Concrete slab for parking a car

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Re-doing the driveway for my Mitsubishi Shogun (weighs about 1.5 tons)
I was planning to lay 8" concrete slab over compacted soil with wire mesh located about 2" from the top.
Someone now says I need 12"!
I thought I was going over the top with 8". The old one was about 4" with the odd bit of wire chucked in and it was OK - I had to take it up to lay water pipes.
Who is right?
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Les Desser
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4" would be adequate. 5" would be excessive. Make sure that the area is well compacted, especially if it had been dug up for reworking the water line.
A good practice would be to dig the perimeter down 8-12" with a single bar of steel. Many slabs break where there is a substantial load placed on corners and edges. ___________________________ Keep the whole world singing. . . . DanG

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Les Desser wrote:

4" for sidewalks, 6" for driveways. 8", and the wire mesh, is unnecessary if you're paying attention to compaction and drainage.
R
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In a previous post RicodJour wrote...

I second this recommendation. As I have said before:
Unreinforced concrete with proper crack control on a properly compacted gravel base. No WWF, no fibermesh, plenty of crack control joints (every 150 sq.ft.).
To reduce cracking use as dry a mix as possible in order to reduce shrinkage. Use a mid- or high range water reducer in order to be able to place the concrete. Some finishers like to use 5-1/2 or 6 sacks of portland cement per CY in order to get a better finish. It takes a little less than 3 gallons of water per sack of cement to make the chemical reaction happen. The rest of the water is for handling and comes out as "bleed" water on the surface or through evaporation through the concrete surface. The concrete changes volume (shrinks) when this happens and cracks form.
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Bob Morrison, PE, SE
R L Morrison Engineering Co
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says...

In our area (northern Ohio), the vast majority of driveways are nominal 4". 6" is extra strong, specified by most cities for aprons. Your vehicle is not particulary heavy. Place the mesh in the lower third of the pour, where it will contribute to the bending strength. If you are doing it yourself, just lay the mesh down and when you pour the concrete , pick up the mesh into the pour as you go using a hook or rake. The aggregate will hold it up.
-- Dennis
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Thank you, DanG and RicodJour for your responses.
[..]

That was my initial thought as well, but after further consideration I think it should go in the bottom.
With a vehicle on the drive (about 3" wider than the vehicle) the weight on each side would be trying to break the slab downwards across the middle.
I.e. trying to crack it open at the top. So the mesh should be located in the top. Not so?
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Les Desser
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Les Desser wrote:

Read the recent discussions on reinforcing slabs in driveways that have popped up on this newsgroup recently. There are differing opinions on the benefit of reinforcement.
Is that dimension correct? The drive is 3 inches wider than the car?
R
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Sorry, slip of the finger - should read 3 foot wider.
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Les Desser
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Further to my earlier query, about an 8" slab, how long should I leave it to dry before parking my car on it?
Temperatures here in the UK in London currently running 10-20 degrees F above freezing
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Les Desser
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If you are buying the concrete from a ready mix plant, work out the details with them.
Keep the water/cement ratio to a minimum, use air entrainment, ask about high early concrete (the term we use here). Plan on curing the concrete either with chemical, visqueen, or flood (continuous sprinkle) as long as you are above freezing for at least 3 days. I would consider 3 days as a minimum, 7 if possible, and 28 if you want the full initial cure. ___________________________ Keep the whole world singing. . . . DanG

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"Les Desser" wrote ...

I missed your earlier query. Why an 8" slab? That must be a very heavy car. I can see 8"-12" around the perimeter if it were a monolithic slab for a garage, but 4" with wwf would be sufficient for parking a car on it. Unless you have a circumstance that you may have pointed out in your original post.
As for drying time, typically in the US concrete cures and reaches max strength in 28 days. The top will be dry enough to walk on the next day if allowed to cure properly but I'd wait at least 2 weeks before putting a car on it.
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Les Desser wrote:

Les-
Here is the ng thread that RIco referred to
http://groups.google.com/group/alt.building.construction/browse_thread/thread/f3ee6bae21e8fa81/42eeb270cac427b5?lnk=gst&q +driveway+rebar&rnum=1&hl=en#42eeb270cac427b5
A thickness of 4" is very skimpy, 6" with wire would be fine
an 8" parking slab for a normal vehicle would even been overkill for me :)
You need "cover" over the wire to protect it from rust.....3" from soil contact, 2" from air.
Just put it in the middle....in a 5 or 6" slab it's hard enouhg to get it in the middle let alone bias it up or down.
I'd make the slab at least 8' wide more like 9 or 10....it's a single concrete parking space and having the slab edge too close to the car will make place the vehicle more difficult AND the edge could be a trip hazard that close to the vehicle
Save the thickness you were considering & go wider (that is, if you have the room)
cheers Bob
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I think the relevant bit from there is
--The weight of a car on the top of a slab pushes down on the --top of the slab, applying compression strength to the top, while the bottom --of the pad is being pulled apart. Enough weight, and the cracks form in --the bottom of the pad and tear through the top. Put the rebar in the lower --half of the slab, and it will hold the bottom together and prevent that --tear through.
That seems to be saying that weight of a wheel would cause the slab directly below it to crack - like standing in the middle of a wooden plank would tend to split it from the bottom up.
I really don't understand that. Unless the slab was suspended on air, the leverage at that point would be close to zero and there would only be compression of the concrete taking place.
I would have thought that the car's weight on either edge of the slab would more likely tend to crack it open in the middle at the top.

