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  • Concrete Pads for Shops

    I might be pouring a pad for a new shop at some point in the future and started looking around at local contractors / costs / etc.

    For houses, it seems that they are putting down R5-R10 rigid foam board on top of the base pack and then pouring the pad. That's fine for a residential house where the loads are relatively light, but I'm wondering about the wisdom of doing something like this for a machine shop. On one hand, the thermal break might be nice if the shop will be heated and it might even help with the condensation problem come spring time (probably not, the mass of the concrete and machine tools is still huge). On the other hand, does it require more reinforcement in the concrete to distribute the loads of typical machine tools (e.g. up to 10k lbs. on a small-ish base). I'm not talking huge power hammers or anything like that - just big mills and lathes.

    What does the group say? Hard pack and concrete or hard pack, foam, concrete?

  • #2
    You cannot seriously be thinking of any building that does not have proper insulation. And don't forget your cold bridging at the edge of the slab has to be avoided.

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    • #3
      There is more to it than compacting the soil.
      I have installed 80,000 pound Boring Mills at work in the die shops, and while home shops are smaller, perhaps this info will help
      Here is what I did in this quick sketch
      Insulation is important, but consider putting it on the outside wall instead of under the pad. See Yellow strip

      1.Yes, compact the soil SEVERAL times as it is back filled (!) Don't wait for the hole to be filled and doing it
      2.Use the foundation with a inside lip for support of the floor and try to use a 5 inch thick Concrete floor reinforced with 6x6 mesh
      3.Do not skimp on the footing , bigger is better and is a cheap additive , especially in wet soils
      4.Key the wall to the footing
      5.Insulate the full wall !
      6.Use Visqueen ( 6 or 8 mil) plastic barrier to prevent floor sweats or moisture transfer
      7.MOST IMPORTANT- Everyone rushes ,even your contractor, but fight the urge to use the floor for 30 days !
      Keep the floor soaking wet for 30 days. cover it with a plastic tarp to minimize moisture loss and every day
      lift the plastic and dump lots of water on the floor then recover. Concrete kept wet for 30 days DOUBLES it;s strength !!!
      So I have the equivalent of a 10 inch thick floor in my shop
      8. Not shown ( but used) in the sketch is a 3/8" expansion fibre placed vertically between the outside edge of the floor and the foundation wall inner lip

      My shop floor is 23' x 23' and is 30 years old and has no cracks or crevices
      Rich
      Click image for larger version

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      Green Bay, WI

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      • #4
        My shop floor was poured in the late middle 70's and is 6" deep with 3/8" rebar every 2' and supported up so it was in the middle. The outside edge of the pad for 18" is 1' deep with extra rebar in it. The dirt was excavated first of course and then sand/gravel fill was added and spread by hand and tamped to the point where we could walk on it and not leave marks from our boots.

        I still have cracks and fortunately just one is very noticeable, the other three are just cosmetic...you can see them but they haven't opened any and are quite short.

        A friend across town has a pad about the same age and it's pristine. The one major difference between the two is the soil itself that the shops sit on so we have come to the conclusion that was why mine cracked and his didn't.

        Back then, it was considered "overkill" but turns out it wasn't.
        Location: Saskatoon, Saskatchewan, Canada

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        • #5
          To add to the above, use a minimum of 4,000 psi mix. The added cost is very small. Also, use a concrete contractor who is qualified to do commercial work and the slump on the concrete should not exceed 4", less it better and it should be vibrated. 5" thickness is good, 6" is better. I would use foam under the slab and I would put radiant heat in it. I prefer a slab that floats independent of the frost wall, but that is IMHO. For a slab the size of most shops, one piece is OK. The concrete should be "placed" not "poured"!!! Residential contractors often screw up the job by wetting down the concrete, getting "cold shoulders" and not getting the wire mesh into the slab. I once held a concrete inspectors license and have seen disasters. Another thing, have someone who is qualified make some test cylinders and crack them. Good evidence of what actually took place. .
          Grantham, New Hampshire

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          • #6
            A follow up note on the foam wall installation. Climate is pretty rough here in Green Bay in Winter
            and the "wall" approach stops the frost heave under the floor. Even friends with slab garage floors will dig down a few feet and place
            foam along the outside footing to protect the inside floor Easy to do and very effective.

