March 1, 2013

Perot Museum: Mistake or Science Lesson?

I visited the Perot Museum of Nature and Science in Dallas with my family this week. Among the fantastic interpretive signage and exciting interactive exhibits, I naturally found time to snap some architecture pics. I thought the smooth, individually sculpted precast panels were masterful, and the prism expressing the escalators whimsical. It was precisely because I was admiring the precast that I started looking at other concrete work and noticed a problem - or was it one more science lesson?



As my daughter danced under the water-molecule presence-detector sculpture, I spotted a lesson the architects may not have intended to teach.

Here is what happens to a neatly ground concrete floor when you only offer it control joints in one direction.
See the cracks running perpendicular to the sawcuts? They are everywhere.

Is this a case of naïve design gone wrong, architects blithely ignoring the limitations of a material for their creative expression? Or a clever demonstration of concrete's need to shrink as it cures? Whether these cracks are intentional or not, you, my architect friend, can still benefit from the lesson. Here's how it works.

Where you have concrete, you have shrinkage. Since concrete's least strength is tension, it has only two choices when it shrinks: break where you've offered it relief, or break where it experiences the most tension. No relief? Then it breaks where it must.

See how these cracks meet the column? That's our concrete bending over the beam, experiencing tension in its top surface. It's not a structural flaw, so your structural engineer won't remind you of this phenomenon: she's given you plenty of rebar to satisfy her duties. It is your duty to hide those inevitable cracks with relief sawcuts or other contraction joint types in vulnerable places. Or, of course, to offer us a building science exhibit.

By contrast, the precast work is very specialized.
Need a key for how to space your contraction joints? A common rule of thumb is to multiply the slab thickness by 24 to 36 for the maximum spacing in either direction. The aspect ratio of the resulting rectangles should be as close to square as possible, but no more than 1.5.  Check out the aspect ratios on the slabs in the Perot Museum example: they're clearly too long for a crack-free surface.

Got a demanding slab design(er) seeking expanses that are just a tad too big? Look into fiber dosing the mix, but beware the dreaded hairy slab that results from long polypropylene fibers too close to the surface.  The Concrete Polishing Association offers more good insights into designing and specifying concrete that will be polished.


This? This is your reward for completing your science lesson!






Photo Credits: All photos by Vivian Volz, copyright 2013.

7 comments:

  1. I don't know about polishing but type K cement might also be good to prevent those cracks. Adds 10% to the cost of the slab though.

    To me it looks like the column is not properly isolated and therefore it would be a structural engineer mistake.

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    1. Thanks, Evan! I had thought about adding type K cement as a possibility, but I need to learn more about it myself. I'll have to follow up next time I have a project polishing a new slab.

      And I certainly appreciate your opinion on what the structural engineer could have done to alleviate the cracking around the columns. Another detail we need to check when we coordinate our documents!

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  2. Great insight into concrete's behavior, Vivian! I love that you point out the elegant simplicity of why concrete cracks: tension. Both you and Evan have solutions that might work to prevent cracking (or at least to extend joint spacing. Macrofibers are a great solution and especially when polished will not be visible (or hairy). Type K (shrinkage comp) works but takes some experience. I do love these precast panels.

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    1. Bill, thanks so much for coming to see! So, I guess polishing trim off the macrofibers at the surface? The "hairy" slabs I've seen were not polished. Thanks for clarifying that issue.

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  3. Excellent blog. Unfortunately, locating contraction joints is as much art as science.

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    1. Thank you!

      You are right, of course. You can provide joints, but you can't make the cracks follow them.

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  4. We're returning to the Perot Museum tomorrow. I'll hunt for some new evidence of concrete mastery and mystery to share!

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