Acceptable construction tolerance for rebar placement

[shaneelliss]

I am a PE and do concrete design frequently. I also frequently am involved in watching the designed concrete members get built in the field and I am constantly stressed and depressed and aghast at the inattention to detail that the concrete contractors seem to have. It seems they never pay any attention to required edge distances, bottom clearances, embed locations, etc. If they pour a footing with some pedestal rebar coming up out of it, it is always off location by a couple inches. Then when they go to place the forms, the forms will sit right against the rebar and they might push the tops of the bar away from the forms, but the bottoms of the bar will still be pushed up against the forms.

I don't make a fuss if the bar in a wall or mat should be on 12" centers and they end up at 11" on one and 13" on the next. I only make a fuss when the depth to the rebar in a bending member is off by more than 1" or so. And that is because I know that if I change "d" in the calcs by an inch, it can be a substantial change in strength. And I make a fuss about edge distances to forms and the the ground. For some reason 3" clear means "just not sitting right on the ground" to a contractor, and even that is a generous definition to some contractors.

If I say something to the contractor about it they look at me like I am completely nuts, and then they tell me that I am completely nuts if I think they could possibly do it any better than how they are doing it. I am told by contractors on every job that my expectations are completely out of line with what is possible. If I had a nickel for everytime someone has looked me straight in the eye and said, "are you f*ing serious?" I would be rich and could retire.

So my question is, is this standard fare for concrete contractors or do I just happen to have a very bad lot of concrete contractors in my area? Or am I really just expecting the impossible? Does anyone else have experience where the rebar gets placed where it should? What should my expectation be when it comes to rebar placement tolerances?

 

[CELinOttawa]

That is completely normal, and while a challenge to deal with, YOU are responsible to see that it is done right. Keep doing the right thing.

Many of the contractors and their staff are plenty intelligent, but have no understanding of what they are building. Better contractors with actual quality control and proper pride do exist, they are just the rare exception.

Keep correcting them, as you must, and nevermind the attitude unless it becomes aggressive or abusive. Brook no opposition to your authority, mind you... If you do they will never correct anything again and simply pour when you're not there, figuring you for a wet noodle who won't do anything about it.

 

[JAE]

I believe ACI 117 is an ACI document on tolerances. You might reference (buy) that and see what it prescribes.

Also CRSI may have some info on that.

Keep in mind there is some degree of latitude in rebar placement - they don't have to be exact and 117 perhaps offers what degree is acceptable.

The phi factors in ACI are partially meant to deal with minor variations in "d".

I would most be concerned with the bar cover.

 

[Ron]

ACI 117 places great emphasis on cover, as JAE noted. Unfortunately it does not give very much on specific bar placement. It refers to a "tolerance envelope" and though it doesn't give you the envelope, it implies the envelope is developed by the designer (so you can decide on your tolerance envelope....do it and stick to it). It also states that in the absence of any other directive, follow the CRSI Manual of Practice.

 

[JAE]

So Ron - there's no bar tolerances in 117? I know there are plumbness, levelness, etc. for the concrete. I just thought there were items in there on bars as well.

I'll have to check my books Monday.

 

[JAE]

This short article: Link seems to imply that 117 does have bar tolerances (see the second page center column).

I did just find a technical note from CRSI - they reference ACI 117 a bit.
Check out this CRSI Document: Construction Technical Note CTN–M–1–11
Link here. Looks like you can download it free....might have to create a logon registration.

 

[Ron]

JAE...I could find nothing in 117 that gave specific tolerances for rebar placement such as....horizontal bars shall be placed within +/- 1/2" of plan dimensions, etc. As I noted, they refer to a tolerance envelope, established by the EOR. Section dimensions and cover tolerances are given. Check your references and correct me as necessary...I ain't proud!!

To some degree, CRSI and ACI have a circular reference going. Each refers to the other and neither gives reasonable tolerance limits. See attached CRSI document...this is a free document from CRSI so not a problem posting...

 

[Ron]

Sorry JAE....noticed you referenced the same document from CRSI and their tendency to circular reference....I should read the entire post..!

 

[JAE]

Ha! Circular referencing.

Sort of like:
Architect shows "see structural" on their detail.
Engineer shows "see architectural" on theirs -
both pointing to the same thing.

 

[BigH]

One spec I have seen:

length splice - +- 25mm
variation in reinforcement
starter bars - one bar dia
slabs and walls - 1/4 indicated spacing
beams and columns - +- 5mm
dimensions of bent bars - +- 5mm

 

[shaneelliss]

Thanks for the input. It isn't that I don't have specs, I just have a hard time getting the contractor to meet them. So I was beginning to think maybe the contractors were right and that it was just impossible to get rebar within an inch of where it was supposed to be. Actually, my specs say within 1/2" in most cases. I was wondering if I was alone in getting the contractors to do it well. I appreciate CELinOttawa actually answering the question I asked.

 

[dik]

For slabs and walls, normally it doesn't matter if the bars are several inches out... I usually look at a 4' or 8' length and the correct number of bars should be in place... the concrete doesn't know if a bar is within a couple of inches... bad workmanship, but not critical... maybe other areas where the workmanship is more critical and a bit of a heads up... as noted above, concrete cover is a more critical item... It may not be practical (other than cover) for rebar to be more or less as spec'd.

Dik

 

[TXStructural]

ACI 117 is the correct document and does cover most specific conditions. CRSI provides tolerances for bending of bar but not currently for placing, since ACI 318 provisions consider the 117 tolerances in the design assumptions. If you use 318 for design, and 301 for construction, you should be using 117 for tolerances.

If you spend a fair amount of time in the field, it may be worth getting a copy of (sales alert) the CRSI Placing Reinforcing Bars book. It explains field practices and is the contractor's reference for bar placement.

 

[JimHudson]

shaneelliss, of everything you mentioned that was wrong the most critical one was the coverage of the rebar the tolerance for that is very important, rebar burning the forms is never acceptable and with the attitude that these iron workers have they should be shot and quartered.