The total volume of the area I am doing, assuming 8" is about 4 cu. yards so the extra 2" is not very significant.

Thanks for that

I take the point, but I would like to understand the physics side. My vote would go for the bar in the top 2" unless someone can explain otherwise.
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Les Desser wrote:

Les-
I'll defer to Bob Morriosn on the mecahnics of a concrete slab on grade (SOG)
If this was an elevated slab (slab on air) the mechanics is very staight forward. The slab is only supported at it's edges & clearly the bottom surface of the slab is in tension thus no top rebar required & you'd want the rebar to be biased towards the bottom of the slab
But where for sure to bias the rebar in a SOG ....I'm not entirely sure. I still think that 5 or 6" is too thin to really get much bias. I'm just happy when the rebar or wire isn't laying on the dirt.
I'm thinking out loud here....the soil & the base prep provides a reaction, a support to the SOG....the soil reaction isn't real stiff but it's a lot stiffer than air.....kinda like placing a 2x4 across a mattress.
The mattress provides some support for the 2x4 ....imagine a 2x4 & matress as wide as your slab. Place a 2x4 across the mattress and load the 2x4 at two places ~ 1.5' in from each edge.
I guess the result is that the ends of the 2x4 will delfect down more than the middle...indicating a postive moment in the center section of the slab. I guess that would indicate the need for reinforcement in the top of the slab.
But again for a 5 or 6" slab the bias is problematic ....but for a thicker (8") slab it would be possbile to place the reinforcment selectively. However you need bottom reinforcement beyond the wheel loading points where the moment goes positive.
All this is pretty much moot ( or at best an intellectual exercise) since Bob Morrison & others suggest no reinforcement at all.
If einforcement is a second order effect (which it provbably is) , then placement of it other than at the centerline would be pretty much third order effect.
Plus biasing it up might compromise the edge behavior.......IMO if you're going to use it
either go centerline
OR bias up in the middle & down at the edges (with some overlap at the loading points)
OR bias it all up & add some bottom reinforcement at the edges (& in passed the loading points by a foot or so)
OR Just leave it out of your 8" slab :)
cheers Bob
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In a previous post Bobk207 wrote...

I must be speaking to myself when I recommend "Unreinforced Slabs on Grade". This an accepted and regularly used practice.
Here's a link to an abstract from the American Concrete Institute:
http://tinyurl.com/ymru2u
Remember, concrete ALWAYS cracks. No amount of reinforcing will prevent it from doing so. The Young's modulus of steel is about 9 times that of concrete. By the time the rebar has had enough strain to absorb much stress the concrete will have already cracked.
The structural model for slabs on grade is a little different than elevated slabs. Assuming you done a proper job of preparing the gravel base supporting soils act like a spring and push back against wheel loads. The purpose of the slab is to give a wearing surface and to spread the load over a larger area.
Here's the basic procedure I recommend for residential parking areas and driveways:
1. Clear of the organics and any top soil to the slab + base depth. 2. Use a roller or plate compactor to compact the soil 3. Spread at least 6 inches of 3/4" minus crushed rock. 4. Use a roller or plate compactor to compact the gravel base 5. Form the edges of the slab 6. Pour at least 5 inches of concrete (6" for heavier traffic areas) 7. Within 12 hours cut crack control joints at least 1" deep. 8. Cure the slab for 2 weeks before loading.
Curing time can be shortened by the use of high early (Type III) portland cement. If soils are soft then it may be necessary to overexcavate and backfill with properly compacted structural fill (or gravel). You can also place a geotextile fabric between the soil and the gravel.
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Bob Morrison, PE, SE
R L Morrison Engineering Co
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Not quite

Accepted. I have thrown away the steel (metaphorically)

[...]
I'll be laying 8" over the base on which the old slab (about 4-5") sat.

The slab is about 15' x 11'. I don't suppose I need the cuts.

Will do.

What would that be for?
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Les Desser wrote:

The geotextile fabric keeps the gravel base from migrating into the soil below.
But since the base material is already in place, probably not worth the effort to R&R.
You're in London area, UK correct?
Bob Morrison.....frost issues? None if his base is well drained?
But why is the old slab of 4 to 5" being replaced? DId it fail? Cause?
Les...it appears that we may have missed some of the details or perhaps you did not include them......I got the impression that this slab was a whole new project, not a redo of a failed one.
cheers Bob
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Agreed
Yes
During installation or use?

Temp here now is about 13 C. Based on recent experience no frost for years to come!

A strip down the centre was cut out to lay new water pipe. I then changed my car to a 4x4 which then cracked a part of the remaining strip.

Sorry - I thought that early on I had given some background - but maybe not.
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Les Desser wrote:

Actually Les, you did say it was a redo....but I'm always a little worried that there can be a missing detail that can change the suggestions
cheers Bob
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In a previous post Bobk207 wrote...

BobK:
You are correct. If the base is well drained then there will be no frost heave. It takes moisture that will freeze and expand.
BTW, you can get frost heave in the floor of a walk-in freezer if you don't make sure the slab is well-drained.
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Bob Morrison, PE, SE
R L Morrison Engineering Co
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