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.|
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.