Structural slab question
Structural slab question
(OP)
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?
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?






RE: Structural slab question
RE: Structural slab question
RE: Structural slab question
RE: Structural slab question
RE: Structural slab question
RE: Structural slab question
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.
RE: Structural slab question
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.
RE: Structural slab question
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).
RE: Structural slab question
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".
RE: Structural slab question
RE: Structural slab question
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.
RE: Structural slab question
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?
RE: Structural slab question
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.
RE: Structural slab question
RE: Structural slab question
RE: Structural slab question
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.
RE: Structural slab question
"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?
RE: Structural slab question
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.
RE: Structural slab question
RE: Structural slab question
Dik
RE: Structural slab question
RE: Structural slab question
In other words, ACI requires 3" when it's cast against earth because you specify 3 and you will get somewhere between 2 and 3. In this context, I'd have to agree that it makes no difference whether you are casting against soil or crushed rock.
RE: Structural slab question
An expert is a man who has made all the mistakes which can be made in a very narrow field
RE: Structural slab question
If the slab then settles under it's own self weight you probably haven't done a good enough compaction job!
RE: Structural slab question
Now another silly question, when sizing the top reinforcing bars (for negative moment over the support) should calculate d using the full depth of the slab minus ¾" cover (for top bars) or should I also include the bottom cover as well (since this concrete may mix with dirt reducing it's capacity)?
RE: Structural slab question
RE: Structural slab question
With a narros strutcural slab, with a small d, the variations in the cover would have a greater effect on d than on a very thick member. So perhaps I'd keep that in mind when choosing what value of d to actually use.
RE: Structural slab question
RE: Structural slab question
you're either going to get 3.625" or 2.25" of cover, depending on which way they turn the brick.
RE: Structural slab question
I did happen to have the book that Bernied was talking about. It was pretty interesting how the author made the argument for using a smaller cover but he pretty much left it up to the engineer to make the decision.
In terms of reducing eff d for the top reinforcing, I decided to take an extra inch off my d to account for earth mixing with the concrete with the sand/gravel. This decision was based more off my judgment that any written text (maybe a bad idea).
RE: Structural slab question
Alternative means of protecting the concrete are permitted by 318. If the barrier (even a plastic sheet barrier) is adequate to stop migration of moisture up to the concrete surface, and will not act as a collector of moisture holding it against the concrete surface, it may reduce the required cover to 2 inches or less.
Since you are on piles, it is NOT a slab on grade, since it will need to span between the piles if there is subsidence - one reason for pile supports. This begs the question of what kind of soil and moisture will be present. If it is well-drained sand, in arid conditions, less cover might be fine. If this is a coastal or other wet/salty environment, more cover or other protection might be needed.
If the soil is clay, is it wise to cast against soil without void forms? If the geotech includes assessment of PVR in the report for the soil, you may need voids.
There is latitude for engineering judgment here, and part of that is determining how much protection from corrosion is sufficient.
RE: Structural slab question
Saline environments I would use an additional 1" cover to slow down corrosion.
RE: Structural slab question
RE: Structural slab question
are you willing to cough up the void formers name?
An expert is a man who has made all the mistakes which can be made in a very narrow field
RE: Structural slab question
Has anyone else had a situation where a cardboard void began to degrade sooner than expected?