[GSBN] Follow up re mesh in clay

Thu Aug 29 13:30:12 UTC 2013

As you will see below, a few of us have had a bit of off-list discussion on steel mesh in clay.  Martin thought this might be of interest to some on the GSBN list, so here it is.

Derek

On Aug 29, 2013, at 12:47 AM, martin hammer wrote:

Hi Derek,

Thanks very much for taking the time to answer Dmitry’s questions with such careful thought.  People on the GSBN might also be interested in what you had to say, but I’ll leave it up to you if you want to post it (probably along with Dmitry’s questions) on the GSBN.

Best.

Martin

On 8/27/13 9:05 PM, "Derek Roff" <derek at unm.edu<x-msg://11/derek@unm.edu>> wrote:

Hi, Dmitry and Martin,

I'm happy to try and answer your follow up questions, although I don't want to give the impression that I am an authority.  I have researched these questions out of my own interest, and revisited some of them in greater detail, in response to Martin's questions.  I have not done any formal research on this myself, but am drawing from my modest building experience, and from Internet research resources.

Your question on testing soil in developing countries is an interesting one.  I don't know how available testing materials would be in various locations.  For testing pH, I think it could be reasonable to transport the needed pH paper.  pH paper is inexpensive, and a single $10 roll could test perhaps a hundred soil samples.  pH paper can be very accurate.  Electronic pH meters are another possibility.  Some are available for under $100.  A problem with electronic meters is that they require calibration, normally with carefully prepared calibration fluids of certified pH.  That makes the process rather more complex for mobile, field operations.

Testing for salts is not so simple as using pH paper, but there are electronic soil salinity meters for around $100-$200.  There are a few that cost less, including this one at $15.59.  http://www.amazon.com/HM-Digital-TDS-EZ-Measurement-Resolution/dp/B002C0A7ZY/ref=pd_sim_sbs_lg_2     I can't vouch for its accuracy, but perhaps it would be sufficient for our building purposes.  Alternatively, soil testing is one of the fundamentals of improving agriculture, so in many locations, it might be possible to have soil tested locally by an agricultural extension agency or laboratory.  This could be a good way to connect with the local economy and researchers.

On your second point, I may have not communicated clearly in my earlier message.  For galvanized steel, the zinc coating will, of course, corrode first, both because it coats the steel, and because it is intended to be a sacrificial coating.  I would think that galvanized steel would always last longer than bare steel, but it will cost more, and not be as widely available.  A separate point that I was trying to make, is that with bare steel in concrete, the rate of corrosion will increase if some other (non-reinforcing) galvanized steel or aluminum (or another metal) is also embedded in the concrete, or attached, nearby.  This can happen when concrete reinforced with normal, bare reinforcing steel also has galvanized or aluminum flashing, anchors, awnings, roofing, decking, or other metal objects embedded in the concrete or attached to it.  These dissimilar metals can set up an electrical potential, in effect turning the concrete into a simple battery.  The tiny current flow will increase the corrosion rate of the bare steel.  This is sometimes called galvanic corrosion.  It would not happen in clay with a fairly neutral pH, that is free of salts and other electrolytes.  It would happen in an acid soil, or one with salts.  However, earthen plasters/bricks/cob are likely to be less electrically conductive than concrete, even when they have a similar salinity, so I would expect the galvanic corrosion to be less in clay.

Increasing the pH of the earthen material in contact with the steel will decrease the corrosion rate.  However, the effect is most significant at really high pH values, pH 11 or above.  Reaching this pH level would require a lot of lime, which would be very expensive.  If the clay soil used for the earthen mixes was mildly acidic, it might be reasonable to add enough lime to get the mix into the alkaline range (above pH 7).  Clays and limes vary, so every local soil and lime mix must be tested to make sure it bonds well without too much cracking.

Your final question is one that I am curious about, in relation to clay plasters with steel.  The research shows that steel corrosion in concrete increases in wetter climates, since it increases the electrical activity of the concrete.  I wonder if the same would be true for steel in clay, assuming non-acid, non-salty clay.  These clays are less conductive and great moisture buffers.  I want to believe that they would protect the steel more, but I don't have any evidence to support this, beyond seeing some very old steel embedded in old adobe houses, with quite modest corrosion levels.

I hope some of this is useful.

Best wishes,
Derek

On Aug 26, 2013, at 5:04 PM, Dmitry Ozeryansky wrote:

Hello Derek,
Thank you for your very informative reply to our question!

May I ask you a few brief follow ups?

  *   If we were to recommend use of steel mesh in clay plaster for developing countries, are there simple tests for salts and pH that we could/should prescribe to determine if local subsoil is adequate?
  *   You say that zinc corrodes faster than steel in concrete, is that also true for clay?
  *   Would you propose adding lime to the plaster mix to further reduce the rate of corrosion?
  *   Do you think that wet climates would significantly increase the rate of corrosion?

Best regards,

Dmitry Ozeryansky

OZERYANSKY ENGINEERING
www.ozerengineering.com<http://www.ozerengineering.com> <http://www.ozerengineering.com/>

2254 Nelson Ave
Memphis, TN 38104
office: 901-305-6540
cell: 510-287-6115

South Main Retrofit Initiative
www.southmain-retrofit.org<http://www.southmain-retrofit.org> <http://www.southmain-retrofit.org/>

Derek Roff
derek at unm.edu<x-msg://11/derek@unm.edu>

Derek Roff
derek at unm.edu<mailto:derek at unm.edu>

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