Reinforcing Concrete Slabs-on-Grade

A few weeks ago, I got a great question regarding the reinforcement of concrete slabs-on-grade, so I thought I’d take a quick minute to share how rebar works structurally when the slab is continuously supported by earth.

First, it’s important to understand that, structurally, no reinforcement is generally required for basic flatwork.  Structurally, if you poured 6″ of concrete on top of a random fairway, troweled it smooth and let it cure, you’d be able to put about 30,000 pounds on a square 6″x6″ with reasonable performance.  That’s enough to jack up a smaller armored personnel carrier to rotate the tires.  (Please don’t try this at home — the greenskeeper won’t appreciate it)

However, one of the three fundamental rules of Civil Engineering is that concrete cracks (usually as it dries out).  When that happens, the most major issue is that it doesn’t look very good.  For this reason, we typically specify some kind of reinforcement to hold the slab together.  This can range from fibers mixed into the plastic concrete to wire mesh to rebar.   (There’s a lot of other stuff that goes into concrete cracking less, but that’s a blog post all its own)

I don’t specify fiber very often, so today I want to talk about wire mesh and conventional rebar.   These are designed (essentially) the same way and have similar rules for placement.  The most critical thing to remember is that, to effectively control cracking, slab-on-grade reinforcement must be more or less in the middle of the slab.  It does no good if it’s at the bottom of the slab when the cracks it’s trying to hold together are at the top.  Similarly, some joints are required no matter what; reinforcement that’s designed to keep the slab together at those points does no good if it gets cut when the finishers make the joints.

Because of that, it’s critical that reinforcement for slab-on-grade gets consciously placed, and that it be supported during the concreting operation.

Don't let this be you!
Poorly placed Welded Wire Fabric revealed during demolition

The photograph at right is of a slab where the Welded Wire Fabric (W.W.F.) wasn’t supported.  Less experienced contractors may try to “hook” W.W.F. into the middle of the slab once they’ve placed the concrete, but before it cures.  This doesn’t work very well — first, even if the finisher isn’t standing on the sheet he’s trying to hook, placement isn’t very well controlled; and second, as the workers move around the slab, their weight pushes the mesh back down (which is called “walking in”).  If the slab is solid enough to support a worker’s weight, it’s too solid to hook.

There are a variety of chairs that make positive placement a snap (in some cases, literally — the chairs “snap” together around bars).  That’s also a blog post of its own, but I’ve used steel wire, bent rebar, concrete blocks, and molded plastic with great results.

The last design consideration is more ergonomic.  Workers are much more likely to follow a specification when it doesn’t get in their way, so it’s critical to either specify a mesh spacing that’s tight enough to walk on (4″x4″ is about as wide as I’m willing to go for that) or wide enough to step between (12″ W.W.F. is available, and our “generic” bar spacing is about 18″).

Properly reinforced slabs-on-grade have much longer design lives and require less maintenance.  EVstudio is happy to help your project last a lifetime.


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