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<div class="moz-cite-prefix">Hi Martin an Derek,<br>
<br>
I just noticed that my steel trowels rust very(!) quickly if the
user forgets to wipe of the earth plaster right after use.<br>
<br>
André, whipe'm clean, de Bouter<br>
<br>
<br>
<br>
Le 25/08/2013 02:57, martin hammer a écrit :<br>
</div>
<blockquote cite="mid:CE3EC0C5.17F3C%25mfhammer@pacbell.net"
type="cite">
<title>Re: [GSBN] Steel mesh in clay plaster (?)</title>
<font face="Arial"><span style="font-size:9pt">Hi Derek,<br>
<br>
A very thorough and thoughtful response, as always. Thank
you.<br>
<br>
Thoughts and information are welcome from others as well.<br>
<br>
Martin<br>
<br>
<br>
On 8/24/13 5:51 PM, "Derek Roff" <<a moz-do-not-send="true"
href="derek@unm.edu">derek@unm.edu</a>> wrote:<br>
<br>
</span></font>
<blockquote><font face="Arial"><span style="font-size:9pt">Hi,
Martin,<br>
<br>
I will start with a quote from the site <a
moz-do-not-send="true"
href="http://www.cement.org/tech/cct_dur_corrosion.asp">http://www.cement.org/tech/cct_dur_corrosion.asp</a>:
"Corrosion of reinforcing steel and other embedded metals
is the leading cause of deterioration in concrete." This
page is a fairly concise, not too technical description of
reinforcing steel corrosion in concrete. The article points
out that concrete contains all the ingredients necessary to
cause corrosion in steel. Concrete itself can function as
an electrolyte, and different locations in reinforcing steel
can act as anode and cathode for inducing corrosion. The
electrical conductivity of concrete is sufficient to support
corrosion. If other metals, such as aluminum or zinc
(galvanized metal) are in contact with the concrete, this
increases the rate of corrosion for the reinforcing steel.
On the other hand, in the galvanized metal itself, the zinc
is a sacrificial layer, which protects the steel, for as
long as the zinc lasts. <br>
<br>
Concrete also contains one significant corrosion inhibitor-
high pH, which helps protect the steel, by aiding the
formation of a thin, passivating protective layer on the
surface of the steel. The author says that the "corrosion
rate [of steel with the passive film protective layer] is
typically 0.1 µm per year. Without the passive film, the
steel would corrode at rates at least 1,000 times higher
[100 µm per year] (ACI222 2001)." [If your mail program
isn't showing the special characters properly, the
measurement units are micro-meters per year, one millionth
of a meter.] Lime also has a similarly high pH. <br>
<br>
The main causes of increased corrosion are salts in or
applied to the concrete, and decreased pH. Salts may be
common in the materials used as aggregate, in the water used
for the mix, or may be introduced after the concrete has
solidified. People add salts to concrete for ice removal
and other reasons, and salts may also be introduced
unintentionally by wind and water, in some locations. <br>
<br>
Decrease in pH can be the result of carbonation in the
concrete, or acids in the environment, both naturally
occurring and applied intentionally. Carbon Dioxide in the
air reacts with water vapor to produce carbonic acid, so a
small acid source is always present. Acid rain can
introduce much stronger acids in greater quantities.
