I have a passive solar off-grid home that works very well with a 2 in. concrete slab that is finished with decorative concrete stain/paint. However, the slab and stain/sealer are very defective and very likely have to be replaced, but one party is proposing a concrete overlay product be put atop the existing slab. The overlay product (ArtCrete Deck Coat, I think it is) consists of two parts--1) Portland cement, aggregate, and a stain, and 2) a modified acrylic latex resin. My belief is this is not good thermal mass (atop the concrete) as the resin component synthetic or at best partly plant material and only earth materials such as stone, clay, water, concrete, provide optimal thermal mass. That in fact the entire slab needs to be removed and the floor rebuilt from scratch to resolve the serious problems and still provide me with optimal thermal mass.

Does anyone have experience or knowledge about this? Using or not using a concrete overlay product for the top layer of a floor for thermal mass, vs. a plain darkly stained concrete slab.

Could you describe how they are "completely defective"?
I don't think you can overlay something which isn't sound in the first place.

I'm not surprised that you aren't pleased with 2" of concrete. Maybe put tile over it.

From a thermal mass standpoint, the overlay product should work fine for thermal mass, as it will just add to the existing 2" concrete thermal mass, assuming it is similar in color (and therefore, similar in absorption of the solar energy). From a structural standpoint, it may not solve the problem of a concrete slab that is "completely defective." That would depend on the details of what is defective in the slab.

Re "completely defective"--my understanding is 10 gal. then 10 more gal. of water were added to the first truck (which came from a good 1.5 hours away), 10 then 15 more to the second, and the trucks sat on site three hours or so, with no retardant added. The troweling and curing processes were apparently not proper as well. It's still questionable whether an appropriate mix was used. The slab has numerous cracks. A few are hairline (to be expected, I know) but some are up to a good 3/8 in. wide and go down the entire depth of the floor. They are all corner cracks--cracks coming off corners (I forgot the term the structural expert used), apparently due to the issues I've mentioned here. Numerous other problems exist with the top layer of the concrete and the paint/dye topping and sealer, but it's a given that at a minimum the floor needs to be ground down 1/4 in. or more at least.

Jonr, the issue is not at all the depth of concrete. Expert research has already been done on this issue. Even 1.5 in. can work fine for applications such as mine.

Very interesting comments. I am confused, though, as from all I've read, only earth products provide optimal thermal mass. So why would synthetic product in the overlay still provide the same thermal mass? Note I need to know if it will provide *identical* thermal mass--not just *acceptable* thermal mass. The house was designed by an expert for a stained concrete floor and I believe anything else will degrade the performance--but I need data to back this up (or refute it).

I answered another comment with more information on the defective slab. If you can read that reply and comment further, I'd greatly appreciate it. The main visible defects are the cracks up to 3/16 in. wide, but I commented on the water added, time the cement sat around before being used, etc. in another reply I just made.

Re your (Lee Dodge) "assuming it is similar in color..." comment, from all I've read, color is not the only key factor in solar energy absorption. If it were, a black rug would be fine thermal mass which it is not. From all I've read, optimal thermal mass storage comes from earth materials only. So my concern is the synthetic portion of the overlay product which is not earth material.

ktot said: "I am confused, though, as from all I've read, only earth products provide optimal thermal mass."

This statement does not match up with my understanding of thermal mass. The total heat capacity of any material is just the specific heat times mass. Water has a very high specific heat, higher than dirt or concrete. Not sure if you consider water an "earth product." Anyway, the heat storage of your slab should be defined by its specific heat, the total mass and the geometry of the slab, the absorption characteristics for solar radiation as modified by the transmissions characteristics of the windows and the area subjected to solar radiation. Simply adding another layer of a concrete mixture on top of the slab should not significantly degrade the thermal mass storage characteristics assuming the solar absorption characteristics are similar. Since you probably don't have an accurate way of measuring the solar absorption characteristics, you might use color as a rough approximation. If you switch from flat black to glossy white, then the absorption characteristics will likely be different. A black rug would absorb a lot of solar radiation, but it has low thermal mass, and would conduct the heat poorly into the slab underneath.

Again, the overlay likely would not address any structural problem with the concrete.

Adding more material can only INCREASE the thermal mass, so the thermal mass will not be the same after, it'll be marginally higher.

But the thermal mass isn't the issue, solar absorption and thermal conductivity is.

Concrete/aggregate/dirt are not exactly refractory materials, but adding an acrylic latex resin for a binder does not substantially alter either it's thermal mass or it's thermal conductivity.

