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Re: purposely permeable concrete

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It seems that you want concrete that has modest but definitely sufficient
compressive strength, has internal voids bigger than water can wick itself
up, and is permeable to moisture but impermeable to ground squirrels.

Somebody who did basement walls of my house 50 years ago achieved precisely
this result in places. Until now I regarded these locations as rock pockets. 

It appears that their secret was to omit the fines and rely on good-sized
rounded stones for all the aggregate, and furnish only enough rather runny
cement-water paste to coat the stones and glom to them by its own capillary
nature where they touched. More paste than necessary to spot-weld the stone
contacts would close up the voids and risk first, water movement by
capillary pull, and ultimately, loss of permeability.

As for finding references on how to do this, it may be easier to find
material on how not to get this result, and selectively disobey what it says. 

If it were me, I would make small trial batches myself to discover what
worked. It shouldn't be too hard to replicate what old time amateurs so
often accomplished with less knowledge or care. The difference now is to be
able to exercise consistent quality control over the intentional
defectiveness sought.

Charles O. Greenlaw, SE   Sacramento CA



At 12:09 PM 6/6/98 EDT, you wrote:
>I'd like to install a retrofit foundation under an existing adobe wall in
>order to shut down a high-rise ground-squirrel condominium.  It need not be
>reinforced, nor high strength; ideally, it would be permeable to prevent the
>formation of a barrier to moisture that enters the wall from above, and to
>keep ground-moisture from rising  by capillarity.  Has anyone had any
>experience with designing a permeable concrete mix by using coarse and fine
>aggregate, but omitting intermediate sizes of aggregate?  Are there handling
>or placement problems with such a mix? 
>
>Nels Roselund
>Structural Engineer
>(Only works on buildings that are older than he is)  
>