Steel helps concrete withstand stress, tension

Tim Carter

Q: My builder is telling me to forget about using reinforcing steel or mesh in my concrete driveway and patio. He says he'll just add an inch of thickness and that's good enough. Would you go to the effort and expense to install reinforcing steel in outdoor concrete?

A: Concrete is very strong if you try to compress or squeeze it. But it's weak if you try to bend or stretch it. When you bend or stretch concrete you're applying tension to it. In fact, normal concrete only has one-tenth the strength in tension as it has in compression.

The contractor might be pouring concrete that has a compressive strength of 4,000 pounds per square inch (PSI). That means it only takes 400 PSI to crack it. Standard reinforcing steel rods have a tensile strength of about 40,000 PSI.

You can use either ½-inch-diameter steel rods in concrete or rolls of steel mesh that resemble a giant piece of graph paper. The grid is 6 inches by 6 inches. In either case, you want the steel to be installed so that the concrete surrounds the steel by at least 1 or 2 inches.

Slabs that have concentrated loads and pressure from a certain direction will require the steel to be placed at a specific spot. Structural engineers will specify the exact placement of the steel in these situations.

You don't need to be concerned about that for most residential work around your home. If you're pouring a 5-inch-thick slab for a driveway, sidewalk or patio, just be sure there are at least 1.5 inches of concrete that will flow under the steel.

I like to place steel rods at 2 feet on center in both directions in my exterior slabs. The benefits are tremendous. Because concrete shrinks inch for every 10 feet you pour, you know it's going to crack. The steel keeps these cracks paperthin. Steel also prevents offsetting, where one part of the slab rises higher than the part on the other side of a crack.