Posted By Lbear on 14 May 2013 11:25 PM />

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


While we all prefer fresh concrete, newer batching technics and admixes give us a much wider window of opportunity then we use to have.
Manual controls include dry batching the mix and adding water to the drum at or near the site. Or sending the product out in a dry batch truck. The dry products are mixed with water on site through the delivery auger.
An experienced batch man will know from the address how long the trip should take and mix a recipe that takes this into account. Just as floor mixes often have accelerators in them to speed up kick over time, long travel mixes often include retarders.
There are many technics available so one should not automatically assume the batch is "old" just because the ticket indicates batch time was more than an hour ago. That said, We still all like fresh concrete!

Posted By Dana1 on 14 May 2013 06:22 PM
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.

This is exactly the kind of information I was looking for--technical, factual data. Thanks so much. I'll save and reread all your comments carefully so hopefully I can digest them.

Posted By jonr on 14 May 2013 06:46 PM
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.

Two concerns exist about tile. 1) The floor may not be able to hold any more weight than it already has with 2 in. concrete. And 2) the floor continues cracking (it's over 2 years old), with some cracks 1/4 in., even 3/8 in. wide--not quite expected hairline cracks, I believe. Some people believe putting tile atop such a severely cracked floor is like putting siding on a house with rotting wood.

Posted By FBBP on 14 May 2013 09:48 PM
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.

More very useful information, thank you. To address some of your questions and comments, 1) there is NO spring in the floor. My builder actually overbuilds houses, this one included. That has been confirmed by an engineering analysis of the floor joists/design, etc. 2) 1.5 and 2 in. elevated slabs are used successfully with minimal cracking when done properly; research and references on this were done prior to my accepting my architect's design of this floor, so a proper slab should not crack at all like mine has; that's one reason for expansion cuts as well. 3) The issues of the stain are not specifically aesthetic at all (though the results look disastrous). The concrete people did not prep the floor properly at all, using none of the required chemicals. Plus they used an overly water dilution of the product--1:8 is the max, they used 1:10. Plus distilled water is required but they used their very hard (I have the technical water analysis results and have had them reviewed by experts) well water which causes problems in the stain as well. Plus they sealed 9 days after pouring without doing adequate testing at all. There's a good 1/4 in. of laitence that is easily scraped off with a putty knife--laitence that should have been removed before the floor was ever stained. Entire rooms have peeled up. To clarify--it's not even stain (which is what I was supposed to get). It's a water-based paint/dye product so is topical, not penetrating.

So at a minimum, the floor needs to be ground down at least 1/4 in. before any new solution is applied.

By the way, these contractors claim 30 years and 16+ years extensive experience in the concrete and decorative concrete industries, respectively--they're not fly-by-nighters.

Sounds like you have large movement from changes in the wood's moisture level and nothing strong enough to resist moving with it. Other than starting over, I think you need something that will move independently, ie slide over the concrete.

Posted By jonr on 15 May 2013 05:01 PM
Sounds like you have large movement from changes in the wood's moisture level and nothing strong enough to resist moving with it. Other than starting over, I think you need something that will move independently, ie slide over the concrete.

Why are you saying it's a wood moisture level? Not a single expert (structural, concrete crack expert, construction, joist design, carpentry, more) has even suggested such a possibility. That's a pretty strange claim, it sounds like to me, or many people's homes, at least those with concrete slabs, would have excessive cracking. Be aware all the cracks are corner cracks which concrete experts know how to interpret correctly.

Relative to the physics of passive solar thermal mass performance, Dana’s words captured this quite eloquently as always. Feel free to use this software for calculating the actual performance:

Borst Passive Solar Thermal Mass Performance

Why are you saying it's a wood moisture level? ... the floor continues cracking (it's over 2 years old), with some cracks 1/4 in., even 3/8 in. wide--not quite expected hairline cracks,

More weak concrete than wood doing what wood does. I haven't seen it and I'm no concrete expert, but what else would explain that you are continuing to get new cracks, years after the concrete has cured? You say that the floor has no spring, so that's not causing cracks. Left alone, cured concrete doesn't crack.

Posted By jonr on 15 May 2013 07:36 PM

Why are you saying it's a wood moisture level? ... the floor continues cracking (it's over 2 years old), with some cracks 1/4 in., even 3/8 in. wide--not quite expected hairline cracks,

More weak concrete than wood doing what wood does. I haven't seen it and I'm no concrete expert, but what else would explain that you are continuing to get new cracks, years after the concrete has cured? You say that the floor has no spring, so that's not causing cracks. Left alone, cured concrete doesn't crack.

What if the concrete never cured properly and/or the mix was very defective? And where in fact did the moisture go if the slab did cure? I couldn't go out the bottom--there's thick insulation under the floor and felt paper above. Once the non-breatheable sealer was applied nine days after the pour, where did the rest of the moisture (the blooms from efflorescence are numerous, still very visible today, over two years later) go while the concrete supposedly cured? In other words, did, in fact, the concrete ever cure out?

