beside a bent bolt and epoxy-in a new one, and move the hole in whatever you are holding down, or add an offset holddown clamp. An hour of engineer time to discuss and sign-off on the salvage modification may save a few $

dsergison · Jan 24, 2019

pic of bolt chamfer tool if unfamiliar.

UNIBURR | Deburring Tool, Saving you time and money

abrasive chop bolt, repair top, use thread die to knock out concrete.

I would buy two core drills. a 4" and whatever barely fits over the bolts such that you don't have to bust off very much?

kenton · Jan 24, 2019

Have you considered using a vacuum base core drill instead of a mag drill? This seems like exactly what they are made for. I know my local Ace Hardware has them for rent.

vacuum base core drill - Google Search

eKretz · Jan 24, 2019

This seems like it would be a great waste of time. I would just core down and remove the old anchor bolts completely and replace them then repour around with some of that high strength stuff. Or core down 6" then torch the old studs and plop in some new ones with a square plate pinned on. Trying to fight to work down below the surface seems like a pain in the butt. And yes, vacuum based core drill is the way to go for sure.

Mr.Smith · Jan 24, 2019

Look into removing the concrete with high pressure water. It will not damage the bolts and will leave a rough excavation for the replacement concrete to bond to. From your description I assume the bolts are all thread. Using water will allow you to inspect the bolts condition as the excavation progresses. Any deteriorated or weakened concrete will be removed automatically. The bolts will not need any further cleaning.

I read that the photo included above is a generic photo that may or may not represent your specific instance. If you do happen to have gang bolts as is shown it is important to create work room so your personnel are not fighting tool access and can work efficiently. Make a hole, cut the bolt with a torch, smooth the top with a cup stone, screw on a coupling and all thread extension, survey for location, and pour back the concrete using a bonding agent where new concrete contacts old.

We did something vaguely similar on a spillway repair. Hydroblasted out the concrete, replaced damaged and missing rebar using couplings and poured back using 20,000 psi concrete to resist future damage from rock impacts. The rebar exposed was undamaged by the hydroblasting.

Thank you,
Mr.Smith

cwilcox · Jan 24, 2019

I (we) are a building full of Engineers so I've got that part covered. Cutting out and replacing with new is not an option. I like the idea of the vacuum drill. Photo provided is not of the actual problem, I can't post images of the job. Anchors are too high tensile to be bent back, there are limits to that and we are well beyond the angle. We have floated a few ideas to the construction team and I shall see how it turns out. Thanks everyone who contributed.

mike 44 · Jan 24, 2019

How did the bolts get damaged? I have seen similar damage caused by cranes and heavy machinery with tracks rolling over the bolts.
The bolts were heated and bent as straight as possible. After columns were set the bolts were welded to the base plates.
Most of the threads were damaged beyond repair.

Hodge · Jan 25, 2019

cwilcox said:
I (we) are a building full of Engineers so I've got that part covered......

I believe I've figured out what the main problem is....

Quite a few ways to repair these as several posters have mentioned. However lack of full disclosure and correct pictures makes it a little more difficult to say the least.

memphisjed · Jan 26, 2019

Your specs are already busted, those aint galvanized b7 rods. a torch and some love with bending forks to straighten them out, red, light orange will not destroy (affect, not destroy) tensile strength. are you jack nutting the base plates or are they direct mounting to concrete? If direct mounting you only need the depth of nut + fudge to have clean threads.
If you are still wanting to coupling nut new rods, why not just chip out concrete a few inches done around the cage, slice with 7 or 4 inch grinder, and add coupling just below grade? no need to clean up cut more than what a bevel motion on the grinder will give you. Given the point of construction in the picture, another option would be to add 4 anchors drilled/epoxied on a bigger pattern with same center point and having the structural shop get the change order for the base plates. This is not uncommon.

cyanidekid · Jan 26, 2019

It’s pretty clear some are replying without a carefull read of the problem, or without useful or practical knowledge, or both. That’s not too surprising, par for the corse!

Yea, super obvious tool to get down into the ‘crete is the coredrill, but the vac needs a smoothish surface for it to work. More for popping a few holes in finished work.

One hundred in a rough slab? With those 1 1/4” dia rods, you are talking a 8” dia. Hole probably 8” deep in order to have the room to jackhammer the concrete out and expose at least 3-4 diameters. Holy crap, Bring in a truck mounted unit! If you can’t get a truck in, get a skid steer or a crawler mounted drill for shits sake. (Hasn’t anyone ever actually been on a construction site?)

How thick is the slab? You are probably best just cutting them out compleatly and casting in new at that point, but if it’s really thick ( over a foot?) and the rods are really deep, then the following may apply;

That takes care of making holes. But shortening the rod? Don’t use a torch, it will really make a mess of it, and is quite dangerous working in a hole in concrete. Both because of simple blowback from cutting a 1 1/4” rod, and also because water is a constituent of cured Portland cement and it can explode violently exposed to that much heat.

