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I’m pouring a small slab for some pool equipment. It will be 4’x8’x3.5”. I expect little to no foot traffic, just a heater, pump, and chlorinator. (Mabye foot traffic just when adding tablets)

Do I need a control joint in the middle? Or do you think my slab is small enough to get away with nothing.

(I have minimal concrete experience in slabs... just things like post holes and filling blocks)

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    It's worth mentioning whether you're using reinforcement. – isherwood Apr 26 at 12:53
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    My rule of thumb is squares up to 12’ don’t need control joints (similar to ecnerwal) but rectangles do when the length is more than 150% of the length or a 4 foot wide pad longer than 6’ needs a control line since this is 8’ I would put one at 4’. Not all control lines need to be visible, you can create a crack line at 4’ in this case by “slicing” down and separating the aggregate on a straight line. After finish the surface and when it cracks it will be on that line. – Ed Beal Apr 26 at 13:25
  • Why isn't that an answer, Ed? – isherwood Apr 26 at 13:42
  • There are some pointers here: concretenetwork.com/concrete/slabs/controljoints.htm. Regardless, would you be more upset at the look of a control joint which was done by you or by mother nature? I don't think two 4'x4' slabs would look bad. – MonkeyZeus Apr 26 at 14:16
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No. My rule of thumb (can't recall where my thumb got it from) is 12 feet/4meters before you really need a control joint.

Reinforcement is always a good idea.

This document has a great deal of vague handwaving, and on page 11 gets to a specific recommendation from the ACI of "24-36 times slab thickness" which in your case is 7-10.5 feet. that suggests that 0 or 1 control joint might be the right number (with the 1 making two 4 foot squares per another recommendation found in the same section of the document that the slab segments defined by joints be square or near square.)

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  • I'd bet real money that a 3-1/2" slab of that size will crack. – isherwood Apr 26 at 12:52
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"Need" is a matter of prerogative. Will a 4x8 slab crack? Maybe. Probably. Do you want the cracks to be straight? Probably. There's your answer: Probably. :)

I'd actually use two lines. Even a 4x4 could crack, especially in that thin a slab. 4" is my personal minimum, and if you're paying for a ready-mix truck 5" probably won't cost you more.

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