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RE: W/C ratio

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The w/c ratio relation to shrinkage was established in a study several years ago by David Whiting, "Effects of High-Range Water Reducers on Some Properties of Fresh and Hardened Concretes", PCA R&D Bulletin No RD061.01T and was cited in "Shrinkage and Curling of Slabs on Grade" by Ytterberg, Concrete International, April 1987.  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. 

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

In the field we see more cracking and poor surface performance of the high w/c ratios, but the problem is not shrinkage.

The study involved a variety of w/c ratios and cement contents with and without water reducers.

Harold Sprague

-----Original Message-----
From:	Bill Sherman [SMTP:SHERMANWC(--nospam--at)]
Sent:	Monday, June 08, 1998 5:36 PM
To:	seaoc(--nospam--at)
Subject:	Re: W/C ratio

Harold Sprague wrote:

>The old theory is that if you lower the w/c ratio you will have less
shrinkage.  A lower w/c ratio will reduce bleed water and reduce
but has not much effect on shrinkage. <

What is your basis for this statement?  I don't think that this is an
statement.  Less water in a concrete mix should translate into lower
shrinkage.  However, if one obtains a lower water-cement ratio by increasing
the cement content, then nothing is gained.  So it depends upon how one
obtains a lower w-c ratio.