Structural slab question 1

[SteelPE]

I am designing a pile foundation (my first) that has a one-way slab. In reference to the bottom reinforcing bars, how much concrete protection should these bars have? I believe they should have 3” of cover since they are cast and permanently exposed to the earth.

A coworker is saying that ACI 318 does not apply in this instance and that a cover of 1”-1.25” is common. What are some other opinions?

 

[vandede427]

the cover requirements are independent of how the slab is supported. don't listen to your co-worker.

 

[Lion06]

If the cover is making your slab too thick, you can spec a crushed stone base. This is not earth and you can justify a smaller cover than 3".

 

[cancmm]

Agreed. Use 3 inches at a minimum in this instance. When you have larger coverages (>3"), be sure to compensate be reducing your bar spacing accordingly. Larger coverage reduces the allowable maximum spacing.

 

[vandede427]

I disagree S-EIT. Earth consists of soil, rocks, tree roots, earthworms, etc.

 

[miecz]

We place the bottom mat 3 inches clear above the top of the piles, as shown in the CRSI Handbook. Sounds like most of you guys weave your bottom mat around the piles?

 

[SteelPE]

Miecz, this is for a one-way slab which is supported by grade beams. We will be placing the reinforcing 3” above the piles in the caps.

The argument that he is making is in regards to how the surface is prepared. He is saying that the material beneath the surface is a prepared subgrade which is properly graded.

 

[miecz]

SteelPE

I believe the prepared subgrade argument supports reducing the cover in a spread footing (no piles) from 3 inches to 2 inches. I don't think this argument applies to pile caps unless you are placing your reinforcing 2 or 3 inches from the bottom of the cap, as opposed to the top of the pile.

 

[Lion06]

vandede-

I would agree that earth does consist of everything you mention, but if you have a prepared subgrade of drainable material.......... I think that is not what ACI had in mind for "cast against and permanently exposed to earth).

 

[Lion06]

vandede-

I would agree that earth does consist of everything you mention, but if you have a prepared subgrade of drainable material.......... I think that is not what ACI had in mind for "cast against and permanently exposed to earth".

 

[vandede427]

where in ACI does it distinguish between soil and aggregate base?

 

[Zambo]

I'm not from the US so don't usually work to ACI 318. I am used to working on piles and pilecaps where the minimum cover is 75mm. But I have never seen this much cover specified for the underside of a slab.

If it was a requirement I would prefer to use 30-50mm blinding concrete (say grade 15Mpa) This will give a good working surface and control the levels well so that the wastage of structural concrete is minimised. I then think 30mm cover would be acceptable.

 

[CTW]

I recall the 3" requirement for cast against earth being explained to me as when concrete is placed directly on soil there is a greater possibility that the soil will mix with the concrete and this area of concrete will be of a lesser quality than the rest of the concrete.

I guess even with a prepared base, there's always the chance of the base material mixing with the concrete.

I've always used the same philosophy as StructalEIT as I've always considered earth to equal soil. But vandede427 makes a logical argument.

Anyone know if ACI has addressed this before and been more specific?

 

[Bernied]

In "Designing Floor Slabs On Grade" (jointly authored by Boyd C. Ringo - past chairman of ACI Committee 360)- Commentary to Section 3.8 it states that the 3" cover for concrete poured on soil is not required.

Unlike a footing which is often poured directly on soil, the subgrade being used as a form to support the slab-on-piles has to be prepared to prevent moisture from penetrating the slab. This is usually a compacted gravel intended to drain to the site drainage system.

A 1" concrete cover is suggested, or maybe 1 1/4" to account for the 1/4" tolerance on the thickness of the slab.

 

[vandede427]

I didn't mean my last comment in a negative way as in "prove it." I'm just not aware of any distinction in ACI.

 

[ron9876]

3" cover when soil can mix with the concrete during placing/vibration. When you place a vapor barrier under the slab it is no longer cast directly on the soil. I usually use a little extra over 3/4"/1" due to difficulty of getting soil level--maybe 1 1/2".

 

[SteelPE]

ron9876

Couldn't placing a vapor barrier directly under the slab cause problems with curling as the concrete dries/cures? I guess a way to avoid that is to make sure the slab is wet cured.... only problem is I don't know what time of year this slab is going to be poured.

 

[bones206]

From ACI 318 Section 7.7 commentary:

"The condition “concrete surfaces exposed to earth or
weather” refers to direct exposure to moisture changes and
not just to temperature changes....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."

Can a prepared crushed stone base be considered equivalent protection to 2" of concrete? Depends on the inspector?

 

[Ron]

The culprit, whether it is "earth" or graded aggregate, is water absorption and exposure to wetting from such contact. In that context, it makes no difference whether it is "earth" or graded aggregate....the wetting is what it is being protected from.

One reason for the difference in cover when concrete is cast against earth or graded aggregate is the open texture in the concrete that results. That's why formed concrete requires less cover...the resulting surface of the concrete has a more closed texture and higher surface density, thus less opportunity for water and other chemicals to affect the rebar.

 

[Bernied]

SteelPE: I wouldn't be concerned about curling of a structural slab tied to grade beams. Slabs-on-Grade can give you curling problems which may indeed be enhanced by putting a vapor barrier directly beneath them.