Concrete for lift

[Jimbo946]

I have a 30x40 garage. The concrete is 6" 4000psi monolithic with 12" wide by 20" deep footings. I do not have rebar or mesh. Want to install a 9000lb 2 post lift. lift requirements range from 4" to 4.25" 3000psi with rebar or fiber mesh. Some of them do not mention fiber mesh or rebar in their requirements. will my foundation be enough at 6" and 4000psi? or did i mess up by not adding the rebar in? thank you

 

[imma_stocker]

I've never heard of a slab without rebar or mesh. Concrete guys (lots of them) told me rebar is the strength while the concrete itself is the just shape. Sidewalks and some driveways go without because they are holding minimal weight and low risk if it collapses compared to a road, home, building, or parking lot.

You might need to cut out large squares for lift columns and tie in rebar to the rest. Go well oversized on the cuts, you'll need to fit a rotary hammer sideways to tie in and you'll need to spread the load as much as possible. Leverage is both friend and enemy in your case.

My 2 cents

 

[ConCretin]

I think you are probably fine to install a lift but obviously check with the requirements of your unit's manufacturer. Rebar isn't generally required for lift installations on existing slabs as long as the minimum concrete depth requirement for the anchors is met.

Rebar or mesh doesn't make a slab stronger from the standpoint of resisting cracking under load. It does restrain cracks from separating thereby maintaining the integrity of the slab. Your slab depends primarily on the ground below to resist the loads placed on it. As long as your base was properly constructed, your 6", 4000 psi slab is plenty stout and will be just fine without reinforcing.

 

[Jimbo946]

I think you are probably fine to install a lift but obviously check with the requirements of your unit's manufacturer. Rebar isn't generally required for lift installations on existing slabs as long as the minimum concrete depth requirement for the anchors is met.

Rebar or mesh doesn't make a slab stronger from the standpoint of resisting cracking under load. It does restrain cracks from separating thereby maintaining the integrity of the slab. Your slab depends primarily on the ground below to resist the loads placed on it. As long as your base was properly constructed, your 6", 4000 psi slab is plenty stout and will be just fine without reinforcing.

Thank you

 

[kctgb]

I’m surprised there’s no rebar. In my area building code requires rebar in driveways and garage floors. The rebar keeps the concrete from shifting or sinking if it fractures. If it was me I would consult a structural engineer, especially when a car/truck is sitting over my head. Your local city building inspector should be able to help you.

 

[wssix99]

I have a 30x40 garage. The concrete is 6" 4000psi monolithic with 12" wide by 20" deep footings. I do not have rebar or mesh. Want to install a 9000lb 2 post lift. lift requirements range from 4" to 4.25" 3000psi with rebar or fiber mesh. Some of them do not mention fiber mesh or rebar in their requirements. will my foundation be enough at 6" and 4000psi? or did i mess up by not adding the rebar in? thank you

What lift are you looking at? Rebar should not be required for a new slab and fiber mesh is NOT a replacement for rebar. (So that point alone, doesn't match up.)

Lift instructions typically have two sets of directives:
- New slab (Continuous slab is needeed. Reinforcing should not be required.)
- Repair slab for a damaged or out-of-spec garage floor (Special cross section with special keying and required reinforcement.)

Sometimes its hard to sort the two out in the instructions. ^

 

[Jimbo946]

What lift are you looking at? Rebar should not be required for a new slab and fiber mesh is NOT a replacement for rebar. (So that point alone, doesn't match up.)

Lift instructions typically have two sets of directives:
- New slab (Continuous slab is needeed. Reinforcing should not be required.)
- Repair slab for a damaged or out-of-spec garage floor (Special cross section with special keying and required reinforcement.)

Sometimes its hard to sort the two out in the instructions. ^

Haven't narrowed it down yet. looking at several in the 9000lb range. Bendpak shows 4.25" at 3000psi and rebar in their requirements . rotary shows 4" at 3000 psi. Others do not mention it.

 

[AC-WC]

If you follow Bendpak guidelines you won't go wrong. I did for my lift and no problems in almost 3 yrs.

 

[wssix99]

Please share a model number and the passage you are looking at. Here's a spec I just pulled from a BendPak 2 post lift. Nothing about rebar...

Concrete specifications. The concrete must be a minimum 4.25 inches thick, 3,000 PSI minimum compressive strength, and cured for a minimum of 28 days. Do not install the Lift on cracked or defective concrete. Anchor Bolts must be more than 6 inches from cracks and expansion joints in the concrete or from a wall.

