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READYMIX DRIVER DAN: “I can add water, but if I do, the strength will go down. Will you take the liability if the Owner complains?”
CONTRACTOR CALVIN: “The Owner won’t find out. He’s a birdbrain — doesn’t know the first thing about construction. Plus, this flatwork should never see any heavy loads. Add the water.”
READYMIX DRIVER DAN: “Okay, I’ll wet it up, but if the *&%# hits the fan, the monkey’ll be on your back.”
CONTRACTOR CALVIN: “Fine, I’ll take the rap. Wet it up good. I want ‘er to flow like water down a crick.”
Clearly Contractor Calvin has never read my “Structural Concepts” book or been to one of my seminars. Otherwise he’d have come across the following multiple-choice question:
Slump:
A. Is a measure of the amount of cement used per cubic yard of concrete.
B. Is an indirect measure of the amount of water in concrete. A normal slump for regular concrete is three to four inches, which translates to a water-cement ratio of approximately 0.4.
C. The more slump, the more water has been added, which translates to weaker concrete. Just a small increase in water-cement ratio, say from 0.35 to 0.53, will cut the compressive strength approximately in half. Not only that, the finished surface wearability will be compromised and it will be prone to excessive shrinkage and cracking.
D. Slump is measured with a device called an “Ice Cream Cone.”
Answer A is false. The best answers are both B and C. Strong concrete has just enough water added to provide a workable mix — exactly what Readymix Driver Dan brought to the site. Regarding answer D, slump is measured with a standardized “Slump Cone.” Ice cream cones are what you eat on a hot day.
Answers B and C assume that no admixtures are used, i.e. just plain old cement, aggregate and water. If, however, you want a more flowable, more workable mix without adding more water, you could use a class of admixtures called water reducers, also known as plasticizers. Talk to your readymix provider about this, though, because water reducers can have side effects such as slowing cure time.
Back to my project, the bottom line is I’m stuck with weak concrete, prone to surface wear and excessive shrinkage. Fortunately, none of it will experience wheel loads, so probably it will be okay from a strength standpoint. My readymix provider stepped up with some sealer to help keep surface wear under control (though he had no obligation to do so). The contractor skated. Oh sure, I could have sued him, but it wouldn’t have been worth the cost and effort. However, I will be sure never to use him again, and also I’ll let others know of his poor performance.
Contractor Calvin needs to learn: A.) Customers know more than you think, and B.) You never know just how far-reaching one dissatisfied customer can be.
Tim Garrison of ConstructionCalc.com, is a professional engineer, author and software producer for the building industry. Send e-mail to buildersengineer@constructioncalc.com. Tim reads every one.
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The views expressed in this article represent the personal views, statements and opinions of the author and do not necessarily represent the views, statements, opinions or policies of the National Association of Home Builders. NAHB does not necessarily endorse any of the views expressed by the author and NAHB is not responsible for any direct or indirect consequences arising out of the views expressed in this article.
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I recently had some concrete flatwork done. The job was straightforward and the contractor well-experienced. How is it possible, then, that he screwed it up?
