Need help with monolithic slab for 1.5 story, 2-car garage 1

[NCEngineer]

I know this forum is mostly for on-the-job engineering questions, but I am planning a 1.5 story, 2 car garage build in the next few weeks and I wanted to get some expert opinions before making any big decisions. I am trying to save costs by doing the drawings myself. I am also want to do the site-prep (dig, set forms, & ready the site for pouring concrete) myself with the help of my two able-bodied sons (17 & 21 yrs old) and my ex-contractor neighbor. We plan to have a contractor come

We have decided to go with a monolithic slab for the 26' x 36' (outside dims) garage to avoid multiple days of concrete pours. Also, the upper 1/2 story will be setup for my wife to use as a crafting room and I plan to use some of the main floor space for a workshop. However, I am a mechanical design engineer, not a structural or concrete engineer, so I feel a little outside my comfort level shall we say...

The ground at the site is "relatively" flat, but it seems to 'dip' in the middle of the 26' edge. The soil conditions are about 24-48" of sandy loam soil on top of very hard NC clay (this is based on other post hole digging and tree planting done in my yard, not on a specific soil analysis). Also, after a torrential rain (say 2"/hr) we will have standing water in some of the area we have marked out for this garage. Granted, this doesn't happen very often, but it does happen.

According to the local inspector that I talked to, rebar is not required, and the depth of the "footing" section of the monolithic will have to go 12" below grade, with a 6" minimum above grade.

Some questions:

1. Do I need a vapor barrier because of the rain issue? We would like to seal or coat the concrete floor in at least the workshop area.

2. What about wire mesh vs. rebar? I understand that cracking should be expected, but I want both to minimize costs as well as maximize the structural integrity of the building/slab.

3. What material should I use for a sub-base? Again, I want to maximize integrity and minimize cost.

4. What about finish? joints? other stuff I have no idea about?

Thanks for any and all help I receive. I apologize for the length, but I hope you understand my situation better.

Sean

 

[briancpotter]

If I'm reading you right, your slab will be 18" thick, which seems excessive to forgo reinforcing. Not to mention 18" x 26' x 36' is 52 yards, or about 5 trucks' worth. How did you plan on pouring that much in a single day?

Brian C Potter, PE

http://simplesupports.wordpress.com

 

[rowingengineer]

1. yes a vapour barrier would be good
2. reo is always good, both in the footing and in the slab, I would get a soils test so you can determine just how much the clay will shrink and expand. This will allow a structural engineer to give you correct depths of footings.
3. depends on the design
4. finish- depends on you look requirements. joints - depends on the amount of reo.

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[NCEngineer]

Sorry, forgot to mention that the thin section of the monolithic will be 4" thick.

 

[NCEngineer]

The monolithic section is 18"x16" on the outside edges (what I called the "footer" section), then it tapers at a 45 degree angle to 4" thick everywhere else (what I called the "slab" section).

According to my rough calculations, the total amount of concrete "should" be just over 21 yards. Is that feasible for one day's pour?

Sean

 

[structSU10]

Do you have a frost depth in your area? You would want the bottom of footing, or perimeter grade beam, to be at that elevation. Do you plan to trench pour some or all of the grade beam? Are you using any crushed stone fill? You plan to pour the grade beam monolithic with the slab correct?

 

[Splitrings]

I would spend a few bucks and get this reference. It includes details of the thicken slab detail you intend to use.

http://books.google.com/books/about/Design_and_Construction_of_Frost_Protect.html?id=dMp4hWfCu20C

My two cents:
1. Use a vapor barrior. Concrete is permeable and will allow ground water to penetrate your slab.
2. Reinforcing a slab on grade will hold cracks that will form tightly closed. It will only give modest strength gains. If you are looking for greater strength it is most economical to increase the slab thickness and not reinforce.
3.Use a base material that is easy to work with, 3/4" max size. If you use a clean stone product it will require less compactive effort. Use only the min. amount require to level up the surface. Again the thickness of the base only contributes a minor amount of the strength that SOG has. Again, it is more economical to use a thinner base and increase the slab thickness to gain strength. For a SOG, uniform support under the entire slab is most important. Any changes in the support provided by the subgrade from one spot to another will likely effect the performane of the SOG.
4. The amount of reinforcing controls the spacing between control joints. For a non reinforced SOG an old rule of thumb is the control joints should be spaced 2.5 - 3 times the slab thickness. As far as finish, don't go crazy with the power trowel if one is used. It will make a very polished smooth surface that can be very slippery.

When you place the concrete, place it has dry as you can and still achieve the finish you want. Don't add any water that you don't need to. It will cause more shrinkage cracks. You might even consider a water reducing or plasticizing admixture. It will make a dry concrete "flow" much easier.