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Re: W/C ratio[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: seaoc(--nospam--at)seaoc.org
- Subject: Re: W/C ratio
- From: "Bill Sherman" <SHERMANWC(--nospam--at)cdm.com>
- Date: 09 Jun 98 20:37:42 -0400
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.
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