impact of 'broken' rebar on concrete strength

[continuousminer]

Dear all,

I have the situation that some 8cm holes (16 in total) have to be drilled in a concrete floor slab containing rebars. It is possible that some of the holes will go through a steel rebar and by this cutting it into two parts. Does anyone have an idea on how this might influence the strength of the slab? Or does anyone knows some literature references where I can find this? I know that the exact impact off course depends on the amount of rebars, thickness, position and form of the concrete slab etc., but it would be of great help if I already had an idea wether the effect is significant or not as it is localised. Most literature that I find concern rebar corrosion in a rather generalised way and not rebars which are broken at a specific point.
Many thanks in advance and best regards.

Jeroen

 

[csd72]

Is this a slab on ground or a suspended slab (i.e. raised above the ground)?

For a slab on ground, the problem is normally only aesthetic.

For a structural suspended slab there could be serious safety issues depending on your exact situation. I would strongly suggest you consult a local engineer to look at it for you.

If this has anything to do with a nuclear reactor then nothing should be done without a structural engineers advice (preferably the original designer).

csd

 

[msquared48]

Hopfully this is not the containment slab...

Mike McCann
McCann Engineering

 

[msucog]

since it sounds like you'll be coring holes in the floor, get someone with experience to do the coring. they can typically tell when the bits encounter rebar. it's also possible to get the rebar roughly located prior to coring.

what sort of loads or new loads are on the floor? ultimately, consult the design engineer.

it is sort of funny the natural reaction when someone that is "nuclear" asks a question that would likely be barely noticed if a "civil" asked the innocent enough question.

 

[Zambo]

csd72 is correct - is the slab a ground bearing slab or suspended?

Next questions are you sure you will be coring the slab not a beam? is the slab prestressed? Are there any heavy superimposed loads nearby - mechanical plant for example?

Of course msquared42 has a very valid point about the use of the slab.

With the answers to these questions the reply may be to proceed and cut the rebar. Coring of slabs is very common in straightforward circumstances.

 

[msquared48]

Zambo:

That's msquared48. Don't push me over the edge any sooner than I want to go! My kids are pushing hard enough!!!

Mike McCann
McCann Engineering

 

[csd72]

Are you still there continuousminer?......

 

[continuousminer]

Yes,
I'm still here, but I was off for a holiday (in our country, we have holiday around the first of november.)
So sorry for the late reply.
Thanks a lot for all the answers!
To be more specific, the slab is not prestressed and it is not a ground floor. It is a floor somewere between two levels and rests onto four sidewalls. (It is indeed a slab in the containment).
The problem is that some in accidental condition, the slab isn't strong enough to withstand the pressures. So it should be made stronger with the help of three steel profiles externally attached to this slab. But for the attachment, rather big holes have to be drilled through the slab. A scheme exist of the position of the rebars, but it is not too precise and there might be differences between the scheme and the as-built (as shown in other cases). So rebars might be hit. They plan to make a small reconnaissance hole, but this is so small compared to the final hole that there is no garantue that they won't hit the rebars.
Jeroen

 

[Rjeffery]

Sounds like Ground Penetrating RADAR (GPR) or Radiography would be the best bet to avoid hitting the reinforcing steel. Pardon the pun but the "Hit or Miss" method is not the best way to go in your situation.

Also, if the steel is to be attached from the underside, you should do a search on epoxy creep...

 

[shin25]

As msucog has mentioned, get some one with experience to do the work.

Usually, if the hole is made with a drill bit, the drill bit tend to deflect as it encounters rebar. An experienced contractor can stop and attampt to drill somewhere else.

I have experience with GPR, it requires, very competent technician and more over, if the rebars are too congested, GPR cannot give reliable result.

 

[csd72]

shin25,

He is talking about 8cm (3inch) holes, these are only practical with a diamond core which will go straight through the rebar.

continuousminer,

Any holes in this slab can be the source of a potential crack, particularly if they cut rebar. That said, these cracks are not usually a strength, rather they are a visible and durability issue.

Where is the steel design coming from for the steel beams? It may be best to get the designer of these to confirm the best way to proceed.

Why the 8cm holes? holes for anchors are usually in the 2 to 4cm range which is achievable with a regular percussion drill (which wont cut rebar as shin 25 pointed out).

