BSI-003 Concrete Floor Problems Building Science Information

BSI-003 Concrete Floor Problems  Building Science Information

BSI-003: Concrete Floor Problems

Here is a pop quiz for you folks that are bored of Jeopardy. California is desert. Florida is a swamp. We build concrete slab-on-grade in both places. Which place has more slab moisture problems? Yes, you guessed it, California. Why? In California they place sand between slabs and plastic vapor barriersin Florida they dont.

Perhaps it was the drug culture of the 60s that turned brains into coleslaw but it is hard to understand the lunatic practice of placing a layer of sand over the top of a plastic ground cover under a concrete slab in California. That this is an often recommended practice by geotechnical engineers results in sputtering frustration among those who know better but can do nothing about it because of the rock star status accorded the geotechnical engineer in California.*

Photograph 1: Florida Slab Note the continuous polyethylene sheet extending to the perimeter formwork completely isolating the concrete slab from ground contact. The saying I have swampland in Florida to sell youis true. But if the concrete is not in contact with the swampland the swampland does not cause grief to floor finishes installed on top of the slab.

Photograph 4: Vinyl Floor Tiles Vinyl floor tiles are vapor barriers that prevent drying of the slab. The concrete does not care if it is wet or if it stays wetin fact it is happy to be wet. The vinyl floor tiles themselves also dont care if they touch wet things. The problem is the adhesive between the vinyl floor tiles and the concrete. Most adhesives today are latex based and very sensitive to alkaline environmentshigh pHlike concrete. Sticking stuff that does not breath to concrete that is wet or gets wet with a latex based adhesive does not work. The adhesive re-emulsifies (goes back to a gooey state) in the wet alkaline environment.

This typically results in moisture problems with floor coverings ( Photograph 3 and Photograph 4 ). The sand layer becomes saturated with water from wet curing (water added to the top of the slab leaks down into the sand layer through service penetrations) or from irrigation (water added to the ground at the building perimeter enters the sand layer from the side or from underneath) or from rain (see wet curing and irrigation).

The sand layer cannot dry downwards by vapor diffusion because of the plastic sheetit is a vapor barrier after all. In fact thats why we have it under the slab. We want a vapor barrier under the slab.** We want to prevent water vapor from coming up from the ground into the concrete slab. But it works both waysit prevents water vapor from going down as well. Bummer.

It gets worse. The water also cant drain out of the sand because it is held there by capillary forces. So even if we perforate the plastic sheet, drainage wont happen. You cant drain water out of a sponge. What if we used pea gravel? Nope, large quantities of water are still held in the pea gravelthink surface area of the gravel and the fines in the pea gravel. Pea gravel does drain, but it retains huge amounts of water even though it drains.

Now for the real ugly part. The sand layer is wetted by liquid phase wetting in a time frame measured in minutes. Whereas, the sand layer can only dry upwards by vapor phase drying in a time frame measured in years. You can do a simple experiment at homeboys and girls. Take a paper napkin and wet it by putting it in a cup of water. Now pull it out. It got wet in seconds didnt it? Now hold it horizontally in the air and wait for it to dry. Takes a long time doesnt it? Now do it again by placing the wetted paper napkin over a plastic sheet and covering the top of it with a layer of concrete. In the real world a 15-minute liquid phase wetting event can take years to dry.

Guess what? No sand layer, no reservoir to get wet and hold and store water to cause grief. No problem. Repeat after me dont ever, ever put a layer of sand between a plastic vapor barrier and a concrete slabdont even think about it.

So where did the idea of putting sand between plastic and concrete come from? Like most bad ideas, it arose from good intentionsit was an attempt to reduce the curl of concrete slabs due to uneven drying. If a slab dries only from the topthe top becomes drier than the bottomduhThe top shrinks relative to the bottom and the slab edges curl upwards. This is not a big movement and is not a big deal except where you need ultra flat floor systemslike in chip manufacturing plants. Slab curl is very easy to control, use a low water-to-cement ratio (less than 0.5) and wet cure the top (wetted burlap worksP hotograph 5 ). Or just use a low water-to-cement ratio and forget about doing anything else.

Photograph 5: Burlap Cure You just cant beat curing concrete slabs the old fashioned way with wetted burlap. This is a project in Sacramento, CA that is obviously supervised by an old guy who knows stuff.

