How Much Concrete?

[DrumrBoy]

Getting an additional garage (detached) and want to get some advice on how thick the floor concrete needs to be to properly support a four post lift.

The contractor says they pour 4 inches of 3000 mix (psi?) over a rebar lattice. The heaviest thing the lift will likely see is the Viper since my other cars are lighter.

Floor specs probably depend on the pads attached to the legs of the lift, but in general, what is recommended floor thickness?

Any lift recommendations are welcome as well. I've perused the previous posts for ideas but more are always welcome!

Woo hoo.

 

[ViperTony]

4" 3000psi with actual REBAR lattice is more than enough. What matters is what the concrete is resting on. If your garage has been filled in make certain its gravel material that has been packed, rolled, tampered really well. I've replaced many a concrete garage floor due to poor filling material and poor compaction. I consider my garage floor overkill...coming from a family of Italian masons I have 4,000 PSI concrete 6" thick with a metal rebar grid expoxied into the foundation that would make most bridge rebar look tiny. Also includes footer beams and two pillars. Its technically a floating floor but my town wouldn't allow me to have a garage under the garage so I had to fill it in. Overkill yes I know but you know Italians...

 

[PatentLaw]

4 inches is very thin in my opinion. I would do 6 inches, rebar in the bottom 2 inches to provide cover between the bottom of the slab and the rebar. You are in a freeze thaw area, so you ask a good question. If we were talking Florida, 4 inches may be ok. Not in Illinios. You are begging for cracks to form.

Also make sure that you put adequate expansion joints in. One pour will most likely result in a cracked floor anyway. Do not pour the concrete at temperatures under 50 degrees. Also try to get the concrete from the mixer to placement within 45 minutes to minimize the amount of water needed to make the mix flowable.

 

[Madduc]

When I had my Rotary Lift installed they wanted 4" minimum.

 

[plumcrazy]

what tony said. its all about the base. you can have 12" of concrete if its on something soft, its gonna crack and be useless.

i personally would go overkill, buts thats me. 6" or more is overkill with rebar done right. and theres a difference in rebar being done right too. make sure they really know what they're doing and have someone competent enough to check their work before its poured and no im NOT talking about the town inspector !!!

 

[PatentLaw]

Perhaps someone else can give you an idea....

http://www.concreteconstructiononline.com/industry-news.asp?sectionID=714&articleID=540592


and really.....what is the difference in cost between 4 inches and 6 inches of concrete? And if you get shorted on the pour in a 4 inch pour, you wind up with 3 inches....

Is that worth it? Really? What you order and what you get can be two different things. Play it safe. It is really not going to cost you any more.

 

[Mr Hemi Head]

I am about to do the same...garage floor is 36x36

Contractor wants to poor fiber reinforced concrete...says no rebar or wiremesh required.

Anybody familiar with this stuff?

The weight of a Viper and lift combined is about the same as large SUV. Why should the floor require special treatment with a lift?

 

[PatentLaw]

Overturning moment from the anchorage of the lift. You would have both a compression load and an overturning moment. The anchors on the outside of the baseplate resist that moment.

While fiber reinforced concrete is ok, I would not use it instead of standard concrete with rebar. Virginia still gets cold weather and the fiber concrete is really not as strong as rebar. Fiber concrete has to be done just right.

And there better be an expansion joint in that 36 by 36 foot pour. It is going to crack without it.

 

[ViperTony]

6" won't hurt and as I stated earlier its what I have in my floor. Not really necessary unless you're going to be parking heavy duty trucks and equipment on it but Viper owners are all about extreme anyway . Freeze/Thaw is a concern for concrete sidewalks and car port slabs but less of an issue for covered garages with foundation walls. I assume you are talking about a detached garage with foundation or stem walls that will be framed and covered? If you're worried about strength with a 4" then go with 4000 psi. The only cavaet I have with 4000psi concrete is that if someday you decided to expoxy your floor it will be a B**** to shot blast the floor for surface prep. Most contractors don't put rebar in a garage floor and go with wire mesh which I believe does nothing for a garage floor with load on it.

At least your contractor went with rebar. Did they specify how far the spacing will be 12" or 24"? 1/2" rebar? How about a vapor barrier? Did your contractor say he was going to lay expansion joints around the perimeter of the floor between the concrete slab and fondation walls? Is he going to score the concrete or install expansion joints in sections? It's OK to pour the whole floor then score it afterwards. So long as he does one or the other. If your garage has been filled in, what did he use for fill material?

You got good advice about the use of water (or limited use) and cold weather. Although its OK to pour concrete in cold weather, even freezing weather, there's a lot of prep and additional work involved to make this happen and there's a risk of the concrete becoming weak. The concrete temperature must be kept above 50 degrees for 5-7 days for a good cure. Contractors often use accelerating curing agents when pouring the concrete in the winter, then cover with blankets and/or use heaters. This all adds up to addtional cost and if the contractor is not a pro at this the concrete will be weak.

