Concrete slab question

[Mastiff37]

I'm working with a company to spec out a metal pole building for a shop. I'll be doing hobby level auto work, maybe with a 4 post lift in the future. They are bidding "plain" concrete with saw cut control joints. The overall concrete extent is to be 32x60 feet. Other companies I worked with bid 6" wire mesh. Nobody has bid rebar. When I asked about this, they are willing to do rebar but gave the impression it was maybe overkill. They suggested some sort of fiber additive to the concrete for improved strength. Everyone has bid 4". So my question is, what is really required for a shop slab? Is it worth $5000+ extra for rebar (I don't know the upper for wire mesh)? I want this thing to last, but my budget is not unlimited. I'm in Tucson, so there's not a lot of water or freeze/thaw cycles, if that matters.

 

[Rusted Nut]

The most important thing is having a good supportive base. If your base is good, 4” slab is fine. What is “regular” concrete? What mix strength, what slump, etc.. What is the layout and spacing of control joints. Read Concretin’s guide to floor slabs, excellent stuff.

 

[Mastiff37]

The most important thing is having a good supportive base. If your base is good, 4” slab is fine. What is “regular” concrete? What mix strength, what slump, etc.. What is the layout and spacing of control joints. Read Concretin’s guide to floor slabs, excellent stuff.

I didn't say regular I said "plain" meaning no reinforcement. But my knowledge of concrete is limited. One of my bids with the wire mesh said 3000 lb concrete. The people I'm working with now didn't specify. Thanks for the reference.

 

[Rusted Nut]

Yes, you said plain, my bad. You definitely need a spec for the proposed concrete. 3500 psi concrete is fine for applications with a good base. Wire mesh will not stop shrinkage cracks, but will hold a cracked slab together. Make sure you have what is required by your lift manufacturer.

 

[wssix99]

Is it worth $5000+ extra for rebar (I don't know the upper for wire mesh)?

Not if you are going to install a 4 post lift.

Rebar is not required and is simply insurance against cracking. Its insurance against random cracks, insurance against inadequate saw cut joints, insurance against shallow saw cuts, insurance against general shoddy workmanship.

Cracks are bigger concern for 2 post lifts, which must be bolted down. Bolting is optional with 4 posts, so cracks in the slab don't cause the same level of complications or safety issues.

Instead of spending the extra $5000, you are better to take the day-and-a-half off work and watch your contractor like a hawk. Make sure:
- You have a well-graded, properly compacted base before work starts
- They give you the full 4" you are paying for. (Most contractors don't... they will treat the depth as a minimum or target.)
- The crew isn't slacking off and needs to throw in in extra water or other odd things to get the job done
- The mesh is on proper chairs so it can't sink to the bottom of the slab (where it does you no good)
- A crew returns first thing the next morning to make the saw cuts (no later, no earlier)
- Make sure the saw cut joints are at least 1: deep!!! (If you aren't watching and aren't insisting, this standard [1/4 depth] will not be followed. Its crucial for crack control.)

Discuss the chairs for the mesh ahead of time - if you don't, the contractor won't show with them. You might even just confirm how many you need and buy them yourself to make sure they are on-site: https://www.homedepot.com/p/Hercules-2-in-Steel-Mesh-Chair-100-Pack-944/315019853

^ Also, don't accept hacked solutions instead of chairs like broken pieces of concrete or bricks. They create weak points in the slab and are not proper substitutes for chairs.

 

[carlaisle]

"Plain concrete" is not a specification. The slump, thickness, reinforcement, and minimum psi need to be specified in the contract. The quality of what is under your concrete is more important than the concrete itself. That should also be in the contract. I would have no problem skipping reinforcement on a sidewalk or even a driveway with an adequate base. I don't think the risk/reward is in your favor with a slab, but it's your wallet. Check with the lift manufacturer to confirm the slab requirements regarding depth and reinforcement. If you haven't selected the lift yet, check with the major manufacturers and go with the most stringent specifications. It's not terribly convenient to modify after the fact.

Reference from our resident expert: https://www.garagejournal.com/forum/threads/the-concrete-undergrounds-guide-to-floor-slabs.382626/

 

[drivesitfar]

I'm far from an expert on lifts, but i've poured more than a few concrete slabs. I'll be following along to learn because I need to either hire out a driveway pour or do it myself in the next few years.

one thing I think is common on garage/shop floors that have lifts is they dug out maybe a foot or more area maybe 6 inch or a foot deep under the slab maybe with some bolts too. not sure if this is needed or overkill, but hadn't seen it mentioned yet.

