Concrete Wicking Moisture

proud_democrat(8)September 23, 2011

My question is this: Does concrete wick or retain soil moisture? I have a great deal of exposed aggregate concrete on my property and in certain spots, I've left holes in it for pockets of green. Currently there are is Miscanthus in some holes and English Lavender in others. I often wonder if weekly waterings with the hose is under- or over-watering them. As a general rule, does concrete wick moisture? Or does it prevent evaporation?

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Belgianpup(Wa/Zone 7b)

It probably does both, to a certain extent; but it isn't like water flows through concrete like through a nylon scrub pad. It was observed at least by 1623 (by Blaise Pascal) that "water seeks its own level". Moisture in general fluctuates the same way, staying fairly equal: if the soil is damp, the concrete is damp; if the soil is dry, the concrete is dry. But since the concrete is also shading the soil from direct sunlight, the moisture should hang around longer since its not exposed to drying wind and sunlight.

May I assume that you left holes in the concrete that extend right into the soil beneath? Unless the soil under the concrete is heavy clay, most of the excess moisture should drain away between waterings.

The question is if the plants need as much as is left. Neither the Miscanthus nor the Lavender really require a lot of water. Personally, I would back off on weekly waterings, and watch the plants for signs of dry leaf tips or wilting, and then adjust.

Sue

    Bookmark   September 24, 2011 at 1:36AM
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novascapes

Moisture movement through concrete is very slow and is dependent on the thickness and makeup of the concrete itself. Wicking is also dependent on the weather. Hot dry wind will wick more up through the concrete. These pocket planting, as well as some enclosed foundation bed plantings, are site specific. You can have non draining clay below and great well drained top soil on top. I would dig a hole for the plant and fill it with water. After it has been absorbed then fill it again. Observe how long it takes for the water to be absorbed. This will give you some idea as to how much water will be needed or what type of plant would do best.
There are so many variables with planting in these pockets that I wonder if a moisture meter would need to be place in the pocket until you are familiar with the water regiment needed. Of course that would change with the weather conditions.

    Bookmark   September 24, 2011 at 7:20AM
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proud_democrat(8)

Thank you both for the feedback. Very good stuff.

    Bookmark   September 24, 2011 at 8:30PM
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Kimmsr(4a/5b-MI)

Concrete does not "wick" moisture from the soil which is why concrete pavement will "explode" during hot weather, since the moisure cannot move through the concrete the sun causes it to move out rapidly which then blows out the concrete that moisture moves through.

    Bookmark   September 27, 2011 at 6:46AM
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novascapes

International Build Code calls for plastic to be placed under concrete within a dwelling or any other public structure. If you install vinyl flooring on concrete without a moisture barrier it will mildew from the bottom.
Soil below concrete parking lots will become dry even when no tree roots are around.
The amount of porosity is dependent on the size of fine sand and cement in the concrete. The new porous concrete being used has no sand, just pea gravel, allowing moisture to travel through rapidly.
Normal concrete is usually air entrained, meaning there are very small bubbles within the concrete. Also concrete is made with water. When the water evaporates it leaves very small voids. The wetter the concrete is poured the more voids or bubbles. These microscopic voids and bubbles allow for the transfer of water, wicking.

    Bookmark   September 29, 2011 at 5:39AM
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Kimmsr(4a/5b-MI)

Water added to the cement mix does not evaporate it reacts, chemically, with the hydrated lime to form the hardened material tha we call concrete. If one wants a strong concrete then everything possible must be done to preven evaporation.

    Bookmark   September 29, 2011 at 6:39AM
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novascapes

"Water added to the cement mix does not evaporate it reacts, chemically, with the hydrated lime to form the hardened material tha we call concrete. If one wants a strong concrete then everything possible must be done to preven evaporation."

I agree. I said it wrong. Sealers are sprayed on or plastic covers it during the reaction phase. If I remember corectly, the water molecule attaches itself to the lime and bonds to form a hydrate.

    Bookmark   September 29, 2011 at 1:57PM
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darth_weeder(z7 NY)

if concrete doesn't wick water/moisture from the surrouding soil, why is my basement so damp?

    Bookmark   September 29, 2011 at 6:11PM
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novascapes

If there is no moisture barrier, which there should be, it could be wicking.
If there is a moisture barrier then it is more than likely condensate building on the surface because of the temperature deferential between the concrete and the air inside.

    Bookmark   September 30, 2011 at 5:18AM
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Kimmsr(4a/5b-MI)

Moisture will condense on concrete and on occassion when the concrete has cracks through it water will seep into the basement, sometimes enough to cause a flood. Becasue of the condensation issue is why many basements have dehumidifiers.

    Bookmark   October 1, 2011 at 6:59AM
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jolj(7b/8a)

"Water added to the cement mix does not evaporate it reacts, chemically, with the hydrated lime to form the hardened material tha we call concrete. If one wants a strong concrete then everything possible must be done to preven evaporation."
If cement mix does not evaporate, then why do we need to prevent evaporation???
It reacts & it evaporate, & moisture move though it, too.
I know of a house with no vapor barrier, that will not hold a paint job, the house is over 40 years old & the paint has peeled Everytime it has been painted.
Those of us who have worked with concrete, know to keep it wet, if poured in the hot summer, so it will not dry out to fast from eveaporation & crack.
I would mulch the plants & check them often for moisture.

    Bookmark   October 4, 2011 at 5:14PM
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Belgianpup(Wa/Zone 7b)

How much moisture travels through concrete is highly dependent on what additives have been used (or not used) in the mix.

Curing concrete is a chemical issue more than an evaporation issue, but evaporation can also take place during the curing, and if too much moisture evaporates, it weakens the concrete, which is why they cover it with plastic. OTOH, too much water added to the original mix will also weaken the concrete. Getting it right does take some experience.

Some additives included in the original mix will slow down the wicking of moisture through the concrete, an important thing when trapped moisture can freeze and expand within the concrete, causing cracks or breakage.

Novascapes' idea of using a moisture meter in the planting hole is a good one! But watching the plant and sticking your finger a few inches into the soil hole will usually have to do for most of us.

Sue

    Bookmark   October 5, 2011 at 8:47PM
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