Cement Thickness and Reinforcement Guidelines?

[SeanD]

Hi,

I am going to be starting a new home build soon and have been going back and forth with the builder on the cement for the garage. I want them to lay the cement for q future lift in the garage. They are putting it in 5" thick and are meeting the 3000 psi strength. Is there anything else I should have them do? What about a particular reinforcement like wire mesh or rebar? Is it worth doing? What does that give me, if I do have them add it? What about this talk of fiberglass? What would this do to the cement pad?

Sorry about all the questions. This builder doesn't seem to get these types of questions about the garage, so I just want to tell them my requirements straight out.

Thanks,

Sean

 

[pattenp]

My 2 post lift specs called for minimum 4" 3500PSI concrete with steel (rebar) reinforcement. I've got 5" 3500 PSI, but no rebar. When I checked on not having rebar I was told it was okay not to have the rebar but the concrete should meet the 4" 3500PSI strength requirement.

 

[SeanD]

What make/model of 2 post lift did you install? I am looking at the Dannmar MaxJax.

 

[SeanD]

If you put in a 4 post lift, is reinforcement really important? I would think there is less stress on the cement with a 4 post lift.

 

[Bob C]

6" at 3500 PSI with Fiber will carry you up thru a 15K 2 post.

 

[sz0k30]

I have an "Aresco" #9,000 lb 2 post lift on std 4" concrete with fiberglass. No wire mesh & no rebar. I've had it for @ 15 years and it hasn't fallen down on me yet!

 

[SeanD]

Is Aresco the company that had a lift on display at the Detroit Autorama the last couple years?

I have an "Aresco" #9,000 lb 2 post lift on std 4" concrete with fiberglass. No wire mesh & no rebar. I've had it for @ 15 years and it hasn't fallen down on me yet!

 

[ConCretin]

If you have a properly constructed sub base i.e. a well compacted, granular, free draining material of adequate depth, you probably don't need any reinforcement.

Fibermesh is designed to prevent early age shrinkage cracks and buy you time to sawcut control joints. It doesn't add anything to the structure of the slab.

Properly positioned steel reinforcing won't do much to prevent cracks but will hold them together and maintain the structural integrity of the slab if they occur. This includes cracks that form in the bottom of your control joints.

Neither fiber or steel add significant weight bearing capacity to a slab - it's the soil below will support the loads placed on it. The concrete just needs to be thick enough to prevent punching shear and distribute the load adequately.

A 4" is probably adequate but 5" will give you some insurance if the sub grade isn't perfectly flat.. I like a 4,000 psi mix for garage slabs because the higher water cement ratio provides a more durable surface but 3,500 is fine.

I'd pay close attention to the sub grade prep and then install wire or bars for insurance. Rebar is easier to support than mesh because it doesn't sag as much. #4's at 18 or 24' centers supported every 4' is a good start. Good luck with your project.

 

[wssix99]

As noted above, you probably don't need reinforcement for the sake of strength, but you may need it based on the layout of your garage. This is a complication I ran in to when planning mine.

Depending on where your contractor puts your expansion and/or control joints in the slab, you could run in to problems later; particularly for a 2 post lift. Those legs will need to stay a certain distance from cracks and joints, so placement of them in your garage could be critical.

From there, you may find that you need to space your control joints out more than the usual 10'. (Depending on the design of your garage.) If you do need to space them out wider, you'll need to add extra reinforcing to control the risk of cracking.

At this point, I'd recommend picking out a specific lift so you can consult the instructions and installation parameters and also asking your contractor for a floor plan with a joint layout.

This is commercial stuff and not common to residential construction, so you'll probably need to guide your contractors around what you need, because slab design and commercial lift installation issues will likely be foreign to them.

 

[Matt The Hammer]

We use fibermesh on bridges now. Like Willysfan stated above, it has its benefits. I know that the contractors we used had some minor issues finishing it.

Wire mesh is cheap. If anything, toss it in now. Way easier now than later.

Like stated above, the rebar/mesh is to keep the slab together.

 

[Justanoldguy]

Hi,

I am going to be starting a new home build soon and have been going back and forth with the builder on the cement for the garage. I want them to lay the cement for q future lift in the garage. They are putting it in 5" thick and are meeting the 3000 psi strength. Is there anything else I should have them do? What about a particular reinforcement like wire mesh or rebar? Is it worth doing? What does that give me, if I do have them add it? What about this talk of fiberglass? What would this do to the cement pad?

