request recommendations for rebar spacing for concrete project

[disney]

Hello, and good morning to all! I would like to hear suggestions/ recommendations for rebar spacing (and #4 vs #5) for a driveway extension I will be working on soon for my neighbor (who is like a father). A class A (front engine gas) motor home will have its rear wheels on this new part. I plan on using 4,000 psi 'crete with fiber from the local cement/'crete company. I plan on the slab to be 6" (or the height of a 2x6) thick. (existing slab less than 4", and this is florida, so freezing should not be an issue with current weather trends....) The addition will be about 14' wide and the depth/length will be on an angle (due to existing driveway) from about 5ft to 8ft. All of these dimensions could change slightly, but that is the general size. I would rather spend a little more now, than say "if only I had....", but I would not like to break the bank putting #8 on a six inch grid either.... Thank you in advance to all that offer friendly and/or helpful comments, and advice.

And for what its worth; I have always preferred working with #5 over #4, unless more than a few bends are required, which there should not be any on this.

Also the rebar chairs are at a height of 2&2.5" I would think that should be fine, as I would think it would be better to have the rebar slightly below center, than at (or above) center.

 

[dodge man]

To tell you the truth, you probably don't need any. 4000 psi mix with fiber should be fine and 6" thick should be good. If you've got the rebar to burn, 18" to 24" spacing should be good, which is based on my seat of the pants and no real knowledge.

 

[/pine]

There is a publication titled "placing reinforcing bars"...you might check your local library...
I did not DL this PDF but it may be of some value?

Placing Reinforcing Bars - Concrete Construction

 

[lakecoastie]

I agree with Dodge on the rebar spacing, just not a fan of fiber.
I am in central Florida and poured a 6" 32x56 slab with 12 x18" footers using 3000psi & NO fiber. I did a 24" grid spaced #5 rebar on 3" chairs. I doubled the footer and edge rebars to make me feel better.
My ground is soft gumbo and wet in rainey season. ZERO cracks in 11 months even with all our rain this year.
If you are able, dig a mini footer on the outer edges & lay in a extra bar. Also compact the dirt, wet, just before laying the plastic & bars.
High psi concrete just gives a better finish. Thickness trumps the psi hardess. For me it wasn't worth the extra $$. Fiber is a waste in my opinion better to spend money on extra thickness. Contractors don't use it either.
>> Best of luck. <<
Android app: concrete calculator by "Chaney Enterprises" is helpful.

 

[disney]

Thank you all for the responses so far. The supplier told me it is 5$/yd for 4000psi, and 6$ for fiber per YD, so at a approx 1.5 yds the 'extras' would not even add $20 to the bill, even if the 4000psi mix and fibers are not 'worth it'/needed, I think I will find value in mere peace of mind.... I may try to dig some small footers, but as the area is already lower than the existing driveway, and likely low enough, I worry that digging my loosen/disturb the soil, and could do more harm than good.

 

[EddieWalker]

I'm not a fan of fiber, but it doesn't hurt anything if you want it.

I am a big fan of rebar. It's tried and proven to work.

Most problems with concrete are from the dirt settling after the concrete has cured and leaving a void under the slab. It's extremely important to have your dirt compacted!!! The other problems come from too much water in the mix. Some contractors will add water to the mix so it flows better and makes their job easier. For maximum strength, you want as little water in the mix as possible. It wont matter what the rating is, if you have too much water, it could easily be half of that when cured.

As for spacing, a 2 foot grid is usually plenty. Sometimes I've gone 18 or 16 inches when I have extra rebar, but I don't think it actually does anything measurable adding extra rebar to the grid on a flat slab.

Having a footer where the vehicle transitions onto the slab and doubling up on the concrete is important. I've never used anything bigger then half inch, also called #4. It wouldn't hurt to use 5/8's at your footing, but there isn't any real advantage on your pad area.

Eddie

 

[ultrarunner]

Just helped a friend replace his driveway in the Oakland Hills...

5/8 bar on a 12 x 12 grid...

I think the grid would have been strong enough to drive over without concrete!

 

[disney]

Most problems with concrete are from the dirt settling after the concrete has cured and leaving a void under the slab. It's extremely important to have your dirt compacted!!!
Eddie

most of our 'dirt' is really sand, which (I have found) is hard to compact. I even own a walk behind compactor (got it really cheap), and all it seems to do in FL is kick up dust, or just dig a hole for itself.

about the water thing, I am having it made/ delivered by a truck, so I hope they get it right!

about the transition part, If I were starting from scratch, yes I would have 10-12" thick edges/footer, but as this is butting up against an existing driveway, I worry that trying to dig deeper at the transition point, could undermine and compromise the existing 3.5" thick driveway.

Again I would like to thank everyone for their input

 

[mikim]

digging a footer along the transition should help rather than endanger the existing slab ...because....you need to drill the existing and install rebar in the holes to tie the existing to the new slab. There's where #5 could really be useful. I would want those holes to be at least 6" deep into the existing then tie the bar good to the bar on the new side.

 

[/pine]

digging a footer along the transition should help rather than endanger the existing slab ...because....you need to drill the existing and install rebar in the holes to tie the existing to the new slab. There's where #5 could really be useful. I would want those holes to be at least 6" deep into the existing then tie the bar good to the bar on the new side.

On this note if OP is opting for a cold joint (rather than an expansion joint) if the existing edge of 3.5" pour is a shear surface (left from a form board) it will help a little by chipping/scoring the shear face...

Also agree about belling the new pour under the edge of the existing...