[GSBN] Steel mesh in clay plaster (?)

Sun Aug 25 14:28:12 UTC 2013

Hi Martin an Derek,

I just noticed that my steel trowels rust very(!) quickly if the user 
forgets to wipe of the earth plaster right after use.

André, whipe'm clean, de Bouter

Le 25/08/2013 02:57, martin hammer a écrit :
> Re: [GSBN] Steel mesh in clay plaster (?) Hi Derek,
>
> A very thorough and thoughtful response, as always.  Thank you.
>
> Thoughts and information are welcome from others as well.
>
> Martin
>
>
> On 8/24/13 5:51 PM, "Derek Roff" <derek at unm.edu> wrote:
>
>     Hi, Martin,
>
>     I will start with a quote from the site
>     http://www.cement.org/tech/cct_dur_corrosion.asp:  "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.
>
>     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.
>
>     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.
>
>     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.
>
>     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 http://www.ncbi.nlm.nih.gov/pubmed/22200075 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.
>
>     The PDF freely downloadable at this site
>     http://bookshop.europa.eu/en/corrosion-of-low-carbon-steel-in-clay-and-sea-sediments-pbCDNA10522/
>     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.
>
>     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."
>
>     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.
>
>     "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."
>
>     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.
>
>     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.
>
>     I hope this is of some help.
>
>     Derek
>
>     On Aug 24, 2013, at 4:39 PM, martin hammer wrote:
>
>         Steel mesh in clay plaster (?)
>         Hello all,
>
>         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).
>
>         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.
>
>         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).
>
>         Thanks!
>
>         Martin
>
>         PS -- I've copied my colleague, Dmitry Ozeryansky, PE
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>
>     Derek Roff
>     derek at unm.edu
>
>
>
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