Concrete Cover to Bottom Reinforcing 2

[dik]

I've gone through a bunch of messages on the site regarding concrete cover to bottom reinforcing.

The reasons given for increasing cover are:
-irregularity of soil surface,
-intrusion of granular base material into slab,
-the possible loss of water to the granular base, having undesirable effects, and
-the contact with a 'wetted' soil will cause the concrete to be saturated.

My comments on the above reasons:
I've not encountered irregularities of the soil; it's usually compacted smooth,
The PEVB prevents intrusion of the granular base into the concrete, and,
The concrete cover to the side allows water into the hardened concrete, so, the 3" clear cover with 1-1/2" side cover serves no purpose for moisture.

I had a recent project where the owner cast a 2' thick slab on grade (to keep weeds down) and then constructed a deck over it. Some of the supports were on screw piles that were sleeved so that there was no load applied. Some of the supports were on the 2' thick slab. Getting the maximum plain concrete moment resistance, I used the full thickness of the slab since there was a PEVB between the slab and fill. Canadian codes require that you ignore the bottom 2" if cast against soil. I chose not to.

I've another project where the excessive bottom cover is posing problems with exposure crack widths. The grade beam will be cast on 'foam void form'. I'm planning to use 2" cover which works for exterior exposure, and, was thinking that 1-1/2" (similar to the sides) could be appropriate.

Any comments would be appreciated...

Dik

 

[KootK]

I've not encountered irregularities of the soil; it's usually compacted smooth,

I question this part. Smooth, maybe. Level and flat like formwork? I don't see it.

I've only been reducing cover when there's mud slab involved.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[dik]

KootK:

darn near flat... differences might be 1/4" max, not 1-1/2" max. If you are using a void form... the 1/4" 'disappears'. Any other suggestions (not being sarky)?

Dik

 

[KootK]

Any other suggestions (not being sarky)?

- Certainly, I agree that there's space here for one to apply engineering judgment as you've done and clearly plan to continue doing.

- I don't feel that vapor barriers or void forms get you anything that I'd consider water proof. As such, I like 1.5"/2" as a min cover number depending on bar size.

- I know that codes around the world differ quite wildly with regard to the amount of cover required for passivation. And I believe that north american codes come in fairly conservative on that spectrum.

- I don't feel that dirt face crack control is a legitimate issue here. Rationally, as long as you have crack control at the depth of the cover required for passivization, I see that as good enough. I'm not sure that I care too much if the sacrificial depths spall apart under freeze / thaw action.

- I personally question whether or not this is the right place to be fishing for extra economies. As you're fond of saying, that game's mostly about intelligent system selection. Still though, if this is a hot button issue for a client that you want to retain, so be it.



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[dik]

I've always wondered why things were done... and, this is just another of my windmills.

Dik

 

[Settingsun]

FWIW/for comparison, the Australian code has concrete cast against non-aggressive soils or concrete on ground protected by membrane in the lowest exposure category with a basic cover requirement of 20mm. If cast directly against ground you add a further 20mm; cast against membrane requires an additional 10mm. The commentary gives only surface irregularity as the reason for this, discounted in the membrane case due to the additional protection from the membrane. These additional cover depths apply to any face against the ground, so the sides as well as the base. I gather from your post that the Canadian code is a bit illogical/inconsistent in this respect.

A very senior engineer once cautioned me that dry ground will suck water out of the outermost concrete and reduce the protection it provides to the steel - this wasn't in the list in your original post.

For plain concrete, we have to reduce the depth by 50mm when calculating strength, once again blamed on surface irregularity (with a note that this is lifted straight from the Canadian code).

 

[dik]

A very senior engineer once cautioned me that dry ground will suck water out of the outermost concrete

good point, and added to the list... I was more concerned about the intrusion of the aggregate into the concrete. I don't know the effects of moisture going into the granular base. With CMU walls, the loss of moisture increases the strength substantially, and is not detrimental.

As far as irregularities, I've not experienced that. Most granular base, I've encountered, is well placed and really quite 'flat'. Undulations can cause a 'hang up' for the slab and promote cracking. I'm currently doing a preliminary design using 2" cover and have updated my drawing notes for concrete cover to include PEVB and 'plastic' void form.

Dik

 

[dik]

I personally question whether or not this is the right place to be fishing for extra economies.

In my lifetime, I've likely designed a kazillion miles of grade beam using 3" bottom cover. I think the only effect of this is to increase crack widths; I've not questioned this before, and, others may gain from this dialogue. I know I have.

It would be nice to know what is being done 'around the world'.

Dik

 

[KootK]

I was just sharing my own thoughts dik, not questioning your right to query the world on this. I'm pretty curiously to hear what the world has to say too. Steve's input was a pretty good start.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[dik]

KootK... that's the way it was considered. If I wasn't welcome to suggestions or comments, I wouldn't have posted the query. The more I thought about it, the idea of 3" cover became less of a 'good thing'.

Dik