My contractor has specified a 4" thick slab on grade for a new garage floor. Is this the accepted thickness? I've read that driveways should be 5" thick and house foundations 8-10" thick. The plans *do* call out reinforcement.
Comments?
4" is standard for a residential driveway, gge slab as well as the house floor.
What about beams to support the walls? Stirrups/vapor barrier? Rebar/mesh? These are equally important IMHO.
Gary
On 14 Jun 2005 13:28:13 -0700, snipped-for-privacy@bigfoot.com scribbled this interesting note:
Four inches is sufficient for the floor of the garage. Of course you will still need beams around the perimeter of the garage (perhaps more in the center, depending upon the complexity of the design), a sand pad to control expansion and contraction under the foundation, a moisture barrier, and reinforcement steel, properly supported so it is in the center of the concrete instead of sitting on the bottom against the moisture barrier where it does no good whatsoever.
If I were building my own garage, say a 20X24, single story, where we live, I'd most likely dig perimeter beams about ten to twelve inches wide, at least a foot below grade and sixteen to eighteen inches would be better, and possibly dig piers on either side of each corner as well. Otherwise, standard pad design, moisture barrier, and steel.
If I got fancy and wanted a two story garage then the beams would be wider and deeper and there'd be more piers. Otherwise the rest would suffice, even the 4 inch floor. But that's where we live where the soil has an unbelievable amount of expansion and contraction. A slab in this area, built to minimum standards, even just for a garage, will fail unless you take extra precautions to keep the soil around the building stabilized by maintaining even moisture content year 'round.
-- John Willis (Remove the Primes before e-mailing me)
Just what are you planning on driving on a 5 inch slab? A Fire Truck?
4 inches is standard, usually 3500 psi concrete. Some use rebar some use steel mats. Saw cut every 10 feet for expansion.Foundations are broken up into parts called footings and stem walls. The thickness is directly proportional to the height of the building. Where I live the footings are minimum of 12 deep x 18 wide about 18 inches below grade. That makes the stem walls 10-12 inches wide and 20-24 inches high. If you live in frost country, other considerations are necessary.
The OP said he read the info. Must have been a handout from the National Concrete Mfger's Association
Your driveway may be expected to support a truck, your garage will not likely ever need to.
For residential structures, I always recommend a 5-inch UNREINFORCED slab on a 6-inch compacted gravel base. Sawcut or tooled crack control joints every 150 square feet max.
Unreinforced = no welded wire fabric, no rebar, no fibermesh
No amount of reinforcing will prevent cracking. The only thing that will do that is pre-stressed or post-tensioned concrete.
Slab strength and thickness depends on extenuating circs, type of subsoils, frost, use of salt on roads, etc. Reinforcing, if properly applied, greatly improves perform of crete, as does limiting the amount of H2O
-- Troweller^nospam^@canada.com
According to SQLit :
Some use fiberglass fiber reinforcement instead. I did ;-)
Yeah, you generally need to get below the frost line. Here, it's at least
4'.
CF&SG is correct in that rebar and lower water content will improve structural performance. However, only the latter will have any effect on crack control.
Most cracks come from shrinkage. Shrinkage comes from having more water in the concrete that is necessary to make the chemical reaction happen.
If left unspecified most batch plants will give you about 6 gallons of water per 94 lb. sack of cement. Only about 3 gallons is required for the chemical reaction. The rest is there so you handle and place the concrete.
There are several ways to reduce the amount of water, including adding more air to the mix which isn't a bad idea for exterior slabs. The added air ("air entrainment") will also improve freeze-thaw resistance.
Don't they measure or test that by measuring the "slump?"
Don't they measure or test that by measuring the "slump?"
************************************************************* Yes they do but if the concrete measures a 6 slump, there is no way to take water out of the concrete. (the only way to stiffen up the concrete is to add more sand, aggregate and cement)
And if it measures a 6" slump and you wanted a 4", then you send the truck back to the plant and ask for one that is to your specifications. You don't just take what they give you.
.... and watch that the driver doesn't add water to move the mix down the chute. The Navy had to rework a three story building because the inspector didn't pay attention. Engineer and lawyers spent a couple of months strugglng over that until a fired employee spilled the beans. TB
It is a little more difficult to send back when the test cylinders don't meet compression days later :-)
That's when the trouble (and overtime for the engineer) really starts!
I have had to require that the contractor tear out the bad stuff and replace with new. Finger pointing gets started in earnest at that point.
But it is always the contractor's fault :-)
Quoting from my recently purchased "Pocket Ref" book, the recommended thickness for driveways is 6 to 8 inches. Garage floors are 4-5 inches.
Putting in a driveway is no small chore. Or expenditure. I tend to make things stronger and thicker than the minimum. A few hundred bucks spent now could save you some major work and expense later. And, making the forms a couple of inches thicker now is no big deal. Materials can be taken out, or the forms merely made a little higher.
Think long term.
DO IT ONCE. DO IT RIGHT.
Steve
"SQLit" wrote
My motorhome is 21,000 #. Now, I don't put it in the garage because it is too tall, but I do drive on the driveway. And when I go visit, I drive it on other people's driveways, too. Hope they have a good one, and didn't get the cheapo economy cut cost save a little money now and pay for it later kind of deal from their contractor.
The way I think of it, I want to know if I ever wanted to drive a fire truck on it, I could.
Steve
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