[GSBN] Steel mesh in clay plaster (?)

Sun Aug 25 00:57:57 UTC 2013

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-sed
> iments-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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