Plain Concrete Design

[CardsFan1]

ACI-318-19 Section 14.5.1.7 states that h shall be taken as 2" less for concrete cast against "soil". Does anyone know if that means that if the concrete is cast against a compacted gravel base, you don't need to reduce by 2"?

 

[phamENG]

I would say yes, you do. As I understand it, the 2" is primarily to account for variations in the excavation surface. You may have a little more control with gravel, but it's still not going to be form work smooth.

 

[dik]

Can you use a 6mil PEVB form? In these environs, compacted granular fill is pretty 'flat'... maybe 1/2" variance...

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

-Dik

 

[CardsFan1]

It is an existing slab that does not mention the existence of vapor barrier.

 

[CardsFan1]

I'm trying to think of a situation where you would have plain concrete that was not cast against the ground. So the term "soil" seems specific.
AND, if I reduce it by 2" then it does not prove out

 

[dik]

then you have no idea of the 'flatness' of the compacted fill... I would suggest the 2" may be appropriate.

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

-Dik

 

[phamENG]

If you read through the plain concrete chapter it describes what it can be used for. Bearing walls and pedestals are the two that would be formed, as well as the sides of a concrete footing if you form those. I agree, though, shallow foundations are the most common use of plain concrete.

Are you going by the concrete strength on an old drawing? If so, it may be less expensive to have a core sample taken and tested to give you a better look at the actual strength. Since it's existing you have no idea how the QA was handled during the pour, I'd be hesitant to take the full thickness.

 

[CardsFan1]

So if I am using the h-2" for the thickness, I assume I need to utilize h-2" to determine bo as well.