[GSBN] Steel mesh in clay plaster (?)

Sat Aug 24 22:51:15 UTC 2013

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:

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
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