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RE: Concrete Slump

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Don,

Not to bicker, but I would say "limit" rather than "eliminate".  As Jay
pointed out, concrete WILL crack no matter how "perfect" is appears to be
or actually is.  More than likely even concrete that has not appeared to
crack is usually cracked, it is just the cracks are small enough to be
really be visible to the naked eye.

And reinforced concrete is designed and assumed to crack...thus, the
reason for the steel reinforcing bars.  If we could truly design concrete
to NOT crack, then the steel bars would not be needed.  But, since
concrete is not so great in tension and will crack under rather small
tension loads/stress (whether from flexure, direct tension, shrinkage,
etc), we put steel in concrete.

If by some miracle the concrete does not crack due to shrinkage issues,
then you would have to have VERY small loads to not have it crack under
load (i.e. keep it such that the tension stresses in the concrete don't
exceed modulus of rupture values).

Regards,

Scott
Adrian, MI


On Fri, 16 Feb 2007, Donald Bruckman wrote:

> I hope this isn't taken the wrong way, but.
>
>
>
> In my experience, there are two causes of concrete cracks:  from bad
> installation and from bad design.
>
>
>
> We spend a ton of time discussing how to construct concrete in ways to
> eliminate, (note I didn't say "limit") cracking.  Low and behold, perfect
> concrete is attainable, but only if you understand what you are doing and
> have an engineer designing it for you that gives the field the chance to
> succeed.
>
>
>
> That said, I am a firm believer that the contractor, and stunningly, from
> concrete subs no less, that repeat the old saying, "There are two kinds of
> concrete, concrete that is cracked and concrete that is going to crack."
> Those are the ones that don't understand concrete.  But, in addition, it
> takes a savvy engineer to fully understand what concrete will do and where
> it will go in-situ.
>
>
>
> I have been privileged to work with a few that truly "got it" about concrete
> and we in the field bowed in awe and blathered, ".we're not worthy, we're
> not worthy." to their consummate expertise, because for many of us, myself
> included, it was an eye-opening experience to actually get it into your head
> that perfect concrete is, in fact, attainable if you pay very close
> attention.
>
>
>
> Don
>
>
>
>   _____
>
> From: The Best Always [mailto:the_best_always(--nospam--at)yahoo.com]
> Sent: Friday, February 16, 2007 1:17 AM
> To: seaint(--nospam--at)seaint.org
> Subject: Re: Concrete Slump
>
>
>
> Thanks Jay.  I appreciate your response to my inquiry.
>
>
>
> The reason that I asked is that I suspect that the slump that the contractor
> uses cause the excessive cracks I  have observed.  The slump that they use
> is reaching as high as 200mm (with concrete additives).  Also, I do not like
> the way they repair these cracks on floor slabs; they do not use injection
> equipment as per product data.  Crack widths are 0.4mm on top and soffit of
> slabs; these are found to be through cracks by flooding the area with water.
> I see no evidence that the repair material fills these cracks completely.
>
> ----- Original Message ----
> From: Jay Shilstone <j.s(--nospam--at)shilstone.com>
> To: seaint(--nospam--at)seaint.org
> Sent: Wednesday, February 14, 2007 3:15:35 PM
> Subject: Re: Concrete Slump
>
> I hope you get a lot of replies on this one.
>
> In a laboratory, when using only rock, sand, cement and water, slump and
> strength (and shrinkage) correlate pretty well. In the real world that is a
> different story. There are several ways to increase slump without affecting
> shrinkage and strength. The reverse is also true. You can impact strength
> and shrinkage without affecting slump. Here are some of the factors.
>
> Water - Total water in a mix is directly related to shrinkage. The more
> water, the more shrinkage. Reduce the water, reduce the shrinkage.
> w/c ratio - I wrote a mini-novel on this subject on the mailing list about a
> month ago. Generally speaking, as you decrease water-cement ratio you
> decrease drying shrinkage, but you can increase autogenous shrinkage and
> thermal shrinkage due to high heats of hydration
> aggregates - There is an optimum combination of coarse and fine aggregate
> that will result in the lowest water demand for the mix (see water above)
> gradation - A well graded aggregate mix typically requires less water than a
> mix with one or more gaps in the grading (see water above)
> admixtures - Generally speaking, water reducers and superplasticizers will
> decrease shrinkage, but that is no guarantee. Some superplasticizers can
> increase shrinkage by as much as 25% (I think that is what ASTM says) and
> still meet ASTM C494. You have to test the concrete to find out what is
> happening.
>
> Self-consolidating concrete - there is a whole new (25 years old) class of
> concrete that is designed to have slump in the 9-11 inch range without
> attendant strength and shrinkage problems. Talk to local concrete producers
> about its availability and suitability to you application.
>
> Generally speaking, most engineers limit slump without superplasticizer in
> their specs to 3-5 inches, but contractors like 4-6 inches better.
>
> Also, cracks aren't just a function of the concrete material. They are also
> a function of the design, concrete configuration, placement and curing.
>
> Sorry you asked? <g>
> Jay Shilstone
>
> At 02:58 AM 2/14/2007, you wrote:
>
>
>
> Hi!
>
> It is my understanding that high concrete slumps would result to excessive
> cracks.  Anyone knows what is the best slump value for a highrise building
> (say 50-storey building)?
>
> Many thanks.
>
>
>
>
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> James M. Shilstone, Jr., FACI
> jay2003.shilstone(--nospam--at)shilstone.com
> President                                     www.shilstone.com
> <http://www.shilstone.com/>
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>
>
>
>   _____
>
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