Yet another slab question

[spazegun2213]

I've read a lot today about the concrete slabs and I figure I should know a thing or two more before I start asking contractors for bids. The garage is going to be 20x20 (its small, I know but thats as big as I can make it). First, the country requires 3.5" 3500psi for the slab. I'm looking at bendpak lifts because they have a short version that will fit under my 10' ceiling (just barely). Their requirements for the concrete are 4" 3000psi. I'm trying to figure out what I should do for footings and since I'm not 100% sure where I want to place the lift, I was thinking about doing a 6'x15'x6" footing for the lift, and the 3.5" for the rest. This would be of 4000psi and I figure with the big footing I have a big area to mount it. Is this overkill? Do I need footings for it?

The garage is going to house my race car, and maybe (if I have the time and money) a project car. Light mechanic work, and nothing industrial.

 

[Jack Olsen]

Is it a two-post lift, or a four post? If 4" is what Bendpak calls for, then I would do the whole thing 4" deep. I would want rebar also, if it's a two-post lift.

With most lifts, the thing you're pouring deep concrete for is not the vertical load it's carrying. It's that you're creating a horizontal structural piece (the floor) which is keeping the lift from tilting over.

 

[Graham08]

I would go for 4" on the entire slab, which is generally standard thickness for concrete floors. The cost difference between 3.5" and 4" will be minimal.

 

[spazegun2213]

Is it a two-post lift, or a four post? If 4" is what Bendpak calls for, then I would do the whole thing 4" deep. I would want rebar also, if it's a two-post lift.

With most lifts, the thing you're pouring deep concrete for is not the vertical load it's carrying. It's that you're creating a horizontal structure which is keeping the lift from tilting over.

ahh good catch, its a 2 post XPR-9. its the shortest 2 post I could find.

 

[rburke65]

Is this a 2 post lift or a 4 post lift, out of curiosity? A 20'x20' floor at 3.5" thick will be 4.3 yards of concrete. Most suppliers have a 5 yard delvery minimum, IIRC, and your going to pay dearly for a "short" load, so you may as well just pour 4" of concrete and be done with it. And if you wanted to real crazy, you ccould place a 6" floor over the entire garage and that would only amount to 7.5 yards of concrete....I'm guessing less than $200 of the original 3.5" floor with the "short load" premium. Call your local ready mix supplier and they can give you prices for 3.5, 5, and 7.5 yard loads.

 

[King Caspian]

Did our shop floor last year. 24X36 with intensions of lift. I went with min. 4" and in some place maybe even 5" and put 3/8" rebar on 2' centers through out. Installed the lift just a couple weeks ago. Bendpak XPR-10A. I would say stay with the 4"min. and include mesh or rebar or both...

Good luck!!

 

[theoldwizard1]

If you don't do rebar, consider something like a 4" square wire grid. If the floor does crack, it will prevent the pieces from moving around.

Keep it wet while curing. A couple of time a day it the outside temp is above 80.

 

[Zeke]

Is this a 2 post lift or a 4 post lift, out of curiosity? A 20'x20' floor at 3.5" thick will be 4.3 yards of concrete. Most suppliers have a 5 yard delvery minimum, IIRC, and your going to pay dearly for a "short" load, so you may as well just pour 4" of concrete and be done with it. And if you wanted to real crazy, you ccould place a 6" floor over the entire garage and that would only amount to 7.5 yards of concrete....I'm guessing less than $200 of the original 3.5" floor with the "short load" premium. Call your local ready mix supplier and they can give you prices for 3.5, 5, and 7.5 yard loads.

I usually go 82 sq. ft for a yard at 4" so you're dead on. But, he has no perimeter footings?

 

[spazegun2213]

I usually go 82 sq. ft for a yard at 4" so you're dead on. But, he has no perimeter footings?

oh, there will be the required footings... so that will add to it for sure. And I'm thinking about having a driveway done at the same time, or close to it.

 

[GeorgiaHybrid]

Sounds almost like my problem that is currently being built. I am going with a 5" thick slab over 5" of gravel. The slab is 4000 psi with #4 (1/2") rebar at 18" on center each way and at the two post locations, I went 12" thick with a lower level of #4 rebar at 12" on center each way for a 4'x4' "footer" to support them.

At this stage of the game, making a change is fairly cheap and the additional 1" of concrete cost was under $200 for my 32x52 slab. The 4000 psi mix was an additional $6 per yard over the regular mix in this area and was another no brainer for me.

Total so far with 20 yards in the footings, another 20 yards in the stem walls, an anticipated 26 yards in the slab (not yet poured) and another 22 yards in the apron (not yet poured), going to 4000 psi concrete will cost me an additional $528 but I will never have to worry about it being "strong" enough to do whatever I want to out there.

 

[Falcon67]

If you can swing it, go 5". That adds not even 1.5 yards to the cost. Then just do the min footings required by your jurisdiction. Rebar 18" OC and you will be good IMHO. Most of the concrete sold should be 3500 or there abouts. If the company is at all reputable, it'll end up 4000+ after 30 days because they won't try to undershoot.

Also look at it this way - last 4 post I looked at used 9" square foot pads. If the race car and lift weigh 5000lbs, that works out to 2224 lb sq/ft.