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San Francisco Fog
04-15-2014, 10:15 AM, (This post was last modified: 04-15-2014, 10:35 AM by eaadams.)
San Francisco Fog
I have an interesting slab and would like any input people have. While it only rained for I think 3 days in the last six months. I have a LOT of slabs this year coming in around 78-85F out in the Sacramento / Central Valley. But in some areas of the SF Bay Area, and what SF is so well known for is the wonderful fog. It is nature's A/C.

I have a project out by the coast where the gym slab has been down since late last summer. It was never rained on before they got the roof in place, 4" steel troweled, Stego directly under. No windows yet.

It is still sitting at 99%. They are just putting the windows on now. Ambient conditions are in the 60F's for temp and slab temp. For humidity 40's - 60's %. But here is the kicker. The GC / Superintendent just pointed out this to me (and I never thought about it because of the location) Every morning the Fog rolls in off the ocean and covers this property. It was like a face palm to the forehead. Obviously this is why this slab isn't drying.

How much moisture can Fog add to a slab? I haven't a clue how to evaluate this. It is obviously keeping it wet. Once we close up the building it should then have the opportunity to start to dry. We in California are usually worried about rain, so I say 'get the roof on' over and over again. I never thought about areas that get fog. Also on most sites, by the time work starts in the summer, the fog has burned off and doesn't return till well after 2:00 quittin time.

Also, does FOG classify as 're-wetting' ?
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04-15-2014, 02:39 PM,
RE: San Francisco Fog
Oh great, now I have to go look up how much water is in fog. Sad I thnk I read it somewhere.

I'd call fog an extreme version of an evaporative cooler caused by dewpoint. So, yes, IMO dewpoint could add water back into the slab more so than high rh.

Time to surf search engines.
Stephen Perrera dba
Top Floor Installation Co.
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04-15-2014, 03:15 PM,
RE: San Francisco Fog
Ok so if the fog hits dew point... wait that would apply to any slab that hits dew point after pour. Arg
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04-16-2014, 03:38 PM,
RE: San Francisco Fog
Hee hee, yea, fog is what happens in extreme dewpoint me thinks. It definetly left moisture on my vehicles, so why not on a slab?
Stephen Perrera dba
Top Floor Installation Co.
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05-03-2014, 06:34 AM,
RE: San Francisco Fog
Continually rewetting the slab will surely slow the drying process down. Fog is air that is saturated with water, the air temp is at dew point, it is essentially a low level cloud.

That steel troweled surface isn't helping the slab dry either....

Good luck Evan!
JD Grafton
Concrete Answers for Flooring Problems
[email protected]
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05-09-2014, 09:14 PM,
RE: San Francisco Fog
(05-03-2014, 06:34 AM)CC Solutions Wrote:  That steel troweled surface isn't helping the slab dry either....

That's the story of pretty much every slab I've tested thus far. We love our helicopter-power-trowelled slabs, until the flooring guys have to charge extra to grind it to porous so they can spray their silicates on Big Grin Nothing gets out, nothing gets in...

I suppose the main thing is the fog makes the slab "equilibrate" with the air, meaning it doesn't- it retains whatever moisture it has.

I didn't know 'Frisco was famous for its fog. I thought it was for bridges and crazy airborne car chases.
The problem with socialism is that you soon run out of other people's money.
- Margaret Thatcher

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