Reinforcement cover 3

[drewtheengineer]

According to ACI, concrete cast against and permanently exposed to earth is to have a minimum cover of 3" (ACI 7.7.1). Does this apply when placing concrete on crushed stone? I cannot find any further commentary on this in ACI nor any other text. What is the common practice?

 

[AseismicGuy]

THe main idea for keeping this thickness is to prevent the reinforcement from moisture, water or some undesired chemicals/salts in soil. Even ona crushed stone layer this thickness should be kept at least 3", I think..Bcs crushed stone is a porous media that does not keep water or other staff away from the concrete surface..

 

[Kramer]

You brought up a good question. Section 1907.7.1 of the 1997 UBC requires 3" of concrete cover for "concrete cast against and permanently exposed to earth". However for formed concrete the cover required is either 1 1/2" or 2" depending on the size of the reinforcing bar. That suggests to me that a portion of the 3" cover requirement is because it is more difficult to assure proper bar placement when you are casting against grade which is usually irregular. This becomes important when you are designing structural slabs on grade. If you are trying to get two mats of steel in the slab, providing 3" of cover to the bottom mat seems overly conservative when you are placing the concrete on a uniform rock base surface.
What does everybody think?

 

[JAE]

If you've ever seen a concrete slab or footing constructed on earth (whether clay, sand, or gravel) you know that there is a natural irregularity that only the most precise contractor can minimize.

In addition to the basic irregularity of the earth, there is also the potential for the reinforcing to "loose" some of its cover due to:

1. Pumping of the earth due to wheel or foot traffic.
2. Sinking of chairs into the earth (concrete puddlers typically walk on the rebar, driving the chairs into the ground.
3. Rocking of chairs due to the irregularity.
4. Movement of the subgrade due to concrete, in mass, flowing onto the earth.


All of the above are more critical to the rebar in that the "exposed to earth" leaves the rebar open to moisture fluctuations that ....well, you know what that does to steel.

 

[Ron]

One fundamental difference in the concrete itself when cast against earth as compared to a form is the paste density at the interface. When cast against earth, there is a natural migration of moisture out of the concrete and into the soil. This allows for a greater porosity of the interface surface, thus allowing a greater return of moisture back into the concrete.

When cast against a form, there is no migration of moisture outward and the paste is comparatively dense, thus inhibiting inward migration of future moisture.

 

[JAE]

Good point!

 

[Kramer]

Those are all good points! Let's take a common situation where you want to upgrade from a 4" (non-structural) slab on grade with welded wire fabric (which doesn't have 3" of cover), to a little thicker slab (say 5" thick) with #3 or #4 bars. This slab might be in a retail store or a residential garage. The reason I might want to put in this stronger slab is because of the presence of mildly expansive soils. In this case I want the steel at mid-depth of the slab (that's where the welded wire fabric was going - theoretically!) to maximize bending resistance for both negative and positive moment. Since the slab is defined as being "non-structural" to start with, is it necessary to provide 3" of cover at the bottom?

 

[JAE]

I would still provide the 3" cover. Typically, we try to place the steel as high up as we can - 3/4" cover on the top, say - to assist in keeping cracks as closed as possible on the top surface (which is the focus of the steel in the first place).

In an expansive clay situation, your 5" slab reinforcing is not going to really help in adding stiffness to your slab to help in structural behavior during shrink/heave cycles, especially if its in the middle. I would still want the rebar in the top to keep cracks closed on the surface, knowing I've got larger crack widths developing in the lower regions.

If I want to stiffen the slab to minimize localized heave, I would use a stiffened slab made up of monolithic concrete ribs, or beams, in a grid pattern under the slab.

 

[Ron]

I agree with JAE...keep the steel in the middle third of the slab, as close to the top of that range as practicable. Also, keep in mind that moisture drive for the slab might be upward, and that capillarity will provide a lot of water to the bottom of the slab in clayey soils. You might want to consider a capillary barrier (fine gravel or sand with a fineness modulus greater than about 2.30)