PCC under footings

[Steel Inspector]

Here were I work,
We are used to pouring a 10 cm plain concrete layer below footings. It's also required to be 10 cm wider in every direction. We do it under isolated footings, strip footings... almost under all types of footings.

Is it required by ACI? and what is it's role?

Thank you

 

[dik]

I don't know about ACI, but we occasionally use it if the soil is unsuitable. Using this thin layer, you should be able to reduce the concrete cover to your bottom steel.

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

-Dik

 

[TigerGuy]

If the excavated surface is at risk of deterioration due to atmospheric conditions (rain, wind, etc), groundwater seepage, or will be subjected to excessive foot traffic by the rebar or plumbing crew, the sacrificial concrete protects the base of the excavation until the structural concrete can be placed.

 

[HTURKAK]

This question has been worked at several threads . Search eng-tips for already answered questions with word Clear cover, blinding layer, mud layer ..

It is not mandatory requirement at ACI and EC .But it is recommended practice . The following is copy and paste from Reinforced Concrete Mechanics and Design by J.Wight ( Sometimes, the bottom of the excavation for the footing is covered with a lean concrete seal coat, to prevent the bottom from becoming uneven after rainstorms and to give a level surface for placing reinforcement.)
When the blinding layer 10 cm wider in every direction, the ftg side formwork will be placed on plain , better workable surface .




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

NEW ENGLAND MAXIM

 

[ALK2415]

"what is it's role"
at first it contribute to foundation bearing capacity enhancing (since material are replaced cement paste instead of soil)
its major role provide clean workspace for steel installation (cutting, connecting and aligning) ... etc

 

[JAE]

I don't think it is "recommended practice" in most areas I've worked at (throughout the US).
Sometimes it is necessary due to:
1. Soft, irregular bearing soil where foot traffic can further affect the uniformity of the base.
2. Possible oncoming rains or drainage which would affect the load capacity of the soils (check with your geotechnical engineer).
3. If the footings are to be formed, it provides a uniform, solid surface to set forms on.
4. Provides a nice, clean, mud-free place to walk for those contractors that suffer from OCD. (or rather - CDO...alphabetical likes it's supposed to be).

 

[swazimatt]

I believe it is called blinding in other parts of the world. In my experience it's main purpose is to create a clean and stable working platform to build your reinforcing and formwork off of. if you didn't have it you could end up with your reinforcing cages slowly working their way into the dirt as you walk on it and/or the reinforcing getting contaminated with mud etc. it may have added benefits in improving bearing capacities, but it is anecdotal and not included in bearing calculations. I wouldn't say it reduces cover to steel because you will have a cold joint between the blinding and the main structural concrete, but perhaps it is considered to reduce the level of exposure used to determine cover, but it has been awhile since i have done structural work