You cant do thatit will never work. I hear that and I just laugh. It works everywhere else on the planet. Yes, but California is different. It is hot, it is dry, its sunny. Really? Check out Photograph 6 its from California. What do you see? A fluted steel deck that is going to get a concrete slab placed directly on it. Steel decking is a pretty good vapor barrier, eh? Anybody ever see anyone put a sand layer between the steel decking and the concrete slabs that are cast over them? So why do I need a sand layer over the plastic for the slab on the ground but I dont need a sand layer for the elevated slab on the steel deck? Apparently the laws of physics change between the first floor and the second floor

Photograph 6: Fluted Steel Deck We seem to be able to place concrete slabs on fluted steel decks without needing a sand layer to control curl.

Okay, so we dont need the sand layer to handle the curl thing. But what about using the sand layer to protect the polyethylene? Hah. The polyethylene does not need protecting. You can poke holes in it, you can puncture it, you can tear it, you can leave gaps in it, and pretty much have your way with it as long as it is in direct contact with the concrete. Huh? But, but, vapor barriers have to be continuous and free from any holes. Actually, no. Air barriers need to be continuous and free from holes, but vapor barriers do not need to be. Lots of vapor moves by air movement, not a heck of a lot of vapor moves by vapor diffusion. The concrete slab is the air barrier, and the ripped and torn and punctured polyethylene sheet is the vapor barrier. Its that Ficks Law thing. Diffusion is a direct function of surface areaif I get 95 percent of the surface covered I am pretty much 95 percent effectiveand the parts that are left I have filled with concrete which is also pretty good as a vapor barrier. I could wear golf shoes and march around the plastic vapor barrier and not do much damage. But put that sand layer in there and you are doomed.

Yeah, but if I dont put the sand layer in there it will take too long to finish the floor. Yes, thats true if you use crappy concrete with too much water in it. The easy answer is dont use crappy concrete. See water-to-cement ratio less than 0.5 above.

We actually have the opposite problem now in California. Real structural engineers are now involved in slab design the adults are now in the gameand they are using lots of very, very good concrete with very, very low water-to-cement ratios and the structural engineers are very concerned that the sand layer under the slab will desiccate the concrete mix and affect the cure. So they are saturating the sand layeryes, you guessed it, wetting it with a hoseso that it does not dry the concrete too quickly. Heres an ideaget rid of the sand and put the concrete right on the plastic sheet so that the bottom of the concrete mix does not dry out.

But what about cracking? If I dont put the sand layer in I get cracking. Use control joints. Duh.

And then when all else fails I get the American Concrete Institute calls for the sand layer argument. Well, thats not true, and they dont. ACI changed its recommendations in 1996. Check out the Guide for Concrete Floor and Slab Construction 302.1 R-96 and go to the Addendum and enjoy.

So what do I do if I have a wet slab because some rocket scientist insisted on a sand layer between the plastic sheet and the concrete? Or what do I do if I have a wet slab and no plastic sheet at all? Epoxy. Thats it? Well.kinda. The nuances are sort of important though.

The problem is pretty fundamental to understand_we have to stick something that does not pass water vapor to a concrete surface that has a high pH. We need to use something that is not affected by a high pHand there are epoxy coatings that can do this ( Photograph 7 ). Or we have to lower the pH and make sure it stays low and then stick something to the top. Easy to say, not necessarily easy to do. Its not that we dont know how to do this, but its that most folks who know how to do this dont want to share information about how to do it because there is big money in knowing how to do this.

BSI-003 Concrete Floor Problems  Building Science Information

Photograph 7: Epoxy Coatings Epoxy coating must either be compatible with a high pH (concrete with an impermeable finish will typically have a pH of near 14) or the concrete must be treated to reduce the high pH in such a manner that it stays low prior to coating with epoxy. Control joints are a separate challengethey must be filled with a completely vapor impermeable sealant to prevent any evaporation from the joint. Evaporation from the joint can lead to salt accumulation and osmotic pressures that can lead to coating blistering.

Finding an epoxy coating that is compatible with a high pH can be a challengeand if we were in a bar I would give you a couple of namesbut the rules dont let me tell you manufacturers names here. Suffice it to say, if they are foreign sounding and very expensive they will probably work. There are also some pretty impressive domestic technologies that involve treating concrete slabs with lithium penetrants that plug capillary pores and neutralize alkalis that then get top-coated with epoxy coatings. What we have learned the hard way is that acid neutralizing does not work nor does using calcium silicate-based penetrants. The acids leave salt residues and the calcium silicates dont completely react leaving behind the alkalis we were worried about in the first place. On a positive note, calcium silicate-based penetrants do clog up the capillary poresi.e. cause arterial sclerosis for concrete and do reduce vapor emissionsbut they should be limited to concrete slabs that do not get covered with impermeable floor finishes. So they are pretty good for exposed concrete floors that will remain exposed.

