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Re: Specifying low w/c

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Jay,

All the studies ever done on concrete clearly show that a low W/C ratio is the best way to achieve durable concrete.  That is, infinitely better than specifying Type II or V cement or any magic additive.  This is especially true for sulfate attack of the cement matrix and to a lesser extent chloride resistance for attack to the rebar.  For generic foundation design a W/C ratio of 0.40 would generally be enough but there are cases where a W/C ratio less than 0.40 might be warranted in a very high sulfate rich situation, such as might be found in a processing plant or structures in hot sea water, or in an extremely brackish ground water situation.  The second best may to achieve durable concrete is to increase the concrete cover.

Thomas Hunt, S.E.
Fluor




Jay Shilstone <j.s(--nospam--at)shilstone.com>
11/02/2006 08:15 AM
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Specifying low w/c





I just got through doing a speech on the fallacies of specifying a low water-cement ratio. I realized afterward that I had never asked an engineer WHY he specified a low w/c. Like most materials people, I assumed it was a combination of "if a low w/c is good, then a lower w/c is better" and a desire to obtain better durability (which can often be obtained through better methods than an extremely low w/c).

So I'm asking the question: "Why specify a w/c below 0.40 if it is not needed for strength purposes?"

About the only other reason I can come up with is to increase the rigidity/reduce creep of a structure, but that also might be better accomplished through other means than a low w/c.

Thanks,
Jay Shilstone

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