Please help! Concrete question

[Penny Pincher]

Hi all,
This is my first time visting this site. My husbands going to love it! We have a small hobby farm in NE Wisconsin. We try to do as many of our own projects as possible with a little help from friends. Our next project is a 30x36 garage that will house a tractor, pick-up truck, etc. We could really use some advice on the concrete. Such as how deep and wide the footings should be, how thick the slab should be, what bag mix should be used for the concrete, and any information anyone would have on re-enforcing, moisture barriers needed etc. Cure time? Poor the footings one day and the slab two days later? Can the entire slab be poured at once? Can you use the ground as your form on the outside edge of the footings? Is it necessary to lay sand or gravel under the cement slab? We DO have friends helping that have knowledge/experience pouring concrete, but I would like to be better informed myself. Any help/suggestions would be greatly appreciated!!! Thank you! :confused2:

 

[MikeInEburg]

First - welcome to TBN. While I admire your goal to DIY, this large concrete project can become a real disaster if not done properly. Especially if you're in a colder climate where frost heave is a possibility. I'd consult a professional.

That said, I can answer a couple of your questions.

Yes, the material under slab must be compressed. Most folks put down gravel for the base. Plastic below slab will keep moisture out. Blueboard under slab will keep floor warmer. Much of the rest depends on your climate and the heaviest load you need the floor to support.
Good luck
Mike

 

[smstonypoint]

First - welcome to TBN. While I admire your goal to DIY, this large concrete project can become a real disaster if not done properly. Especially if you're in a colder climate where frost heave is a possibility. I'd consult a professional.


Mike

I'm with Mike on this one. Mistakes prepping, pouring and finishing concrete are very expensive to correct. Unless your friends have a lot of knowledge/experience and are willing to put in the time to supervise and help you, I would hire a pro.

Just my two cents worth.

Steve

 

[yooper1]

Hello and welcome to TBN

 

[Egon]

Welcome.:thumbsup::thumbsup:

Consider checking what your local regulations call for in regards to footings, reinforcement etc.

Check out "The Portland Cement Association" on the internet. They have lots of information.

Check with some local contractors for pricing. Especially for just the pour and finish.

Note: Pouring concrete may entail a lot of work and a sub par appearance and performance if done by folks with no experience. Not saying it can't be done but it can be difficult.

 

[Carl_NH]

Welcome to TBN.

I built a 24x36 Barn/Garage 10 years ago with a floating slab setup. Essentially you have a level area sited, remove the loam/topsoil, add 3-4" of crushed stone on the pad, then dig the perimeter trench.

In our area (NH) code was 24" W x18" D trench around the outside so I excavated 24" deep all around added crushed stone to 18" then vibratory packed the trench and pad areas.

The concrete guys came and formed around the perimeter with 2x12"s and stakes, then added 1/2" rebar in the trench looped around and tied into the rebar in the pad area. In addition rebar was laid on the bottom and top of the looped rebar in the trench. THe last thing they laid wire mesh in the pad area. This took most of a day doing the rebar and tie in's then the next day they poured concrete - about 30 yards if I recall.

Total job on the prepared pad was $2700, including finishing, concrete sealant, and cutting scoring lines every 10' to prevent cracking.

So far for 10 winters (below zero some times) no problem.. Good Luck and as Egon says, check your local codes..

Carl

 

[jeffgreef]

"Such as how deep and wide the footings should be, how thick the slab should be, what bag mix should be used for the concrete, and any information anyone would have on re-enforcing, moisture barriers needed etc. Cure time? Poor the footings one day and the slab two days later? Can the entire slab be poured at once? Can you use the ground as your form on the outside edge of the footings? Is it necessary to lay sand or gravel under the cement slab? "

1- Depth of the footing depends on your frost depth in your area. If your footings don't extend below the frost depth, heaving may crack the foundation and tweak the building. Find recommended depth from local contractors or building department and go at least that deep. Width depends upon the expected weight of the building, plus snow load that is possible. You can get an engineer for that, or an experienced builder can give you a rule of thumb. If you can afford to hire an engineer and have it all checked by the local building department in the permit process, you will be almost certain to get a trouble free foundation.

2-Slab thickness- 4" so long as it is not taking any structural weight, or very heavy machinery. Cars and CUTs are not too heavy. Using rebar in the slab in stead of wire mesh gives an extra measure of security against cracking that lifts and offsets. Cracks will occur, but if they don't offset they are not a problem.

3-Bag mix- use a standard 2500 lb. foundation mix unless your engineered design requires more, rare for a small building.

4-Reinforcing- an engineered design will show you that, or your wise local contractor will use a rule of thumb given the size/design of the building. The amount of rebar and size of footings depends entirely on the design of the building.

5-Moisture barrier- most engineers call for several inches of gravel, a moisture barrier, then a few inches of sand below the slab. A plastic layer for the moisture barrier is all that's needed, it will reduce or eliminate water coming up through.

6-Cure time- Always let the concrete set up a few days before you put weight on it. Temperature must be above 50 or so when you pour- if it's below when you pour you can screw up the concrete. Don't pour in winter. Keep it moist- it cures by reaction with water, not by water drying out.

7-You can pour footings and slab separately, or all at once. The latter is harder to accomplish, but finishes up the job in one step instead of two.

8-You can use the ground as your form anywhere unless the dirt is so soft it caves in. Make sure the bottom is firm native soil, never fill in the bottom where you will pour concrete with dirt. Just use more concrete on firm native soil if you excavate too deep.

I suggest putting in underfloor plumbing for a toilet room, sink, washing machine, whatever. It's real easy to do it beforehand, a big pain in the rear to do it later. You might never use it, but it won't be in the way.

Professional concrete finishing is a learned skill. Getting a slab flat, smooth, and sloped evenly toward the door to drain any water away is not easy. If you do it yourself, study a lot beforehand. I would hire a professional concrete person or two with the right tools for the pour to be sure it looks nice.

If a permit is required for your building in your area, and you don't get one, and later an angry neighbor wants to mess with you, all they have to do is call the building department and complain. Set your mind at ease and get a permit. This will also help you guarantee that your design is well done. They might let you design it yourself without paying for an engineer. In that case find a local contractor to, at least, design the foundation.

Concrete work is brutally hard labor. Hire a crew.

Good luck!

JG- Building Inspector