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?

 

[dik]

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

Dik

 

[dcarr82775]

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

 

[frv]

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.

 

[rowingengineer]

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

 

[Bernied]

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!

 

[SteelPE]

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)?

 

[JAE]

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.

 

[JAE]

...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.

 

[oneintheeye]

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.

 

[vandede427]

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.

 

[SteelPE]

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).

 

[TXStructural]

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.

 

[asixth]

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.

 

[dik]

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

 

[rowingengineer]

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

 

[asixth]

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?