Mono pour of foundation and slab
I’m going to start on my shop this spring. I had planned on pouring footing and stemwall first, then pouring the slab, but for the sake of efficiency I’m thinking about one pour. Obviously there would be a perimeter form. There are a couple of piers in the middle with column bases. The entire thing is over structural fill and foam insulation board. From those of you that have done this, any tips you can share on how to form? How to approach this in general?
Doing it in one pour is the main attraction for me, since I have to stop work and borrow a couple of my friends for the day to place mud. Also seems like less work to form and strip the thing. Any other pros or cons that I am not seeing? All the concrete I’ve ever done has been foundation first and slab later.
Replies
It's pretty straight forward. Site prep is important, digging clean trenches for the turndown is also important.
I wish I knew how to draw on the computer, but I'll try to desribe the form as I know it. If this isn't clear I'll try again.
If you take a whole bunch of 2x4 scrap that is anywhere from 2' to 4' long and cut a slight miter (maybe 5 degrees) in one end and nail that beveled end perpindicular every few feet to a long 2x6 you have a long 2x6 with all these little short pieces sticking out like outriggers at a right angle and down a little bit.
If you use two 3/4" steel nail stakes on each of those 2x4s you can set the 2x6 form board around the outside of your turndown and adjust the height quite accurately with those nail stakes. So now the form board doesn't have anything sticking up in your way and it's easy to get straight.
Then if you tack your rigid foam to the inside of the 2x6 and shovel some dirt to the outside of the foam as the concrete comes up you can minimize the waste in the turndown.
If you are not careful a monoslab can be a spectacular waste of concrete, this technique helps.
How high do you want the stemwall to come above the slab?
I want the wall and slab flush, everything about 8" above grade. Nothing sticking up except anchor bolts around the edge. The footprint is 24x32. There's one bearing point about 10 feet from one end, along the centerline, for a post that's contained in a wall.
That sounds like a slab with a thickened edge, David. Is this on a flat spot, or is there slope involved?
The site is about 18" slope over 32' and I will have my excavator backfill on the low side so that the building is sitting on a flat spot when done. I want a uniform 8" exposure of concrete above grade all the way around when done. The concrete truck can get to three sides of the site so I'm sure there won't be any need to pump.
One of the things I am not clear about is how to place and compact the gravel base for the slab area. I am assuming 6" of 5/8 or 7/8 crushed rock set with a plate compactor under the slab, and 4" insulation board over that. Very easy to install all of that nicely when the stemwalls are in place, but harder to imagine when you have a perimeter ditch for the turndown with that nice 45 degree cut into it. Easy to knock dirt and gravel in there while doing the slab prep, unless you do that first.
I like Brownbagg's idea of placing the footer first, then coming back on top with the slab, one pour. Hell, I like all the ideas. I have a hard time committing myself to something unless I know exactly how it's going to go... especially when I have ~25 yards of concrete involved at $150 per.
If you e mail me your mailing address, I'll send you a drawing of how I'd form it, David. Piece of cake. This has got to be the easiest monopour imaginable. No way I'd pay short load prices for mud for a footing for a job that size.
As to your fill compaction, a lot depends on the surface you are compacting on, the material you're compacting, and the equipment you have available. You can use pit run gravel for fill a tractor bucket at a time as long as you have plenty of time to soak it. I have used a garden sprinkler for a few days and you'd be amazed at how well the water displces the air, then runs out itself or evaporates, going a long way toward compaction.
If you use a plate compactor, I'd be carefull to only fill and compact a couple inches at a time. Otherwise you can whack the crap out of the top few inches, leaving a lot of loose fill beneath. I only use them for the very top, so as to not waste concrete.
You're in the San Juans, aren't you? I have no idea what the ground is made up of there. Is it rock? Volcanic deposits? Here on the eastern side of Puget Sound we are mostly glacial till and after 25 years I'm a lot more comfortable working this type soil than advising someone what they should do wherever they happen to be.
Jim, the top 12" or so are topsoil. It's so black I can sell it, literally. Then we get to a brownish gravelly soil that's very hard. When I put the foundation under the house I dug down to the brown stuff, soaked it, tamped the loose crumbles in, and poured.
For the shop I assume I'll remove all the topsoil from the footprint of the building, trench a little ways into the brown stuff for the footings, and build up fill and foam insulation under the slab.
Equipment available is basically everything. My excavator has three machines, a plate compactor, a jumping jack.
There are no short load charges here so I'm not opposed to doing two pours, but like I started thinking.... one would be easier in some ways, and maybe less time consuming. There seems to be some negative connotation to mono pours, but I can't tell if it's due to the difficulty of forming it accurately or some other quality issue.
It will take a LITTLE longer to form a monopour for your job than it would take to set the same forms up on top of footings. But you'd be saving the time of forming, pouring, and stripping the footings. Pretty easy decision for me. But like someone once said "to each, their own."
Hey, something else I was thinking of this morning. We are debating building a new shop when we move to our new place this summer, so I've been mentally kicking ideas around. If I ever did build another shop with a slab floor, I'd at least look into heating the floor somehow. Especially here in our climate.
Anyways, you were talking about a slab floor, in your new shop, and I thought maybe it was worth mentioning.
I'm pretty sure I'm gonna put tubing in the slab. I've been crunching the numbers and we can barely afford this project as it is. I could open up some room by cheapening it quite a bit--comp roof instead of metal, vinyl windows instead of wood/clad, something besides decent wood siding, etc. Still, the tubing can be there for later and I suppose the worst that happens is I never hook it up. We had radiant floor in our last house and it's worth pretty much whatever you have to do to get it.
mnon pour. 2+3=7
"I suppose the worst that happens is I never hook it up."