Solar absorption rates may change with the solar reflectivity related to it's color would still have a bigger effect than any miniscule change in thermal mass. Solar reflectivity/absorption is completely orthogonal to the mass, and to it's thermal conductivity. The black rug has high absorption,very little thermal mass, but has comparatively very LOW thermal conductivity. Latex acrylic resin may not have much mass on it's own (but it has WAY more thermal mass & conductivity than the air in the 3/16" gaps), but it is also not very insulating, and in the thicknesses we're talking about here will not have much effect on either the absorption rate or the total heat stored in the slab. If you were adding 5" of this stuff rather than a skim coat it might be measurable, but probably still OK. The other materials embedded in that resin have comparable thermal mass and conductivity as the concrete, and only add to the existing thermal mass. Even if you added highly insulating aerogels into the mix it would have little effect on the rates of solar absorption at these thicknesses.

The resin is required to bond it together and to the pre-existing concrete, but it would take some fairly sophisticated laboratory testing on a removed section of slab to be able to measure the thermal difference between the before & after treatment performance of the slab, and it would still come down to it's color relative to the incident spectrum.

"Optimal thermal mass storage" would probably be water, not earth materials, but it's a bit hard to walk on water at room temperatures. :-) I'll take the skim coat of acrylic latex bonded stuff rather than a puddle for my walking experiments.

Thermal mass is a measurement of how much heat is stored per degree per unit weight.

Thermal conductivity is a measurement of the rate of heat moving through the material per degree of temperature difference.

Solar reflectivity/absorption is the amount of incident light gets retained by the material rather than reflected.

They are not the same thing, but all will affect the thermal performance of the slab as a solar absorber, heat emitter, and heat storage. Thermal mass is all about the heat storage end. Solar reflectivity/absorption affects how much of the solar energy gets absorbed directly and how fast it can radiate (rather than conduct/convect) the heat back out. Thermal conductivity affects primarily the rate at which the heat at the surface gets stored in the rest of the concrete.

But the primary thing the overcoat will change is still the solar reflectivity, which has absolutely NOTHING to do with the thermal mass or thermal conductivity of the overcoat, only it's color relative to the incident spectrum.

Posted By Lee Dodge on 13 May 2013 06:22 PM
ktot said: "I am confused, though, as from all I've read, only earth products provide optimal thermal mass."

This statement does not match up with my understanding of thermal mass. The total heat capacity of any material is just the specific heat times mass. Water has a very high specific heat, higher than dirt or concrete. Not sure if you consider water an "earth product." Anyway, the heat storage of your slab should be defined by its specific heat, the total mass and the geometry of the slab, the absorption characteristics for solar radiation as modified by the transmissions characteristics of the windows and the area subjected to solar radiation. Simply adding another layer of a concrete mixture on top of the slab should not significantly degrade the thermal mass storage characteristics assuming the solar absorption characteristics are similar. Since you probably don't have an accurate way of measuring the solar absorption characteristics, you might use color as a rough approximation. If you switch from flat black to glossy white, then the absorption characteristics will likely be different. A black rug would absorb a lot of solar radiation, but it has low thermal mass, and would conduct the heat poorly into the slab underneath.

Again, the overlay likely would not address any structural problem with the concrete.

This is very helpful. Yes, my understanding is water is an earth product. It is great for thermal mass which is why people use black barrels of water in a greenhouse to release daytime heat at night.

The color issue is totally irrelevant in this discussion (relative to my problems). It's a given whatever the solution is, the floor will be dark, as that's fundamental to passive solar for absorption, so there's no need to discuss the black to white issue (relative to my problems, at least).

However, on the material of the top layer, I agree a black rug would absorb a lot of solar radiation but it has low thermal mass as you say but WHY? What makes this true, since you say it's not an earth issue? I need a technical/engineering/detailed explanation of this. A table showing different thermal masses of different materials, with specific data (numbers/values) would be great, for example.

You can google for both "specific heat" and "thermal conductivity".

If this is just concrete as a topping, the time left before pouring is less important but concrete really should be poured within 1 hr, and ideally within 45 minutes so a couple of hours is really not great. It won't affect the heating of the slab but it would have been rejected if it was for structural uses.

Expert research has already been done on this issue.

Maybe you can point us to this research. I'm having a hard time seeing how a 2" slab is going to be workable/durable in a passive solar situation. How was the previous slab reinforced?

Posted By MikeSolar on 13 May 2013 08:40 PM
If this is just concrete as a topping, the time left before pouring is less important but concrete really should be poured within 1 hr, and ideally within 45 minutes so a couple of hours is really not great. It won't affect the heating of the slab but it would have been rejected if it was for structural uses.

Are you saying it should be poured within 45-60 minutes AFTER it leaves the concrete plant and is in the concrete mixer truck?

