Concrete slab for parking a car
Les Desser
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?
--
Les Desser
(The Reply-to address IS valid)
DanG
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
"Les Desser" <News...@dessergroup.com> wrote in message
news:u1VqS+Et...@dessergroup.com...
RicodJour
Les Desser wrote:
> 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?
4" for sidewalks, 6" for driveways. 8", and the wire mesh, is
unnecessary if you're paying attention to compaction and drainage.
R
DT
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
Les Desser
<dthomp...@SPAMwowway.com> Thu, 9 Nov 2006 07:54:03 writes
>In our area (northern Ohio), the vast majority of driveways are nominal
>4". 6" is extra strong,
Thank you, DanG and RicodJour for your responses.
[..]
>Place the mesh in the lower third of the pour, where it will contribute
>to the bending strength.
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?
RicodJour
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
Bobk207
Les-
Here is the ng thread that RIco referred to
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
Bob Morrison
> 4" for sidewalks, 6" for driveways. 8", and the wire mesh, is
> unnecessary if you're paying attention to compaction and drainage.
>
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.
--
Bob Morrison, PE, SE
R L Morrison Engineering Co
Structural & Civil Engineering
Poulsbo WA
bob at rlmorrisonengr dot com
Les Desser
RicodJour <rico...@worldemail.com> Thu, 9 Nov 2006 06:25:35 writes
>Is that dimension correct? The drive is 3 inches wider than the car?
Sorry, slip of the finger - should read 3 foot wider.
Les Desser
Bobk207 <rkaz...@gmail.com> Thu, 9 Nov 2006 07:14:01 writes
>Here is the ng thread that RIco referred to
>
>http://groups.google.com/group/alt.building.construction/browse_thread/t
>hread/f3ee6bae21e8fa81/42eeb270cac427b5?lnk=gst&q=14+driveway+rebar&rnum
>=1&hl=en#42eeb270cac427b5
>
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.
>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
> :)
The total volume of the area I am doing, assuming 8" is about 4 cu.
yards so the extra 2" is not very significant.
>
>You need "cover" over the wire to protect it from rust.....3" from soil
>contact, 2" from air.
>
Thanks for that
>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 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.
Les Desser
it to dry before parking my car on it?
Temperatures here in the UK in London currently running 10-20 degrees F
above freezing
DanG
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
"Les Desser" <News...@dessergroup.com> wrote in message
news:$qPe1YUA...@dessergroup.com...
hawgeye
"Les Desser" wrote ...
> 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
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.
Bobk207
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
Bob Morrison
> 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
>
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:
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.
J.C.
"Bob Morrison" <SpamF...@junk.com> wrote in message
news:MPG.1fc25449b...@news.west.earthlink.net...
> In a previous post Bobk207 wrote...
>> 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
>>
>
> 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.
Those aren't cracks. It was a rookie pour and they're cold joints. <G>
Les Desser
Bobk207 <rkaz...@gmail.com> Sun, 12 Nov 2006 18:40:13 writes
>OR Just leave it out of your 8" slab :)
You and Bob have convinced me!
Les Desser
Morrison <SpamF...@junk.com> Mon, 13 Nov 2006 18:07:25 writes
>I must be speaking to myself
Not quite
>when I recommend "Unreinforced Slabs on Grade". This an accepted and
>regularly used practice.
Accepted. I have thrown away the steel (metaphorically)
>
[...]
>
>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)
I'll be laying 8" over the base on which the old slab (about 4-5") sat.
>7. Within 12 hours cut crack control joints at least 1" deep.
The slab is about 15' x 11'. I don't suppose I need the cuts.
>8. Cure the slab for 2 weeks before loading.
Will do.
>
>You can also place a geotextile fabric between the soil and the gravel.
What would that be for?
Bobk207
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
Les Desser
Bobk207 <rkaz...@gmail.com> Wed, 15 Nov 2006 01:49:01 writes
>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.
Agreed
>
>You're in London area, UK correct?
>
Yes
>Bob Morrison.....frost issues?
During installation or use?
> None if his base is well drained?
Temp here now is about 13 C. Based on recent experience no frost for
years to come!
>
>But why is the old slab of 4 to 5" being replaced? DId it fail?
>Cause?
>
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.
>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.
Sorry - I thought that early on I had given some background - but maybe
not.
Bob Morrison
>
> Bob Morrison.....frost issues? None if his base is well drained?
>
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.
Bobk207
Bob Morrison wrote:
> In a previous post Bobk207 wrote...
> > You're in London area, UK correct?
> >
> > Bob Morrison.....frost issues? None if his base is well drained?
> >
>
> 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.
>
I get frost heave in the "floor" of my mom's freezer...... :)
sometimes she forgets to close the door completely & the moisutre
overwhelms the defrost cycle .....the moisture freezes & "mounds" the
aluminum panel that is the "floor" of the freezer
I remove the floor panel & defrost with warm water & shop vac....good
as new
cheers
Bob
Bobk207
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
Les Desser
Morrison <SpamF...@junk.com> Wed, 15 Nov 2006 15:34:22 writes
>You are correct. If the base is well drained then there will be no
>frost heave. It takes moisture that will freeze and expand.
I was actually wrong in my last post - the air temperature here in north
London was about 63 F. Today it was a cold 52 F. Tonight they are
expecting lows of 45 F.
Last year I think the worst we got was some mornings with frozen car
windscreens.
The chance of frost below an 8" slab here is not very high I suspect.
In any event the water table is at about 9' and there is very little
open soil to catch rain in the area.
-------
In any event, many thanks for all the posts and advice given here.
Bob Morrison
> was actually wrong in my last post - the air temperature here in north
> London was about 63 F. Today it was a cold 52 F. Tonight they are
> expecting lows of 45 F.
>
> Last year I think the worst we got was some mornings with frozen car
> windscreens.
>
> The chance of frost below an 8" slab here is not very high I suspect.
> In any event the water table is at about 9' and there is very little
> open soil to catch rain in the area.
>
Les:
I believe our climates have similar temperatures. Mostly maritime
moderate with an occasional freeze.
Our local building codes call for building foundations to be a minimum of
12" below finished grade. So, there is some potential for frost heave but
not much. Again, a well-drained based will go a long way to preventing
the problem.
Cretedude
You only need 4 to 5 inches of concrete for a driveway. Been d oing c
concrete for 46 years
--
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