Sorry about all the questions. This builder doesn't seem to get these types of questions about the garage, so I just want to tell them my requirements straight out.

Thanks,

Sean

Cement is only a powder ingredient of concrete.
You may as well call it water.

 

[brownbagg]

i love fiber mesh, i use it in all my slabs, rebar is not needed except on a loaded slab, rebar is a weight factor, most slabs are not load factors. want put it in, go ahead, but it doesn't do anything till a load is apply. car truck, they don't weigh anything.

 

[Dan in Pasadena]

Cement is only a powder ingredient of concrete.
You may as well call it water.

EXACTLY. Cement is to concrete as flour is to cake batter.

PS: Put friggin rebar in. You're building a garage for Pete's sake don't cheap out on the foundation.

 

[joes169]

i love fiber mesh, i use it in all my slabs, rebar is not needed except on a loaded slab, rebar is a weight factor, most slabs are not load factors. want put it in, go ahead, but it doesn't do anything till a load is apply. car truck, they don't weigh anything.

Maybe you didn't read the OP but there would be a load applied via a auto lift, especially if it's a two post lift. Not sure what benefit you've seen firsthand with standard fibermesh, but after 20 years or so of seeing it here, I believe it's a waste of money for most slabs-on-grade.

For the OP, as LLY mentioned, 4000 psi will finish much better and have a far denser surface. We install #3 rebar in every garage floor we pour. There's just too much potential for settlement in new construction now to IMPO. I've replaced A LOT of garage floors that would have performed for decades longer had they originally been constructed with $50 worth of steel........

 

[wssix99]

i love fiber mesh, i use it in all my slabs, rebar is not needed except on a loaded slab, rebar is a weight factor, most slabs are not load factors. want put it in, go ahead, but it doesn't do anything till a load is apply. car truck, they don't weigh anything.

This is not entirely true. Reinforcing can be used in slabs for two purposes:
- resistance to bending
- crack control

For a normal residential application, even with a lift, bending is not an issue (and reinforcing is not required) if the base is properly prepared, as mentioned above.

The need for crack control reinforcing depends on the type of concrete used, the other other controls put in place, (expansion joints, control joints, etc.) and the quantity of other controls put in place.

 

[SeanD]

Thanks everyone for your input. I talked to the contractor and they talked to their concrete people and they don't refer to the strength in PSI but in MPa. There is a conveter for it though. They said they will be putting in 32 MPa concrete, which comes out to 4641 PSI. I have asked them to quote rebar.

 

[Justanoldguy]

We use MPA here in NZ as well.
Most garage floors are only around the 20 to 25 MPA here.

 

[Big Bad Dad]

If the specs call for a 4" MINIMUM, why not spend a little more and go to 5" or 6" for added strength and peace of mind? And not necessarily the whole garage. The concrete should be placed over a 3 to 4" thick base of crushed stone. Why not just have the ground scooped out a couple of inches deeper in the area of the lift and have the stone put in place with a "depression" in the lift area. That would give the extra thickness and should not add very much to the cost.

 

[sz0k30]

Is Aresco the company that had a lift on display at the Detroit Autorama the last couple years?

I go to Autorama every year and there is always a company showing a 4 post lift. Probably Aresco, but I/m not sure.

 

[wssix99]

If the specs call for a 4" MINIMUM, why not spend a little more and go to 5" or 6" for added strength and peace of mind?

If you are using reinforcement, you need more for a thicker pad to get the same resistance to bending and/or crack control. So, on a reinforced slab, it not only increases the concrete cost, but the steel cost as well.

I'm still a fan of adding a little. I specified 5" and found that I was only getting 3.5" during the pour. (For a number of reasons.) Fortunately, I was there and was able to correct the problem!

Why not just have the ground scooped out a couple of inches deeper in the area of the lift and have the stone put in place with a "depression" in the lift area. That would give the extra thickness and should not add very much to the cost.

For a reinforced slab, this can cause more cracking because the the thicker slab will develop more tension (relative to the strength of the reinforcement) from shrinkage than the thinner section. This is one of the reasons why deeper machinery footers/slabs are supposed to be isolated with expansion joints. For a lift, this is not necessary. They are designed for a flat slab.