 

[70runner]

Tough call. I've done several installs and tend to go well beyond the usual 4" - 3Kpsi spec many manufacturers recommend. This is a MINIMUM spec. Your 6" - 4Kpsi pad is beyond that spec, though I'd be concerned about lack of rebar. Most recent install, new 5" - 3.5Kpsi pour, had a 4'x12' slot in the lift area at 12" depth, with an additional layer of rebar tied to the pad rebar. Used Hilti HY200R V3 epoxy system with 9" fasteners. The structural engineer who designed my 12ft high retaining wall used the word overkill when he looked at my design. Additional peace of mind standing underneath 2-3 tons.

 

[MikeC55]

I couldn't agree more. Don't cheap out when it comes to anchoring your lift.

 

[mikedodge]

Lifts are installed all the time in older buildings with unknown concrete and they don't usually cut out and re-do anything for it unless there are known problems or something comes up during instalation like the slab isn't thick enough or it's not strong enough for the anchors. Yours meets what most manufacturers list.

 

[Hobby_Man22]

Lifts are installed all the time in older buildings with unknown concrete and they don't usually cut out and re-do anything for it unless there are known problems or something comes up during instalation like the slab isn't thick enough or it's not strong enough for the anchors. Yours meets what most manufacturers list.

I've heard that concrete continues to harden over the years even after the 28 day wait period. 28 days is just the time it takes to get to whatever the design strength is. Whether its 4000 or 6000psi. So in theory youre in good shape putting it in an older building. Now this info could be total bs and not correct. Just what I've heard on the internet

 

[CV428]

I've heard that concrete continues to harden over the years even after the 28 day wait period. 28 days is just the time it takes to get to whatever the design strength is. Whether its 4000 or 6000psi. So in theory youre in good shape putting it in an older building. Now this info could be total bs and not correct. Just what I've heard on the internet

28 days gets you to 90-100% of the design strength depending on environment. Concrete continues wet curing above 125% for well over a year. I've had people argue with me over this, but try busting out old concrete vs concrete less than a year old.

 

[Mikes61]

I think for a 4post lift you’d be fine. For a 2 post lift I’d cut out and reinforce with rebar in the area where the anchors are going. Definitely do overkill.

It’s not much work or expense to do now. Just think about the car that’s going to be 6’ in the air with you standing underneath it. If the concrete or anchors fail, whats the car going to fall on? Another car? Your kid?

This is nothing to shortcut on or say “it should be okay.”

If you don’t do this right now, you’re always going to wonder.

 

[coldh2o]

For those saying rebar is needed, please explain the engineering behind this requirement.

 

[ConCretin]

For those saying rebar is needed, please explain the engineering behind this requirement.

Don't hold your breath. lol.

The lift manufacturer's engineers are fine with an un-reinforced, 4" slab. The GJ engineers declare the 6" unreinforced floor must be torn out and redone with 12" of reinforced concrete. We love our overkill round here.

 

[CV428]

For those saying rebar is needed, please explain the engineering behind this requirement.

If a section of slab is removed to dig deeper, the new pour should be rebar "pinned" to the existing slab at minimum, but a full rebar lattice is not required.

As for rebar reinforcing on a new pour without tying into existing, it's going to add a safety factor but is not 100% necessary. A good, actual, uniform 4" pour (many "4-inch pours" are 2.5"-4" thickness, as I have found many times in my career) is fine for most lift applications after 28 days. I went with 18" thick concrete w/ rebar in my lift bay because I have astronomically horrible luck and wanted that safety factor (yes, 18", sectioned and pinned, long story, very overkill). Then, I see backwoods redneck shops lifting full size diesels on 3" cracked concrete daily and they somehow manage it just fine for years...

Concrete is strong in compression loading, but not shear or tension. That's where rebar is required- IF those forces exceed the strength of the concrete. Not all forces are compression, it depends on the distribution, moment loading, shock, etc. There's a lot that goes into it.

Even though a few manufacturers have posted "engineering studies" that claim this isn't true, a 2-post lift has compressive, shear, and tension loading zones in the concrete beneath it. The moment inward creates upward forces behind the lift, even if it's minimal on smaller lifts with a narrower footprint. A top cross-bar typically isn't structural, and is only there for looming cables or hydraulics. Typically where there is a massive lift failure, the concrete was already compromised, lift installed prematurely (before 28 days), or had dangerously thin areas from lazy prep work.

