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