Concrete cover 2

[Tstruct]

Referring to ACI 318-19 table 20.5.1.3.1 or any older version with respective clause regarding concrete cover, I have few questions:
1. What is the difference between the two types of exposures namely "CAST AGAINST AND PERMANENTLY IN CONTACT WITH GROUND" and "IN CONTACT WITH GROUND"? Which member lies in which exposure category?
2. Generally foundations don't directly rest on ground rather on lean concrete, should I consider foundation bottom exposure as not in contact with ground?

 

[WesternJeb]

If you have a stem wall, it is usually cast between forms but then backfilled around it later after the forms are removed. This is an example of "in contact with ground" but not "cast against ground".

Wall footings or Mat foundations are cast against and permanently exposed to earth.

I am not too familiar with lean concrete mats under foundations, are you in a high frost depth area? I would still specify cast against and permanently in contact with ground to be conservative but I am not sure what the standard is for this.

 

[milkshakelake]

Wall footings or Mat foundations are cast against and permanently exposed to earth.

Where I practice, wall footings are in contact with ground, not cast against and in contact with ground.

I think that the reason more cover is needed for "cast against ground" is because the ground might be uneven, which would have disparity in values of cover. To encompass for tolerance in ground level, the cover is increased by an inch, because that's the expected variation. So if it's cast against lean concrete, it would be an even surface and I'd consider it "in contact with ground." I recommend OP to read the commentary in case it sheds some light on this.

 

[HTURKAK]

WesternJeb (Structural) explained "in contact with ground" and "cast against ground" . I would like to add that "cast against ground" in general used for underground structures ( e.g. thrust blocks ..) if the foundation is cast directly on the ground the same cover (3 in ) should be provided .

In case of lean concrete seal coat , ( blinding layer ) this will give a level surface for placing reinforcement and protect reinf. from dirt .The term ' blinding layer' is meaningful that is , blinding the surface to get a clean working environment. In German , this is called 'Sauberkeitsschicht' which could be translated as ' cleanliness layer '. But if the foundation has an insulation layer ( so water cannot raise with capillary action, and concrete protected from sulphate attack ), my opinion is cover 2 in . is OK .

I would like to remind Clause 14.5.1.7,
( When calculating member strength in flexure , combined flexure and axial load , or shear the entire croo section shall be considered in design, except for concrete cast against soil where overall thickness h shall be taken as
where overall thickness h shall be taken as 2in. less than the specified thickness .).

I would like to hear the thoughts of others on this subject.







Use it up, wear it out;
Make it do, or do without.

NEW ENGLAND MAXIM

 

[WesternJeb]

HTURKAK, I appreciate the intent of respect but please remember that not all of those on this forum are males.

 

[dold]

Where I practice, wall footings are in contact with ground, not cast against and in contact with ground.

Ditto but that only applies to the vertical edges of the footing. But the bottom face of the footing is surely cast against ground, no? Unless you're using a mudmat (lean concrete, etc) underneath, in which case I'd agree with HTURKAK's assessment for 2" cover there.

 

[milkshakelake]

But if the foundation has an insulation layer ( so water cannot raise with capillary action, and concrete protected from sulphate attack ), my opinion is cover 2 in . is OK .

I agree. This is the code commentary:

Alternative methods of protecting the reinforcement from weather may be provided if they are equivalent to the additional concrete cover required by the Code. When approved by the building official under the provisions of 1.4,
reinforcement with alternative protection from the weather may have concrete cover not less than the cover required for reinforcement not exposed to weather.

I also agree about not assuming male/female; I fall into that myself.

 

[milkshakelake]

Ditto but that only applies to the vertical edges of the footing.

Agreed. Vertical edges in contact with ground, including the vertical edge of a strip footing. Horizontal edges meeting soil are cast against ground.

 

[dik]

I often use a sheet of extruded polystyrene as a layer to avoid this 'cast against the ground'.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[WesternJeb]

Dik, this is common for slabs on ground in my locale. Not common for footings.

 

[dik]

Concur... but have occasionally used DOW Hi60 under footings to protect against frost heave...

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[HTURKAK]

Dear WESTERNJEB ,

Thank you for your warning and i have edited my respond . English is not my first language and correct me if i am wrong ,but AFAIK, he, his mr . could be used non gender specific . Perhaps my intention to use Mr . from other languages .Any gender discrimination at least could not be my thought.

By the way, i liked your and Milshakes's posts and given BIG PINK STARS..




Use it up, wear it out;
Make it do, or do without.

NEW ENGLAND MAXIM

 

[Tstruct]

I am not too familiar with lean concrete mats under foundations, are you in a high frost depth area?

No, I am not in frost area. But it is common practice here to provide 6inch thick lean concrete under every foundation. So for us, nothing is cast directly against ground.

Thank you everyone for your very helpful responses. This forum and experienced professionals here have been really very helpful specially for young engineers. Thanks again.

 

[BulbTheBuilder]

I am not particularly sure about this, but European code or Western world (excluding North America) recommends a 2" (50mm) blinding layer before casting foundations.

 

[dik]

All my use of 'guys', 'gentlemen', etc. are binary, for political correctness.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[milkshakelake]

@dik Maybe something like "fellow engineers" would have less ambiguity? We stopped with Mr. and Ms./Mrs. in emails to clients, since we had an incident where a client got offended. I'm not either one myself and don't get offended either way, but some people take it seriously.

 

[dik]

I got some eMail from a local plan examiner a couple of weeks back signed (he/she)... my response included (he/haw), and I'm not a redneck.

I don't intentionally offend people (my late wife always said that I lacked tact). I'm old and wobbly and did not grow up in a politically sensitive time period, and I have no intention on changing. Taking it seriously can definitely lead to stress.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[dold]

I got some eMail from a local plan examiner a couple of weeks back signed (he/she)... my response included (he/haw), and I'm not a redneck.

That seems like a fantastic way to have the plans examiner throw the book at you.

I don't intentionally offend people...

You sure about that? lol.

 

[lexpatrie]

Every footing detail I've ever seen has 3" cover. Bottom and side. Look at CRSI tables, too.

CRSI 1957 from Archive.org