In my case, I went to install anchors and the concrete thickness was so inconsistent that I couldn't wedge several anchors in place. When we tore up that section, it varied from 2" to 4". We dug down through #57 that wasn't properly compacted and re-bedded it. It was like $50 extra in concrete to make it stupid thick, why not, just spanned the whole width of the bay.

 

[WNYflyer]

Typical slab on grade is designed as un-reinforced concrete. In other words the slab thickness is determined based upon eliminating cracking of the concrete in tension due to bending, etc., with an appropriate safety factor. Steel reinforcing is often designed and added simply to help control shrinkage cracking, allow larger control joint spacing and is assumed to add no tension strength to the concrete.

28 day strength of concrete is simply an arbitrary target length of time established long ago by some dudes around which typical concrete mixes are established as well as a standard utilized for concrete design.

For an existing concrete slab on grade or for that matter a new concrete slab on grade that is typically designed as un-reinforced, location of any cracking whether due to shrinkage, structural failure, man made control joints, etc. is critical. Only the two post lift manufacturer would know the slab footprint relative to the lift location that must be crack free. That said without that information initially I would probably be looking at no cracks within 4' on either side of the centerline of the lifts, 2' to 3' behind a post and none located between the posts to get a feeling if I'm screwed. But as I said, in reality only the particular lift manufacturer knows the correct answers on the zone of no slab cracks relative to the lift location.

 

[Hobby_Man22]

28 days gets you to 90-100% of the design strength depending on environment. Concrete continues wet curing above 125% for well over a year. I've had people argue with me over this, but try busting out old concrete vs concrete less than a year old.

Does it stop at 125 percent? I thought it continues to harden indefinitely. I guess that doesn't make sense, it has to stop eventually

 

[tomshep]

Don't know if it is possible, but does a metal detector pick up in concrete? That may give you some answers.

Tom

 

[C-S-H]

Does it stop at 125 percent? I thought it continues to harden indefinitely. I guess that doesn't make sense, it has to stop eventually

The design strength fc' usually has to be met by 28days per specification, but sometimes we use 56 or 90 days for slower curing concretes.

At 7 days, strength is about 75% of fc' or higher.
At 28 days, strength is about 130%+ of fc' for lower strength mixes and 115%+ for higher strength (new mix designs).
At 28 days, strength is about 100% times fc' + 3 standard deviations (well established mix designs).
At 56 days, strength is still strongly gaining in lower pozzolanic mixes.
At 90 days, strength is still strongly gaining in higher pozzolanic mixes like mass concrete mixes.
At 2 years strength gain is mostly over except in extremely cold areas. Strength at 200% fc' is not uncommon in lower strength mixes.

 

[C-S-H]

For those saying rebar is needed, please explain the engineering behind this requirement.

A 2-post lift applies a vertical force to the slab (small effect in this case), and bending moments about 2 axes to the slab. The moments twist the slab in contraflexure and will crack the slab. So, the slab needs to be designed to have moment strength post-cracking. Let’s say that the slab has been designed sufficiently strong to take the shear force along the inside edges of the lift’s base plates, and the anchor bolt connections have been designed correctly. If you design the slab and bottom rebar to take all of the moment coming from the base plates in the plane of the lift posts, then you calculate the required bottom steel area in the design strip using strength design like this:

9000# truck/2

* 1.25 (dynamic load factor)

* 36” (eccentricity from critical moment section to lift pads)

* 1.7 (load factor)

/ 0.9 (resistance factor)

/ 4.125” (internal moment arm in 6” slab)

/ 60000psi (yield strength of steel rebar)

/ 0.20 in^2 per #4 rebar

= 8 - #4 GR60 rebar distributed across the design strip, continuous under both base plates.

Now design the rest of the required rebar in the bottom mat (both directions), and the top rebar mat (both directions), including all the bar lengths. Design the top mat rebar and slab to resist half of the applied moment in each direction.

I checked this 6” slab as plain concrete for this loading, and it was operating above the cracking moment at service load. A safe plain concrete slab designed for this loading would be much thicker than 6”. I would only use a slab designed to modern structural engineering standards. Else, you are unnecessarily risking your life. We figured out how to design safe concrete and steel structures many decades ago. No guessing!!!