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Re: Specifying CMU block weights

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Jim:

Lighter weight block is generally more absorptive and more permeable than
normal weight block.  This is due to the use of less density lightweight
aggregates or due to the more porous structure if a foam agent is used to
get the ligher weight.

Over all, the more absorptive nature of light weight block can affect the
durability of the block.  So, if the block is going to be subjected to
many freeze/thaw cycles than you should use normal weight block.  Also, if
moisture penetration is an issue, then normal weight is the way to go.

Light weight block will tend to shrink more than normal weight block, but
to get really "accurate" you should get current shrinkage data from the
block manufacturer that is (potentially) supplying your project.

And if block strength is an issue, while you can get high strength light
weight block, generally normal weight high strength block will be more
economical.

I would say that if you are dealing with a cavity wall where the block
will largely be protected from moisture, then light weight block could be
used as long as you account for the shrinkage issues.  The other thing
that you will likely want to think about is whether you want Type I
(moisture-controlled) or Type II (nonmoisture-controlled) block.  I would
say for exterior applications (potentially including cavity walls
depending on your "warm fuzzy feeling" tolerance) you would want Type I.
It is used where you need higher strength, resistance to moisture
penetration, and resistance to severe frost action.  This would generally
be things like retaining walls or basement walls or exterior walls where
the block is directly exposed, but could include exterior cavity walls.

HTH,

Scott
Adrian, MI


On Wed, 26 Jan 2005, Jim Wilson wrote:

> What are the do's and dont's for specifying CMU block
> weights in the northeast?  I am under the impression
> that normal weight blocks are to be used for exterior
> exposed CMU walls due to water resistance.  Is water
> the governing factor, or are there other design
> components to this decision?
>
> Would medium weight or lightweight block then be
> specified only for interior partition walls, or would
> they also be specified for exterior walls that are
> protected behind a vapor barrier, such as behind a
> brick face?
>
> I am guessing that it would it be too confusing to mix
> block types on a project for interior and exterior
> walls?  Seems safer to use normal weight block unless
> medium or lightweight are explicitly required for some
> reason.
>
> TIA,
> Jim Wilson
>
>
>
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