# concrete sub base



## smaj100 (Oct 17, 2012)

I have a question for some who have been there and done that. I am prepping to put concrete in our garage. My dilema is this. The slope of the packed dirt floor from front to back of the garage a distance of 20' is about 4' front to back. I've dug a footer along the sides and back, set rebar in the concrete and stacked cinder block with vertical rebar out of the footer and filled the cinder block core to make what i think is a really strong retaining wall. I left approximately 4" of each cinder block core empty at the top to pour the slab into to help bind everything together. By question is this apparently gravel prices in my area have gone through the dang roof. So it is no longer economical for me to fill with gravel up to the 4" level for the slab. 

Can I use sand compacted with a plate compactor to fill in the majority of the dead space and then put 6" of gravel on top of the sand pack that and then pour concrete? Or would I be better off getting fill dirt and packing it down then covering with gravel?


----------



## bbqjoe (Feb 10, 2017)

Are you talking about a 20% grade on your garage floor?
20 feet long with a 4 foot difference from front to back?

I might have that figured wrong.


----------



## TheLazyL (Jun 5, 2012)

smaj100 said:


> ... I left approximately 4" of each cinder block core empty at the top to pour the slab into to help bind everything together...?


Front porch was wood floor and wood framing way pass it's better years. I tore it down and used the wood for campfires.

Then I had a Contractor put a concrete footer and with cement blocks for the foundation. Before he poured the cement floor he covered all the cement blocks with roofing felt to keep the concrete out of the cores when he poured the floor. I was expecting rebar or at least pouring the floor concrete into the block cavities (like your planning on doing) to tie it all together. Contractor said no. You want the concrete floor to float on the foundation. Otherwise every time the foundation moves the stress will cause the floor to crack. After ten years the porch floor had one hairline crack across front to back. Don't know if I would had more cracks if it was "all tied together" or the Contractor was blowing smoke.

New house they used washed sand to bring the garage floor and driveway to grade. They hosed it lightly with water and than ran a mechanical compacter over it.


----------



## phideaux (Nov 7, 2015)

This ^^^"You want the concrete floor to float on the foundation. Otherwise every time the foundation moves the stress will cause the floor to crack"

I been there...*compact at least every six inches you add.*

Jim


----------



## smaj100 (Oct 17, 2012)

Thanks everyone for the info. Yes it a slop e difference from 0" to 4' over a 20' span. I never thought about letting the slab float instead of tying it into cinder block walls. I had heard to pack the sand or dirt every 4-6" lift. I can get river sand for 8 ton vs 15 ton for 3/4gravel.... So it's almost a no brainer to me or am I looking at this wrong?


----------



## TheLazyL (Jun 5, 2012)

smaj100 said:


> Thanks everyone for the info. Yes it a slop e difference from 0" to 4' over a 20' span. I never thought about letting the slab float instead of tying it into cinder block walls. I had heard to pack the sand or dirt every 4-6" lift. I can get river sand for 8 ton vs 15 ton for 3/4gravel.... So it's almost a no brainer to me or am I looking at this wrong?


Me thinks sand is easier to pack then gravel. Just so there is no way for the sand to wash out from under the concrete and leave a void.


----------



## Woody (Nov 11, 2008)

Definitely want the slab to float. Why not use fill dirt and compact that? Heck, a local contractor might just give it to you rather than have to truck it somewhere else to dump. Worth at least asking around. Lots of contractors would love to have a short haul to get rid of it. If there is an immediate need, then this might not be an option. If you have a little time, it might be.


----------



## smaj100 (Oct 17, 2012)

Ok i've decided since i have plenty of land and dirt. I'm going to cut the grass and sod off the top of a section way out in the back 40 and dig up the dirt mostly clay to use as fill for my garage floor. My question is I know it needs to be tamped. I've read before that the jumping jack tampers are better for this type of compacting than the plate compactors. Also do I need to add any moisture for dirt/clay fill during compaction like you would sand?


----------



## phideaux (Nov 7, 2015)

I have done a few of those bases, using clay/dirt, and I've always had good , non cracked, slabs , using the Plate compactors,

But I always did about every 4 to 6 inches and kept it moist , not wet, 
but definetly not dry .

Take your time packing , 



Jim


----------



## TheLazyL (Jun 5, 2012)

smaj100 said:


> ...the dirt mostly clay to use as fill for my garage floor. ...


Clay is not good for fill. Expands and contracts too much with moisture.


----------



## phideaux (Nov 7, 2015)

This is very true, ^^

You gotta be careful with having too much clay in the dirt .


It will become like rubber.


Jim


----------



## smaj100 (Oct 17, 2012)

well crap, back to the sand theory. thanks guys.


----------



## Woody (Nov 11, 2008)

It's always something, isn't it?


----------



## azrancher (Jan 30, 2014)

smaj100 said:


> well crap, back to the sand theory. thanks guys.


Well actually if you can find it... Crusher Fines are the best for compaction, no round corners to roll against each other, which is why concrete makers avoid beach sand and wash (this is an AZ term) sand. I did a geothermal copper exchanger in a 8' deep pit (50'X50') and they wanted crusher fines because when the copper is hot it expands and pushes the material away, creating a void, voids are no good for thermal heat/cold transfer.

I can no longer find crusher fines, I think they use it all in concrete.

*Rancher*


----------