Thermal mass of materials is measured by their specific heat, in  BTUs per degree-F per lb.  Density matters, so not all materials of the same specific heat will have the same storage capacity per unit volume, even if they do by weight. "Earth products" are neither identical by specific heat OR density.

http://www.engineeringtoolbox.com/s...d_154.html

The whole notion that there's something magic about "earth product" is silly. Asbestos & mica are earth products with decent thermal mass characteristics (asbestos more so than mica), but have comparatively poor thermal conductivity (like your fuzzy rug), making them somewhat lousy materials to use for thermal mass if the thickness is too high. Vermiculite (another earth product) has the same specific heat of concrete, but is VERY insulating.  Pumice does too, but it's also very insulating and even lower density. Rubber has more than 2x the specific heat of concrete and pumice, but is NOT an earth product, and has many times the heat storage capacity of pumice by volume.

You'll note that plastics have a very HIGH thermal mass in terms of BTUs per degree-pound, but they are fairl low density compared to basalt or portland cement so in terms of heat storage per unit VOLUME it's comparatively crummy but it's still better than pumice or vermiculite.

But we're also talking miniscule volumes, volumes that are the interstitial spaces between the denser portland cement and sand aspects of the slurry.  There's not enough thermal mass in the whole overtopping mix to even MATTER from a thermal storage capacity point of view, but it doesn't negate the thermal mass of the pre-existing concrete the way a half inch of foam insulation might. The thermal conductivity through the overtopping is about the same as the concrete, with or without the acrylic latex. 

If you could make something behave as a significant insulator at that thickness even when it's the binder for not-so-insulating sand our thermal storage problems would be over, because we would have discovered a super-insulator like none ever seen before. As I said in my previous post, even if you replaced the sand with aerogel granules (some of the best insulation currenly known), it would still not have an appreciable effect on the use of the slab as passive solar storage.

A dark porcelain tile will provide better thermal conductivity and specific heat than concrete. Probably at a lower cost than other fixes. And you will know exactly what it will look like when done.

The tile would be set with thin set which is close to identical to the overlay material.

The question here is should OP accept the overlay as a repair or does he want the supplier/contractor to rip out and start over.

If we assume that the 2" material is laid over a suspended wood floor, it will likely crack again even if it is replaced.
The main cracking appears to be shrink cracking, based on the fact that it is cracking at corners i,e, narrow parts of the floor. The big areas of the floor shrunk as they cured and the weakest point (the narrows) is where the concrete cracked. Probably no reinforcement as it is not common or all that useful in this thin a slab. Fibres might have been used to control shrink but probably not in this case. OP indicates that water was added, but we don't know if this was within design or actually "extra". If extra it would have contributed to the shrink factor.

If we had moved this discussion to the radiant forum, I believe that our resident experts would agree that the overlay would have very little impact on the slab from a heating perspective and since this is both in and out, I think it would behave similairly for the solar aspect. Dana has explain much of the reasoning behind this.

If we agree that the concrete will probably crack again (though maybe not so much) if it is replaced, then applying overlay, which is much more elastic than straight concrete, might be a win/win situation. The existing concrete has probably cured by now and will show very little future cracking if any unless the existing floor is not ridged enough. Even if there is some bounce to the floor the overlay will handle most of it without cracking.
OP mentions that the stain/sealer is not good but not what is wrong with it. If it is poorly done from an artistic point of view, than bear in mind that it is much easier to control the final result with overlay than in raw concrete. If the issue is poor adhesion then it will have to come off.

Should OP decide to go the overlay route, then it is best to chip out the cracks in a small bevel about 1/2" deep and repair them with a product like Ardex Feather Finish. Check with the overlay supplier to see if it is compatible with the existing sealer or if the sealer should be removed.

Posted By Lbear on 14 May 2013 12:19 AM

Posted By MikeSolar on 13 May 2013 08:40 PM
If this is just concrete as a topping, the time left before pouring is less important but concrete really should be poured within 1 hr, and ideally within 45 minutes so a couple of hours is really not great. It won't affect the heating of the slab but it would have been rejected if it was for structural uses.

Are you saying it should be poured within 45-60 minutes AFTER it leaves the concrete plant and is in the concrete mixer truck?

Yes, it normally starts setting up within 45 minutes and I've been on some jobs where the truck sat around for an hour after it left the plant and was rejected. Of course, this was for critical structural reasons in this case and there is leeway  for less critical work like ICF or but ideally, the sooner the better. I've been with site foremen and engineers who regularly reject old concrete.

2" of concrete on a possibly bouncy floor ? Hard to keep it from cracking given the circumstances. 

Posted By MikeSolar on 14 May 2013 10:03 PM

Yes, it normally starts setting up within 45 minutes and I've been on some jobs where the truck sat around for an hour after it left the plant and was rejected. Of course, this was for critical structural reasons in this case and there is leeway  for less critical work like ICF or but ideally, the sooner the better. I've been with site foremen and engineers who regularly reject old concrete.

What do they do when the concrete plant and the job site is at least a 45 minute trip or longer due to traffic?