That combination, puddle of molten steel, exploding concrete, can equal a very, very bad situation. DO NOT TORCH CUT IN A HOLE IN CONCRETE.

At 1 1/4” dia, I’m thinking do everything possible NOT to have to cut below the surface at at all. Re-design the footings to accommodate the extra hight of a coupling nut and rod extending above the slab.

If you must cut below, cut that hole eccentric to the rod, (blast out with jackhammer being carefull of the threads below the cut) trim and clean up ends with a die grinder and cubitron cutoff wheel. Cutting the rod is the easy part, cleaning the threads is the tough part. The high pressure water may come in handy here.

SAG 180 · Jan 26, 2019

If the exact locations of the rod isn't critical and the concrete is deep enough, couldn't you drill beside it and epoxy the replacement next to the damaged rod to make a non contact lap splice?. The correct epoxy is going to be much stronger than the concrete so long as high temperatures aren't involved.

Edit: I see Magneticanomaly has already suggested the same fix.

247951d1548497467-ideas-solicited-how-shorten-anchor-rod-concrete-lap_splices.jpg

Greg White · Jan 26, 2019

I cant help with the engineering of the work, but can/will show pity
on the labor crew thats going to do the work, I rode shotgun on a cement
cutting truck, we did whatever came at us, that isn't a pretty sight you have there.
Please come back and tell us (generally) the fix you came up with.
Good Luck
Gw

cwilcox · Jan 26, 2019

cyanidekid said:
It’s pretty clear some are replying without a carefull read of the problem, or without useful or practical knowledge, or both. That’s not too surprising, par for the corse!

Yea, super obvious tool to get down into the ‘crete is the coredrill, but the vac needs a smoothish surface for it to work. More for popping a few holes in finished work.

One hundred in a rough slab? With those 1 1/4” dia rods, you are talking a 8” dia. Hole probably 8” deep in order to have the room to jackhammer the concrete out and expose at least 3-4 diameters. Holy crap, Bring in a truck mounted unit! If you can’t get a truck in, get a skid steer or a crawler mounted drill for shits sake. (Hasn’t anyone ever actually been on a construction site?)

How thick is the slab? You are probably best just cutting them out compleatly and casting in new at that point, but if it’s really thick ( over a foot?) and the rods are really deep, then the following may apply;

That takes care of making holes. But shortening the rod? Don’t use a torch, it will really make a mess of it, and is quite dangerous working in a hole in concrete. Both because of simple blowback from cutting a 1 1/4” rod, and also because water is a constituent of cured Portland cement and it can explode violently exposed to that much heat.

That combination, puddle of molten steel, exploding concrete, can equal a very, very bad situation. DO NOT TORCH CUT IN A HOLE IN CONCRETE.

At 1 1/4” dia, I’m thinking do everything possible NOT to have to cut below the surface at at all. Re-design the footings to accommodate the extra hight of a coupling nut and rod extending above the slab.

If you must cut below, cut that hole eccentric to the rod, (blast out with jackhammer being carefull of the threads below the cut) trim and clean up ends with a die grinder and cubitron cutoff wheel. Cutting the rod is the easy part, cleaning the threads is the tough part. The high pressure water may come in handy here.

Your description is closest to the direction it looks like we are going. This isn't in a slab but the foundation actually. This is big stuff at a major construction site. Regardless I'll try to post back in a few weeks with the ultimate repair procedure. But remember this isn't hobby and isn't something that can be discussed freely on the public domain so it may not have specific details.

LKeithR · Jan 26, 2019

Something that hasn't been mentioned is who's dime is paying for the repair? I've seen situations on construction sites
where things were wrong or there was serious damage and the only solution was to break up the foundation and start over.
Not desirable at all but sometimes you've gotta bit the bullet and do the right thing. Depends a lot on the size of the job.

If it's possible I'd be looking at stepping over a certain distance and installing new wedge anchors or using epoxy to install
new studs. Both are accepted methods of doing things so long as your footprint allows room to do so. If the structural steel
is already on site there are portable punches that you can use to make new holes.

I have racked my brain trying to think of a tool that would cut those anchors off cleanly below the surface of the concrete and I
can't come up with anything. And that's the key; creating a flat, square end on the anchor that requires a minimum of cleanup
to fit a coupler nut. If this is a foundation--raised above the floor level--would it be possible to use a core drill to make a
space around the stud and then cut a window in from the side of the foundation big enough to allow you to get in there with
a zipcutter and nip off the end, Once you have access cutting a 1-1/4" bolt with a zipcutter is a piece of cake. If the anchors
are in a floor the same idea would work, you'd just need to break out more concrete--creating a sort of keyhole shape--but in
the long run doing that might still be the easiest solution...

MattiJ · Jan 26, 2019

Ugh. some shitty case you have.
Bridgeport head or SOB magnet drill mounted to heavy ad-hoc XY table? use forklift to position over every hole.

Someone mentioned spliced joints... depending on code and whatsnot you would need 40 to 130" inches splice lenght for 1 1/4" rebar :crazy:

Ideas solicited on how to shorten an anchor rod in concrete

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