specifying concrete for slab pour to get the right look


we are pouring 4000psi concrete for a structural slab that will be a finished floor in a single family residence, trying to get a rough idea of what is usually done to get that darker, varied look, more like an old factory floor rather than a tract home garage(homogeneous), like this maybe?

I know the cement can affect the color, we are on the west coast which I've heard generally has a darker cement from most suppliers than on the east coast but I don't really know. Thanks

Mar 22, 21 1:42 am
Wood Guy

The easiest way to darken the color is to let it damp-cure slowly. When concrete (or other cementitious material) dries before it can cure it will be lighter than if allowed to cure slowly. Slow-curing is also a good way to ensure the design strength is actually achieved, and will minimize cracks. FYI, 4000psi concrete is more prone to shrinkage cracks than lower-psi concrete, assuming standard, equivalent mixes otherwise.) 

You can add a dye to the concrete mix. Usually powdered but liquid is also available. You can use plain black, but mixing in a bit of blue can make a richer-looking mix. 

You can specify an aggregate with a dark color instead of using whatever they have the most of. You'll have to ask what is available at your local batch plant. 

You can polish the concrete after it's cured, which is really a light grinding with a diamond wheel that removes the surface cream and exposes the aggregate. Since the aggregate takes up more space than the cement (which is the glue, or binder), with polished concrete the aggregate color matters more than if you don't polish the surface. 

If you're going to polish the concrete, you can specify a maximum 3/8" tumbled stone, which will have a much finer look than the typical 3/4" crushed stone--more like the Terrazzo finishes in old public buildings. 

You can acid stain the concrete after it's cured (and polished, if going that route). The acidic stain has a chemical reaction with the strongly alkaline concrete and the results will be organic and somewhat unpredictable. 

You can seal the concrete with a finish that "enhances" the color--masonry trade-speak for making it dark, like oil on wood. 

There are probably other tips but that's all I can think of for now. 

Mar 22, 21 9:00 am  · 
3  · 

This is a very informative post from Wood Guy. 

A few other observations based on experience: 

  • Consider additional rebar loops at outside and re-entrant corners. 
  • Consider designing a nice (and practical) saw-cut pattern to control cracking. 
  • Consider stair nosings, treads, and contrasting visual and tactile indicators at stair treads (depending on code). 
  • If the installers are placing the concrete with a hose, you can frequently get concentrations of aggregate that become really obvious once you remove the cream finish - consider specifying some hand-seeded aggregate before they level and float the slab.
Mar 22, 21 10:49 am  · 
t a z

Probably worth pointing that the majority of the ArchDaily photo references are likely not the structural slab, but rather an architectural concrete topping slab (or screed).

For the maximum control of the finish

Mar 22, 21 12:00 pm  · 
2  · 

I highly recommend a mockup or several to see what you can accomplish in terms of finish as well as check the skill level of the concrete crew.

Mar 22, 21 11:37 am  · 
2  · 
atelier nobody

The best way to get a perfect polished concrete floor is to spend 20-50 years putting down and stripping multiple flooring types with multiple adhesives, let some vehicles drip fluids on it, bolt down and then remove various heavy equipment, drop a few big block engines on it for some nice chips, then scrub it as clean as you can get it and polish it. (Can you tell I've done a few retail TIs?)

For new construction, polishing will expose a little more mottling than the uniform gray of the trowel finish, but if you want the heavy mottling in some of those ArchDaily pictures, you'll need to use some stain.

Mar 22, 21 2:21 pm  · 
1  · 

we've done acid etching or grind/polish

integral color is basically always required in our area because we get such variable colors out of the plant typically

we did an integral deep black for our studio heated slab that was gorgeous after curing but before sealing...they sealed it with a gloss product mistakenly and then ground/polished it to correct. We were pretty sad at the lost finish, but it's been pretty good otherwise

Mar 22, 21 2:30 pm  · 
1  · 

what a lot of great responses, thanks a lot! Atelier Nobody, totally agree, the actual floor I was aiming for was the floor of SCI-Arc which is basically exactly as you describe :)

sciarc floor (not the exhibit) :)

Mar 25, 21 2:00 am  · 

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