            Rich
            Green Bay, WI

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            • #7
              Most pump pea gravel and it cracks. Put too much water in it and you get cracks. If the truck can get close enough to use the chutes, order 3/4 rock , 6 sack mix. Good chance it wont crack. Pumping concrete has its problems.
              Last edited by Fasturn; 09-23-2021, 12:46 AM.

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              • #8
                I built my 46’x100’ Shop in 2010 and used 1-1/2 Styrofoam under the heated slab 4500 psi that’s 8” thick with 5/8 rebar on 16” spacing.Been very pleased how it turned out,there are a few hairline cracks but nothing big.If I recall correctly the styrofoam was tested by building a 4’x4’ Form 18’ high and filled with concrete,the styrofoam failed when the depth hit 16’.The Walls are 8” thick with Roxil Insulation plus 1-1/2” styrofoam inside and out giving it a R rating close to 40,R 60 in the ceiling.I can’t stress enough the importance of rebar,have poured over 500 cubic metres of concrete on our farm in the last 40 yrs. and the older concrete slabs that were poured prior to that major downfall was lack of rebar.


                I watched a very interesting documentary on the Pentagon Design and how many life’s were saved in 9/11 by it’s robust design.They showed CAD drawing of the columns which were originally designed to support Military Vehicles and Supplies but by the time it was completed was consumed by Administration Offices.They really focused on Rebar Placement through out the entire structure and with it being severely over built saved a lot of floors from failing.

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                • #9
                  Fasttrack,

                  I built my shop in 2010 and did the same as the fellas suggested with a bit of an additional thing that you should consider. DO put a vapor barrier and the rigid foam on the bottom too, I did the sides also and this combination will keep your shop at an even temp even in the winter, as the slab will retain some heat when it's not giving it away to the ground underneath.

                  The other thing that you should consider for a minimal cost is putting in radiant floor heating. I just installed 3/4" PEX tubing in 3 loops around the shop and it's a 24'x34' floor. It cost next to nothing for the tubing and then when you can afford it put in the heating system later. It's well worth the time to install the PEX directly to the wire mesh or the rebar which ever you choose to use. I used the 6" square mesh and made sure it was on risers to make sure it was in the middle of the 6" slab. I did the heating system just a year ago and what a difference it makes. Even heat in the whole shop and once the slab is warm it doesn't run very often and is comfortable all day long.

                  I have a 4 post 8k lift with a car on it and one on the floor under it, a Bridgeport mill, Miller Syncrowave 250 welder that ways a lot, and have a 1/2" plate steel welding table that is 6'sq and on wheels so that's a heavy point load, and other heavy things on the floor with no cracks at all. I did have the concrete contractor divide the room into quarters with stress lines and then do a smooth finish on top of them, not broom finished and there are only hairline cracks where the stress lines are but not even continuous. This allows for things to roll around without any issues, so do consider a smooth finish vs the usual broom finish.

                  OK that's it...!

                  TX
                  Mr fixit for the family
                  Chris

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                  • #10
                    If you are getting a building permit, you don't get to decide a lot of details. The building codes specify insulation levels and where to place it, thickness and reinforcement of slabs, and compressive strength of concrete mix. Most concrete will crack from thermal expansion. Contractors will saw control joints every 20 ' so the crack will follow the saw cut. There are differences between heated and unheated buildings and floating slabs and slabs supported by a frost wall. You can have an unheated building on a shallow foundation that will not heave when properly insulated.

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                    • #11
                      Thanks everyone - some really useful advice here.

                      I won't get into all the details, but I'd be looking at building a barn that matches the historic character of the surrounding area. As a barn, the local building codes are pretty relaxed - no insulation is required and the concrete is up to me... usually the barn would have a dirt floor. There is some question as to whether or not I would pour a footer and go with a platform framed building or concrete piers for a pole-barn.

                      Chris, excellent point about the cost to add the tubing for radiant heat. Certainly doesn't hurt to plan ahead and throw a few loops of orange PEX down.

                      Tundra Twin Track, yep! My only experience pouring concrete was helping pour pads for grain bins and pivots. Tied a lot of rebar! But that's what has me questioning the foam - for bin pads we would compact the subbase in 6" lifts of hardpack using a trackhoe. Some of the pads built this way are from the 60's and still completely free of cracks. Foam, being somewhat compressible, seems like the wrong thing to put under a pad with a heavy load on it, but I guess as long as the pad is strong enough to distribute the weight of the machines, there would be no problems.