What if the concrete never cured properly and/or the mix was very defective?

As far as I can see, you have not yet described your installation or given a full account of how you understand it to be defective. Is this a slab atop a wood structure and subfloor?

What if the concrete never cured properly and/or the mix was very defective?

Then you have weak concrete, but I thought that that was already well established.

Concrete does not "cure" by drying. Totally saturated concrete can be fully cured. To much water in the unset mix leads to a number of problems but once the mix is set it should be kept damp to wet to get the best strength.

To simplify the process, consider that the lime in the portland cement is baked to remove all moisture. The curing process is the reverse. The lime absorbs as much water as possible. If there is not enough moisture the design strength is not reached. If there is to much moisture it is ignored. The majority of this absorption occurs in the first 3 to 4 days. After that, it is not as important to keep it wet and the balance of the free water would evaporate off a 2" slab in a couple of days. So normally the sealer applied to the slab after nine days would have been fine. Most slabs are poured over poly or some other low permeant material to keep the water/lime (grease) in the slab so your felt is not unusual.

The fact that there is loose sealer etc. indicates there is a problem. This can indeed be the result of moisture lifting the finish but there are many other possibilities as well. Most likely an incompatibility problem between the stain/paint and the sealer or as you mentioned a reaction of the local water with the finish.

I am confused by the fact that the slab continues to crack. Concrete really only cracks for two reasons. Shrinkage and movement. As the slab is two years old, it is not possible that the slab is still shrinking. Even very weak concrete will not crack unless movement/deflection occurs. This concrete is reinforced externally by the floor structure so compressive strength is not likely the issue. Of course stronger concrete will withstand more movement but we need to find out where the movement is coming from.

I assume the slab is internal to the building envelope so it is unlikely that the movement is due to thermal expansion or contraction. If there is a 20º difference in temps, hairline cracks might open and close but you would need a much greater difference to produce new cracks.

That only leaves non uniform up/down movement as the cause of cracking. Again stronger concrete is more resistant to movement cracks but it still requires movement. At one point you indicated the floor is very substantial and at another you indicate that it might not withstand the additional weight of tile. This is contradictory.

Ideally you should remove a piece and send it to a lab for analysis. Two at home tests. Take a light hammer and hold it by the end of the handle and let it drop from about 3". The sound should be crisp. If you have another slab, say a garage slab, you could compare the two. If the 2" sounds more muffled than the good slab you might suspect issues. Also you can take a sharp awl and attempt to penetrate the slab.

You indicate there was "a good 1/4 in. of laitence that is easily scraped off with a putty knife". This indicates a problem. A properly finished slab will have a smooth hard finish with very little chalking. Not being able the see the actual surface, I am just guessing but the first thing that comes to mind is that the finishers got on the floor to soon. The weep water may still have been on the surface of the slab and was then reworked into the top 1/4". This will usually produce a chalky white soft surface even though the rest of the slab is fine. This would explain the bad staining/sealing but not the cracking.

Posted By ICFHybrid on 16 May 2013 07:59 AM

What if the concrete never cured properly and/or the mix was very defective?

As far as I can see, you have not yet described your installation or given a full account of how you understand it to be defective. Is this a slab atop a wood structure and subfloor?

Yes. New construction, extremely well built house (other than main floor concrete products of course).

Posted By jonr on 16 May 2013 08:59 AM

What if the concrete never cured properly and/or the mix was very defective?

Then you have weak concrete, but I thought that that was already well established.

So what are the implications, the possible long term results, of weak concrete in an elevated slab with radiant tubes inside and decorative finish on top (or possibly concrete overlay topping, or natural or ceramic tile topping in the future)?

The finish will crack, unless it floats/slides over the weak concrete.

Posted By jonr on 16 May 2013 12:29 PM
The finish will crack, unless it floats/slides over the weak concrete.

Perfect. I need an authoritative source that states that. Can you point me to any?

Step 1: Document the floor structure and subfloor to show that it is adequately designed and constructed to support a concrete slab and that it is approved for the weight of such a slab.
Step 2: Verify that the exact product used to lay the "concrete" was installed as per manufacturer's specs.
Step 3: Take photos and samples of the slab to show how it was defective.

If those steps are insufficient to elicit a response from either the contractor, his bonding agency or his insurance, then you can consider suit.

In the meantime, you can use the above steps to figure out where the whole thing went wrong and what the most appropriate fix will be.

Some possibilities or combination thereof;

1. Improve structure to stabilize slab.
2. Remove/replace slab.
2. Use Schluter Ditra or other decoupling product to assure a good tile installation over newly stabilized slab.

