A better workshop foundation?

[Lee Schierer]

You don't indicate what part of the country you live in. Years ago when I built my pole barn.
barn.jpg
I was advised to put in a compacted gravel floor. It worked great the first summer and winter, however, in the spring when the ground and gravel thawed it became spongy and the boat trailer left ruts. When I paved the driveway, I had the garage floor cemented and have had no further issues.

You might want to consult your local building codes to see what is permitted in your area. You will likely have to pull a building permit to make your shop.

[Bill Jobe]

This thing has been one pain in the butt after another. Now the guy I hired to do the sonotubes has backed out.
I'm considering a hail Mary and hauling in gravel, compacting it myself, then framing it out and pouring a slab. Just too late to do anything else. My second choice for the job must have been upset that I didn't hire him because he isn't answering my phone calls.
So, having never poured a slab, how deep do I need for a gravel base? Also, with night time temps dipping into the sub-freezing numbers i'm wondering if I still have time for a slab. And there's nothing but rain forcast through the next several days. I understand they can mix something in to heat the concrete but know nothing about it.
I honestly do not know what to do at this point. I've decided that i'm fine with the 5' setback. I'll lose some patio footage but I can live with it.

Open to ideas of any kind.

[Bill Jobe]

The next bid in line wants to pour a 5" slab on compacted gravel with no footings around the edge.
Bad idea?
If I raise the slab a couple more inches than planned the gravel base should drain well. Will that do?
Also, I'm going to tell him not to add more water when the concrete arrives, nor any lime. But i'm trying to decide whether to have the concrete source to add fiberglass or airiated (can't recall the term used) .
Which do you think would be better?
It's supposed to get back up in the 50s next week and that's the time he planned to pour.
Any other things i've missed?

[Reed Gray]

Okay..... Personally, I would have a 4 inch thick slab in the center, and 6 inch deep by maybe 1 foot wide on the exterior perimeter. Yes, you could do a solid 5 inch thick slab if you want. The 'fiber mesh' in the mix do work similar to rebar for holding the slab together after it is poured. You can end up with hair balls on top of the slab which can be burned off with a torch. I would do the rebar, #4 or half inch. One piece around the outside, and then 2 ft. grid on the inside. Best to suspend it rather than pick it up. If you can find some knock outs from the cinder blocks, or broken cinder block, that works as well as the little cubes that they sell with the feed sack wires in them. You do need to watch to make sure the concrete does not push it up against the forms as you pour. The cubes can be pushed over as you pour. You want a 4 to 5 inch slump for your concrete, and most dispatch companies will send it out at a 4 inch slump which is a bit difficult to push around, but you can add a little extra water. You don't want it soup mix slump, which can happen if you pour in the afternoon and they are trying to get rid of some thing that no one else wants. It is common to add 'air entranment' in areas that get a lot of freeze thaw type weather. Works better than standard mixes. We use it some times here in Oregon. Not really necessary for interior slabs because they don't get down to below freezing temps when on the inside. It does make it a bit more difficult to work as the mix gets kind of sticky. Most of the time in the winter, we would add hot water to accelerate the set up time. It is a little help, but cold ground and cold water and cold aggregate can cancel most of that. Salts (calcium chloride) is a common additive to accelerate the set up time. Never did like that stuff. I had heard that it can rot out the rebar sooner as salt and metal don't mix well... You will need to plan to insulate the slab for up to a month. Black plastic and straw work well, or they have fancy insulating blankets. Concrete is supposed to reach 90% of PSI strength in 10 days, and be fully cured in 30 days. A bit faster in warm/hot weather. If the building is lifted by crane onto the slab, then safe at 10 days, or even a week. If you are sliding it on, then wait 2 weeks at least, depending a bit on weather. You can drill in anchor bolts (one brand was 'red heads') which work okay. Just letting it sit will be okay unless there is a tornado, and you are in tornado country.

