Precast concrete slabs

jeepinmatt

jeepinmatt

#1 WEBWHEELER
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Stanley, NC
Im looking at floor options for the garage/aparment Im building. I will be filling about 6ft at the rear and 2-3ft at the front, but I hate to waste all that space by putting dirt in it. Im trying to find out more about the cost and load capacity of precast concrete slabs such as Hollowcore. Any of you guys know about it, or even who I could call?
 
You can suspend a poured slab by using a network of steel beams and metal decking. I have designed a few with the help of a structural engineer and a few poured wall companies. Not sure about the precast floors, but have used precast walls. I did one with a clear span of about a 20'x30'. It was like a bomb shelter. Pretty cool.

Carl Welch
 
I'm no structural engineer but you could go w/ something similar to what cuz1 said w/ some form of steel or concrete columns/joists/etc... just dont see how it would be anywhere near economical.

If you do back fill 6' of soil keep it free of organics and place it in lifts and compact it very well or you'll end up with some differential settlement over time. I'd go with something like 1' loose lifts and compact, then repeat...


As for the precast/prestressed concrete slabs.. I'd be prepared to spend some coin (no experience w/ them though... just the extras/engineering involved makes me think it would be atleast double what a backfilled/poured slab would be).
 
instead of backfilling with dirt, you could maybe use some stone and not have to worry with compacting it near as much as dirt.

I'd think for a slab dirt backfill with 6" to 1' lifts with compaction of 95% would be sufficient

for stone, I'd prob. pour it in and run with it without compaction as it'll compact very, very little to none
 
The amount of money you will spend is hard to justify for the space you will gain. Fill it with dirt in lift's like said above. You are looking at a lot more time, work, and money for the little bit of storage.
 
Fawk backfilling w/clay. You'll never get the compaction right on your own, it'll settle and the floor will crack all to shit. Use stone.

The load capacity on a precast slab is fine, but it's not worth the expense.
 
saf-t scissors said:
Fawk backfilling w/clay. You'll never get the compaction right on your own, it'll settle and the floor will crack all to shit. Use stone.

The load capacity on a precast slab is fine, but it's not worth the expense.
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Yeah obviously it'll be harder to get compaction but it's possible... however 6' of stone tapered to 3' up front is going to be $$$$$$...

I'd backfill w/ soil, put down a woven geotextile then put 1' of stone on top...
 
uncc civilengineer said:
Yeah obviously it'll be harder to get compaction but it's possible... however 6' of stone tapered to 3' up front is going to be $$$$$$...
I'd backfill w/ soil, put down a woven geotextile then put 1' of stone on top...
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Compared to repouring the floor?

What are the specs on the walls and the footing? Did you include the slab in the wall design?
 
Stone isn't that expensive. last time I priced it, it was less than $300 for a dump truck load. But, thats non-washed #57
 
Thanks for all the good input so far, thats why I love this forum.

I figured around 85 yards of stone. My wonderful county says anything over 1' of dirt or 2' of stone has to have compaction and an engineering signoff. I was going to use stone just for simplicity, but then I found that out.

I can get dirt for free (well, the cost of my fuel), and I will buy a tamp off craigslist for $300-500, so that is probably the cheapest option, but a whole lot of work.

What about sand instead of dirt?
 
What are the specs on your wall and footing?

Has an engineer reviewed them?
 
Damn, that sucks on the engineering signoff. I'm sure the engineer will want to sign off on the proctor tests as well, which will add some $$$

You may could just rent the tamp?

85 yards seems like a lot of fill for a garage floor.

sand still has to be compacted.

I'd spend a few $$$ on stone to cut back on the compaction. The compaction rate of stone over dirt is much, much less. For stone, it'll be more of a formality as most engineers and construction testing companies know the stone will not compact much if any. The dirt may be free, but you'll have tons of time compacting that dirt and getting a engineer to sign off on the compaction percentage and you'll have the materials testing 3rd party out there more often and for longer doing their tests. Ask them to try to do a proctor test on stone :D They won't do it. It'll prob. just be a witness of compaction versus a proctor test on stone
 
saf-t scissors said:
Compared to repouring the floor?
What are the specs on the walls and the footing? Did you include the slab in the wall design?
Click to expand...

hmm... possibly...

I mean, if he's looking at something like a 25x20 garage... that's probably around 74CY or 100 - 110 tons of stone (just assuming 4' depth across the board and 1.5tons/CY)... It still needs compaction IMO (and I would not use washed stone)... Not sure what stone's going for now but say $20/ton and you have $2000 in your base course...

To each his own but yeah I'd say it could easily cost more than re-pouring the floor... And if completed correctly that slab will be fine if filled w/ fill dirt.

A sturdy geotextile goes a long way and is comparably cheap ($0.50/sf or so)...

EDIT:

Testing will be simple... if done in lifts probably 1 test per lift ... so a total of maybe 6 tests... not much money... There is no way I would use sand. You want your soil to be free from organics also...
 
Well, until he tells us anything about his wall and footing design, it's a moot point.
 
6 tests but thats 6 different trips the testing agency has to make unless he can get it all done in 1 day. that much dirt with 1 tamp, I'd suspect at least 3 trips by a testing agency. maybe get testing done in the AM, then work through the day and get one in at the end of the day.

but yeah, its all irrelevant until we know the specifics
 
The fill will likely be balanced for at least 3 feet, more if I need. I've got a lot of time and no money. Washed stone is $25/ton. I just redid the math, it will be about 96 yards, not sure how many tons that is, but definitely not cheap. I haven't laid any block yet because I want to determine what will be in the middle before I do.
 
jeepinmatt said:
The fill will likely be balanced for at least 3 feet, more if I need. I've got a lot of time and no money. Washed stone is $25/ton. I just redid the math, it will be about 96 yards, not sure how many tons that is, but definitely not cheap. I haven't laid any block yet because I want to determine what will be in the middle before I do.
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Why would you want washed stone (#57)? washed stone is good for drainage... ABC or Crusher Run is good for bases and compaction and should be cheaper..

Also 96CY seems like a lot (probably around 135 Tons of material)
 
rockcity said:
instead of backfilling with dirt, you could maybe use some stone and not have to worry with compacting it near as much as dirt.
I'd think for a slab dirt backfill with 6" to 1' lifts with compaction of 95% would be sufficient
for stone, I'd prob. pour it in and run with it without compaction as it'll compact very, very little to none
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X2 . You can put gravel in in two or three inch lifts when you get done put a sprinkler on it for a hour or so for a couple days. It will tighten up solid and you want have to worry about it .
 
I would call your inspector and ask what he wants to see because there are code issues with back fill heights. He will tell you what code requires unless the engineered that sealed your plans specified certain materials and backfill heights. I know the plans I do are sealed by a structural engineer and have details for foundation wall thickness based on back fill. Not sure what they will require in your area. Each municipality is different.
 
uncc civilengineer said:
Why would you want washed stone (#57)? washed stone is good for drainage... ABC or Crusher Run is good for bases and compaction and should be cheaper..
Also 96CY seems like a lot (probably around 135 Tons of material)
Click to expand...
I've been told from multiple sources that washed blue stone (#67 I think) requires no compaction. 24x24x3 + 24x24x1.5 is the area to be filled. Thats 64+32, about 96 cubic yards. I want it to be less too, but it just isn't.
 
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