concrete cover 1

[Steel Inspector]

hello
ACI 318 specifies minimum concrete cover, is it possible that we might encounter certain instances where a corrosive atmosphere or a severe exposure requires a concrete cover higher than that specified in the code?

also is there anything regarding using different concrete covers for the inner face and out face of a member? like using a higher concrete cover for the outer exposed side of a beam or column than the inner side?

 

[dik]

Definitely. The difference in cover affects the neutral axis. I'm not aware of any papers on the extreme differences in concrete cover.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Steel Inspector]

this is an example in the ICC concrete manual, explaining two different concrete cover valves for internal and external faces of a concrete wall!!!
which is what led me to ask about it? and where in the code could such a thing be mentioned?

 

[Agent666]

Well in the case of a different environment on the inside/outside of a member or on different faces it can result in different exposure on each face, and hence different minimum cover requirements.

For example, in the example you noted for a wall, one face might be an exterior face of the structure subject to the external environment, while the other might only be subject to an internal air-conditioned environment. In this case the minimum cover required by the code may/will be different, so you can set both faces at different covers if you desire.

The other common example would be foundations, the top of the foundation may have a different minimum cover requirement if it was not poured against the natural ground like the base or sides may have been.

For a column or beam I would not expect to see differences as it would be too hard to manage on site potentially, but definitely for walls or precast cladding panels I see it all the time.

https://engineervsheep.com

 

[dik]

SI... the link is broken...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Steel Inspector]

sorry link in not working, i tried a number of times

but it is just like agent666 described.
now that is permissible by the code? in beams and columns you aren't able to do just because it is not practical? it have no negative effect on the member? like it would not make the member unbalanced? cause some sort of eccentricity, specially in columns, walls, and beams?
is there any mentioning of these in the code?

 

[Steel Inspector]

one more question,

i would like to have more clearance ( or specificity ) on the code classification of exposure condition:
- cast against and permanently in contact with the ground
- exposed to weather or in contact with the ground
- not exposed to weather or in contact with the ground.

the first is clear, i have confusion between the two last. my understanding is:
- exposed to weather or in contact with the ground: includes, any member the is in contact with the ground, and any member that is exposed to external weather: like footings, raft foundation, columns on the perimeter of the building, beams on the perimeter of the building, last roof slab if not covered, and similar
- not exposed to weather: internal building members. like internal columns, intrnal beams, internal slabs ...etc

am i correct?

 

[HTURKAK]

Dear Steel Inspector (Structural),

The code specifies the minimum concrete cover and it is not reasonable to use different covers for interior and exterior face of the columns and beams.

I would suggest you to look also CHAPTER 19 durability requirements . When you look to the Exposure categories and classes ( 19.3.1.1 ) what are the categories could be assigned for F,S,W,C ?..

This subject is not only providing minimum cover .( cement type, permeability, concrete class , Corrosion protection of steel ...)

I witnessed protection paint of concrete piperacks, concrete tanks etc in a Northern Europe country. In past , we also painted the concrete elements at marine environment.

 

[Steel Inspector]

dear HTUKAK,
concrete cover is intended to protect the steel, i.e a requirement for reinforcement durability, while the exposure categories in chapter 19 address the durability of concrete itself. two different subjects.

as for different internal and external concrete cover, it is mentioned in a book published by ICC. i don't think they mentioned it by mistake. what i want to know is if it is mentioned in ACI code? or else where with more detail. the example in the book didn't get into details.

 

[dauwerda]

The concrete cover requirements for steel reinforcement is covered in section 20.6 of ACI 318.

i would like to have more clearance ( or specificity ) on the code classification of exposure condition:
- cast against and permanently in contact with the ground
- exposed to weather or in contact with the ground
- not exposed to weather or in contact with the ground.

the first is clear, i have confusion between the two last. my understanding is:
- exposed to weather or in contact with the ground: includes, any member the is in contact with the ground, and any member that is exposed to external weather: like footings, raft foundation, columns on the perimeter of the building, beams on the perimeter of the building, last roof slab if not covered, and similar
- not exposed to weather: internal building members. like internal columns, intrnal beams, internal slabs ...etc

am i correct?

Yes, it sounds like you have a good grasp on it. Keep in mind if the columns, beams, etc are covered by some sort of facade they would not be exposed to weather.

 

[Steel Inspector]

thank you dauwerda

 

[Steel Inspector]

code commentary:
the condition exposed to weather or in contact with ground refers to direct exposure to moisture changes and not just to temperature changes.

can you elaborate plz?

 

[dauwerda]

Here is the full excerpt:

The condition “exposed to weather or in contact with
ground” refers to direct exposure to moisture changes and
not just to temperature changes. Slab soffits are not usually
considered directly exposed unless subject to alternate
wetting and drying, including that due to condensation
conditions or direct leakage from exposed top surface, run
off, or similar effects.

I take it as they are trying to make it clear that "weather" does not just mean freeze-thaw cycles, but also wet-dry cycles. To be considered protected from weather the elements in question would need to be protected from both.

 

[Agent666]

Repeated wetting and drying is worse than say immersing a member in water in terms of durability.

Exposure to weather or contact with the ground means just that. For example the soffit of an exterior slab would only be subject usually to environment temperature change and not necessarily subject to the wetting and drying effects of rain for example.

https://engineervsheep.com

 

[HTURKAK]

Dear Steel Inspector,
I looked to your Internet Country Domain ( AE).. If the question is for that zone, you should consider both durability of concrete and reinforcement.
I have past experience for KSA . If the concrete is buried ( foundation, basement , underground str. etc. ) you should provide not only the clear cover but protect the concrete with membrane waterproofing, bitumen epoxy paint etc.
For the exterior concrete elements , you may protect with cladding, painting etc.

If the question is for a specific project, you may provide some details to get more specific responds.

Good Luck ..

 

[Steel Inspector]

dauwerda, Agent666
thank you.

HTURKAK,
thank you, i didn't say i shouldn't consider both. i said that my question is only regarding concrete cover.

yes we have high levels of sulfate concentration in soil. but that does not directly effect steel corrosion, it effects concrete first.
sure when concrete detoriorates then steel is left unprotected. but when addressing sulfate or freezing and thawing for example, the code doesn't require any special concrete cover, instead it requires better concrete. the reasoning for that in my opinion is that it is meaningless to request more cover when you know that concrete it self will deteriorate. the right thing would be to improve concrete itself to be able to withstand that exposure.