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-   -   Basement Slab Thickness Question (http://www.houserepairtalk.com/f17/basement-slab-thickness-question-15091/)

MoreCowbell 11-13-2012 06:47 PM

Basement Slab Thickness Question
 
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Those who read my other posts know that I am in the middle of a crawlspace-to-basement conversion project. A few days ago I placed the concrete for my retaining wall footing (the retaining wall is L-shaped and runs the length of two adjacent foundation walls.

I screwed up on about a 10' wide section where the footing is about an inch too high. My plan was to place a 3.5" floor slab. If I keep that same floor level, the slab will taper up to only 2.5" over that 10' section of wall footing. Here is a sketch of what I'm talking about:
Attachment 4536
My column footings have a 3.5" curb in the center where I am planning to screed my floor slab level with. I'm real nervous about tapering up to 2.5" though.

My question is would this be a real bad idea to have part of the slab go that thin? The other option I'm thinking of is to raise the finish slab surface up a 1/2" and add thickness to my column footings with a 1/2" of thinset to match the floor. That would give me a 4" slab that tapers to 3" in that section.

Any thoughts on this? Thanks.

Wuzzat? 11-13-2012 07:10 PM

Sidewalks are about 4" thk; this seems to be a minimum but maybe because of the height of a 2x4 and not so much on compressive and bending strength of the concrete.
I guess if you go thin but sure to use reinforcing wires.

BridgeMan 11-13-2012 10:32 PM

Your concrete won't know it's only 2.5" thick over the footing. It will perform just fine, especially if you first chip off the top front footing corner, such that there's more of a gradual transition from the thinner to the thicker portions of the slab. Doing so eliminates the stress-raiser at the (otherwise) re-entrant corner.

MoreCowbell 11-14-2012 06:03 PM

Thanks for the replies. I feel better about forging ahead with my intended floor height. I'll just make sure to compact my gravel base real well and use a good amount of wire mesh reinforcement. I'll also order a higher than normal PSI mix from the concrete contractor.

itsreallyconc 12-01-2012 04:16 AM

paved lots of hgwy & airport conc but never understood wire mesh was good other than: 1, adding strength while the ' green ' conc was in tension ( initial set / cure ) & 2, holding the broken pieces together when it cracked :D all the engineer's w/whom i worked thought the same however we got paid for placing it per contract - fiber is no substitute for timely joint pattern NOR will it prevent random cracking,,, higher compressive strength makes conc more likely to resist heavier loads - if it were me, id want higher flexural strength :beer:


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