Crusher run thickness under concrete slab

[Norman]

Good day everyone. I have a lean-to shed that I will be putting some crusher run into in order to make a better surface for the tractor and implements. Eventually I may want to poor concrete in there. So I think that normally around 4" of crusher run is typical beneath a floating slab like this. The issue I have is that one corner of the shed floor is significantly lower, about 12", than the others. Would it hurt to have that much space taken up by crusher run under the slab on that corner? If so, what is the solution to fill that space?

To be clear, the corner that needs filling is probably 7'x7'x9.8' triangle of the shed that is 14'x14' square. I will be building a retaining wall in that corner to hold in the crusher run (or whatever material is best).

 

[North Idaho Wolfman]

You should be fine to fill with that.
Do you have something to compact it?
You'll need to do that before you pour concrete.

 

[Norman]

You should be fine to fill with that.
Do you have something to compact it?
You'll need to do that before you pour concrete.

I’ll rent a compactor for sure. Probably not going to add concrete right away either so time will be on my side.

 

[North Idaho Wolfman]

You can add some fines to get it to tighten up too.

 

[GreensvilleJay]

nowadays they use HPB ( 3/8 stone,clear and washed) under concrete in garages. Good old crusher run works fine IF you power tamp real good, every 3-4" AND add water(heavy misting).
When you think you've tamped enough,please spend another hour or two..you've got ONE chance to do it right,unlike the guy who did the garage I have. Front,left corner has 'shrunk down'..hmm same corner the retaining wall is, hmmmmm...

 

[ruger1980]

The only thing it might hurt is your pocketbook.
As stated compact it and the subgrade well.

 

[GreensvilleJay]

14 x 14 slab(4") will be about 2.5-3 yards of concrete.. probably over $600 ?
When you form up the slab, also form a pad or ramp at the front doorway. That way the excess concrete gets used and not wasted

 

[mikester]

Good day everyone. I have a lean-to shed that I will be putting some crusher run into in order to make a better surface for the tractor and implements. Eventually I may want to poor concrete in there. So I think that normally around 4" of crusher run is typical beneath a floating slab like this. The issue I have is that one corner of the shed floor is significantly lower, about 12", than the others. Would it hurt to have that much space taken up by crusher run under the slab on that corner? If so, what is the solution to fill that space?

To be clear, the corner that needs filling is probably 7'x7'x9.8' triangle of the shed that is 14'x14' square. I will be building a retaining wall in that corner to hold in the crusher run (or whatever material is best).

Your aggregate thickness selection should be based on your sub soil type. Hire an independent engineer to get a more balanced opinion on this if you aren't sure, you can't always trust the contractor to make sensible recommendations.

The biggest potential problem with more aggregate is if your installer is ignorant and lazy and doesn't compact it properly then your slab will be unstable. Then good luck trying to catch the bugger after he runs away with your cash and you are left with an expensive heaving slab and crooked floors.

 

[Norman]

Your aggregate thickness selection should be based on your sub soil type. Hire an independent engineer to get a more balanced opinion on this if you aren't sure, you can't always trust the contractor to make sensible recommendations.

The biggest potential problem with more aggregate is if your installer is ignorant and lazy and doesn't compact it properly then your slab will be unstable. Then good luck trying to catch the bugger after he runs away with your cash and you are left with an expensive heaving slab and crooked floors.

Heavy red Georgia clay is my subsoil, and it’s not shallow.

 

[radas]

I've had good luck using a jumping jack to tamp the clay and then use geotextiles over the subbase prior to using 4-6" of limestone with fines and plate compacting in 2" lifts. This method has worked very well for me on two paver patios over very dense clay soil in MI where we experience significant freeze thaw cycles every year.

Wouldn't hurt to add drainage pipe behind your retaining wall (under your slab) and drain to daylight as well. Not a bad practice for any retaining wall.

