Need a book? Engineering books recommendations...

Return to index: [Subject] [Thread] [Date] [Author]

Re: Specifying low w/c

[Subject Prev][Subject Next][Thread Prev][Thread Next]
If you go all the way back to Duff Abrams, he states there are 2 components to obtaining durable concrete:

1) Reduce paste content by maximizing aggregate
2) Reduce the permeability of the paste.

Permeability can be reduced by low w/c, but there are other methods, such as including silica fume or other pozzolanic materials, that can also reduce permeability.

One of the problems with specifying a low w/c is that, in order to obtain a low w/c, the producer must add cementitious materials. This increases heat of hydration, which increases thermal expansion and results in cracks when the concrete shrinks, and also increases autogenous shrinkage, which is the result of the reaction between water and cement producing a smaller combined product. It doesn't do any good to have concrete with a permeability of 200 coulombs if the concrete cracks every 5 feet due to shrinkage. Yes, the steel restrains the cracks, but the cracks open a path directly to the steel.

The main portion of my speech was that the 0.26 w/c ratio normally associated with the w/c required for cement hydration was determined after grinding cement with water for 4-5 days (Bulletin 29 from PCA). Normal concrete doesn't go through this process. In fact, once w/c goes below about 0.40, there are not enough water filled capillary pores for hydration products to form and so cement hydration stops. The lower the w/c below 0.40, the greater the quantity of unhydrated cement.

The real key to durability with regard to sulfates and chlorides is lower permeability, not just a low w/c. If we can get lower permeability at a higher w/c, we eliminate a lot of problems, not the least of which is sustainability and the current cement shortage.

I guess that part of the problem is that the design community often doesn't know what permeability is NECESSARY for durability (except as shown in ACI documents), and much of the ready mix industry doesn't really know what permeability their concrete actually has. This situation is probably being changed by the move to performance specifications.

Maybe I should amend my original question to:

"What reasons would a designer have for specifying low w/c, besides strength and durability?"

I thank you for your response and hope you don't feel I'm being argumentative. I am just exploring different points of view.

Jay Shilstone

At 10:55 AM 11/2/2006, you wrote:

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
Please respond to seaint
To
<seaint(--nospam--at)seaint.org>
cc
Subject
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

James M. Shilstone, Jr., FACI                 jay2003.shilstone(--nospam--at)shilstone.com
President                                     www.shilstone.com
The Shilstone Companies, Inc.                 214-361-9681
9400 N. Central Expy., #105                   800-782-8649
Dallas, TX  75231                             FAX: 214-361-7925
If our new anti-spam filters block you, send an email to jshilstone(--nospam--at)gmail.com.




-----------------------------------------------------------------------------------------------------
The information transmitted is intended only for the person
or entity to which it is addressed and may contain proprietary,
business-confidential and/or privileged material.
If you are not the intended recipient of this message you
are hereby notified that any use, review, retransmission,
dissemination, distribution, reproduction or any action taken
in reliance upon this message is prohibited. If you received
this in error, please contact the sender and delete the
material from any computer. Any views expressed in this message
are those of the individual sender and may not necessarily reflect
the views of the company.
-------------------------------------------------------------------------------------------------------

James M. Shilstone, Jr., FACI                 jay2003.shilstone(--nospam--at)shilstone.com
President                                     www.shilstone.com
The Shilstone Companies, Inc.                 214-361-9681
9400 N. Central Expy., #105                   800-782-8649
Dallas, TX  75231                             FAX: 214-361-7925
If our new anti-spam filters block you, send an email to jshilstone(--nospam--at)gmail.com.