My architect girlfriend claims that concrete leaches (sp?) or in her terms “wicks” water from the ground. We were getting a storage facility and she insisted we put everything on a shelf for this reason. (I wanted to put everything on a shelf too, but to avoid pests and possible flooding, etc.)
She further says that this explains why the (concrete) floor in her bathroom is cold even on warm days, even though she’s on the third floor.
I have seen concrete objects (used for shore retention) that were half submerged in water. The top half was clearly dry, at least on the surface. This to me argues against her statement.
But, I’m still stuck with an explanation for why third floor concrete is cool on a hot day in a hot apartment on the third floor.
I am NOT a hacker!
Water migrates through concrete. The outside surface of a poured concrete basement wall is typically sprayed with a tar or silicone substance to act as a water barrier. When finishing a basement, you also need to add a moisture barrier between the concrete wall and the framed wall you’re adding.
I doubt water is migrating up to the third floor, however. The floor is cool because concrete has a large thermal mass, and it would take a long time and a lot of energy to heat up a concrete slab. Even if it reached the same temperature as the rest of the room, it would still feel cool to your feet because concrete is a better heat conductor than carpet or wood, and it has that large thermal mass, so it sucks the heat right from your feet.
Your concrete retaining blocks probably are wicking water up, but the water is continuously being evaporated by the wind and sun, so the surface stays dry. Try taping a plastic sheet to one of the blocks, and after a while, I bet you’d get moisture build-up below it.
“The lad(d)y doth protest too much, methinks.”
It is too clear, and so it is hard to see.
Your girlfriend may or may not be right. Concrete does indeed act somewhat like a sponge and take the moisture from the soil it is on. It also tends to mirror the moisture content of the air. So if you have a high average humidity in your area, the 3rd floor slab could well be damper than your carpet.
It can even weep moisture, interfering with things like subfloor bonding and flooring adhesion. Impress your girlfriend by calling this, “at or below grade hydrostatic pressure”. Zens plastic wrap test may work in extreme cases but pros do it thusly;
Place one ounce of calcium chloride on the slab. Rim a shallow glass dish with grease (petri dish works well) and press it over the pile of crystals. After two days observe, look for sweating, partial or total liquification of the crystals. If they remain unchanged, there is no moisture problem with the slab.
Concrete, tile and marble tend to be a few degrees cooler than the air in the room. Bakers love a granite or marble countertop for rolling out dough for this reason. I have never investigated the physics involved in this, but you have piqued my curiosity.
And if you really want to impress your girlfriend, refer to “at or below grade hydrostatic pressure” as “head.”
You can set a post in dry concrete. Just set the post in the hole and pour dry “sackrete” in the hole. the concrete will wick enough moisture from the ground. I used this method several times when setting up satelite dishes.
As for moisture in concrete it will give up moisture for a long time. I’ve heard that concrete doesn’t reach it’s maximum strength for 100 years
They also use this in setting up pole buildings.
As far as bakers go-I believe they use marble because it is not porous and the dough doesn’t stick.