 

[Henro]

Good day everyone. I have a lean-to shed that I will be putting some crusher run into in order to make a better surface for the tractor and implements. Eventually I may want to poor concrete in there. So I think that normally around 4" of crusher run is typical beneath a floating slab like this. The issue I have is that one corner of the shed floor is significantly lower, about 12", than the others. Would it hurt to have that much space taken up by crusher run under the slab on that corner? If so, what is the solution to fill that space?

To be clear, the corner that needs filling is probably 7'x7'x9.8' triangle of the shed that is 14'x14' square. I will be building a retaining wall in that corner to hold in the crusher run (or whatever material is best).

Well, what I would do is save the money it would cost to pay a professional for advice, and just put a minimal amount of crusher run in that corner, and make the concrete thicker there.

Simplistic I suppose, but in reality, what is the difference if the space is filled with loose stone or concrete? For a small space anyway.

Don't over think it. It is only 14 x 14 feet of area for the whole shed.

 

[GreensvilleJay]

re: Simplistic I suppose, but in reality, what is the difference if the space is filled with loose stone or concrete? For a small space anyway.

if it's loose stone,water WILL eventually erode and they'll be a gap between the concrete slab and the ground. If the concretes a thick uneven slab it WILL eventually crack at the 'thick-thin' line.
it's super important that ALL the 'fill' be COMPACTED before concrete is poured.

saw a TV show years ago, guy took hammer to tap the basement floor, even I, with 24/7 tinnitus could here the difference of where the 'hollows' were.

 

[Ridelght]

re: Simplistic I suppose, but in reality, what is the difference if the space is filled with loose stone or concrete? For a small space anyway.

if it's loose stone,water WILL eventually erode and they'll be a gap between the concrete slab and the ground. If the concretes a thick uneven slab it WILL eventually crack at the 'thick-thin' line.
it's super important that ALL the 'fill' be COMPACTED before concrete is poured.

saw a TV show years ago, guy took hammer to tap the basement floor, even I, with 24/7 tinnitus could here the difference of where the 'hollows' were.

Thats news to me. When you guys say "chrusher stone " is that the ground up concrete stuff they sell ? Here they call it base. I used some under a paver patio that I rented the Tamping thing from Home Depot to compact. Never thought about under concrete that's a great idea.

 

[Henro]

re: Simplistic I suppose, but in reality, what is the difference if the space is filled with loose stone or concrete? For a small space anyway.

if it's loose stone,water WILL eventually erode and they'll be a gap between the concrete slab and the ground. If the concretes a thick uneven slab it WILL eventually crack at the 'thick-thin' line.
it's super important that ALL the 'fill' be COMPACTED before concrete is poured.

saw a TV show years ago, guy took hammer to tap the basement floor, even I, with 24/7 tinnitus could here the difference of where the 'hollows' were.

Tapping a hammer on the floor makes no difference if the floor supports what it holds. Not saying that a solid floor is not better, just saying that if it is good enough, it is good enough.

OK...but I assume there will be a roof on the shed, so water coming in should be a non issue...or minimal at worst. Maybe...depends on the lay of the land.

I think I had a similar question with my house. BUT after 30 years plus, no issue at all...but there is a roof to keep water away...

Edit: By the way, I know the claim is that if concrete thickness changes, the concrete will likely crack at the point where the thickness changes. BUT how can this happen? Only if something settles on the low side of the transition. So...if that is the case, how can extra concrete settle, as compare to what material might be put there before the concrete is placed?

Time to analyze the overall picture I think...

 

[GreensvilleJay]

re: I think I had a similar question with my house. BUT after 30 years plus, no issue at all...but there is a roof to keep water away...

The 4' kicker pipe to get water away from my house on south side 'magically' removed itself one day....spring rainstorm/thaw... I have water IN my basement, 1st time in 20 years.arrgh... so 'weird stuff' can happen.

neighbours nnext conncession up, lost ALL their water when quarry blasted and ,oopsy, hit the acquifer ! never saw so many concrete trucks SPEEDING to plug the friggin hole. so 'weird stuff' can happen

I know, it's 'just a shed' but build it right ONCE means the grankids can enjoy it when they own the house too !