What Slab Size Relative To Thickness ? ? ? - Building & Construction - Page 2 - DIY Chatroom Home Improvement Forum

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Old 04-09-2009, 01:07 PM   #16
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concrete slabs/pads

Hey guys, I wanted to possibly pour a concrete porch-I love the low long wide concrete porch i've seen on many craftsman style homes-I dont ever see expansion joints or saw marks for control on those. Am I right in reading that after pouring and sawing the control joints-you fill those cuts with caulk (if so what kind?) and then epoxy paint the whole slab so it looks solid?

Originally Posted by concretemasonry View Post
In a typical slab, you generally do not have expansion joints unless you have street or highway!!! - You have control joints to control where the concrete cracks and can be maintained. Concrete does not expand beyond the lenth it is when it is poured. - The curing shrinkage is greater than the temperature expansion.

Joints between a slab and a structure are used to separate two different types of structures.

The problem occurs when the joints get full of "junk" that prevents the slabs to expand back to where they were. - Complicated by the movement up and down caused by loads (especially by repetitive highway loads). For a patio or a driveway, properly sawed joints that are cleaned and calked withing a month or so of pouring minimize movement, dirt and maintenance.

No matter how thick the slab is, it will shrink during curing the same amount. Rebar and wire mesh will decrease the amount of shrinkage cracking if used properly. Fiber mesh is really only good for micro surface cracking and is subject to the correct plant mixing times and placement.


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Old 04-09-2009, 01:15 PM   #17
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the avg porch floor's thickness is 4" here,,, that'd mean no,,, slab larger'n 8',,, eg, if you ave a 16' deep porch, you'd need to either form OR sawcut a jnt from side to side then slice 'n' dice 8' wide,,, if you haven't seen 'em, there're crks instead,,, we use silicone install'd per mfg's recommendations over either backer rod OR closed-cell backer rod,,, imo, there's no PAINT that'd be permanent,,, howzabout acid-stain & sealer ? ? ?

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Old 04-09-2009, 02:13 PM   #18
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There is no rule of thumb that relates the square footage of a slab on grade to its thickness.

If a slab is a one way structural slab, spans only one direction, then the depth will be dictated by the length of the span and the loading applied.
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Old 04-09-2009, 03:33 PM   #19
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therefore a slab-on-grade can be unjointed & as large as possible ? ? ? 100' x 1000' x 4" ? ? ? you lost me & a bunch of other guys who're aci members,,, i never saw a slab that only spann'd 1 direction,,, if i'm placing a clear-span structural slab, i'm using steel - usually 2 mats of # 6 depending on length of span,,, knew i shouldda save those bdge plan i'd think a porch is much different, no ? ? ?
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Old 04-09-2009, 03:46 PM   #20
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Boy, this thread is getting confusing! I think the origonal ? pertained to the spacing of control joints relative to slab thickness, I think? Agan, my understandig has always been 24:1, & an aspect ratio of 1.5:1 maximum, meaning no square shall be more than 1.5 times longer than it is wide, or vice versa.

Last edited by jomama45; 04-09-2009 at 03:51 PM.
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Old 04-09-2009, 06:08 PM   #21
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you're right & that was my original ? as i'd forgotten but i thought it was 20 therefore a 4" slab should be no larger'n 8' ( thickness x 20 = ( answer in decimal ft ) 8.0,,, keep things as square's possible, too, to prevent random cracking


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