concrete 'mold', rammed earth in Mexico
November 20, 2009 7:27 AM   Subscribe

Why does so much of the concrete work here eventually mold? (I am in the Yucatan) If that is the right word for this action. The discoloring. Is there an admixture missing? Something that is used elsewhere? how much is to be blamed on the humidity? People elsewhere are experimenting w/ new mixtures (i. e. fly ash) to make less wasteful, lighter etc.; in this area, is anyone experimenting w/ new formulas? What is the difference between exposed formwork say, in the US, and here? Oh, and one more. Could 'rammed earth' construction be done here? What is the raw ingredient necessary? There is alot of limestone here.
posted by ebesan to Technology (10 answers total) 1 user marked this as a favorite
Mold is a natural product of a place to live, and moisture to grow. You probably cannot keep your concrete dry, because the humidity in the air is sufficient to grow mold in some places. Instead, you could deny the mold a place to live by adding a smooth coating to the rough concrete, closing all of those little holes. An appropriate paint may do this, along with adding a mildew killer to the paint before applying it.
posted by Midnight Skulker at 7:53 AM on November 20, 2009

Without seeing pictures, it's hard to say. It could be efflorescence or any number of things. The color of the sand and the aggregate influences the color, but that shouldn't change over time, unless there something weird going on.
posted by electroboy at 8:00 AM on November 20, 2009

The old stone walls work fine if allowed to 'breathe'; i.e. absorb the moisture from the ground, and release thru the walls.
If painted, the paint will eventually peel away due to moisture build-up, unless it is the traditional type called 'cal' that allows breathing.
So it needs sealing from all sides? Sounds impossible. why doesn't this occur in say, New England or Tokyo?
posted by ebesan at 8:07 AM on November 20, 2009

For rammed earth it is best to use "clean" material from a quarry. What you want is called "quarter-inch minus" (at least here in the U.S.). It can be any type of stable rock (such as granite) but must include all particle sizes from dust up to quarter inch. You then mix this with 8-10% (by volume) of portland cement, and wet it down until it is just wet enough to form into a clump but not dripping wet (it looks like a clean soil at this point and not tiny rock fragments).

Rammed earth is very labor intensive and the "best" way to do it is by using modular concrete formwork, pneumatic tampers, and a skid steer -- but this requires money to rent the equipment. You can do it cheaper by using on-site dirt, but this requires a careful analysis of the properties and strength of the material. Tamping by hand takes a LONG time. But if you have access to a lot of friends and relatives who are willing to work, doing it all by hand is the way to go.

There is a good book by David Easton called The Rammed Earth House that goes into a lot of detail.

This group has built several rammed earth structures in Nicaragua. You might want to contact them for advice on doing it on a budget from on-site materials.
posted by bengarland at 8:14 AM on November 20, 2009

Google for "concrete sealant"

There are many different kinds, and they serve different purposes. Some breathe, some don't. If your structure did not include proper moisture control details during the design/construction phase, you'll need to be even more careful when applying any sealant lest you just make everything worse.
posted by aramaic at 8:32 AM on November 20, 2009

Is it not cured correctly?
posted by ebesan at 11:04 AM on November 20, 2009

or just efflorescence, not mold, like electroboy suggests?
posted by ebesan at 11:06 AM on November 20, 2009

In my experience looking at houses in Merida, it's both efflorescence and mold.
Here's an example.
The efflorescence comes from poorly prepared footings drawing moisture up into the walls. Rising damp.
The mold comes from the humidity.
I haven't researched these problems enough yet to offer any remediation advice, but the folks on the Yolisto board would have direct experience and some advice.
posted by Floydd at 11:37 AM on November 20, 2009

To expand on what Floydd said, typically a layer of gravel is placed underneath a foundation or slab to provide drainage. If you just pour concrete right on the ground, and if that ground is something that retains water well, like clay, you're likely to have a perpetually damp piece of concrete.

Regarding fly ash, it's used here as well, in lightweight concrete, flowable fill and mostly nonstructural applications. Otherwise, people are experimenting with concrete all the time. There's slag concrete made with blast furnace waste, polyester and metal fibers, all sorts of admixtures to make it more plastic. It really depends on what your goals are.
posted by electroboy at 2:51 PM on November 20, 2009

Is it not cured correctly?

Improper curing usually results in cracking and low strength, not discoloration.

I'm not sure what you mean by formwork. In the US, a structure like Floydd linked to would probably be concrete block and stucco. There are all concrete buildings like tiltups, but those are usually business park type structures.
posted by electroboy at 2:59 PM on November 20, 2009

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