If your slab is bad because of structure issues, it all has to be replaced.
If it is simply because it was a bad mix, I doubt you can "top" it successfully because the underlying slab will continue to be unstable, although you could try to use your pictures to get a company like Schluter to see if their product could successfully go over a demonstrably bad slab.

Posted By ICFHybrid on 16 May 2013 08:45 PM
Step 1: Document the floor structure and subfloor to show that it is adequately designed and constructed to support a concrete slab and that it is approved for the weight of such a slab.
Step 2: Verify that the exact product used to lay the "concrete" was installed as per manufacturer's specs.
Step 3: Take photos and samples of the slab to show how it was defective.

If those steps are insufficient to elicit a response from either the contractor, his bonding agency or his insurance, then you can consider suit.

In the meantime, you can use the above steps to figure out where the whole thing went wrong and what the most appropriate fix will be.

Some possibilities or combination thereof;

1. Improve structure to stabilize slab.
2. Remove/replace slab.
2. Use Schluter Ditra or other decoupling product to assure a good tile installation over newly stabilized slab.

If your slab is bad because of structure issues, it all has to be replaced.
If it is simply because it was a bad mix, I doubt you can "top" it successfully because the underlying slab will continue to be unstable, although you could try to use your pictures to get a company like Schluter to see if their product could successfully go over a demonstrably bad slab.

I'm about two years ahead of your suggestions, so many of your comments aren't too helpful at this point. However, you say "...it was a bad mix..." How is that determined? We have what was in the mix but it appears a combination of the mix (not necessarily inherently bad until way too much water was added onsite), too long sitting on the truck, inadequate cure time, etc. caused the problems. Some people say cracks 1/4 in. and 3/8 in. wide can be filled and a topping over them will be fine. Others say cracking will continue into the future. Obviously no one can predict the future with certainty. The contractors are saying 1/4 in. and 3/8 in. cracks are very normal (even for concrete poured for decorative staining).

However, I'm unclear what you're saying in your Step 2: "exact product"? What product to lay the concrete are you referring to? The process they used to trowel and finish the concrete? What manufacturer's specs?

So that's really what the current question is: how can I determine with strong certainty that either a topping over the slab will yield a top quality result to match the rest of the house, or that to ensure top quality of the floor, the slab needs to be removed. I'll stress again the floor structure and subfloor have already been documented as more than adequate--with hard data (calculations) as well as inspection by experts.

Fine. If your structure and subfloor is documented as proper, then the existing slab is probably just suffering from too much water and maybe a poor cure due to too much time on truck. Too much water will cause excessive shrinkage and cracking during cure, but all that should be over in the first few weeks. All you have to do is check the concrete for minimum compressive strength necessary to lay a floor over. I can't tell you what that is offhand, but it isn't much as some very soft materials like gypcrete can be used to successfully underlay tile floors.

Fill the cracks, top it, if necessary, to achieve a better surface, use an isolation layer like Schluter Ditra under the tile and you should be good to go. If you still want a stained and finished concrete surface, I would NOT keep the existing slab. Start over.

By "exact product", I was referring to the possibility that a bagged product was used. If it was only Portland cement, sand and aggregate, then what you need is the mixing plant records which should show if the mix started life properly.

PS - 1/4" and 3/8" cracks are not normal in any properly laid and reinforced slab, particularly for a residential floor.

Posted By ICFHybrid on 17 May 2013 01:16 AM
Fine. If your structure and subfloor is documented as proper, then the existing slab is probably just suffering from too much water and maybe a poor cure due to too much time on truck. Too much water will cause excessive shrinkage and cracking during cure, but all that should be over in the first few weeks. All you have to do is check the concrete for minimum compressive strength necessary to lay a floor over. I can't tell you what that is offhand, but it isn't much as some very soft materials like gypcrete can be used to successfully underlay tile floors.

Fill the cracks, top it, if necessary, to achieve a better surface, use an isolation layer like Schluter Ditra under the tile and you should be good to go. If you still want a stained and finished concrete surface, I would NOT keep the existing slab. Start over.

By "exact product", I was referring to the possibility that a bagged product was used. If it was only Portland cement, sand and aggregate, then what you need is the mixing plant records which should show if the mix started life properly.

PS - 1/4" and 3/8" cracks are not normal in any properly laid and reinforced slab, particularly for a residential floor.

Thanks for clarifying. By the way, I know 1/4 and 3/8 aren't normal, but the contractors keep claiming they are, so your comment is useful confirmation. One request, though--I am due what I paid for, which was stained concrete (not tile), so can you elaborate on why you "would NOT keep the existing slab. Start over."?

What I do know about the mix at this point (more information is being requested) is it's a 7 pea gravel mix (which I was told was more expensive but would minimize cracking (!)) and has some Ultrafiber 5000 or something like that in it.