Only 2 guarantees you can give about concrete, It will crack, and no one is gonna steal it.....

robo hippy

[John K Jordan]

The next bid in line wants to pour a 5" slab on compacted gravel with no footings around the edge.
Bad idea?
If I raise the slab a couple more inches than planned the gravel base should drain well. Will that do?
Also, I'm going to tell him not to add more water when the concrete arrives, nor any lime. But i'm trying to decide whether to have the concrete source to add fiberglass or airiated (can't recall the term used) .
Which do you think would be better?
It's supposed to get back up in the 50s next week and that's the time he planned to pour.
Any other things i've missed?

I personally don't like a slab of any size without at least a turn-down on the edges. A lot depends on the slope and drainage. I've seen too many quickly poured things like sidewalks wash out under the edges or corners.

The fiber does work but I really prefer to add rebar even with a small slab, at least around the edges. For a larger slab I get serious with the rebar and weld every intersection. 1/2" Rebar is cheap. (I was an inspector and I've seen too much.) I run an extra piece of rebar near the bottom of the turn down. You cannot put too much steel in concrete. Enough rebar will hold the slab together even with some ground settling. If you use rebar make SURE it is not pushed to the bottom where it is useless - lots of guys doing the pour don't care. I like to block it up to the right height with broken pieces of cinder block - the plastic "chairs" can break down when walked on. This is the end of my shop with the slab designed to support my tractor and bobcat:

concrete_rebar.jpg

If wiring a sub-panel in the building you can clamp a heavy solid ground wire to one of the pieces of rebar and bring it out the side of the slab as a secondary ground.

Make sure the edges are rodded. I always cover the green concrete with plastic and let it cure for seven days. In freezing temperatures we put insulation on the top.

Reed, you sound like you were in the concrete business! I was an inspector at one time, the guy who showed up at the job site with a slump cone and forms for test cylinders.

JKJ

[Bill Jobe]

Thanks everyone.
Robo, your comment on no one is going to steal it reminds me of when I removed the chain link fence and put up a wood privacy fense.
The metal posts were buried in concrete and some of them broke off right at the top of the concrete. I put an add in the paper for the fense, posts and top rails. A carload of guys showed up and said they'd take it all.
Thinking, dang those posts that broke off. Now i'll never get rid of the posts or the ton of concrete stuck to the bottom.
But they meant, literally, all of it. They loaded up huge concrete chunks with the end of a broken post and all. Having just a car they tied all of the top rails ( about 200 ft.) to the roof of their car with bailing twine and away they went, rails splaying every direction. I was so tickled that they wanted everything that I suggested they load the posts - broken and unbroken-- into my truck and I'd haul them home for them.
They were tickled to death and offered me money, although I had advertised them for free.
I just couldn't take the money.
What they ever did with all that mess I'll never know.

[John K Jordan]

Only 2 guarantees you can give about concrete, It will crack, and no one is gonna steal it.....

My concrete guy gives three guarantees:
- It will set up
- It will crack
- No one will steal it.


My 24x62 shop floor has shallow relief cuts every 12'. Four years later we had to get down on the floor with a bright light but we found the cracks - in the grooves but so tight they were almost invisible.

JKJ

[Bill Jobe]

My concrete guy gives three guarantees:
- It will set up
- It will crack
- No one will steal it.


My 24x62 shop floor has shallow relief cuts every 12'. Four years later we had to get down on the floor with a bright light but we found the cracks - in the grooves but so tight they were almost invisible.

JKJ

You cut instead of pressing a groove? Do you think it's the best way? I've never messed with concrete.
So, 1 cut across the middle cross ways should do it for my 12x20 or should I give it 2?
What width and depth do you think is best, John?

[Bill Jobe]

I don't think fiberglass is very expensive. Would adding it to the mix as well as rebar give a stronger bond? This guy hadn't intended to do either.
Also, I have a few lengths of 1/2" rebar left over from making a koi pond cover frame and some trelleses and flower pot hangers. It is considerably rusty. Can it still be used if fully suspended in the concrete?