I forgot to add earlier that I mixed synthetic fiber in my concrete mix to eliminate shrinkage cracks but this was an experiment and not needed for my floor with all of the rebar. I'll go back to step 1...proper subgrade preparation. Make certain you've got good fill material and its been compacted. I replaced my neighbor's garage floor 2 years after his house was built. It began to sink and crack and became uneven. After we jack hammered the floor out we found no rebar, no mesh and the garage was filled with loamy material instead of sand and gravel. It wasn't compacted and compression tests on the concrete showed less than 3K psi which was probably caused from way too much water in the mix and it was poured in the winter and froze before it was fully cured. The floor was basically sitting on a sponge.

 

[Mr Hemi Head]

Overturning moment from the anchorage of the lift. You would have both a compression load and an overturning moment. The anchors on the outside of the baseplate resist that moment.

While fiber reinforced concrete is ok, I would not use it instead of standard concrete with rebar. Virginia still gets cold weather and the fiber concrete is really not as strong as rebar. Fiber concrete has to be done just right.

And there better be an expansion joint in that 36 by 36 foot pour. It is going to crack without it.

Control joints every 12' were mentioned...is an expansion joint something different?

 

[ViperTony]

Control joints every 12' were mentioned...is an expansion joint something different?

Control and expansion joints are the same, different terminology. Concrete shrinks as it cures and temperature changes as well as moisture will cause the concrete to expand and contract. Without control joints you'll see cracks throughout the floor as there's no relief for the contract. We don't want cracks caused by expansion/contraction. You'll either see a saw cut in the floor every so many feet or expansion material made from foam/plastic about 1/4 - 3/8ths in thick. Around the perimeter of the slab, between the slab and foundation wall I use expansion material. I use saw cuts for control joints in the floor itself. A general rule of thumb is space the joints 2-3 times the thickness of the concrete in feet. For a 4" slab space the joints every 12 feet. 6" slab every 18 feet, etc. However, in my garage floor I have no control joints 30x26 and I can get away with this because of the overkill rebar system I installed. I'll take pics of the joints from my basement and garage floors and post tomorrow.

 

[rraulston]

my 2 post has a 6" with rebar. no issues....

 

[Dave149]

I have 2200 square foot garage all with 5" floor. That's what the contractor said I should have to be safe. It get's pretty cold up here. I also had concerns about the hoist pads, so I had the floor poured deeper (about 8") in 2-two foot square areas where the hoist pads would be. This seemed like a good cheap solution to the problem.

Hope this helps.
Dave

 

[Mr Hemi Head]

Thanks Tony
The Viper forum is fantastic when members share information like this.

The center of the garage floor is solid VA clay but the footers were overdug, due to all the rocks, and will have to be filled in.

30 ton of #57 was needed to prep the breezway (30 X 14) and probably just as much will be required to fill the footer area of the garage floor.

I will look into the vibratory plate.

 

[2001 GTS]

I don't think you will get shorted any concrete as long as the prep is right

 

[ViperTony]

Thanks Tony
The Viper forum is fantastic when members share information like this.

The center of the garage floor is solid VA clay but the footers were overdug, due to all the rocks, and will have to be filled in.

30 ton of #57 was needed to prep the breezway (30 X 14) Probably just as much will be required to filll the footer area of the garage floor.

I will look into the vibratory plate.

Curious about the footers...were the footer forms set and poured on top of the final footer grade or were they set above grade? I've seen situations where contractors dug too deep but instead of pouring an extra foot or two of concrete in the foundation walls to make up the height, they instead built the footing forms a foot above the grade. This one case was extreme. They duge 12 - 16" too deep. They then poured the concrete and let is spread out under the form and let is spread on either side and let it cure like that. Imagine a blob of concrete just left of cure in an unformed state...This created an unstable and weak footing. I wish I took a picture of this. The result: the home owners great room sank 2" toward the back of the house. The footing crumbled from the stress of the house. All because the contractor was too cheap or too stupid to fix his mistake. What's worse? The inspector gave the footings a passing grade...

 

[AFL in NJ]

I let my Civil Engineer papers lapse about 9 years ago when I went into IT, but if it were my garage, I'd have the contractor use a sonotube form to put at least 12" x 12" cylindrical footings where you plan to place the lift. These footings can be under the 4"-5" of reinforced concrete floor you put in. Also, I'd want a 3/4" gravel mix of at least 8" under the floor compacted with the very least a "jumping jack" or "plate tamper" and that gravel should be moist to ensure maximum compaction.

But that's me....ViperTony and others said pretty much the same thing.