A less impressive, more primitive, but very effective technology is to install a ceramic tile floor over a damp concrete slab. The grout joints in the tile floor allow the slab to breathe. This approach has limitationsreally trashed slabs with high vapor emissions cant be fixed this way. One of the main reasons is that most ceramic tile floor adhesives are modified. This means they have latex polymers in them that are very sensitive to highly alkaline environments. Oops.

Even new assemblies get into trouble specifically vinyl composition flooring ( Photograph 8 and Photograph 9 ). When these types of highly impermeable flooring systems are installed over concrete slabs cast over steel fluted decks high alkalinity at flooring slab interfaces can cause debonding of the flooring. The best strategy is to coat the top of the concrete slab with an epoxy coating prior to the installation of the flooring. Alternatively, dont use fluted steel decksuse removable forms so that the slab can dry downwardsobviously not a solution for slab-on-grade construction.

Photograph 8: Vinyl Composition Flooring The best strategy is to coat the top of the concrete slab with an epoxy coating prior to the installation of the flooring. Waiting for the slab to dry is typically not an option as this may take a year or more. Alternatively, dont use fluted steel decksuse removable forms so that the slab can dry downwardsobviously not a solution for slab-on-grade construction.

With really trashed concrete floor systems a pretty much bullet proof repair approach (and not too shabby for new construction either) is the use of dimpled plastic sheet membranes ( Photograph 10 and Photograph 11 ). These membranes use pressure equalization ( Figure 3 ) to address vapor transmission. An airspace is provided over the top of a wet concrete slab. This airspace is bounded by a vapor impermeable chamber (outward evaporation is not possible) and equilibrates with the slab. The vapor pressure in the airspace tracks the vapor pressure in the slab and stasis is achieved. So how to stick this dimpled sheet membrane to the wet slab? Use cement. Concrete is cement with aggregate. And apparently cement is compatible with cement. Who knew?

Figure 1: Slab-on-Grade Done Right Concrete slab is isolated from ground contact with sheet polyethylene. Note that the polyethylene extends under the grade beam and upwards to grade. Latex paint over the exposed portion of the slab prevents liquid phase water from being absorbed by the concretebut allows vapor phase water to leave the slab thereby facilitating drying. Also note the capillary break isolating the wood framing from the slab.

Figure 2: Wood Floor Wood floors can be installed over damp slabs coated with epoxy coatings.

Figure 3: Vapor Pressure Equalization An airspace is provided over the top of a wet concrete slab. This airspace is bounded by a vapor impermeable chamber (outward evaporation is not possible) and equilibrates with the slab. The vapor pressure in the airspace tracks the vapor pressure in the slab and stasis is achieved.

* A good geotechnical engineer is worth more than a good surgeon or a good bookiethey are to be cherished and reveredjust try building anything anywhere without understanding the dirt under your building. Unfortunately, some of them have no idea about water control associated with slab constructionbut they cant control themselves and offer opinions anyway. Just tell me about the dirt. Dont design the slab for me. The structural engineer will do the concrete slab design. Leave the water management of the assembly to the architect and the builder. You are not being paid to comment on this anywaywhy would you want to take a risk you are not being paid for? Memo to structural engineersthat goes for you toojust stick to that structural thing you dodont talk to me about vapor barriers because you just dont know. And dont get me started about building officials.Where in the code does it say there has to be sand between the plastic and the concrete?

** The plastic sheet also acts as a capillary break. We can use crushed stone as a capillary break if there are no fines in the crushed stonesbut the stones dont act as a vapor barrier. So it is common to use crushed stones with no fines as a capillary breakand then add a plastic sheet over the top of the stones in contact with the concrete as a vapor barrier. Or just use the plastic sheet to do bothbe the vapor barrier and the capillary break. So why not just use plastic and not stones? Well the stones can also provide drainage (i.e. be a drainage pad) and facilitate soil gas control (i.e. radon venting) by creating and extending a pressure field that can be coupled with a vent to the atmosphere. Depending on where you are you will often see just plastic or plastic with stones. Below grade slabs tend to get the stones and plastic approach. Slab-on-grades tend to get the plastic only.

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