[John K Jordan]

The concrete installers I know wait a week or so then run a big concrete saw over the surface but it should be done sooner in hot weather. (Most of mine has been in cold weather.) I didn't measure the grooves, but maybe 1/8" wide or so. I think the cut is supposed to be 1/4 of the way through the slab but I found some very shallow and they still worked. Concrete shrinks as it cures and dries so the saw cuts create weaker areas where the cracks can form instead of cracking at random. That said, a small 10x20 slab may be fine without a saw cut. A 12x24 slab for my new peacock shelter back in the spring of this year didn't get any grooves and there are no cracks. I did keep it covered with plastic for about three weeks and occasionally hosed water under the plastic to keep it sopping wet. (I use overlapped 4' wide strips.)

As with most things, there is controversy about rust on rebar. As long as the rust is on the surface and the iron is not corroded significantly I personally don't think the rust matters. Nearly all rebar I've seen at big constructions sites was left outside in the weather and had some rust on it. I've used very rusty rebar pieces to rod wet concrete near the forms and the concrete stuck to it so well it was difficult to break off with a hammer. The ribs on the rebar are what the concrete grips.

Some people don't put steel in a small slab. Lots of people just use welded steel mesh made for reinforcing concrete, available at any building supply. (Lay it right on the gravel, pour the concrete over it, then use a hook to pull the mesh up into the wet concrete. On small slabs around the farm I sometimes use left over pieces of galvanized fencing and add what pieces of rebar I have on hand around the slab, maybe a foot from the edges. For a slab that a portable building sits on the concrete I don't thing the concrete is too critical - most of them sit on gravel or cinder blocks. However, I'd want it done the best way possible - the slab might be used another way someday.

BTW, I like to slope a slab slightly for drainage, sloped more if it will be exposed to rain or need to be hosed down occasionally. This shouldn't matter for yours since it's going to be covered. It's still best to put good drainage around the edges (e.g., french drain if needed), especially if the ground is sloped so water won't freeze under the slab and break it. I just dug a ditch around the back of my peacock house.

I think it's worth getting someone with experience to pour and surface the concrete. And if this needs to be inspected, ask the building inspector about all this. Nothing here needs or gets inspected.

I am certainly no expert on making concrete slabs, all I know is what I've done which has worked for me. I hope to put one this winter down one side of my barn where the llamas and donkeys like to hang out, maybe 10'x40' and another 12x24 for a shelter.

JKJ

[Frank Pratt]

The construction company my brother works for has developed a system for concrete slabs with no control joints at all. The biggest they've done is somewhere in the 10s of thousands of square feet. Can't remember all the details of how they do it.

[Bill Jobe]

The construction company my brother works for has developed a system for concrete slabs with no control joints at all. The biggest they've done is somewhere in the 10s of thousands of square feet. Can't remember all the details of how they do it.

If you think of it please post how they manage to do that.

[Frank Pratt]

I know they start with a meticulously prep'd base that is perfectly flat & heavy layer of poly under the pour. The idea is that the entire slab shrinks as it cures & that base allows it to move freely. As far as mix & such, much of that is proprietary. I seem to recall he said they use glass fibers for reinforcement. It's not always a sure thing though, & sometimes there are small cracks that appear.

[Bill Jobe]

I know they start with a meticulously prep'd base that is perfectly flat & heavy layer of poly under the pour. The idea is that the entire slab shrinks as it cures & that base allows it to move freely. As far as mix & such, much of that is proprietary. I seem to recall he said they use glass fibers for reinforcement. It's not always a sure thing though, & sometimes there are small cracks that appear.

Doesn't the gravel base have to be sloped as well, to keep water from pooling up under the slab?

[Frank Pratt]

Doesn't the gravel base have to be sloped as well, to keep water from pooling up under the slab?

Don't believe so. They use a pretty dry mix so I doubt that water under the slab is an issue. I'm sure drainage of the base gravel is addressed though.