Regards,
Aaron

 

[cayman]

Two post lift mounted on 2 year cured 3000 psi 4 inch thick concrete with remesh only. I have left 5,000 plus pound vehicles up on the lift for weekends at a time with no cracking, base mounting bolt pulls, etc. I've been using this setup for three years now. Maybe I'm living on borrowed time but I don't think so. If you figure the PSI the concrete is subjected to with the weight of the lift posts plus a 5,000 pound vehicle sitting on the surface area of the lift post bases, it is less than the psi the concrete is subjected to with the weight of the car just sitting there on the concrete on all four tires, considering the square inch footprint of the tires and the weight of the vehicle. Worse yet, what about regular hydraulic floor jacks that even have less of a footprint square inch-wise that we all have used in our garages to jack up half the car without any thought about whether or not we have strong enough concrete under it for proper support? But hey.....the more concrete the better. Better to be on the safe side.

 

[RAYSIR]

I asked my good freind who is the AK distributor for Rotary lifts. A 4 post only needs 4" of support since it has no leverage pull on mount bolts and doesn't even have to be bolted down.A 2 post needs 8" in a 3' x3' pad surrounded by 4" the rest of the flor and should be rebarred 8" into 4". Need the 8" for the anchors bolting down the lift. Hope this helps clear it up.

 

[plumcrazy]

not enough difference in cost from 4"-6" of concrete. why wouldnt anyone do it? you only have one chance with concrete dont be stupid/cheap.

and rebar is the best, then wire mesh then ya have fiber reinforced which id use along with mesh if it were me. but the mesh would be epoxy coated.

AFL had a great suggestion on the sonotubes at the spot where the lifts gonna be...

 

[DrumrBoy]

Outstanding info folks....thanks for taking the time to share it!

I moved to Atlanta (haven't changed the avatar/profile) yet so hard freeze is a rare thing.

They expect to pour within a couple of weeks so the temps will probably be between 75 and 85 - good there.

The garage will be 22 x 44 and the additional cost for 6 inches over 4 is only 700 bucks.....perhaps overkill but at 700 its cheap insurance.

Thanks too for the education on rebar and expansion joints.... I'll be sure to explore this with the contractor and spec in the proper work.

This forum is awesome!

 

[AviP]

4" is more than enough. If you are still concerned, then spec out the lift and have pillar posts at the 4 corners where the lift would meet the floor. 6" is a waste. It adds 50% more material. And yeah, the base for the foundation is always key. Keeping the water away from the foundation is important.

 

[past ohio]

I am a voter for the 6 inches at least, with the rebar in the two inch lower range, at least 4000 psi and also be sure and either use a saw or put in expansion joints appropriately, IF you ever want to put in a swing arm lift, you need at least 6 inches for the bolt-anchors to bite ...my $.02

 

[GTS Dean]

I'm a highway and bridge contractor and built my home and garage foundation. I went with 3000 psi concrete at 5" thick on top of well-compacted subgrade with #4 rebar on 12" centers. Concrete slump was limited to 5" and everything was consolidated with vibrators. The slab was wet-mat cured for 7+ days and I don't have a control or expansion joint in the entire 55x25 slab. It has a few very minor shrinkage cracks but I'm not worried about them in the slightest. I dug T-shaped grade beams 18" deep and 24" wide at the top for lift anchorage parallel to and between the parking spaces. The perimeter has a 30" deep x 12" wide grade beam all the way around.

Concrete is not supposed to be self-leveling. Cracks are a bigger problem the more water the finishers put in the mix (to make their job easier) and control joints become necessary.

Your mileage may vary.

 

[2001 GTS]

Outstanding info folks....thanks for taking the time to share it!

I moved to Atlanta (haven't changed the avatar/profile) yet so hard freeze is a rare thing.

They expect to pour within a couple of weeks so the temps will probably be between 75 and 85 - good there.

The garage will be 22 x 44 and the additional cost for 6 inches over 4 is only 700 bucks.....perhaps overkill but at 700 its cheap insurance.

Thanks too for the education on rebar and expansion joints.... I'll be sure to explore this with the contractor and spec in the proper work.

This forum is awesome!

Just wondering, who are you getting the concrete through?

 

[plumcrazy]

what dean said. watch the finishers bigtime. let them know right off the bat about curing and finishing

 

[Mr Hemi Head]

Curious about the footers...were the footer forms set and poured on top of the final footer grade or were they set above grade?

Yes Tony the footer forms were set and poured on top of the final footer grade. They were shooting for 8" of concrete but it varied from 9" to 15" in some areas. The forms were custom built on site by a fussy carpenter and followed the contour of the grade, even over rocks that could not be excavated.

As a result concrete was confined to the forms.

There are numerous step footers due to huge holes cause by excavated tree stumps.

I realize this is after the fact but should the lower step footers be connected to the higher footers? Not sure how this could be done.

 

[Paul Hawker]

How about pouring footers where the supports will go. That way you can make it extra strong and less possibility of cracking.