Woodshop Concrete Slab Question

[Motown]

Hey all,

I am hoping to get some opinions/help as I am remodeling my garage/workshop. A little background: I've replaced the roof, foundation footings (I live in the bay area) and adding sister joists so I am now ready to pour my slab.

I am planning on pouring a 6" slab with 1/4 inch rebar (16OC) and will pour on top of compressed base rock and Stego vapor barrier. I am also using 1/2 inch expansion joists around the perimeter/footings, etc.

My two questions: if I am using expansion joists will I need to also use/cut in control joints in a 20x20 space? As its my workshop I'd like to have a smooth surface to move equipment. Also, I am undecided on an Epoxy finish on the floor....any recommendations on how to best prep for that in the event I decide to go that way?

Thanks in advance, any help would be much appreciated.

Motown

 

[Clweed]

how big is the slab?

 

[Motown]

20x20

 

[Clweed]

20x20

No you don't need control joints, CJ are typically spaced in 20 x 20 grids as a general rule.

Lee

 

[dougb]

The general rule for contraction joints is that they should be spaced 2 to 3 feet for every inch of floor thickness.  So for a 6" slab, joints would be every 12 to 18 feet.  The floor will crack eventually; I'd want it to crack along a predetermined line.

 

[Motown]

Thanks guys. Sounds like there is two schools of thought.

My assumption (you know how that can go) is that I would be creating a 1/2 inch of space of all sides of the area so that the concrete could 'breathe'...expand/contract.  It sounds like cutting in control joints is a safer way of avoiding cracks. Would love to hear more opinions. Thanks again.

 

[magellan]

I've poured many yards of concrete in my years.  In your climate I do not believe you need the control joints but, if you feel you would like to control the possibility of a crack occurring you could cut the control joints and then fill them with a caulking or one of the many epoxy resins that are available for concrete.  Your slab is virtually floating as you described your plan.  Your best plan might be in the curing to avoid cracks.  After you finish it cover the concrete with plastic and then some Styrofoam for a week leave the heat in it.  Since it is an enclosed area you can control the curing.  If you had access to concrete blankets which I know you most likely don't that is one way to control the curing.  Many schools of thought on concrete.

 

[Mort]

The concrete will crack. That's what it does. Control joints just make it look better when it does.

I'd do two of them, right down the middle in both directions. That way you have 4 10x10 squares. Concrete slabs will usually crack every 10 feet so you're covered.

 

[Cheese]

In the Midwest concrete always seems to crack. Between my self and a couple of buddies we've tried rebar, rebar & steel mesh and finally steel mesh with rebar and glass fibers. Sooner or later they all cracked.

I think that's the reason they now cut the slabs with saws while the concrete is still green. The path of least resistance...if it cracks, it will crack in the saw kerf.

 

[rst]

Cutting control cracks in your slab will not hind machinery movement unless the slab actually cracks and heaves.  In your case this should not happen.  I have control cuts in my shop and have no problem moving anything and everything I own is mobile.  The cuts should be 1/3 the depth of your slab but you do not need to be neurotic about the depth. I cut 1 3/4" cuts in my 6" slab (the depth I got from my worm Skil, its 7 years old and has never moved or cracked.

 

[travisj]

There are two types of concrete: cracked, and hasn't cracked yet

 

[Klemm]

control joints have my vote in your 20x20 space.
Floor finish: I've done EPOXY after a Muriatic Etch and thorough rinsing/vacuum pick up. Let dry thoroughly. Roll on first coat of finish material with some additional dilution after the catalyst had a chance to react (induction period) - not before.
One word of caution: Epoxy becomes really slippery when wet - risk of falling!!. I would go with Urethane. Buy quality industrial material. All this is expensive and time consuming but after 20 years I can walk around my shop floor in my socks, control joints don't interfere with moving all the equipment I have on casters. Do it right in the first place. The first price paid is the smallest price in the long run.

Oh, and don't let anyone move any motorized equipment into the newly finished floor - not even one hour - keep it super clean.

 

[Motown]

Thank you all for the advice, it was very helpful. I think I'll cut two control joints and follow some of the additional procedures recommended. Much appreciated.

 

[mark60]

Control joints are the safe bet. You can always fill them later with foam backer rod and top them with Sikaflex in the future, epoxy or not.

 

[WoodButcher402]

  If you had access to concrete blankets

If you wanted to go with concrete blankets, you can usually rent them from places that handle construction equipment; United Rental, Sunbelt Rental et. al. Just check with a few places in your area.

 

[Clweed]

  If you had access to concrete blankets

If you wanted to go with concrete blankets, you can usually rent them from places that handle construction equipment; United Rental, Sunbelt Rental et. al. Just check with a few places in your area.

You can also use plastic sheeting to cure the concrete, it will trap the moisture and stay wet for days

 

[DB10]

  If you had access to concrete blankets

If you wanted to go with concrete blankets, you can usually rent them from places that handle construction equipment; United Rental, Sunbelt Rental et. al. Just check with a few places in your area.

You can also use plastic sheeting to cure the concrete, it will trap the moisture and stay wet for days

That is how we do it, leave it covered for at least a week maybe longer if possible, slows down the curing, most cracking problems are from drying out too quickly.

 

[Grakat]

You can also spray water onto it for about a week, that helps the slab retain its moisture while it cures. I just poured a 20' X 12' slab and cured it that way. It's now been down for about a month.

Soon I will put the shed up on it.

 

[Tinker]

I like the two cuts giving four 10x10 squares.  Slow cure is the best insurance against future cracking along with particular care in compaction of the base material.  You have used good material in your sub-base and indicated it was well compacted.  Good.

It is also a good idea to be sure the drainage is good around the structure.  Is the shop going to be heated?  Heating will prevent frost damage (if you are in an area where freezing is to be expected) and is god insurance. 
Tinker

 

[Marven]

Don't forget to run conduit for electrical to the center of the slab or wherever you will need it for workbench and table saw.