Cracking up

We’ve all heard the jokes about concrete slabs and cracking. There are only two kinds of concrete—concrete that’s already cracked and concrete that’s about to crack. My beef with this is that it makes people feel that we aren’t in control of the material. It’s as if the slab is going to do whatever it feels like and we can only stand idly by and watch. The truth is that we can indeed make concrete slabs that don’t crack. At least, we can make concrete that only cracks where we want it to. The notion that concrete always has lots of random cracks is one that I’d like to see dispelled.

Of course, that means we need to actually place lots of concrete that does not end up cracked. If, as I insist, we know how to do that, then why is there still so much cracked concrete around? In most cases, it’s simply ignorance. The guy who designed the concrete or the guy who produced the concrete or the guy who placed the concrete simply didn’t know enough to prevent random cracking.

Now, in most cases random cracks do not impair a concrete slab’s serviceability. Cracks in flatwork are only detrimental to performance when there is lots of traffic that breaks down the edges. Nonetheless, it looks bad and justifiably leads the public to feel something is wrong. If we can keep the cracks inside the control joints, everyone feels we have things under control. In fact most people don’t even think a joint is a crack when of course that’s all it really is. But it’s a crack where we want it, a controlled crack—thus the term control joint.

Today we can even build concrete slabs that actually don’t crack—or that have such thin cracks that they can’t even be seen when the concrete is dry. Jerry Holland has shown that post tensioning or high doses of steel or synthetic fibers can result in crack-free slabs even with geometries that would normally be sure to crack.

So tell the world! Concrete doesn’t crack! Then go out and prove it to them.