new concrete slab with radiant heat that will be polished for finished flooring

1. dowel the slab into the foundation or leave it unattached with expansion joints? 
   —No, I wouldn’t
2. pex running through expansion joints?
   —I would loop it under.
3. we have some interior foundation grade beams that the pex might need to run above for the ideal boiler and manifold location
4. control joints: tooled vs saw cut?

—Tooled. You’re going to gets some cracking no matter what you do. And you’re relying on someone to be perfect with layout and execution of saw cutting.

1. My research: saw-cut control joints are generally preferred over tooled control joints for heated polished slabs (due to their deeper cuts, more visually appealing on finished surface b/c less wide/collect less dirt and debris)
2. Are there other options I should be considering (such as “zip strips”)?
3. pex running under control joints? —-I think this would be best—-
   1. take preventative measures: using conduit or pipe wraps where pex runs under control joints
4. concrete mix advice - does a mix with the following components seem reasonable? Am I missing or misunderstanding anything?
   1. 3/8" pea gravel
      1. If we follow the rule for 3 times the aggregate size above the tube [ref post], and with awareness that smaller aggregate does end up being a bit weaker mix;
      2. does a 7 sack mix, and/or other admixtures (liquid fibers) compensate adequately for this?
   2. Liquid fibers 
      1. Assume this refers to cellulose-based microfibers that are designed to disperse extremely well in concrete mixes and are often invisible in the finished product, even after polishing. Liquid fibers (specifically cellulose microfibers like UltraFiber 500 by Solomon Colors, or similar products from other manufacturers) are highly recommended for polished concrete. Unlike traditional polypropylene fibers, which can sometimes "fuzz" or become visible on the surface, these liquid fibers are engineered to be non-obtrusive.
   3. Water reducer (assume this is the same as “plasticizer”).
   4. Fly ash
      1. I've read that fly ash can cause issues when grinding and honing, so it’s better to avoid it.
   5. Integral color (to achieve a consistent darker grey color in finished product)
      1. Opinions I have heard: add it for consistency; not worth the cost; add a small amount of black to achieve a grey and minimizing cost. I personally would do smallest aggregate. Hot Rod would be the expert on this.
5. Insulation strategy
   1. I was thinking we should taper the top of the edge insulation
      1. similar to this diagram, though our sill is flush with the stem walls, and don’t think we would put insulation on edge below the bottom of slab (I am in climate 3) Standard detail, looks good to me.

we had a concrete expert at an RPA convention yeas ago talking specifically about radiant and slabs. His suggestions

When you pour a slab within 4 concrete walls you can get external restraint cracking. Always have a bit of an expansion strip around it. They have a dense rubber foam strip at the concrete suppliers for this1/2 wide 4” deep.

The larger the aggregate the stronger the concrete mix, go with 3/4 or larger.

The mix leaves the plant properly mixed. Every gallon of water added to a yard of concrete reduces the psi by about 100 lbs. use a plasticized if you need it to flow better. If they add5- 10 gallons when the truck arrives to make it easier to “pull” your odds of a crack increase.

Rebar, heavy remesh, nothing guarantees a slab will not crack. The steel keeps the slab from shifting where a crack goes through a slab. A crack is a free control joint :)

The subgrade below the slab has a lot to do with the integrity. On commercial work they do compaction tests. I have seen a few jobs fail that test and the subgrade had to be reworked,

I would use a base rock at least 4” and compact it.

I had a finished, colored slab in two of my office/ shops, color was mixed in the batch at the job, and no, they do not like doing that and may charge extra! Actually the color sort of wore through where we walked the time all you would start to see some aggregate.

How large is the slab? Control joints are on a 10x10’ grid. I have poured jobs without any control joints for custom basement colored slabs.

Know that when you saw cut a colored slab you will see white stripes as the aggregate is not colored! Usually you caulk those saw cuts with a colored silicone.

Ideally a control joint is 1/3 the slab thickness, which is not doable with tube. Best you could gamble would be a 1” deep cut if you have tube 2” below the top. But a crack will travel across a 1” cut. I’m not convinced any type of control joint is 100% able to prevent or stop a crack. It the tiny little cracks, called road mapping, that drive me crazy in finished slabs. Prepare your self for those.

A troweled in joint will not look good at all. It would be a wide joint to caulk nicely?

While the mesh is great for tying tube it does little to reinforce the slab, especially if it is not lifted.

Yes the fibers look like dog hair on the finished slab, but if they power trowel it melts them off and they are not noticeable.

The recommendations from the speaker were to increase the fibers to 3 lbs per yard, do not water the mix, add agents if it is extremely hot, as a fast dry day can be an issue. On hot summer pours they would spray a coating on them as soon as the finished troweling, supposed to slow the water from leaving the top.

The slabs that looked the best to me over the years were stamped or stenciled ones, they hide cracks well. I did one job where it was stamped to look like wood planks! It was a little rough on the feet however.

Hoping to get a perfect, long lasting crack free polished slab is risky business. I doubt any plant or concrete finisher would guarantee that. It is a hard unforgiving surface, in kitchens especially. But tile is also should you drop anything on it.

I would tube it 6” oc to operate at the lowest SWT, minimize expansion/ contraction. 3/8 is easiest to work, but 200- 225’ loops. You get a tad more concrete coverage.

Pros and cons to high mass vs low mass radiant. Slabs need to be set and left circulating imo. They do not ramp up or down quickly.

I will say in a shop in a cold climate, nothing beats a warm slab. Concrete is a tricky medium to work with, and it is pretty final. Concrete is the most consumed product in the world. Looking at some of the structures and roadways the build in China, they must be the largest consumer?

Just to emphasize a couple of @hot_rod 's excellent summary. He's right. Fibers won't be a problem if you power trowel. You won't notice them. And use enough. Also, his comment on don't add water. Just don't. As he says, if for some reason you need it to flow better, specify adding plasticizer — and let the ready mix company do it.

Last tidbit. Keep the concrete moist or at least damp for at least 72 hours. That is really truly critical, and a lot of the problem surface cracking one sees is from not doing this. Perhaps I can explain why. Cement is the binder in concrete — it's what sticks to the aggregate and holds it all together. Cement does NOT dry. It cures by reacting with water to form a crystalline solid. This takes time — and enough water. If there isn't adequate water, not all the cement can react and the cement won't cure. This is particularly true at the surface of, for instance, a slab. Too much water is almost as bad. Concrete mixes are very carefully designed!

you can also cover the slab in tarps or plastic once it sets enough that the tarps won't mark it to help keep it wet so it cures properly.

you would definitely want to run outdoor reset to mix the slab heating temp with the combination of the high mass and your range of about 90 degrees of outdoor temp where you need heating. the better you insulate the less your heating load will vary with outdoor temp.