Cob: Re: Cob & Steel

[Matthew HALL(SED)]

ANovelli at aol.com writes:
>One point I have not seen discussed is a corollary of the thermal 
>transmission of steel. It is well known in bale circles that rebar pins 
>"sweat", and over time create cores of rot around them, potentially weakening 
>the steel and of course, destroying any resistance they offer to movement. 
>While you are using a much lower percentage of straw in a cob mix, the clay 
>may not help solve this problem.
Steel rebar is commonly used in abobe and rammed earth walls to increase flexural strength
particularly against high winds. Steel that is embedded in earth with a suitable amount of cover
will not corrode significantly as it is kept away from pockets of air and moisture. The problems
you speak of in a lightweight highly aerated material such as strawbale are not the same as in
condensed subsoil where there is no provision for air pockets etc. As with reinforced concrete the
key to corrosion resistance is adequate depth of cover, cement will also enhance this. This is all
commonly used in western australia and has been for the past 20 years since cyclone tracy struck
and people sought to increase the wind resistance of walls. The technology is well understood
there; i see no reason why it cannot be translated to cob walling. If the steel columns are
embedded to give adequate cover and the thermal mass is not interrupted by a cold bridge i think
the idea works well.
Regards
Matthew

____________________________________________________________________
Matthew Hall BSc (Hons) GradBEng
PhD Research Student
Centre for the Built Environment
Unit 9 Science Park
Sheffield Hallam University
Pond Street
Sheffield S1 1WB
England

Tel: +44 (0) 114 225 3200
Fax: +44 (0) 114 225 3206
E-mail: M.Hall at shu.ac.uk