Foundation replacement questions

[swampratt]

Ok so they call it floating slab foundation?
Since I was a kid I had watched them pour the outer perimeter of a house foundation ,, then later pour the inside and not attach it to the slab.

I thought this was crap then and I still think it is crap now.
Bugs can get between the slab and outer wall .. and it does not seem strong at all.
The way I was taught to pour any type of concrete was like this. With lot's of rebar all the way through the footing and the slab.
Dad and his friend poured a 30x? garage and did it like this with rebar and welded wire both in the 6" slab.

Here is a picture.. and they did not cut any expansion joints and in 25 years it did not crack apart.

 

[Boehlertaught]

The bottom line to all of this is that builders either don't know how to build a house to last or don't want to spend the money. And, of course, most people that have not had foundation or slab issues are not in-tune with the fact that a well build concrete slab costs a lot of money. If you're building a new house and you want a slab, the only way to help it stay put and in one piece is to put a HUGE footing around the slabs perimeter and to put a lot of piers in the slabs middle. All of the concrete for the footing, piers and slab should have rebar and the cement should have fiber in it. The typical footing for a house seems to be 24" deep an a foot wide. I didn't go the slab route. We have a crawl space for the house. The detached garage is a slab. Both buildings have footings that are 48" deep, 36" wide. Nothing has moved yet. Yes, over built but it will last...I hope.

 

[NightShade]

The bottom line to all of this is that builders either don't know how to build a house to last or don't want to spend the money. And, of course, most people that have not had foundation or slab issues are not in-tune with the fact that a well build concrete slab costs a lot of money. If you're building a new house and you want a slab, the only way to help it stay put and in one piece is to put a HUGE footing around the slabs perimeter and to put a lot of piers in the slabs middle. All of the concrete for the footing, piers and slab should have rebar and the cement should have fiber in it. The typical footing for a house seems to be 24" deep an a foot wide. I didn't go the slab route. We have a crawl space for the house. The detached garage is a slab. Both buildings have footings that are 48" deep, 36" wide. Nothing has moved yet. Yes, over built but it will last...I hope.

I personally abhor the slab foundations. I grew up where it was always a deep 6 foot foundation without a basement (crawlspace) and generally 8 foot with a basement. Most of the slab houses I have been in have had area's where the slab has broken and wants to start sloping downhill in the middle of a room. If you have a slab foundation doing the piers is a must as well as making sure the grade is compacted very well before concrete is poured. Most of the builders I have seen do the slabs bring in some fill use a plate compactor and then pour a day or two later. It's good enough for a sidewalk that takes no weight other than a person walking around not a 10 ton house. Without compacting the ground where you intend to build after stripping the sod then filling and compacting every four inches with a sheepsfoot the ground will compact on it's own over time.

I worked road paving as well as grade crews running a sheepsfoot around ALL day long while on the grade crew and even with that heavy bugger compacting the soil I watched the loaded 651 cat scrapers squish the soil two to three inches as they drove past. If you are planning to build and think a little plate compactor is going to do the job right I have a vacation home on the moon that I can sell ya. It's move in ready

 

[CHenry]

The slab is not a structural support and as long as the base is good, the slab will be fine without any piers. Piers would be a huge mistake. Test holes should be drilled to see the soil type and find bedrock and then piers under a wide footing is a good idea. My house sits on a rock (literally almost) and is 15 years old and I see no cracks in my brick or sheetrock from settling. Guess I got lucky but I also knew there was shallow rock there.

 

[DeeJay]

................. Test holes should be drilled to see the soil type and find bedrock and then piers under a wide footing is a good idea. ..................

^^^^^THIS!!!!^^^^^
The usual problem is not the foundation or slab strength, or lack thereof. The problem is the soil (red clay) with its huge shrink and swell characteristics. When it swells, it will lift an enormous amount of weight (ie, footings). When it shrinks, it will create cavities under the slab and allow the slab to bow down in the middle or the footing to sink.
The best answer is usually a good solid footing around with grade beam footings spanning across, all with rebar. And all supported with piers belled at the bottom and with rebar "cages" installed in each one prior to pouring and tied to the footing. Keeping everything from being pushed up is as important as keeping everything from sinking.

 

[CHenry]

^^^^^THIS!!!!^^^^^
The usual problem is not the foundation or slab strength, or lack thereof. The problem is the soil (red clay) with its huge shrink and swell characteristics. When it swells, it will lift an enormous amount of weight (ie, footings). When it shrinks, it will create cavities under the slab and allow the slab to bow down in the middle or the footing to sink.
The best answer is usually a good solid footing around with grade beam footings spanning across, all with rebar. And all supported with piers belled at the bottom and with rebar "cages" installed in each one prior to pouring and tied to the footing. Keeping everything from being pushed up is as important as keeping everything from sinking.

yes, a floating slab is a proven technology and the stemwall is the only supporting foundation so if you put it on a clay base, its a fail. If you plant oak trees next to the house, expect so see heaving of the house in that area in 25 years or less. The soil can't move or you will have issues, period.
By the way shrink swell soil is also a reason we have shity roads, or so I am told.
The sub base is the key.