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I'm wanting to add on to an existing building. The reason I'm asking this is the current building that I'm adding on to, the concrete floor is terrible. Really out of level, cracked everywhere, so i don't want to have the add on do the same thing. I had the building built by the low bidder here in southern Illinois, lesson learned! I've asked around a couple of contractors about how deep to dig for the floor, 8 inches was the answer I got. I've got a 3039r & a tiller to dig it out with. So 4 inches of rock compacted , then 4 inches of concrete ought to work . I've also been told to dig out for the floor first , then use pto post hole digger for posts. The post hole digger I have available to use is only 9 inches. Going to stack 3 - 2x6x12 for the posts. I'm thinking the hole is going to be too little .Going to fill in around posts with concrete. Any opinions or thoughts?
 

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I used a 2x6 form so it is 6 inches + or - with 1/2 rebar every 16" to 18". The other thing is I have the redi-mix plant make my concrete with 6 bags of cement, much stronger concrete.

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I'm wanting to add on to an existing building. The post hole digger I have available to use is only 9 inches. Going to stack 3 - 2x6x12 for the posts. I'm thinking the hole is going to be too little .Going to fill in around posts with concrete. Any opinions or thoughts?
In our county, the required footing diameter (and thus post hole diameter) is dependent on building width. I added a 15 ft wide add-on to an existing structure last year, and they let me get by with a 12 inch diameter post hole (with 6x6 posts on 10 ft centers).

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Discussion Starter · #5 ·
In our county, the required footing diameter (and thus post hole diameter) is dependent on building width. I added a 15 ft wide add-on to an existing structure last year, and they let me get by with a 12 inch diameter post hole (with 6x6 posts on 10 ft centers).

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I left out the measurements , its going to be 20ft x 30ft with posts on 10 ft. centers. Thank you.
 

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SIL just had his floor done, he put a 6mi plastic, then 2 inches of foam, in floor heat, 1/2" rebar every 16" on center, then 5" of concrete. 40 x60 barn with a rat wall 24" deep 16" wide took 48 yards.
 
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4” of rock is a minimum amount of rock. If your existing slab is failing the go atleast 6”. Use a woven geotextile fabric under the rock and compact it really well with a plate compactor. Make sure you soak it good with a hose before you compact it. Use rock with fines in it. A 5/8” minus works well, but I’d use a 1 1/4” minus (minus means it has fines in it around here) on top of woven geotextile fabric. Auger your holes after the slab prep is done so you don’t fill them with rock. As driveshaft mentioned, reinforce your slab. That, on top of a good base will last a long time.
 
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I’m not a contractor or expert barn builder but from my experience your post holes should be below your frostline (48” for me in central Indiana) and the bottom of the hole needs to have a concrete floor, I think >4” thick. I wouldn’t set the posts in concrete. The posts should have some 2x4s scabs nailed around near the bottom to resist wind lift.
If you’re using a 3pt PHD I would advise to straddle your wall line with the tractor and drill all your holes for each wall with the tractor in same orientation. Don’t back up to the wall, drill a hole and then continue that process these length of the building. PHD don’t always dig straight down perfectly vertical. If all the holes are in the same orientation it’s easier to get the posts aligned to each other.
A box blade with the rippers down followed by a bucket then more boxblading to level it will make quick work of topsoil removal with less disruption than a tiller.
 

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How much topsoil do you have to take off? I wouldn't go less than 6 inches of well compacted base. You can drive a semi truck on 6" of concrete. Have you considered fiber concrete?
My shop is 18" of base compacted on compacted clay. The concrete is fiber 6 bag, 18" inch on center 1/2" rebar, minimum thickness is 6". I have 1 hair line crack coming from a drain. It's about 2 foot long, that's it. The floor is 2 years old.

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things that make for "terrible" concrete are

poor base under the slab
poor soils under the base under the slab including moisture issues
frozen soils and base under the slab (frost heave)
poor weak concrete mix , improper mix
lack of concrete reinforcement
improper concrete curing before ussage

you are getting lots of information about mitigating these type issues above so i wont add any but i will mention that the stronger the mix the quicker it will set up, 6 bag (4000spi) mix is that i would use but it does set up quicker than lesser mixes so plan accordingly, and consider each of the issues above and how your dealing with each
 

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bang for the buck crushed concrete works well and is usually cheaper than limestone if available. I would go 6 inches on the base compacted 3 and 3. full 4 inches of concrete. compact it with a vibratory plate compactor, when you think it is compacted do it again doesn't take all that long. Do not concrete the posts backfill with dirt or gravel. if you need to raise the grade at all make your pad 2 foot bigger all the way around for stability
 

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Unfortunately, the correct answer to your floor does not come from a formula. It is circumstantial. Since we don't know what the existing soil is like, and what is underneath it we can only say what usually works. Soil that drains well and is well compacted or an abandoned dump site? Since you already had a fail I would want to be sure to get it right this time.

I would start with you backhoe and dig a few test holes to see what you have. If you are finding a lot of organic material or foreign debris, keep digging. It needs to be removed. If you are finding clean and solid soil you will be in much better shape.

The reason your floor cracked and sank in the first place is most likely due to settlement under the slab leaving it unsupported. A bad concrete mix is unlikely and for concrete to sink it needs a void underneath to sink into. A stronger floor (thicker with more rebar) can take care of some settling but the best, and cheapest approach is to start with a clean, well compacted soil. When I poured my 2,000 SF pole barn floor I first put down 6" of compacted gravel and then poured 5" of concrete with a grid of rebar.

Building codes often specify the minimum amount of base and concrete but that is never a guaranty it won't fail. Silt, soft soil, organic material like tree trunks or garbage will always lead to a failure down the road. Any concrete contractor worth their salt tell you the same. Make sure you know what you are starting with before you get too far down the road.
 

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Discussion Starter · #13 ·
Unfortunately, the correct answer to your floor does not come from a formula. It is circumstantial. Since we don't know what the existing soil is like, and what is underneath it we can only say what usually works. Soil that drains well and is well compacted or an abandoned dump site? Since you already had a fail I would want to be sure to get it right this time.

I would start with you backhoe and dig a few test holes to see what you have. If you are finding a lot of organic material or foreign debris, keep digging. It needs to be removed. If you are finding clean and solid soil you will be in much better shape.

The reason your floor cracked and sank in the first place is most likely due to settlement under the slab leaving it unsupported. A bad concrete mix is unlikely and for concrete to sink it needs a void underneath to sink into. A stronger floor (thicker with more rebar) can take care of some settling but the best, and cheapest approach is to start with a clean, well compacted soil. When I poured my 2,000 SF pole barn floor I first put down 6" of compacted gravel and then poured 5" of concrete with a grid of rebar.

Building codes often specify the minimum amount of base and concrete but that is never a guaranty it won't fail. Silt, soft soil, organic material like tree trunks or garbage will always lead to a failure down the road. Any concrete contractor worth their salt tell you the same. Make sure you know what you are starting with before you get too far down the road.
Very informational from everyone!! My opinion as to why the concrete cracked everywhere is just too thin. Outside you can look at it , approximately only 2 to 3 inches max. That's what you get from low bidder. Here's something else that was interesting Fill That Post Hole With Concrete | Porters Building Centers. I was thinking to set the posts & backfill with concrete ,guess thats another can of worms to open huh. Thank you everyone!!
 

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I’d go with 6” of concrete. The 4” of gravel underneath it might be enough if the soil is decent.
 
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