Concrete floor for workshop questions

[Jag]

Every thing in the posts looks good. I would reccomend using a plastic vapor barrier on top of the sand. Also if you are using a grade beam or edge beam, keep about 3 inch of cover (concrete) from the outside form or earth to the reinforcement (required by most codes). Yes I am a structural engineer.

 

[W5FL]

jag,

Guess different areas of the country do things differently. In Texas, if you use a vapor barrier, the concrete will crack all over the place. I have not seen one used on any residential work in over 30 years. I have no idea why this happens. The water table is not a problem here and the moisture penetration through concrete is not bad. Another thing that really matters is the concrete mix and I don't just mean the sacks of cement in the mix. Dryer concrete is harder to work but does not seem to crack as bad as concrete that is poured too wet.

 

[ejb]

I knew this was the right place to ask the question...

Wen, thanks for the very detailed response, but I have re-read your post about 3 times and still don't get it (or can't quite picture it).

I live in New England, so the frost line is about 3 1/2-4 ft I believe..I will need to verify before I start, but I know it is more than a few inches.

I think I agree with you about the pit, I scanned a few books last night and they said a few very negative things about them...not just about liquids (i.e. gas) collecting in their, but als supposedly even the fumes are heavier than air and will tend to collect in a pit, just waiting for a spark to ignite them....scratch the pit.

Now for the floor:

I can't quite picture what you mean by the "beams", my floor is 24X30, my frost line is going to be about 3-4 ft. first reading of your post, I was picturing a 12" X 18" X 30 feet long pour, below the frost line, and that if they were going to be 8-10 feet apart I would need 3 or 4 of them. Since that didn't make any sense to me at all, now I think maybe you are talking about what I would call a "post", in order words, dig 18X12 inch holes (maybe I could use sono-tubes instead), every 8-10 feet in each direction, so I would need about 16 of them (4 rows of 4 post, 1 at each corner and then evenly spaced more or less the rest of the area. The posts would get dug, the rebar would be added to the posts, and they would be poured and allowed to harden.

The rest of the floor, I was going to use the sides of the building (it has concrete walls up the first couple of feet and then turns to wood) as the form, so only by the entrance would I need to put in a 2x12. Then I would dig out the entire floor about 12 inches deep, put in 3-4 inches of sand, use chairs to hold rebar in place every 18 inches (is that layed in a grid pattern, or all in one direction? grid would probably be better).

Is the concept that the floor itself is supposed to sit on top of the posts? If you had a really thick slab, say 6 inches heavily reinforced with re-bar, do the post really do any good? The slab is going to be sitting on the ground anyway, right? If the slab was suspended on the posts, then I could see how they would prevent the floor from moving with the frost, but since it is going to be lying directly on the ground, won't it move when the ground moves anyway?

Do I have that right? I think I understand the rest, but I am going to pickup a book at home depot anyway...their is more to this than I realized, and would rather do it right first.

 

[MarkV]

EJB-Sounds like you have gotten good advice from those who have posted already. I build down south and no frost problems here so things maybe differnt. We would not feel the need to run grade beams or post pads/footings in a slab unless it were a load bearing point for the building. If I understand, your building is up and you are infilling the existing floor with concrete. I would go along with the other advice given for a slab. Like MarkC said, we have had great luck with fiber reinforced concrete. It has proven to be very strong and not hard to work with. I think I would talk to a local concrete person and find out the local norm.

Wen, a 50x90 slab with hand mixed concrete. Glad I missed that one./w3tcompact/icons/smile.gif

<P ID="edit"><FONT SIZE=-1>Edited by MarkV on 6/14/00 07:57 AM.</FONT></P>

 

[JimBinMI]

EJB & others,

I had a Pole Barn built last summer and plan to put the concrete in this summer/fall, so I'm interested in this discussion. My township requires footers, 4' deep if the barn/garage is attached to the house. My neighbor built a beautiful barn last 2 years that could be a house. It is not attached and is stick built, he was required to only have 16" x 18" footer poured all the way around, he then came up with 2-3 rows of cement block, then stick built on top of that.

Since my building is Pole construction, all posts had to be minimum 4' deep and sit on top of cement in bottom of hole. I'll be pouring a slab inside the 32' x 48' barn. {Should make implement attachment much easier!} Most people doing this here just pour 4" all the way around. Some have suggested that I pour the edges to a foot or so as a Rat Guard (That ought to keep Paddock out!/w3tcompact/icons/smile.gif) to keep animals from digging under the edges...not necessarily for structure of the floor.

Comments Welcomed!

JimBinMI

We boys and our toys!

 

[TomG]

We live in a small place, and our building inspector lives just up the road. I probably would take out a building permit even if I didn't need one. Permits usually are based on the value of the construction, and should be cheap for that job. Inspectors know something about building in their areas and also have pro forma examples of various building applications. For not much money, you'll find out what works and also if any codes are applicable.

I believe it's common to build 'slab on grade' for non-residential structures in most areas. The construction is designed to float on the ground. What is important is not so much how deep the frost goes, but how much frost heave is present. Heave depends on temperature, soil and drainage. The amount of heave your neighbour has can be very different from yours.

The combination of heave and load determines a building technique that won't destroy itself in a few years. Whether a structure will be heated or unheated also makes difference in building technique. The roof and drainage also make a difference. Frost heave can exert enormous pressures. Basically, your structure is design to float, or its heated. It's not realistic to build strong enough to stop frost heave. There is an unused restaurant/garage near here that is pretty much destroyed by frost heave. Roof drainage failed, which let water collect underneath a slab on grade floor under part of the building. As near as I can figure, the frost heave was enough to snap a 2x4 post on a stairway as well as crack concrete buttresses on the other side of the building. Of course, there are cracks everywhere and you can see outside through some fairly recent repointing of concrete block joints. It's a real good idea to make sure that slab on grade construction is on a well-drained site. A house on the same property had a corner of the foundation crack from being left unheated for one winter.