I'm guessing it doesn't freeze much in Tuscon, but I bet it's pretty hot at times so having fresh concrete wetted down fairly often might be another thing to figure in?

 

[wssix99]

one thing I think is common on garage/shop floors that have lifts is they dug out maybe a foot or more area maybe 6 inch or a foot deep under the slab maybe with some bolts too. not sure if this is needed or overkill, but hadn't seen it mentioned yet.

This is not a recommended practice. Its not in any lift manufacturer's instructions and is not instructed by any industry design standard.

Any deviation from a slab with a consistent cross section can lead to differential stresses (particularly if there is reinforcing steel or mesh inside) when the slab shrinks, which can induce cracking. A thickened section can also act as an anchor (like a pipe or other penetration) and constrain the slab when it shrinks. This can also induce cracking.

Beyond the risks, the practice is unnecessary. Modern lifts are designed for the concrete to act as a flat plate sitting in constant contact with the ground.

 

[drivesitfar]

good to know. maybe on the ones I saw the slab wasn't part of the lift foundation? another thing that might be nice is some below surface mounts in the floor (with covers) to use for body work or maybe other uses?

I know that when I do pour a driveway since i'm on a bit of a slope that i'll probably spend the extra to have rebar and 5 inches thick, but I haven't figured that project out 100% yet.

 

[Mastiff37]

Dumb question, but why would a four post lift with a car on it be any more issue than a slightly heavier car? I get that two post lifts are bolted down, but the fours just sit there and their load is no more concentrated than a car.

 

[drivesitfar]

in my experience there are not DUMB QUESTIONS if you are trying to learn. like I mentioned I'm far from an expert on lifts and maybe the ones that had extra concrete were in earthquake zones or maybe that's how they stored their extra cars so 2 cars would be in that area at the same time. I think your part of the world should be ok according to the last poster's information, but i'm pretty positive if you pour when it's hot out that you'll have to hire a good crew or work fast and keep it wet.

 

[wssix99]

Dumb question, but why would a four post lift with a car on it be any more issue than a slightly heavier car? I get that two post lifts are bolted down, but the fours just sit there and their load is no more concentrated than a car.

Great question.

When a car is on a 4 post lift, the slab "sees" exactly what you are describing - A slightly heavier car. A car sits on four tire contact patches. The 4 post lift lift sits on for steel base plates. The contact patches and the plates are roughly the same size, hence the divided weight between the patches/plates is roughly the same and the pressures the slab sees are roughly the same.

2 posts have a bigger base plate, so the downward pressure also ends up being the same when that larger plate is accounted for. The 2 post lift has twisting moments to it, (side-to-side and forward-to-back) which are counteracted by the bolting.

 

[wssix99]

good to know. maybe on the ones I saw the slab wasn't part of the lift foundation? another thing that might be nice is some below surface mounts in the floor (with covers) to use for body work or maybe other uses?

Auto lifts don't have a foundation at all. They just sit on and/or are bolted to a simple slab. (Many people are confused as to why and how a lift works with a simple slab, so its common that they will try to make a "foundation" even though the instructions don't tell them to do so.)

You can search for "Floor Anchor Pots" to find devices to sink into the concrete for auto body work. They also make similar fittings to chain down airplanes at airports, but the garage anchor pots have nice covers to keep the shop floor cleaner when not in use.

I know that when I do pour a driveway since i'm on a bit of a slope that i'll probably spend the extra to have rebar and 5 inches thick, but I haven't figured that project out 100% yet.

I grew up in a house with a driveway on an aggressive slope. Over decades, the slab slid down the hill a few inches. Thickened edges of the slab might have helped slow this down.

My father poured the slab before I was born and it never cracked. Rebar was not used, but he made sure to have a good base and used a really dry/quality mix.

 

[AC-WC]

Follow these guidelines from Bendpak. It's irrelevant which manufacturer you buy from. They all recommend same or very similar. This is what I did for my retrofit in existing concrete. For new slab they have new slab recommendations as well.

For a 4 post the requirements are less than 2 post.

