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Re: Minimum (or zero?) Temperature Reinforcement

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Walter,  

Research to provide low cost durable housing for low income folks is 
important and laudable.  I hope you find the effective means of accomplishing 
this in your country.  It's challenging if you are working in reinforced 
concrete.  I have some thoughts on the nature of the challenge.

A number of innovative architects designed and built reinforced concrete and 
reinforced masonry homes in California in about the 1920's.  Frank Lloyd 
Wright and Rudolph Shindler are two of the more well known of those 
innovators.  About 50 to 60 years later, serious problems developed in many 
of their projects due to corrosion of reinforcing steel.  These and a number 
of other observations have led to my maxim that reinforced concrete has a 
serviceability period of 100 years plus or minus a few decades.  On the other 
hand, unreinforced masonry structures can last millennia without suffering 
similar effects -- but, on the other-other hand, they may not survive in 
earthquake zones.  Nevertheless, the use of steel reinforcing has inherent 
limitations when long-term serviceability is important among your goals.  

Inadequate concrete cover over reinforcing steel is a major reason for early 
corrosion damage due to carbonization (a change in the pH of the concrete 
brought about by the reaction of atmospheric CO2 with the constituents of the 
concrete -- CO2 penetrates slowly into the concrete lowering the pH as it 
advances).  In 4" thick sections, a means of keeping the reinforcing in the 
center of the section will be vital to the long-term serviceability of the 
structures you are developing.  

Portland cement protects embedded reinforcing steel against carbonization for 
many years, but the slow process of carbonation eventually reduces the pH to 
a level at which the protection is lost.  I'm concerned about the low 
strength (= low cement content) that you are proposing  -- it may result in a 
shortened serviceability period.

Education to provide an understanding of the importance of moisture control 
as a defensive strategy should be another component of the system you develop 
for the folks you are hoping to help.  Roof leaks, plumbing leaks, poor site 
drainage, irrigation near the house -- these all can let moisture into the 
structure that may leach away protective cementitous components of the 
concrete and accelerate the corrosion process.

Coatings that exclude CO2 and moisture may be something that you should look 
into.  However, if moisture cannot be completely excluded, its better not to 
use a sealant at all -- moisture trapped inside a coating quickly leads to 
deterioration of the concrete.  I believe that Sika makes a product that is 
supposed to block CO2 while allowing water vapor to pass, but that may be too 
exotic for your low-cost program.

Many of the California houses suffering corrosion problems have well-to-do 
owners, but, even for them, the problems that they face in restoring their 
corrosion-damaged homes are overwhelming.  The heirs of the builders of the 
system you are developing may have to face similar overwhelming difficulties. 
 If you could find a way to not only minimize, but eliminate the temperature 
steel, while maintaining seismic resistance, you would have a most satisfying 
system.  Have you tested structural components that have no reinforcing?  On 
the other hand, I know that I'd be reluctant to live in a house that has 
unreinforced concrete overhead.  Are any of the nonmetallic fiber 
crack-control products available to you?  What other materials with tension 
strength are at hand in Peru that you could test or for which you might seek 
out test results from other researchers?

Are you familiar with the research by Julio Vargas Neumann and Juan Bariola 
at the Pontifical Universidad Catolica del Peru into earthquake resistant 
house construction methods using locally available materials (adobe and 
quincha)?  I suppose that you certainly are, but if not, they may be onto 
something that could be helpful to your project.

I hope your small-house development project is a success.

Nels Roselund
Structural Engineer