This issue is really far too complex to be resolved over a forum, as has been said above you really should talk to a structural engineer about the specifics.

csd

 

[continuousminer]

@ zambo: it is indeed a slab, not a beam, and not prestressed.
as csd72 said, it is indeed a rather too complex situation for the forum, that's why I tried to keep the post as simple as possible. But I think it is clear that the impact on the strength of cutting through a rebar should not be neglected (sound obvious when you put it like that but it was difficult for me to judge the sevearness of cutting a rebar in half and I found no literature about tests done on residual strengths or something like that).
Radar seems indeed a good suggestion, but apparently not possible in this case. I will talk this over.
The holes are cut with a diamond drill and one cannot feel whether a rebar is hit or not.
Any way, thanks for the suggestions and replys! There are some structural engineers looking at the problem but the impact of cutting through the rebar was apparently not considered initially as a problem.

Jeroen

 

[DarthSoilsGuy]

if the slab is exposed, GPR should work well and give a good tell where steel is located. we would use it for forensic work with 8" CMU structural masonry (rebar and grout where they should have been). The indications were fairly obvious. We would subcontract our GPR and the guy was pretty famous in the business (seemed like he was always in the newspapers looking for dead people). so maybe his equipment was just that good.

and am i missing something here about coring slabs/walls. Shin, csd, msucog, and continuousminer are talking about the same kind of core barrel, right? perhaps the size of the hole may be too small for a tech to have had a lot of practice with that size barrel to get a good feel on it. But there are people that only nick, not cut, rebar and pull out. There are other people too (I'm one of the others )

I had a rebar locator that looked like a telephone available to use. it had some weird name that i don't remember. I never trusted the thing though.

out of curiosity, how thick is the slab and what size rebar & spacing is there?

 

[DTGT2002]

The HILTI corporation makes a fairly accurate and reasonably priced (~$500) rebar locater. I does need calibration to your materials to be best, but usually decent calibration can be established without too much trouble.

In work on non containment support structures in the past, I have had good sucess locating with it. Further, you can require the use of a drill stop device which will cut power to the tool being used with it upon encountering a grounded rebar or live wire. This does require either a grounded rebar system or access to a section of rebar common to the slab. Alot of ifs in post installation work, but still possible.

Assuming the design exists (which I assume it does if it is know to be deficient), analysis of worst case scenerios could be run to see the effects of the lost steel. Is it possible to include the greatest potential lost capcity in the retrofit such that none of the rebar in the affected zone is required? While no budget is unlimited, spending on containment structures should be allowed a bit of liberty.

Are multiple mats of rebar involved, and is the top and bottom surface exposed? Exposing some rebar may be another option to verify the inplace location of at least the outer mat.

Best of luck and be careful,

Daniel Toon

 

[Zambo]

As stated earlier it is best that the structural engineer providing the stel beam solution takes a look at the possible cutting of rebar. It will be easy using the spacing of the bars in the original slab design to work out the worst case of how many bars could be cut with an 80mm diameter hole.

I think it is fairly likely that coring and cutting the rebars will turn out to be a satisfatory solution.

 

[PEinVA]

Cutting rebar isn't necessarily a bad thing. It really depends on a number of factors:

1. Is this top or bottom bars?
2. Is this located in a neg. or pos. moment area?
3. What is required amount of steel in the slab vs. the actual steel in the slab? (if you have original design documents)
4. Rebar may not only be tensile or compressive reinforcement but could also be a shear head over a column. This is more dangerous when within critical areas and development lengths of reinforcment.

With all that said, it is safest to GPR, and if possible avoid hitting any bars, if this is not possible, it is possible to cut bottom bars in column strips in most cases and top bars the further away from the column strip. Again, these are quick rules of thumbs in determining what the scope of work should be for the specific job. These are just some quick rules we use.

Anyone else use these rules of thumb for scope of work?

RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke

http://www.tbastructures.com

 

[civilperson]

If the same bar is cut multiple times vs many different bars cut one time, a change in load sharing may be found. ACI allows bars to be spaced as (Eq 10-4, 10") or by (Eq 13-3, two times the slab thickness). If cutting the bars exceeds this distance for the spacing of the remaining intact bars, then additional strengthening will be required.
This can be done by externally adding steel members below the slab.

 

[csd72]

continuousminer,

Here are a couple of links showing the typical issues with opening in slabs. That said, I dont thinks any of these specifically covers your situation.

http://www.concrete.org/General/CI2807mota.pdf

http://www.allbusiness.com/manufacturing/chemical-manufacturing-chemical-product/4074555-1.html