Carbonation is usually slow for good, thick concrete made
with pure materials, but may occur much more quickly in less
pure concrete mixes and thinner applications, such as
plasters. Carbonation is more rapid in lime mixes than in
concrete. Cracks in the concrete or lime, of course,
increase the rate of corrosion. <br>
<br>
Clays are highly variable, but are unlikely to have the high
pH that helps form a protective layer on reinforcing steel
in concrete and lime. I found a statement that natural
clays can vary between pH 2 and 10. Within the pH range
that is common for clays, neutral to slightly basic mixes
will have the lowest corrosion rates, according to the
websites that I checked. On the other hand, many clays will
not act as an electrolyte. If an electrolyte is lacking,
the rate of corrosion will stay low. This site <a
moz-do-not-send="true"
href="http://www.ncbi.nlm.nih.gov/pubmed/22200075">http://www.ncbi.nlm.nih.gov/pubmed/22200075</a>
contains an abstract on the use of clays "to impart
remarkable protection against corrosion to galvanized
steel." Salts may or may not be present in the clay,
depending on the local conditions, water, geology, and the
clay mix. Clay is a much better buffer for moisture than
concrete is, which would usually help steel in clay resist
corrosion. Clays are not subject to carbonation. Lower
temperatures will reduce the rate of corrosion. <br>
<br>
The PDF freely downloadable at this site <a
moz-do-not-send="true"
href="http://bookshop.europa.eu/en/corrosion-of-low-carbon-steel-in-clay-and-sea-sediments-pbCDNA10522/">http://bookshop.europa.eu/en/corrosion-of-low-carbon-steel-in-clay-and-sea-sediments-pbCDNA10522/</a>
contains several interesting quotes, which are somewhat
divergent from each other, and not identical to the
conditions of reinforcing steel in clay plasters. The
authors were concerned about steel immersed at high
temperatures (90 degrees C) in sea sediments. While other
sites have suggested that more water increases the rates of
corrosion, this article finds the reverse, which they
attribute to the lack of dissolved oxygen in the sediment
zone they investigated. <br>
<br>
With no mention of the amount of water involved in the
referenced studies, the authors say, "In literature, data
can be found on corrosion of mild steel in clay. Exposing
ductile iron or carbon steel [H. Tas SCK/CEN Mol, Personal
communication] directly to clay at room temperature gives
rise to general corrosion rates ranging from 10 to 50
µm/yr." <br>
<br>
However, their tests and references show a much lower
corrosion rate of only 8 µm/yr in one study with steel in
clay under unspecified conditions, and from another study,
2-10 µm/yr at 25°C, in bentonite clay. <br>
<br>
"Tests in deaerated substitute seawater were conducted at
Harwell at 90°C [G.P. Marsh et al. - Corrosion assessment of
metal operpacks for radioactive waste disposal - European
Appi. Res. Rept. - Nucí. Sc. Technol., vol. 5, pp. 223-52
(1983)] which give, after a stabilization period of about
2.000 h a corrosion rate of about 8 µm/yr. Another series of
tests was performed in which low carbon steel sample were
embedded in bentonite saturated with a basic synthetic
granite groundwater at 90, 50°C and at room temperature
[K.J. Taylor, I.D. Blaid, C.C. Naish, G.P. Marsh - Corrosion
stu dies on Containment Materials for vitrified Heat
Generating Nuclear waste AERE G - 3217 (1984)]. After a
stabilization period a corrosion rate ranging between 20-37
µm/yr at 90, 9-32 µm/yr at 50 and 2-10 µm/yr at 25°C was
apparent." <br>
<br>
Based on the references that I could find, the rate of
corrosion for steel in clay is substantially less variable
than for steel in concrete. (I can't think of another
example where a property of clay is less variable than an
industrial product.) Steel deeply imbedded in excellent
concrete, and protected by a passivating layer, will have a
corrosion rate that is a tenth or less of that for steel in
clay, according to the figures that I found. Steel imbedded
in an average Portland cement plaster with some cracks, in
which the passivating layer is absent or compromised, might
have a corrosion rate fifty times higher than steel in a
clay plaster. <br>
<br>
As with so many things in building, since testing reveals
such a range of potential variability, it would be useful to
test the materials under local conditions. <br>
<br>
I hope this is of some help. <br>
<br>
Derek<br>
<br>
On Aug 24, 2013, at 4:39 PM, martin hammer wrote:<br>
<br>
</span></font>
<blockquote><font face="Arial"><span style="font-size:9pt">Steel
mesh in clay plaster (?) <br>
Hello all,<br>
<br>
Can anyone weigh in on the use of steel mesh in clay
plaster, in terms of corrosion of the steel? In
particular if it is susceptible to a higher rate of
corrosion than steel mesh in lime or cement plaster (or
what an expected service life might be). Laboratory
tested evidence is especially welcome, but so is anecdotal
evidence (pro or con).<br>
<br>
I know there has been concern expressed about this for
many years. I’ve heard theory, but I haven’t seen hard
evidence that it is actually a problem.<br>
<br>
I ask this in the context of a Strawbale Tutorial I am
co-authoring for the World Housing Encyclopedia. The
tutorial is meant as guide for constructing small houses
in seismically active regions of the developing world.
Thus the desire for a reinforced clay plaster as the
in-plane lateral resisting system. Darcey Donovan has
used nylon fishing net in her system with PAKSBAB in
Pakistan (which was shake table tested) but I am looking
to use other mesh materials where such fishing net might
not be available. Metal mesh seems to be readily
available in most of the developing world. (We are also
considering natural fiber mesh, but these may have
strength and degradation problems). <br>
<br>
Thanks!<br>
<br>
Martin<br>
<br>
PS – I’ve copied my colleague, Dmitry Ozeryansky, PE <br>
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<font face="Arial"><span style="font-size:9pt"><br>
Derek Roff<br>
<a moz-do-not-send="true" href="derek@unm.edu">derek@unm.edu</a><br>
<br>
<br>
<br>
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