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Structural slab question

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?
 

RE: Structural slab question

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

 

RE: Structural slab question

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".

RE: Structural slab question

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.   

RE: Structural slab question

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

RE: Structural slab question

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?

RE: Structural slab question

(OP)
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.
 

RE: Structural slab question

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.

RE: Structural slab question

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).   

RE: Structural slab question

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".   

RE: Structural slab question

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

RE: Structural slab question

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.

RE: Structural slab question

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?

RE: Structural slab question

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.

RE: Structural slab question

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.

RE: Structural slab question

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".

RE: Structural slab question

(OP)
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.

RE: Structural slab question

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?   

RE: Structural slab question

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.

 

RE: Structural slab question

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.

RE: Structural slab question

If a structured slab, then I'd use voidform and cover to a formed surface, not earth.

Dik

RE: Structural slab question

If it is on a void form I would use 1.5" or 2" depending on bar size

RE: Structural slab question

I remember one of my professors mentioning that the additional 1" requirement (2" vs. 3") of cover for concrete cast against earth had more to do with the inability to properly chair the rebar..  

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

I would only use void form if the soil below was clay and expansion was an issue. Typically I put two layers plastic sheeting down and reduce my cover.

An expert is a man who has made all the mistakes which can be made in a very narrow field

RE: Structural slab question

I find most decent GC's use concrete bricks to hold the required cover to slab bottom reinforcement. This seems to eliminate the possibility of slab bolsters from punching through the subgrade or vapor barrier.

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

(OP)
After reviewing some other drawings I have found in the office I have decided to use a 3" clear for my bottom reinforcing.

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

I've used the full 3" cover for determining d for the top bars.  I think it is fair to say that it averages out.   

RE: Structural slab question

...plus - the φ factor is intended to account for minor variations in the strength parameters, which includes variations of d.  

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

I would say your h will be actual thickness minus bottom cover and the eff d is h minus top cover minus half diamater. Since the concrete section starts at the top of the pile head for hogging. This is only applicable for a very small region however around the pile so I would not deduct it and use the d based on the thickness minus top cover minus half diamater.  

RE: Structural slab question

did anybody ever actually answer this question, or did we just get off on a tangent about voidforms and Appendix D?

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

(OP)
I work in an office where I have the luxury of seeing drawings from other engineers.  I was able to locate a job with a pile foundation.  They used 3" as cover for their strucutal slab on grade. So after reading the opinions on this forum and finding another set of drawings I decided a 3" cover was best.  I also detailed my vapor barrier below a compacted sand/grave layer.

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

If it is structural concrete, as opposed to slabs intended to remain on grade, and is cast against soil or permanent fill, the requirement is 3 inches, because:  1) soil formed surfaces are ALWAYS irregular, 2) bar supports may embed into the soil reducing minimum cover (use sand chairs or precast blocks), 3) soil carries moisture to the concrete.  The cover is intended to reduce corrosion rates, therefore maintaining adequate cover is imperative where it is cast against and remaining in contact with soil.

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

Footing beams I always provide a minimum of 2" clear cover. How does everyone find using void forms. I had a really bad experience with cardboard void formers that literally did not hold there form and the void collapsed 5" between placing the concrete and the concrete setting. I will post photos of the construction when they come to hand.

Saline environments I would use an additional 1" cover to slow down corrosion.  

RE: Structural slab question

one of our clients has a 3" cover requirement as well as galvanised bars for all reinforcing... due to the high chloride content...

RE: Structural slab question

asixth,
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

The product used was bildavoid. It was more site practice than the quality of the product, I'm sure this particular manufacturer has compliance certificates that state the product had been sufficiently tested. I cannot say too much at the moment for privacy purposes but I do intend on writing a post in the future regarding what happened for reference to future engineers. Too many structural failures are swept under the rug. This is a great resource to collate data to better further the standard of engineering.

Has anyone else had a situation where a cardboard void began to degrade sooner than expected?

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