Any concrete pros care to give some input?

[Dynasty]

I am having a metal building put on the property. It is one of the prefab galvanized 2" tube frame carports with the sides and ends enclosed, so not real heavy. Comes on a trailer and a few guys slap it together in a day or two. It will be installed on a concrete slab. My county wants me to put footers 42" into the ground due to local codes and weather (cold winters, hot summers - OHIO). Unfortunately, if I do what they want, it will price me out of my own project. I had a few concrete contractors over and they all laughed when I said the county wants me to put footers for this application. They all said its NOT necessary for this type of building.

So, with no footers I would like the concrete to be as strong as possible for long life without spending the extra thousands that footers cost. I was thinking 4" thick concrete, 4000 psi, fiber reinforced, AND rebar. This is in additon to having the ground heavily compacted to avoid moving and busting up the concrete as the weather changes.

My questions are:

What specs should I aim for as far as the aggregate base to be used?

What specs should I aim for in terms of the rebar diameter and rebar spacing? The slab dimensions are 27' x 31'

I do not have much experience with concrete, so I am open to suggestions.

 

[Finally]

I am in residential construction in Florida, I have no idea about an aggregate base, but we pour the slab concrete directly on the dirt (though covered with 20 mil plastic) All rebar is #5 (about 5/8" thick) for a typical slab on grade. I am not sure what you are referring to with spacing, but you would typical run 2 rows of rebar around the perimeter of the footer, spaced evenly so as to be a minimum of 3" away from the sides of the footer. This allows for the concrete to fully encapsulate the rebar. You would also need rebar chairs to hold the rebar the 3" off the bottom of the footer.

You would only "need" rebar in the footer where the load is. You can always add pieces on the corners to keep the curing cracks to a minimum. If you are using fibermesh, rebar within the body of the slab is not required (unless you are in earthquake zones).

 

[rustyjames]

I would install a 4" base of clean, crushed stone for the slab prep. No fiber, and WWF instead of rebar.

 

[theoldwizard1]

Absolutely no organic material in the base ! Use a good vapor barrier. Insulation if it is in the budget.

If the inspector says you need a foundation, I don't know how you can get away with not having one. I some areas of the country a perimeter foundation is called a "rat wall" because it prevents small animals from borrowing 7nder the slab.

 

[3rdgendslmech]

When I did my barn i stripped all the top soil, removed enough dirt for a 4 inch base of recycled concrete. 2" at a time and wetted it down pretty good before I compacted it with a really heavy plate tamper. Concrete was 4" of 4000psi. I've got a 10K baseplate lift and i've picked up 8500 lb diesel trucks everything is still good.
There's one problem with your statement though.....you said "the county wants" if you're doing this by permits you might want to do what they say otherwise it could create problems. If you dont need a permit then a 4 inch base of RC6 and 4 inches of conrete should be good enough.

 

[brownbagg]

sonotube under the legs

 

[kaymccampbell]

Can't you do an Alaskan slab under it? Around here, most standalone garages, prefab or not, sit on a variant of an Alaskan slab.

 

[Bretny]

I have done some concrete work and my carport slab with no footings and a 48in frost depth. A good compacted stone base at least 4in is needed, concrete underlayment, don't skip this step. You an also use mesh and would use it over rebar. Dont slip the underlayment step because you can get under slab water leaking up into your building and theres really no easy cure. Mesh over rebar because the fact is the slab is going to crack. Mesh will help hold it together just the same as rebar but it's way easier to install.

Also I would tend to go 5in thick and possibly 6-7in thick at the garage door.

 

[Retroman]

I would hire a structural engineer to draw a slab/footing detail and present it to the county when you go for your permit and see if they will accept it. That will give you all the info you need on base material, vapor barrier, Steel reinforcement, slab thickness and PSI of concrete.