I'm no expert (as noted, I take out building permits to get cheap advice), but I think that a sand bed and rebar reinforcement is almost unavoidable. Foam insulation under the slab and vapor barrier also are good ideas, especially if it's going to be heated. The foam provides additional cushioning against frost heave. I think of 3.5" section slab as sidewalks, but who knows, maybe it would be heavy enough.

An appropriate thickness and reinforcement should be a decision based on considerable building knowledge. As noted, what works is related to the slab and load as well as the building and the soil and temperature at your specific site. If I was depending on 'rules of thumb,' I think I'd want to seriously over-design the slab.

 

[Von]

ejb,
Thought I would chime in. I built my shop about 5 yrs ago. and it has two floors, both are concrete. Top floor is on 2" bridge decking like they use on the roads. Is 7" + 5" thick. (bridge deck is corragated) and has both rebar and mesh. (weights in at 27K lbs.) The bottom is a floating slab 5" thick (the block walls go down to below the frost line) Has 6" of washed stone and drain pipe on the outer edge, also both rebar and mesh. Both hold up very well. Just leave room for expansion on the outside edge. I did all the setup and then paid someone to pour and finish the top. I have drains in both floors and I wanted the slopes and finish to be right. It was well worth the money to have it done right the first time! I think I would have been mad everytime I saw any screwups that I made in the finish by being cheap. Like someone said you need to know how to smooth it out right and the tools to do it. At My old house I built a new garage on a floating slab, the outside edges were 14" deep and the center was 6". All over 12" of stone. Its still standing with no cracks after 15 yrs. I did that one my self and thats why I hired out the next one. I knew I wouldn't be moving in the near future.

Von

 

[W5FL]

Yes, the rebar is in an 18 inch x 18 inch grid.

We do not have frost heave to any great extent in Texas. Beams below the frost line are inexpensive to pour here. You can make the slab 3 ways. One is to just use cushion sand and pour the reinforced slab on top of the sand as flatwork. It will probably crack, but will be ok for a workshop floor. Yes, it moves with the ground, the trick is to get it to stay in one piece when it moves! /w3tcompact/icons/smile.gif

You can put 16 inch pier holes down to the frost line around the exterior every 6 to 8 ft (yes 16 posts). Put 4 pieces of 5/8 rebar in the pier holes and pour a 6 inch thick reinforced slab on top of that but you must pour all of the piers and slab at the same time. Should use 4 to 6 inches of cushion sand under the slab.

You can float a slab with a permimeter beam about 12 inches wide and 18 inches deep with another beam 30 feet long down the middle and two more across the long dimension with 4 pcs 5/8 rebar in each beam. This also requires 4 to 6 inches of cushion sand under the slab. If frost pushes it up, you hope it all comes up in one piece since you must pour all of the beams and slabs at one time.

Concrete required is roughly 11 yards with the flatwork, 21 yards with the posts, and 20 yards with the beams. The flatwork approach is probably suitable, but would check with someone in your area. Either piers or beams make the structure much stronger, but cost a little more.

It is cheaper to build things in Texas, but you knew that anyway. We also have NO codes outside of incorporated areas, so you better know what your subs are doing as there is no building inspector. Just the tax man and he only cares how big it is.

Hope that is clearer than mud. /w3tcompact/icons/smile.gif

 

[Budinohio]

ejb, I've read all the replies, and wish I had all the advice that you are getting. At the time when I poured my 42x48 shop I asked alot of questions but diddn't get many possitive replies on exactly what way to go. This is how I did mine, it sits on solid ground (not loose fill) with a 7-8 in. fine stone base (screenings). I ran a 4" perimeter tile 2' away from outside of building, I went with floor heat (1750' of tubing in floor) so I put vapor barrier down with 1" blue board for insulation on top of that with 2" blue board around perimeter of wall. I then fastened tubing to floor. I did not go with any kind of footer or rat wall, and I poured 6" of fiber reinforced 6 bag mix concrete on top of tubing. I insulated walls with nu-wool blow on with 3/4" osb to finish walls, and the same in the ceiling.( R 28 in walls R 40 in ceiling ) If I was doing it over I would have a footer and 2" blue board under floor. It's a pole barn by the way,and as far as footers I can still do so I've been told just harder and more exspensive to do now but 2" blue board in floor is obviously a no go. I just did get insulation done so no report on how it heats yet. Good luck on your project, bud in ohio.

 

[Wingnut]

I've had a bit of experience too (and my father spent some years as a cement contractor) ... but mine experience is for the cold latitudes. The frost line in mid-Alberta is 6 feet ... so we didn't worry about pouring supports that deep.
As Wen stated, the fill is of prime importance. Any time I needed to put in a floor that I wanted to last, the secret was a good smooth base, 6 inches of gravel, 2 to 6 inches of samd, lots of tamping to get it settled, and then a 4 inch concrete pad with adequate rebar. Forget the welded wire fabric - go for the real stuff.
(The most interesting floor I ever worked on was a liquor store floor in the high arctic. Because of the permafrost, the floor was actually 3 feet off the ground, supported on pylons. Steel subfloor, and then 6 inches of concrete with 2 layers of rebar.)
Forget the pit ... besides sump problems, they're a B*tch to keep clean and dry, hard to get in and out of and darned dangerous to boot. Just think of the fun of trying to get your tractor out when a front wheel "misses"

too bad that common sense ain't