There is one thing worse than not hooking up radiant heat - drilling the floor sometime and puncturing a tube. I like radiant floor heat myself but take the time to do a careful placement layout and stick to it. Then take as many photos of it as you can stand before the pour, especially anywhere tubes are near planned walls and doors. In those critical locations I like to put a tape measure in the shot to give scale in case I need to drill holes later on.Lignum est bonum.
What do you mean if you ever build another shop? You get out there and build a new shop and make it bigger than the house like you are supposed to. And make sure it has radiant floor heat.
Build your forms out of 3/4" ply and 2x4s. That will keep your stakes 4 1/4" away from the trench.
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Nail a cleat to the form for each anchor.
At the post footing, set a 4' square 2x4 scree frame, the top of which is at grade. Use a 4' 2x4 guide for each pair of anchors. Pour it first, and scree it off. Run a Fresno over it. 'Crete will take a crumbly set in 10-15 minutes. Go back to the near end and start pouring towards the post footing, when you get to it, carefully pull the screes and stakes, but leave the anchor guides. Vibrate (or rod) the stake holes to break the crumbly set. When you are finishing pull the anchor guides. A little extra hand finishing there and . . . all done.
SamT
mono pours are easy. Its the only way we do placement here. Pour your footeres first and then come back and put your slab on top. that way you can pour your footer dry for the extra psi add a little water then float the slab.. 2+3=7
i did mine this summer in a mono pour,formed up similar to whats been describe. went pretty good,nice thing was when your done pouring your done. but there are some downside.one being no matter what it's going to take more concrete becuase you can,t make 90 degree on the inside edge. i did kinda stake up a 5" high pc of masonite,then i poured around the foundation and up on the masonite a inch or so,then jerked the masonite out of there.helped keep a little more sand in the floor part. but my biggest problem was,had all the forms set, rebar done etc. called for inspection,by the time inspector got here it looked like rain. if your set for a mono and it rains your had. you have to pulled all the rebar clean out trench and reset.i got a cement truck within 20 mins of ok of inspect. and poured.had to tarp it later in afternoon but no rain in trenches!! by the way how big are you doing? mine was 24x24 1 morning had trenches dug,set forms,next day got all rebar set and sand spread and packed. 3rd day insp. and pour. just me except on pour,had a pro cement guy and me. good luck larry
hand me the chainsaw, i need to trim the casing just a hair.
Well my aversion to monopour is well known, although that's more for footing/wall combos. Still, for smaller buildings like sheds and detached garages, thickened-edge slabs can be OK - if care is taken with form accuracy.
You mentioned a couple of pad footings to carry columns, so the project sounds like a reasonably large one. Hence the temptation to do it all in one shot when you have a crew together. And you have a very mild climate where you are, I'm guessing the required frost coverage is 18", maybe 24" at the most, so the slab edge forms don't have to be very tall.
I'd still recommend a two-part pour. The key is good elevations. You can pour the footings and get away with floating elevations due to stakes that moved. If that happens during a monopour you won't even know about it until the puddles form on the slab after the first rain. Set the slab edge forms on a footing and now they have something solid to sit on and cannot change height during the slab pour. The same thing applies for straight edges. The slab edge forms will be lower because of the thickness of the footing.
Monopour can be done accurately if you are painstaking with line and elevation, using lots of stakes and braces to hold it all in place (probably about the same as separate footing and slab pours). But if you slap it together fast like most of the monos I've seen, the results are ugly. And then you spend more time shimming and fooling around with framing to get level top plates out of a decidedly less-than-perfect bottom plate.
If you have a site that allows tailgating the footings, you only need one helper. One guy runs the chute and the other vibrates the concrete level to the top of the form. Unless there are many hundreds of feet or the weather is unusually warm, you'll have plenty of time to place and then go back to strike off and dig out a keyway.
Finally, does the engineer require cast-in-place anchor bolts for the columns, and are they to be located in the footing or the slab? If they're to be in the footing, then obviously you need to pour the pads earlier and then block out for late column placement. However if you are permitted to place the bolts or saddle after the fact, then don't even bother with a pad footing form. All you really need is a thickened slab in that area/s. If a separate mat of rebar is called for, install it first before the main slab mat goes over the top.
Good luck!
It sounds like you are recommending the footings in one pour, then the slab on top in another. Assuming the slab is 4", would you pour the footings to within 4" of the finished elevation of the slab, or would the slab have a thickened edge sitting on top of the footing?
Sorry, didn't make myself clear. Thickened edge slab with the slab edge forms fastened onto the footing. You can snap lines on the footing and make sure everything is exactly square and to correct dimension before the forms go into place. I've done a lot of commercial buildings this way. Big box stores use this a lot - not that that makes it right per se, but it is sound engineering.Lignum est bonum.
I happen to have a pic of what catskinner is talking about...
Nice photo -- thanks.
Monopour of slab and footing
David:
Have you considered using fabric for your edging form? Here is a typical project that used fabric in Florida for the edging. Uses minimal lumber and the fabric protects all the lumber from concrete damage. Naturally is it important to properly brace the top screed board and strongly attach the screed to the stake as the fabric tries to torque the screed board off the stake.
http://www.fab-form.com/projects/Fastfoot/Residential/terra_group.html
This is a green solution that is worth considering. I work for Fab-Form, so take this into account in your review. However the contractor in Florida has no relationship with Fab-Form.
Sincerely
Richard Fearn.