                      Edit: 46x100 <salivating> That's a proper shop. I got spoiled working on my BIL's farm - he turned a 60x200 hog building into a shop by lifting the roof 20' to get clearance for machinery. You farmers have all the big toys ​ I'll be limited to a fairly modest 35x50 or smaller. Current shop is 30x40 and definitely feels a bit cramped... need to sell some machines!

                      Rich, CPeter, thanks for your professional opinions. We get some cold weather up here in NH, too, and frost heaving can be a problem. I do have a limited budget so I'm weighing the cost/benefits of a proper foundation (like I'd use for a platform framed garage) and just pouring a floating pad and concrete piers. I know the full foundation would be better, but it's also more costly, both in site work and material.
                      Last edited by Fasttrack; 09-21-2021, 01:00 PM.

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                      • #12
                        Originally posted by Fasttrack View Post
                        I do have a limited budget so I'm weighing the cost/benefits of a proper foundation (like I'd use for a platform framed garage) and just pouring a floating pad and concrete piers. I know the full foundation would be better, but it's also more costly, both in site work and material.
                        A rather significant factor is if your intent is to live there the rest of your life, or might you be likely to move in the not-too-distant future. If you are only a temporary resident, there is little point in paying for a mack-daddy slab/building (you won't get your money back out and most people wouldn't know/care about the difference). If you DO intend to live there for the rest of your life, it would be silly NOT to go 'all in'. Most realities end up in a 'best guess' balance, of course, but it is a consideration worthy of contemplation.
                        Location: North Central Texas

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                        • #13
                          So much of this depends on the nature of the soil in your area. Anyone giving advise with no knowledge of the soil, which must support anything you build, is just guess work.

                          Talk to your local building department. They have that knowledge.
                          Paul A.
                          SE Texas

                          And if you look REAL close at an analog signal,
                          You will find that it has discrete steps.

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                          • #14
                            Just watched something on TV the other day about a rail station in the UK that had to raise some upper level tracks 5 feet or so. Simply filling with concrete was too heavy for the old viaduct that supported things so they filled most of the space with foam and capped with concrete. The foam was able to support the tracks, platforms and other structures. Must be some foam materials that can handle the compressive loads of a workshop. Probably the expensive ones😀
                            George
                            Traverse City, MI

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                            • #15
                              Originally posted by Fasttrack View Post
                              Thanks everyone - some really useful advice here.



                              Tundra Twin Track, yep! My only experience pouring concrete was helping pour pads for grain bins and pivots. Tied a lot of rebar! But that's what has me questioning the foam - for bin pads we would compact the subbase in 6" lifts of hardpack using a trackhoe. Some of the pads built this way are from the 60's and still completely free of cracks. Foam, being somewhat compressible, seems like the wrong thing to put under a pad with a heavy load on it, but I guess as long as the pad is strong enough to distribute the weight of the machines, there would be no problems.

                              You’ve got a lot of knowledge with use of rebar which is great,Grain Bin Pads need good base to start.Hopper Grain Bins need substantial more concrete & rebar considering the small foot print of the legs.My nephew works on heavy equipment D11 Crawlers and Rock Trucks,there Shop Floor is on Styrofoam.
                              Originally posted by Joel View Post
                              A rather significant factor is if your intent is to live there the rest of your life, or might you be likely to move in the not-too-distant future. If you are only a temporary resident, there is little point in paying for a mack-daddy slab/building (you won't get your money back out and most people wouldn't know/care about the difference). If you DO intend to live there for the rest of your life, it would be silly NOT to go 'all in'. Most realities end up in a 'best guess' balance, of course, but it is a consideration worthy of contemplation.
                              Very good point Joel,if staying long term ‘all in ‘ good way to put it.
                              Originally posted by George Bulliss View Post
                              Just watched something on TV the other day about a rail station in the UK that had to raise some upper level tracks 5 feet or so. Simply filling with concrete was too heavy for the old viaduct that supported things so they filled most of the space with foam and capped with concrete. The foam was able to support the tracks, platforms and other structures. Must be some foam materials that can handle the compressive loads of a workshop. Probably the expensive ones😀
                              I mentioned earlier they tested foam with 16’ deep wet concrete before Styrofoam failed,don’t know what that would be in PSI being 4’ square.

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