Wednesday, October 15, 2014

Which concrete reinforcement method is best?

When we were talking with our home builder about concrete, we found that he used several different methods to reinforce the concrete, depending on the application.  I had my own ideas about what I wanted to use, but after talking more with our builder, he's won me over.  Here's what I've learned.

Concrete is a very popular construction material all over the world.  It's very strong under compression, but it's not so great under tension, and it doesn't bend well.  To help with those latter two situations, it can be reinforced by inserting other materials inside the formed concrete slabs or walls.

Most people are familiar with steel reinforcing bars, or "re-bar."  These are steel rods about a half inch in diameter that are placed inside the concrete to add strength.  They help the concrete resist breaking and separating.  Re-bar is the strongest form of reinforcement.  It's what's used in structural concrete, sometimes taking up as much as half of the volume of a load-bearing concrete beam used in large construction projects.  In a home, re-bar is typically used in the foundation walls to keep the house from falling apart if the earth surrounding the foundation moves in an undesirable direction.  Single pieces of rebar can span between two separate pieces of concrete to help join them together (like a foundation wall and a garage floor, for instance).

Our builder uses re-bar in the foundation walls, footings, and the large, wrap-around, covered front porch slab.  Because the porch slab is outdoors, but still underneath a roof and part of the house structure, the rebar was deemed necessary to keep it from breaking & separating.

A lesser form of reinforcement is wire mesh.  Wire mesh is made of steel wires, roughly 1/8" in diameter, welded together into a rectangular mesh with approximately 6-10" rectangular openings.  It runs across the entire middle of the concrete slab.  Wire mesh doesn't provide as much structural strength as re-bar, but when cracks form in the concrete, wire mesh will help keep the pieces together so they don't separate and form ledges or large gaps.

Our builder uses wire mesh in the driveway and garage horizontal slabs.  These (nearly) outdoor slabs are outside the home's drain tile and heating envelope, and therefore the ground under them is more likely to move around during weather extremes.

Both wire mesh and re-bar are inserted into the concrete form before the concrete is poured in.  When done by a competent installer, they are set up on metal stands (called "chairs") so that once the concrete is poured, the steel will be in the middle of the slab or wall.  A careless installer may just lay the mesh on the bottom of the form and pour the concrete on top of it, which means the steel will end up under the concrete rather than inside it.  This obviously won't strengthen the concrete at all.

A third form of reinforcement is fiberglass mesh, or GFRC.  Thin strands of fiberglass, a few inches long, are mixed in with the concrete as it's being mixed together.  When the concrete is poured into its form, the strands end up lying in all different directions, much like a scoop of cooked spaghetti noodles on a plate.  This provides a lot of shear strength to the concrete, making it more resistant to hairline cracks.  If you can reduce the hairline cracks, you'll reduce the larger cracks and ledges.  Fiber mesh isn't nearly as strong as wire mesh or re-bar, but it's also a lot cheaper and requires far less labor to install.

The ends of these fiberglass strands will be exposed on the surfaces of the dried concrete, making the concrete look like white dog hair was shed all over it.  Fortunately, those can be burned off with a torch once the concrete has cured.

Our builder uses fiber mesh in the basement floor flab.  With the insulated house above the slab and drain tile surrounding the perimeter of the foundation, the earth underneath the slab doesn't usually go through the freeze and thaw cycles that cause exterior concrete flat work to crack and heave.  While some builders use wire mesh in the basement slab (and I initially lobbied for that in our house), other builders use no reinforcement whatsoever in the basement slab.  Our builder feels that wire mesh is overkill there, but fiber mesh provides relatively cheap insurance against premature cracking.

So, the finally tally is that we'll have re-bar in the foundation and front porch slab, wire mesh in the garage & driveway slabs, and fiber mesh in the basement floor slab.  Time will tell how well that works out.

If you have any experience--positive or negative--with various forms of reinforced concrete, please share it in the comments below!

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