 

[Jlbc212]

Any place where there is stable soil if the organic (loam) soil is removed and replaced with a good compacted stone base, then that base alone would be sufficient to hold up just about any vehicle. A properly mixed and not watered down 4" layer of concrete added on top simply provides a hard, durable surface that can be easily cleaned and is nice to work on. Neither of my two garages have any rebar or mesh in the concrete. Both are free of cracks. A concrete driveway I had installed at my previous house back in the 1980's still looks good today. My former neighbor's driveway installed in the 70's also still looks good. Those also have no rebar or mesh. This is in Massachusetts.

 

[ConCretin]

If you have a few minutes, take a look at my Guide to Floor Slabs in the link below. It addresses your questions and might be helpful as you plan your slab.

I've never really under stood the concept of thickening slab edges for frost protection that is common in some regions. It assumes the frost is not going to get under the slab itself which is integral with the 'footing'. This seems like a recipe for differential movement and cracking. We would never think about building that way up here.

A 6" to 12" thickened edge provides some extra capacity where building loads land and where you drive in and out but going down 42" for frost seems senseless.

 

[Walter_TA]

If you add fiber to concrete it will give it the strength to not crack as it cures. Rebar not required. A thickened edge of 8 inches deep, 12 to 18 wide is good. I had my dirt guy did one bucket wide around to edge. Make sure you have vapor barrier 10 mils. Pour at a slump of 4. Do not let them add water(If you can). Keep the forms on for a week. Keep the top wet for a week. I used burlap with a sprinkler on a timer. Do not let the top dry out, it will not cure. 4000 PSI is good. If it is poured to wet it will crack. You need a good base, that is covered above.

 

[wssix99]

Who is the manufacturer of the building? What specs do they require for the install? That is the detail that you need. The building will be designed either to be placed on a flat slab or on footings. If the building is designed for one and you do the other, you are going to have problems, no matter what your installers or your county says.

If you are in a freeze zone, your county folks are going to default to a "footer" answer if they don't have all the engineering details, so you can take that with a grain of salt. Does your county require that you have this inspected? If not, you can ignore them anyway and just follow the instructions. If they do require that you get this inspected, you should be able to provide them with the required manufacturer's/engineering details and they will come to the right answer. (footings or no footings)

So, with no footers I would like the concrete to be as strong as possible for long life without spending the extra thousands that footers cost. I was thinking 4" thick concrete, 4000 psi, fiber reinforced, AND rebar. This is in additon to having the ground heavily compacted to avoid moving and busting up the concrete as the weather changes.

My questions are:

What specs should I aim for as far as the aggregate base to be used?

What specs should I aim for in terms of the rebar diameter and rebar spacing? The slab dimensions are 27' x 31'

I do not have much experience with concrete, so I am open to suggestions.

You should hold off on all of this until you answer the questions above. If your building requires footers, the slab will still break apart even if you make it "stronger." Likewise, making the slab "stronger" than spec is just throwing money away.

All the answers to your questions and your problems should come from your carport manufacturer. (We can help you translate things for your county.)

Beyond that, if you are looking to do work under the car port (vs. just using it for storage), you might have goals for quality of the floor. If so, we can help with that also. (Control joints, reinforcing, cement content of the concrete, fibers, etc. all impact the finish and quality of the slab, which will impact how it looks, feels, and wears for you.)

 

[Walkers]

Can't you do an Alaskan slab under it? Around here, most standalone garages, prefab or not, sit on a variant of an Alaskan slab.

Okay, I’ll bite, what’s an Alaskan slab?

 

[Walkers]

Contact the company and have them send you an engineered drawing for the slab, they should have one on file for this. Other option is to have an engineer make a drawing for you.

 

[Neggy]

Alaska concrete slabs; concrete in alaska Archives - Hansen Buildings

 

[kaymccampbell]

Okay, I’ll bite, what’s an Alaskan slab?

A monolithic slab with a roughly 18" thickened edge, extra rebar around the perimeter, and sometimes an insulated skirt around the outside. It was used in cold climates and just floated on the surface. I'm sure there are other names for it by now, but back in the day that's what we called them.

