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

[LoterGarry]

I posted here previously and received so much guidance and support (thank you all), and now getting back on track. Because of damage to joists and foundation, we ended up having to do some structural work (joists, foundation). I have searched around this forum for answers and advice. I found some pieces I’ve incorporated in this post.

I have a radiant installer that I am ready to sign a contract with. He has provided layout of pex runs, heat loss, etc. Installation is in joist with heat transfer plates for 2nd floor, and in slab for 1st floor. Pex is specified as 1/2".

Now I’m getting prepared for the next phase: new slab preparation. I am actively speaking with concrete contractors to prepare, pour and finish the slab (class B, level 2 semi gloss finish). I know that the concrete mix design is very important for specialized applications like a polished slab with radiant heat. One source I have been getting info from is delgrossodesign website, and the owner has provided me additional info via email. The owner is outside our area and price range, so ultimately we will need to hire someone else for the finishing of the slab.

Initial plan for slab was 4” slab with #4 rebar 12” OC doweled into foundation at 24” OC. I will be going back to my engineer to confirm the direction we want to head. He has not been great, and signed off on things during inspections that later caused problems. However, we didn’t want to start fresh with a new engineer (due to cost and need to resubmitting plans) so we decided to keep him to complete the project.

I’m looking for practical recommendations, informed by what folks have seen work best in the field for our situation. While our Engineer of Record (EOR) will, of course, need to review and approve the final agreed-upon mix design for compliance, I value folks first-hand experience in this site.

One concrete contractor I received a quote from recommended the following in his estimate:

• 5” slab with the slab unattached from the foundation and using expansion joints between the slab and interior foundation walls (to better minimize slab cracking).
• 6” of compacted gravel, vapor barrier, insulation, 5” slab
  • 1” insulation of slab (will specify 25 psi insulation, likely XPS Dow or foamular 250); could go thicker.
• Pour 7 sack 4000 psi with ⅜ pea gravel; admixtures would be specified by working with the concrete supplier company (he mentioned: liquid fibers, water reducer as admixtures to consider)
• He said he would only create tooled control joints. My research on control joints:
  • 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)
• He mentioned that I should mist the slab post pour a couple times a day to help minimize cracking during curing

The questions I am looking for advice on (or anything else folks care to offer):

1. dowel the slab into the foundation or leave it unattached with expansion joints?
2. pex running through expansion joints?
   1. we have some interior foundation grade beams that the pex might need to run above for the ideal boiler and manifold location
3. control joints: tooled vs saw cut?
   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”)?
4. pex running under control joints?
   1. take preventative measures: using conduit or pipe wraps where pex runs under control joints
5. 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
6. 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)