Thoughts on this slab

[thejudges69]

I think we made an error in pouring my slab. It's a poured foundation, 8" slab 4500 psi rebar reinforced slab. When we poured it the slab didn't pour all the way down to rest on the foundation wall. I now have this void undery slab. It's a couple inches tall, and only across the 14' door openings. It recesses I'm about 2 inches. I'm wondering if this is an issue. My concern is coming in loaded will eventually crack the end of the slab. Under the slab is compacted limestone, from that's visible if you look into the voided area. This is tough to explain. I'm thinking if I clean out the loose material and fill this void with 4500 PSI quikrete that maybe a better fix. Thoughts?

Oh don't mind the pea gravel, that's temporary.. limestones going in there.

 

[like2wheel]

Seems like the pea gravel is spilling out from under the edge?

 

[thejudges69]

Seems like the pea gravel is spilling out from under the edge?

Naw the builder poured it there for the french drains and access to the slab so you could get in the building.

 

[quickfarms]

Are you going to pour a concrete slab or apron in front of the doors?

 

[thejudges69]

Are you going to pour a concrete slab or apron in front of the doors?

I've considered that simply because of this. The budget says I can't though lol.

 

[coldh2o]

I'm having a hard time understanding how this happened. It looks like you didn't have a form that extended down to the foundation, only the top half of the slab?

 

[thejudges69]

I'm having a hard time understanding how this happened. It looks like you didn't have a form that extended down to the foundation, only the top half of the slab?

How it happened it irrelevant. How I'm gonna correct it is what's important.

 

[mcbane]

A 2” gap at the edge of an 8” slab seems superficial. Maybe check with the person who calculated the need for 8”. The extra thickness may have nothing to do with wheel loads - could be necessary for other loads like pallet racking anchors.

 

[thejudges69]

A 2” gap at the edge of an 8” slab seems superficial. Maybe check with the person who calculated the need for 8”. The extra thickness may have nothing to do with wheel loads - could be necessary for other loads like pallet racking anchors.

It was my call to go from 6-8". I wanted to make sure the slab would hold anything I brought in for weight..

 

[mcbane]

Then don’t sweat it. When you build up an exterior apron with compacted rock or concrete, that small void will be filled.

 

[thejudges69]

Then don’t sweat it. When you build up an exterior apron with compacted rock or concrete, that small void will be filled.

That's what I thought as well. This is just a first for me. I wasn't on site for my last shop so this had me worried.

 

[topp64]

So just to make sure I understand what you’re talking about here, you’ve got 2” of compacted limestone between the bottom of your slab and the top of your 8” wide foundation wall where the overhead door will be? If that is the case I would be more concerned with moisture in that 2” of limestone freezing and expanding in the winters. I would probably clean the rock out and use some patch like you mentioned. But instead of using basic quickcrete I suggest getting some type of V/O (vertical/overhead) patch material.

 

[thejudges69]

So just to make sure I understand what you’re talking about here, you’ve got 2” of compacted limestone between the bottom of your slab and the top of your 8” wide foundation wall where the overhead door will be? If that is the case I would be more concerned with moisture in that 2” of limestone freezing and expanding in the winters. I would probably clean the rock out and use some patch like you mentioned. But instead of using basic quickcrete I suggest getting some type of V/O (vertical/overhead) patch material.

Yes that's basically what the issue is. I don't do construction, so I'm not sure how to explain it better. But If I fill the void, wouldn't the freeze area move to the point of least resistance, so basically out away from the building into the driveway area?

 

[topp64]

Yes that's basically what the issue is. I don't do construction, so I'm not sure how to explain it better. But If I fill the void, wouldn't the freeze area move to the point of least resistance, so basically out away from the building into the driveway area?

Typically you separate the exterior concrete, a driveway or an apron, from a floor with an expansion joint. You want to allow movement of the exterior slab, that’s subject to the expanding and contracting of its sub base from freezing and thawing, without damaging the floor that doesn’t move. If the floor is reinforced with rebar that has been drilled and epoxied into the foundation then eventually something has to give.

 

[ConCretin]

Since the slab is 8" thick I don't think I'd be particularly concerned as long as there is no way for the stone base to subside from under the slab and even then, the concrete could withstand quite a load. I don't see any difference with regard to frost or freezing whether you fill the void or not.

 

[Jtrudel90]

What is the difference having stone between the slab and the foundation wall (at the door openings) and having stone between the slab and the ground on the other 2xxx square feet. The rest of the slab is on compacted fill its not resting on the footing or stem wall. Your going to have a gravel/stone entrance going into the shop, its not like the stone is going to wash out from under the Pad.

 

[joes169]

I sleep better at night knowing we pour every floor and stoop all the way down to the foundation. If not, what's the point of the foundation under the doorways if not to support the slab? We might do it differently here, though, as we utilize concrete ledges around the entire interior perimeter of the building, as well.

I'd dig some of the stone back (at least 4-5" deep), set up a 2x12 or the like the entire width of the door, and vibrate bag crete between the two.

 

[thejudges69]

I sleep better at night knowing we pour every floor and stoop all the way down to the foundation. If not, what's the point of the foundation under the doorways if not to support the slab? We might do it differently here, though, as we utilize concrete ledges around the entire interior perimeter of the building, as well.

I'd dig some of the stone back (at least 4-5" deep), set up a 2x12 or the like the entire width of the door, and vibrate bag crete between the two.

That's what I wanted and I think it was a miscommunication. The slab has been backfilled against. I don't think that it will ever be an issue. I'd thought about the bagged 4500 psi stuff but didn't get to it.