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# Concrete slab as finished floor - residential

Stillfighting

Hi world,

Wondering about concrete slabs right now at midnight.

For my lack of knowledge, I can only think of T. Ando's projects when I think about just pure, raw concrete house without any insulation since it was probably not required back then. I also understand that now days it is common practice to add insulation above the slab with additional finished flooring materials.

But, I just wanted to know if there is any built project out there that used the concrete slab as the finished floor? With or without insulation.

Many thanks!

K

Non Sequitur

we add insulation below the concrete slab.  Don't think I've seen it added above before.

Stillfighting

I see. Right, it is typical for basement and crawl space. But, you know any project done it for the main floor? Also, just a follow up question on that. Wouldn't the weight of the slab or the live load compress the Insulation though? Wouldn't it sink a bit? Or deformed? I just don't know the limitation of the pink material I guess. Thanks!

Non Sequitur

They make high compression rigid insulation specifically for under slab applications.

It doesn't even need to be that high compression. 25 psi is pretty typical for commercial underslab use. You can get the "pink stuff" in 100 psi compressive strength if you want to. Get out your calculator and some scratch paper and you'll probably find that 25 psi stuff is more than enough.

Non Sequitur

My clients often drive fork lifts and trucks on the slabs... so high-density is justified but I agree, you don't need high-density in most applications.

NS, here's an example I saw when I was doing a quick google for my earlier response:

"Forklifts to be used in the building impart 8,000 pounds of force at the wheel, which has a 6-inch by 10-inch tire footprint on the slab. If the designer assumes that this load distributes at a 45-degree angle through the slab, the 8,000 pounds ends up distributed over approximately 396 square inches [(6 + 6 + 6)(6 + 10 + 6)] of the insulation’s surface, for an average pressure of 20.2 psi.

The article is actually saying that's a conservative approach and doesn't need to be that high. Using the approach the article is recommending, the load on the insulation is calculated at 1.85 psi.

Non Sequitur

Neat. Thanks. I have a private car/boat/helicopter collector garage on the books heading to construction this summer. I'll take a look at the u/s insulation and see.

Wood Guy

I use concrete as the finished floor regularly, on high-performance homes. I agree with EA that 25 psi is plenty. I actually use 15 psi in most cases, with 25 psi under footings, or 40 psi under columns supporting 15 kips or more. I use EPS or GPS, not XPS, for environmental reasons but structurally either one is fine.

I have an ADU under construction now, and two new homes about to break ground, using this forming system: https://warmform.com/. The ADU slab will be covered with a floating wood floor so the finish didn't matter, but the other projects will have exposed concrete. The least expensive approach is to just seal the finished surface, but you can also grind the surface to remove the cream and expose the aggregate; you can add dyes to change the color, you can acid-stain the surface to get rich, leather-like patterns or you can use paint or solid stain to cover the concrete.

Stillfighting

Learned so much! Thanks a lot for the knowledge everyone. By the way that warmform thing is so freakin cool!

Almosthip

Keep in mind that concrete cracks.  You cannot avoid it, it WILL crack

Wood Guy

That is sort-of true. There are many things you can do that will greatly limit how much cracking occurs. One that is often overlooked is counterintuitive--the higher the design strength, the greater the chance of shrinkage cracks. Go with code-minimum requirements or a little over. You can specify low water content, allow slow curing by keeping the surface damp, use rebar appropriately, welded wire mesh and/or micro-fibers to limit fine cracking, ensure a well-compacted, well-drained sub-slab prep, don't backfill walls too soon. Use supplementary cementitious compounds such as Pozzolans or fly ash to replace some of the Portland cement and to densify the concrete. Provide proper drainage. Provide air entrainment to prevent spalling. Etc..

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