Rebar Placement Tolerance

[Greatone76]

I have a foundation plan that has a foundation that has a top and bottom rebar mat. The plan just says that the rebar has to be minimum of 3" of clear cover. I have the contractor telling me that the dimension is just a minimum, so they have the top mat as far as 6" from the top of the foundation. Since ACI-117 only had a minus tolerance for clear cover, how do I get the contractor to place the reinforcements where I'm sure it was designed to be 3" from the top? Is there something else in ACI-117 that covers additional cover? Is there another industry standard that I can use for rebar placement?

 

[hokie66]

There should be different requirements for the top and bottom mats. What type foundation is it? If there is flexural tension in the top, the bars should certainly be located closer than 6" from the top. Why don't you ask the engineer who designed the footing or slab to sort this out?

 

[JAE]

Simply check with the engineer/designer on the mat to see if a reduced "d" for the top steel is acceptable.



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[BigH]

Does your specification not have a tolerance for placement of steel? Some require that steel be placed within 5 or 10 mm of its intended location; there is also placement tolerances bar to bar. This should all be detailed on the bar bending schedule.

For back-up, see ACI 117-90 reapproved 2002 (this is the latest that I have). Section 2.2 covers placement of reinforcing bars.

 

[Canuck67]

As a general rule, concrete cover should not be specified as "minimum concrete cover", it should be specified as "concrete cover".

Excerpt from the standard in my jurisdiction re placement tolerances for steel reinforcement: "(a) concrete cover: ±12 mm (however, the concrete cover shall in no case be reduced by more than 1/3
of the specified cover);" Note that there are several other tolerances for items such as beam reinf., hooks, etc....

 

[TXStructural]

If you are using ACI 318 for design, ACI 301 is the governing spec, unless you state something else. They are developed to work together. ACI 117 is a consolidation of the permitted tolerances from these documents. Cover in 318 and 301 are "specified cover", not minimum. The minimum is specified cover less the tolerance, which generally results in something on the order of 2/3 of specified.

ACI 301-10 contains the provision:
3.3.2.3 Concrete cover—Unless otherwise specified,
concrete cover for reinforcement shall conform to Table 3.3.2.3.
Concrete cover tolerances shall comply with ACI 117.
Position tie wire ends away from exposed concrete surfaces.

As far as maximum cover, there is no such value. The critical dimension is obviously flexural depth (d), and adding more cover in footings simply provides better protection from moisture. There is some disagreement between designers about how thick is too thick for cover. The concern is that at some point, one could experience spalling of the cover as a unit - principally due to shrinkage or thermal effects.

It is common to fill over-excavations with concrete, resulting in "excess" cover under or to the side of reinforcement. There is generally no problem doing this.

 

[flight7]

ACI 117-10 also has provisions for reinforcement location (as opposed to cover):

2.2.1 Placement of nonprestressed reinforcement
When member depth (or thickness) is 4 in. or less.... +/- 1/4"
When member depth (or thickness) is over 4 in. and not over 12 in.... +/- 3/8"
When member depth (or thickness) is over 12 in.... +/- 1/2"

Unless the different provisions are mutually contradictory, the contractor needs to meet both the location requirement and the cover requirement. Six inches cover is way too much unless the member was designed for it.

 

[darthsoilsguy2]

Using that logic, Johnny Contractor could have tied the two mats together and had them both just 3" from the top.
There is a saying about wasted time making plans idiot-proof that is applicable here.
the contractor either knows he is wrong and is being a ####
or
the contractor has a dangerous base assumption that needs to be corrected, if not for this job, the next.

send an email to the Structural and CC the contractor and your client and ask the Structural to clarify. Tell the SE that you understand these bars are supposed to be at the bottom from reading the plans.... OR since this is probably cast.... write a discrepancy notice or some other written document to communicate the work was not installed per plans for the SE to review. Whatever it is... do it now if not done already. If you get some blowback on it... just let whoever know that you couldn't believe it was the SE's intent to let the contractor put it anywhere they wanted to and that you are not a REFEREE for a Game, but an Inspector for Building Safety.

 

[darthsoilsguy2]

Now if you are the SE, or work for the SE...

i'm not disagreeing with the people have said that the language shouldn't say minimum here in this thread.... but believe me... there are many many many SEs out there that use that language, and it is enforced as the specified cover. so there is industry standard in your toolbelt... and... yes you could change those standard details for the next one.