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Re: Concrete Mix Design Variations

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The added cracking is because in order to obtain a 0.35 w/c ratio and to be
able to pour the concrete they add more cement and hence more water.  The
net result is you have more drying shrinkage even if you ignore thermal
issues.  If you want to limit drying shrinkage you want to limit the water
and not worry about the w/c ratio.  A low water cement ratio is desirable
in many servicability conditions but it is not always essential.

On the subject of performance specifications for concrete first we need to
be honest as to what characteristics we want from the in-place concrete.  
We then need to agree on a method to measure those characteristics and then
there needs to be some real accountability that will motivate the
Contractor and his supplier to do what is necessary to achieve the desired
results.    Unless we can address these points there will be no
improvement.

Mark Gilligan



Message text written by INTERNET:seaint(--nospam--at)seaint.org
>
To: <seaint(--nospam--at)seaint.org>
From: Jay Shilstone <j.s(--nospam--at)shilstone.com>
Subject: Re: Concrete Mix Design Variations

There is a big move afoot in the National Ready Mixed Concrete Assn. to try

to switch the concrete industry from prescriptive specifications to 
performance specifications. There are advantages and disadvantages to both 
methods.

There are problems on both sides of the fence - both design and 
construction. You cite the problem with the construction side, but there is

an engineering problem as well.

You are in Phoenix. I don't know first hand, but if Phoenix is like other 
places I have been in the U.S., there are engineers specifying a .35 
water/cement ratio, sometimes with straight cement, for concrete cast in 
the summer. They then wonder why their concrete is cracking, since a lower 
water/cement ratio is supposed to be more durable than a higher 
water/cement ratio.

In this case, the problem is that the engineers have forgotten that, while 
low w/c concrete has less drying shrinkage, the higher cement factors 
result in more heat of hydration and a higher initial thermal expansion. 
The result is thermal shrinkage cracks.

I have seen all sides of the concrete process, from designers who don't 
understand the material they specify, to concrete producers who admit to 
lying about what they put in their concrete, to contractors who "give the 
concrete a drink" when the lab isn't looking, to laboratories who don't 
have the foggiest notion about the right way to run the tests they are 
hired to perform. And don't forget owners who want the Taj Mahal for the 
price of an outhouse.

There are two keys to the solution - education and accountability. If we 
are not willing to make the effort and pay for both, we will continue to 
get what we are getting right now.

Stepping off that soap-box and looking at your situation, here are a few 
questions:


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