Concrete Slab Moisture In High Humidi...

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Author Message Ronald L. Geren, RA, CSI, CCS, CCCA, SCIP Senior Member Username: specman Post Number: 433 Registered: 03-2003 Posted on Friday, March 30, 2007 - 03:57 pm:    I'm stepping into new territory here (Pacific Northwest--Seattle, to be specific), so I'm focusing my question to those in that area. Situation: 7,500 SF T.I. in existing shell building with no interior slab-on-grade (SOG). The concrete SOG will be provided under the T.I. contract. Architect wants to install vinyl flooring, but is concerned since the owners have experienced problems with vinyl flooring in the past. I almost choked when the architect said they were going to lay plywood on top of the slab to allow "better adhesion" for the vinyl. There're a few factors that are affecting the work: short construction period, interior concrete placement, and high humidity environment (to me, every place is a high humidity environment when compared to Phoenix). I suggested a surface-applied vapor retarder such as Aquafin SG3, but cost may be a factor. What have you all done in similar conditions? Anne Whitacre, FCSI CCS Senior Member Username: awhitacre Post Number: 500 Registered: 07-2002 Posted on Friday, March 30, 2007 - 04:35 pm:    I hate to ask who the architect is who suggested plywood... in the northwest, it would be typical to have a Class A vapor barrier directly below the poured slab. However, since concrete up there cures about 1/2 inch per month, you're going to need a topical treatment on it, and you don't want a vapor "retarder" but an actual barrier and it should be one that the vinyl folks and their adhesive will work with. Floorseal is the one I've heard of the most, but I havent' used it. www.floorseal.com I've never heard of using plywood in that condition, so I'm guessing the architect came from "somewhere else" where they don't understand wet concrete either. Anne Ronald L. Geren, RA, CSI, CCS, CCCA, SCIP Senior Member Username: specman Post Number: 434 Registered: 03-2003 Posted on Friday, March 30, 2007 - 05:11 pm:    Anne: Thanks. You're right, even the Aquafin product is called a vapor barrier. The architect is also from Phoenix, so there's your reason. Water in this area is what you carry around in those little plastic bottles. Some architects, who have experience limited to this region only, don't have the appreciation of what water can do to buildings. John Hunter Senior Member Username: johnhunter Post Number: 21 Registered: 12-2005 Posted on Friday, March 30, 2007 - 06:06 pm:    There should probably be a two-pronged approach: First, the SOG done under the TI Contract should have a good vapor retarder under it (Stego, or equal). I understand this is Phoenix, but it is also my understanding that the relative humidity immediately below a slab on grade approaches 100% in virtually every climate and a vapor retarder will help to keep that moisture from migrating further up through your slab. Second, a topical vapor emission control system installed on top of the concrete. We've used Creteseal, a day-of-the-pour product, in California and have had positive experiences with it. It is among the least expensive treatments I'm aware of. We've also used Floorseal that Anne references and have had success with that as well, although it is a more involved installation (bead-blasting the concrete, etc.) and therefore somewhat more expensive. John Hunter Senior Member Username: johnhunter Post Number: 22 Registered: 12-2005 Posted on Friday, March 30, 2007 - 06:09 pm:    Apologies. Misread the reference to PNW vs Phoenix, but the approach would still apply. Don Harris CSI, CCS, CCCA, AIA Senior Member Username: don_harris Post Number: 122 Registered: 03-2003 Posted on Monday, April 02, 2007 - 08:02 am:    Stupid question time again... If there is a barrier on the bottom and a barrier on the top, where does the vapor go? Does the concrete ever cure properly? Margaret G. Chewning FCSI CCS Senior Member Username: presbspec Post Number: 123 Registered: 01-2003 Posted on Monday, April 02, 2007 - 08:35 am:    If I remember correctly from materials class in college, the water in concrete is used in a chemical reaction with the portland cement. That's why you need to keep it moist with barrier above and below for a specific period of time for the hydration to work properly. If too much water evaporates out too quickly, the cement doesn't set up and is weakened. Mark Gilligan SE, CSI Senior Member Username: markgilligan Post Number: 150 Registered: 05-2005 Posted on Monday, April 02, 2007 - 11:41 am:    While we need to control loss of water early in the life of the concrete, the concrete mixes used for buildings have much more water than is needed to obtain desired strengths. Typically curing of concrete need not extend longer than a week. Anne Whitacre, FCSI CCS Senior Member Username: awhitacre Post Number: 501 Registered: 07-2002 Posted on Monday, April 02, 2007 - 02:02 pm:    I asked the "where does the water go" question a few years ago. Concrete can stay wet indefinitely -- its used for outdoor slabs and underwater construction, after all. so, the water doesn't go anywhere but it also doesn't hurt anything. concrete cures by a chemical reaction, not by evaporation (as does cement plaster). if it dries out too quickly, the cure won't happen properly, which is why you have to keep it damp or covered up for the curing period. as for "what happens to the rebar" if the concrete is wet" -- rust typically happens in reaction to oxygen. the materials that have the biggest problems are those that are wetted and then dried out, especially if they are wetted and dried out over and over again -- construction in tidal zones is particularly at risk for that reason. Stuff that stays underwater indefinitely usually isn't a problem. Steven Bruneel, AIA, CSI-CDT, LEED-AP Senior Member Username: redseca2 Post Number: 42 Registered: 12-2006 Posted on Monday, April 02, 2007 - 04:00 pm:    The first time I personally experienced a finish flooring adhesion failure due to slab moisture content and pH problems was in the early '90's on the renovation of a historic hotel in San Francisco. I mention those details because the slab where the finish floor adhesion failed was poured in 1908, and you would think that it would have "dried" by 1992 or so. That is when I realized that all concrete contains moisture. If you found some concrete that had "Zero" moisture content, it would probably crumble into powder. Anne Whitacre, FCSI CCS Senior Member Username: awhitacre Post Number: 502 Registered: 07-2002 Posted on Tuesday, April 03, 2007 - 01:19 pm:    I bet that 1908 slab had no vapor barrier under it, and had enough capillary draw to pull up any moisture below grade into the slab. there is inherent moisture in most building products -- on the west coast, its pretty hard to get a building below 20% relative humidity, after all. Moisture goes from greater relative humidity to lesser, and if the ground is wetter than the building (or the air is wetter than the interior) its going to go in there. the big relative humidity problems occur not in the pacific northwest, which is rather cold Mediterrean climate, but in the southeast, which is humid both summer and winter. that's the part of the country known as the "mildew belt". Steven Bruneel, AIA, CSI-CDT, LEED-AP Senior Member Username: redseca2 Post Number: 43 Registered: 12-2006 Posted on Tuesday, April 03, 2007 - 02:01 pm:    Anne, No, that slab was at the ground floor over several basement levels. Although the slab probably got wet a few times with holes being made in the building envelope, this seemed to be a case of moisture migrating to an old dry slab from the air. On site walks during CA, that old building seemed to hold the damp, with fog sometimes visibly inside the building. Then all it took was smart architects like ourselves to decide that sheet rubber flooring (from that Italian tire company) was the right finish for back-of-the-house spaces.