All kinds of youtubes out there for examples. NOTE-babysit your concrete sub for the pour! Pouring for lifts is more of a technical requirement than pads, garage floors, driveways etc. Do not trust them to do it right. Mine did not have experience with this, I gave him the instructions and he was still going to do it wrong. Did great with the concrete but again clueless about lifts/requirements.

https://www.bendpak.com/media/wysiwyg/downloads/Concrete-Slab-Footer-Recommendations-BendPak-2025.pdf

 

[ConCretin]

They are bidding "plain" concrete with saw cut control joints.

So my question is, what is really required for a shop slab? Is it worth $5000+ extra for rebar (I don't know the upper for wire mesh)? I want this thing to last, but my budget is not unlimited.

The previous posters have done a fine job as usual but I'll weigh in to expand a bit on your original question. If the budget is not as you say, "unlimited", that $5,000 could be better spent elsewhere. Rebar and mesh do not make a slab stronger in terms of preventing it from cracking under load. A slab on grade transfers the loads placed on it to the soils below. Steel reinforcing simply holds the slab together after it cracks. Nice to have but not necessary if you pay attention to your base construction to prevent cracks in the first place.

With regard to fiber, it is a defense against a different type of cracking brought on by shrinkage. Concrete shrinks quite a lot as it shrinks, causing tension, which results in cracks. If you cut your control joints early enough the concrete will crack where you want it to making the fiber unnecessary. Ideally this should be done immediately after placement with an early entry or 'soff cut' saw. You might ask your contractor when he plans on cutting your joints. The next day may be too late.

So with all that said, an unreinforced or plain concrete slab is just fine if you pay attention to the details.

 

[Cairo94507]

We poured 4300 PSI, 6"-7" thick for our 44x36 garage. It was then power hard trowled and saw cut for relief joints. We also formed pits for a mid-rise scissor lift and the garage will get a 2-post lift as well. Not a crack anywhere. Our concrete crew have worked together for over 20 years and it was obvious. We watched the entire pour to make sure no water was added. We are very happy with the finished result.

 

[nadogail]

We poured 4300 PSI, 6"-7" thick for our 44x36 garage. It was then power hard trowled and saw cut for relief joints. We also formed pits for a mid-rise scissor lift and the garage will get a 2-post lift as well. Not a crack anywhere. Our concrete crew have worked together for over 20 years and it was obvious. We watched the entire pour to make sure no water was added. We are very happy with the finished result.

My Concrete Guy tells me that he guarantees that his slabs will eventually crack, but nobody will steal one after the check clears.
Nobody has not yet complained that their slab didn’t crack. He has not yet been asked to replace a stolen slab.

 

[ConCretin]

We poured 4300 PSI, 6"-7" thick for our 44x36 garage. It was then power hard trowled and saw cut for relief joints. We also formed pits for a mid-rise scissor lift and the garage will get a 2-post lift as well. Not a crack anywhere. Our concrete crew have worked together for over 20 years and it was obvious. We watched the entire pour to make sure no water was added. We are very happy with the finished result.

I'm glad you are happy with your slab but if you check the bottom of your control joint, I'm pretty sure you'll find a few cracks. Control joints hide cracks, no force known to man will prevent them.

 

[Rusted Nut]

I'm glad you are happy with your slab but if you check the bottom of your control joint, I'm pretty sure you'll find a few cracks. Control joints hide cracks, no force known to man will prevent them.

 

[Cairo94507]

OK, good point. I should have said aside from the control joints, there are no cracks in this floor.

 

[firebirdparts]

Dumb question, but why would a four post lift with a car on it be any more issue than a slightly heavier car? I get that two post lifts are bolted down, but the fours just sit there and their load is no more concentrated than a car.

It's not, it's just that you might have 2 cars sitting there in a one-car-sized space. The loads for a 4 post lift are trivial compared to 3000 psi hardness, but if the base under the concrete is bad, that weight of 2 cars there might break the slab. Somewhere it's sitting over a void. Probably not something you should be worrying about. Instead, worry about not having a void.

 

[Mastiff37]

It's not, it's just that you might have 2 cars sitting there in a one-car-sized space. The loads for a 4 post lift are trivial compared to 3000 psi hardness, but if the base under the concrete is bad, that weight of 2 cars there might break the slab. Somewhere it's sitting over a void. Probably not something you should be worrying about. Instead, worry about not having a void.

I forgot that most people just use them for vertical toy storage. I only plan to use mine for working on vehicles. But hell, the kinds of cars I see doubled up are a hell of a lot lighter than a full sized pickup, and I'd sure hope I could drive my pickup with a load of gravel into the shop without cracking the floor.