Concrete control joint question

[rademamj1]

Around my area in Texas, we have a nasty clay formation right at the surface, probably 8-10 feet thick and covers hundreds of miles. They have now stopped using gravel base before the foundation pour, because over the years, the gravel being heavy will slowly work itself downward and into the underlying clay and thus rendering it ineffective. The number of foundation repair companies that have sprung up is incredible, and only a small number are truly qualified.

They have now moved to a select fill base, which is 80% fine grained clean sand and 20% silt. This is spread and compacted before the foundation pour and 10' beyond foundation limits. Over time, the select fill being same density does not drop down into the clay. Alot of foundations are now being done post tension cable, and so no rebar of any type is laid down. The verdict on using post tension is still not in yet. The theory is concrete in tension is strong, stable and crack resistant. So up to 48 or more 1/2" plastic sheathed cables are laid out at 16" spacing placed directly in middle of the concrete, one end anchored and the other end is extending 2' beyond the foundation. Three weeks later after the concrete is poured, a tension crew comes by and stretches those cables, placing the entire foundation in full tension. No expansion joints or saw crews are needed.

My building was done this way last June, and there were cracks in the foundation before the tension crew showed up. After they tension the slab, I noticed all visible cracks had closed. I preferred the rebar, but after a soil test my builder refused rebar and would only give warranty on Post Tension Slab.

 

[3Ts]

I have 2 buildings, one is a rebar slab, the other is post-tensioned slab. Both have cracks in them. The rebar slab cracks are about what I'd expected from concrete, the post tensioned cracks are much much smaller. They are visible, but I'd put them into the hairline category.

 

[mikester]

With the above mention of foam...

I can easily understand the benefits of a thermo break below the concrete...

But logically, a layer of foam added above the gravel seems to be less solid than the typical gravel foundation used directly under the concrete?

Is there risk of foam not being as stable as gravel and potential for less support under the slab?

The concrete "floats" on the soil. That might sound weird but it's true. You can even make a boat out of concrete!

If your load is evenly distributed the system floats over the sub soil.

 

[yanmars]

Talking of foam, some of the ESP rigid foam boards supposedly will support 40 or 60 pounds per square inch. That is a lot per square foot, should support a fair amount of concrete.