 

[p00p]

I was just talking with someone that did a similar structure. They did something very different than mentioned in the above posts for a slab.

They had a few inches of asphalt laid down, leveled out on top with a few inches cleaned gravel, put down forms, used fiber mesh, & floated in 4" worth of concrete. Didn't really probe for details, & don't know if they did this on a "Sunday" , but I would like to know how it'll hold up after a few years. If I ever run across that person again, I'd like to ask more questions.

 

[rustyjames]

A monolithic slab with a roughly 18" thickened edge, extra rebar around the perimeter, and sometimes an insulated skirt around the outside. It was used in cold climates and just floated on the surface. I'm sure there are other names for it by now, but back in the day that's what we called them.

FPSF:

Frost-Protected Shallow Foundations: Saving Energy and Construction Cost with Foam-Plastic Insulation - Green Building Solutions - Green Building Solutions

Featured Featured Recent Blog Posts Select Page Frost-Protected Shallow Foundations: Saving Energy and Construction Cost with Foam-Plastic Insulation Jay H. […]

www.greenbuildingsolutions.org

 

[Uncle murph]

I am having a metal building put on the property. It is one of the prefab galvanized 2" tube frame carports with the sides and ends enclosed, so not real heavy. Comes on a trailer and a few guys slap it together in a day or two. It will be installed on a concrete slab. My county wants me to put footers 42" into the ground due to local codes and weather (cold winters, hot summers - OHIO). Unfortunately, if I do what they want, it will price me out of my own project. I had a few concrete contractors over and they all laughed when I said the county wants me to put footers for this application. They all said its NOT necessary for this type of building.

So, with no footers I would like the concrete to be as strong as possible for long life without spending the extra thousands that footers cost. I was thinking 4" thick concrete, 4000 psi, fiber reinforced, AND rebar. This is in additon to having the ground heavily compacted to avoid moving and busting up the concrete as the weather changes.

My questions are:

What specs should I aim for as far as the aggregate base to be used?

What specs should I aim for in terms of the rebar diameter and rebar spacing? The slab dimensions are 27' x 31'

I do not have much experience with concrete, so I am open to suggestions.

I’m guessing it’s not bearing load as much as the very real possibility that the thing could wind up airborne at some point and possibly kill someone.

 

[matt_i]

If you google "hud frost protected shallow foundation" you will get a .pdf document that's a "cookbook" to design your frost protected shallow foundation for basically all of north america and also for unheated buildings. I would submit the HUD guide and also your own specific calculations for your foundation design when permitting outside of the classic 42".

If you do get funneled into 42" despite other efforts you can do a trench foundation, 8" wide (earth-formed) and pour the foundation and slab as a monolith with the only forms being right at the surface of the ground.

 

[jmdirk]

Lots of interesting suggestions. But wssix99 nailed it. The foundation needs to meet the requirements of the building supplier AND the county. Many times the county will tell you to do something overkill, or you get an engineer to design it.

There a few slab engineers around here (Ontario Canada) that will do up a slab design for a few hundred dollars. They'll ask for the building drawings are requirements and then design the slab to meet those. Once you get an engineers stamp on the drawings, then you'll probably not have any issues getting the permit. Without, the county will just make you build for the absolute worst case scenario.

 

[welder4956]

sonotube under the legs

^^^This, 42" deep. Pour the floor after the carport is installed.

 

[billconner]

Interesting this 6 month old thread came back to life today. I wonder if the building is built?

The IRC, the most commonly adopted building code, allows 600 sf without frost protection. So one states amend this. I think PA is 2000.

If the building is to be heated, the shallow frost protected foundation makes sense. I recommend the ASCE 32-01 standard over HUD, as it's a little less restrictive and what is referenced by the model building codes.

It it's never to be heated, the SFPF gets expensive but still possible. I think if unheated, I'd go with welder on piers at the columns to below frost depth, and pour a slab floor after, maybe with a pressure treated 2x8 or 10 at edge projecting 4 to 6 inches above slab inside metal siding.

Curious what you did.