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RE: Concrete Slump

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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. 




From: The Best Always [mailto:the_best_always(--nospam--at)]
Sent: Friday, February 16, 2007 1:17 AM
To: seaint(--nospam--at)
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)>
To: seaint(--nospam--at)
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:

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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