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Re: W/C ratio

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Harold Sprague wrote:

>... The w/c ratio has little effect on shrinkage.
>As a sample comparison, one mix had 658 LB/yd of cement
>and a w/c ratio of 0.35 resulted in a 9 month shrinkage
>of 0.055%.  Another mix had 376 LB/yd of cement and a
>w/c ratio of 0.70 resulted in a 9 month shrinkage of
>0.055%.  This is one of several examples in the study.

In the above sample, the water content is similar for each
tested mix.  This is consistent with my previous statement
that increasing the cement to reduce the w/c ratio will not
help.  Effectively the water content in the mix is the primary
factor for shrinkage - if one reduces the w/c ratio by reducing
the water content rather than increasing the cement content,
lower shrinkage should result.  (Ref. PCA "Design and Control
of Concrete Mixtures", 13th Edition, Chapter 13.)

>The study indicated that higher cement contents generally
>resulted in slightly higher shrinkage.  Jim Shilstone had
>similar observations in his studies.

Interesting - PCA claims the opposite in the above noted
reference.  PCA shows test results where cement content was
varied between 5 and 8 bags/cy while maintaining a relatively
constant water content, and the resulting shrinkage was
actually slightly less for the higher cement contents.  (The
above sample noted is consistent with these results.)  PCA
concluded that cement content has little effect on shrinkage.

On the other hand, PCA says "despite reduction in water
content, water-reducing admixtures can cause significant
increases in drying shrinkage."  Thus admixtures are not
the cure-all.