Concrete Coverage Maximum 1

[INCORANGER]

Is there code or standard practice that regulates maximum coverage between rebar and forms. I know ACI 318 specifies minimums but I do not see any maximums.

 

[DTGT2002]

The tolerances for cover are now contained in ACI 117-06. Only the tolerance for the "d" dimension remains in ACI-318-05 to the best of my knowledge.

In ACI 117-06, the tolerances for cover are give only with a negative sign, indicating that too little cover violates the tolerance, while too much cover does not.

Depending on which side of the arguement you are standing on, you may or may not want to press further into 117.

As a responsible engineer, you would push on and find that in section 4.5 - deviations from cross sectional dimesnions are presented and here you might find more useful information which will limit how much extra cover could be provided based on not exceeding the tolerance allowed for increasing thickness.

The painful part of all this is the fact that while a reasonable amount of concrete could be placed without violating the dimensional tolerances, there is no simple way to check your "structural" tolerance - that is, how much added concrete is too much at the point it impacts the self dead load of the structure etc.

Hope this helps, the "new" ACI 117-06 is worth buying, in my mind, as I feel it clarifies many aspects of the old document.

Good luck,

Daniel Toon

 

[INCORANGER]

Thanks Daniel,

I am the field engineer for a construction company. We have been told we must keep the spacing between 2"-3" rebar to formwork. We have had spots where there was 4". All are foundations 3' below grade. I just felt that 2"-3" was asking a lot.

 

[msquared48]

I think you are touching on the crack control issue here and how much cracking you are willing to see at the surface of the concrete. That is covered by ACI too.

Mike McCann
McCann Engineering

 

[civilperson]

The maximum cover effects the crack size, the minimum clear cover may be increased in harsh and corrosive environments. An occasional extra inch can be tolerated in clear cover except by overly anal building inspectors. The crack control issue should not apply to below grade concrete.

 

[DTGT2002]

While excessive cover does not violate the cover tolerance, it could violate the tolerances for geometry of the member.

What value is provided in the drawings for the required cover?

Daniel

 

[INCORANGER]

No value on drawings... General requirements just refer to ACI 318. That is the problem, on-site QC hard to get along with. We will comply this time but I will definitly have it ironed out before we start the next time.

Thanks for all the help

 

[DarthSoilsGuy]

assuming the bearing surface is cast against earth, 4" cover does not sound bad at all. Or we're all in for some major trouble.

 

[INCORANGER]

Darth,

We used EFCO forms but the QC guy thinks we are building a watch. I think you are only in trouble if this guy ends up on your jobsite.

 

[DTGT2002]

If there drawings and/or specs don't call for a cover requirement, by what basis is the QA reviewing work?

For structural members, you are fighting many dimensional issues, but the most basic, to me, is the struggle between d, the distance from the center of steel to the end of the compression area versus the acceptable cover.

Too much cover can reduce d and reduce the flexural capacity of the member. Too little cover and the rebar is compromised and d is out the window if there is no steel to resist tension.

I have worked with designs in the recent past where the layout and geometry would not allow for the construction tolerances of ACI and these restrictions were not illustrated in the structural drawings.

The tolerances we are provided by ACI try to strike a common point between what is acceptable in the design community and what is consistently possible in the field with reasonable means. The definition of reasonable is hard to come by, but with a reputable contractor and a smart owner/design team, a good relatioship should be possible. Most of the time. Or at least some of the time.

Best of luck,

Daniel Toon