Concrete cover to rebar

[ajk1]

CSA A23.1 specifies the minimum required concrete cover to rebar, for corrosion protection, based on the exposure classification of the concrete specified for the element. For example, if the specified concrete exposure classification for a concrete wall is CSA C-1, then the minimum required concrete cover is 60 mm for the rebar in both faces of the wall, even though only one face of the wall may be exposed to chlorides.

Question:

Which ACI Standard gives the minimum required corrosion protection to rebar, and how is it specified? Is it based upon the exposure that the surface is exposed to (example exposed to weather requires a certain cover, etc.)?

 

[dik]

From a website:

http://richardson.eng.ua.edu/Former_Courses/CE_331_fa10/Notes/Beams/Concrete_Cover.pdf

Dik

 

[TehMightyEngineer]

ACI 318 is the primary reference for clear cover, the two main variables will be exposure and whether the concrete is cast-in-place or precast.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[ajk1]

Thank you both.

dik - Is what you sent the latest edition of ACI? If not, do you know which year edition that it was?

Thanks.

 

[dik]

Attached is 2005 edition... I may have a later ACI edition at the office.

Dik

 

[ajk1]

Dik - thanks very much. Much appreciated. I have ACI 318 in the office but not here at home where I am in my retirement most of the time, so your attachment is just what I need.

 

[briancpotter]

FYI, these values are the same in the latest version of ACI (318-14), though they've confusingly rearranged everything.

Brian C Potter, PE
Simple Supports - Back at it again with the engineering blog.

 

[dik]

brian... you pay good money for these books... there's money to be made in publishing... Who would buy the 'same' book, if it were identical to the last one you bought...

Dik

 

[ajk1]

The CSA A23.1 Standard which governs work here in Canada specifies the cover in terms of the "CSA exposure classification" of the concrete in the element. For example, a wall with exposure Classification C-1 concrete would require 60 mm cover to all the rebar in the wall, even though only one face of the wall might be exposed to corrosive elements. A318 wording is perhaps a little better, but I think that the best wording was in the very old Codes (such as NBC 1965) which specified the required cover in terms of the "surface" exposed to the environment. So for example, a perimeter wall of a building with one surface exposed to the exterior and the other surface in the conditioned space of the building, would have different covers required for the interior and exterior face rebar. I do nt think the current CSA way of specifying cover based on the exposure classifcation of the concrete is good..it is overly conservative in some cases. Even the ACI wording sees not as clear as it could be. Any comment?

 

[briancpotter]

The ACI wording is a little confusing, for the exact reasons you say - the cover is defined in terms of "members", even though a single member will have different exposures on different surfaces. Beyond this, "exposed to weather" is sort of doing a lot of heavy lifting. It's never been quite clear to me if something on the outside face of a building, but covered with some surface (roofing, etc.) counts as exposed to weather or not.

It'd be nice if it were clearer, as small details like this have a big impact in how thick your concrete members have to be.

Brian C Potter, PE
Simple Supports - Back at it again with the engineering blog.

 

[briancpotter]

Also, if your concrete is going to be used in an environmental application (water treatment plant, etc.), it'll be governed by ACI 350, which has an additional set of (more burdensome) cover requirements. I'm not familiar with marine applications, but I assume the code for that (ACI 357) also has some fairly beefy cover requirements.

Brian C Potter, PE
Simple Supports - Back at it again with the engineering blog.

 

[ajk1]

briancpotter - thanks for your comments. I looked up my old 1963 edition of ACI 318 and it specified the cover in terms of the "surface": "Section 808 (b) The concrete protective cover for any reinforcement at surfaces not exposed directly to the ground or weather shall be not less than...". So it appears that the National Building Code of Canada (NBC) used to follow the wording of ACI, but for some reason it appears that both NBC and ACI dropped the important word "Surfaces". I don't know why. Perhaps the thinking was that if a member was exposed to an environment that would cause corrosion of reinforcement on one face, the corrosive agent would eventually work its way through to the other face, either by leakage through cracks or by permeation through the concrete. That is probably true for many members such as floor slabs, but is not true of all members, walls being a good example of where a lesser cover could safely be used on the side of the wall that is in the conditioned building space. If no one on this forum can think of an answer, then I should write to ACI. Seems strange though that this has not been picked up...am I missing something?