# Monolithic slab with turned down edges



## Kroy1497 (Apr 14, 2013)

Hey folks!

I am about to pour my first monolithic slab for a carport I am rebuilding and expanding. It was 12x24 and will be expanded to 24x24. My questions are regarding the perimeter where it will be "turned down" for added strength, thicker than the typical 5" slab in the "field" portion.

I was planning on making the outer portion that will be turned down 12" wide, all the way around the perimeter and 10" deep. Is that good enough?

Because I need to back fill the center (the "field") with gravel and compact it, can I place wooden forms 12" in from the perimeter, fill with gravel and compact? AND...can I leave these form in place and pour a monolithic slab over them? 


Appreciate anyone's opinion on this, just not something I have done before!


----------



## raskolnikov (Mar 10, 2008)

Kroy,
I wouldn't leave the forms in after the pour, why not pull them after the fill is in place and then call for 'crete? 

D.


----------



## xcmason (Mar 20, 2008)

Kroy1497 said:


> I was planning on making the outer portion that will be turned down 12" wide, all the way around the perimeter and 10" deep. Is that good enough?


It's hard to say, not knowing what part of the country you are in. Around here bottom of footer needs to be 36'' below grade if you want to comply with codes.


----------



## Kroy1497 (Apr 14, 2013)

I was worried the edges of the compacted gravel would mush over into the outer section if I pulled the forms. Maybe I'm over thinking this..haha!

xcmason - I'm just south of you in the DC metro area. Here they let us pour slab on grad (the carport roof is not supported by this slab in any way - it has columns just outside the slab. What are your thoughts about leaving the forms in?


----------



## xcmason (Mar 20, 2008)

Well if this is just essentially a floating slab with no structure sitting on it then you should be good on depth. As far as the forms, I wouldn't leave them in either. Typically in situations like this most people form the perimeter and let the gravel grade fall naturally (approx 45 degrees) from the center slab grade to bottom of footer. Just my $.02:thumbsup:


----------



## thom (Nov 3, 2006)

Can't leave those forms in. Termites love buried wood. 

It is illegal to leave wood forms in place here, to tell the truth I don't know if its local or IBC but we cannot leave lumber behind in the earth.


----------



## CJKarl (Nov 21, 2006)

Personally I'm not crazy about thickening the perimeter. I would be more inclined to pour a bond beam then a slab over it or just a good beefy slab all the way out.


----------



## fjn (Aug 17, 2011)

Also,if you do go the route of thick edge,make sure your transition from thick to thin is at a 45 deg. angle or it will crack at the step 100 % of the time. Even with the precaution,chance for crack there is great. I would follow CJKarl's method.:thumbsup:


----------



## Kroy1497 (Apr 14, 2013)

CJKarl said:


> Personally I'm not crazy about thickening the perimeter. I would be more inclined to pour a bond beam then a slab over it or just a good beefy slab all the way out.


Thanks for the advice. Time for me to profess my ignorance re: a bond beam. I'll have to do some research on that. Again, thanks for the help guys.


----------



## CJKarl (Nov 21, 2006)

It's not like you are worried about frost heave. I would spend the time and money you would save by not thickening the slab and spend it on a good compacted base.


----------



## CJKarl (Nov 21, 2006)

Heck, I have a 5 ton pizza oven and will have a 2 ton table sitting on 6" reinforced slabs.


----------



## Kroy1497 (Apr 14, 2013)

CJKarl said:


> Heck, I have a 5 ton pizza oven and will have a 2 ton table sitting on 6" reinforced slabs.


Yeah man, I checked out the thread on your oven and table, that is SWEET!!:thumbsup:


----------



## Kroy1497 (Apr 14, 2013)

CJKarl said:


> Personally I'm not crazy about thickening the perimeter. I would be more inclined to pour a bond beam then a slab over it or just a good beefy slab all the way out.


CJ- Is this what your talking about?

http://www.mtidry.com/hyperspecs/09CM02SW002/09CM02SW002_image.pdf


----------



## dbrons (Apr 12, 2010)

I don't know if that's what he was talking about but that is one cool looking pdf ! 
Dave


----------



## jomama (Oct 25, 2008)

If you're only trying to hold back 5" of gravel, you won't need anything to keep it back. Just compact in lifts, and the last thing you do is clean the horizontal grade beam out to width. Hand-tamping the angle on the transition will help as well.

The only time we need to use material to "form" the inner edge of gravel is if the grade beam is 16"+ high. At that point, if you don't use something (usually we use hardware cloth or the like) in these scenario's the grade beam gets huge........


----------



## Rio (Oct 13, 2009)

As pointed out above no interior form necessary but when and if one is going to leave a form in place, depending on if it can be supported by backfilling, gyp. board (drywall) can be used and left in place.


----------

