Concrete Patching Rebar Mesh

by David
(Ottawa, CA)

Q: I am replacing a large patch of my concrete pool deck about 6 feet long and about 18 inches wide. One of the long sides of this patch is right up against the steel coping of the pool wall (i.e. I have taken the concrete out right up to the edge of the inground pool structure). The pool deck is 3 feet wide therefore the other long side of the patch buts up against the existing concrete deck.

Does the rebar used to pin the patch to the existing concrete have to be a half inch or can it three eighths?

Do I need rebar and wire mesh for this job or will rebar suffice?

Thank you for your kind reply.

A: Great question, David.

You didn't say how thick the concrete slab is... and that will help determine whether rebar or wire mesh will be your best bet. If the slab is 5 inches thick or less, then you may do better with wire mesh only. But if it's 6 inches thick or thicker, then rebar is probably the way to go. In either case, you probably do not need 1/2 inch rebar (also called #4 rebar). The rebar pins that you use to pin the new slab to the old concrete can be 3/8 inch (#3 rebar).

A general rule of thumb is that for columns, walls, or piers, you need #4 rebar, and for footings you should use #5 rebar, which is 5/8 inch. But for driveways and patio flat work, #3 rebar is usually fine.

Say your slab is 5 1/2 inches thick. Personal preference is going to play a role here. There are concrete masons who do not like unrolling wire mesh into slabs because - well, frankly because it's a bit of a pain unrolling it and properly positioning it in the middle of the slab. But since your slab is only about 6 feet long, you can buy a sheet of wire mesh and have it cut to fit. If you elect to use wire mesh, which usually has 6 inch squares and comes in sheets of 5 x 8 feet, overlap the sections by at least 6 inches.

If you put a control joint in your slab, you'll want to leave an open seam of around 4 inches where the control joint will be located. (Control joints should be spaced in feet no more than 2 to 3 x the slab thickness in inches. In other words, a 4 inch thick slab should have joints located 8 to 12 feet apart at most. Joints should be cut to 1/4 the depth of the slab, so a 4 inch thick slab should have control joints 1 inch deep.)

The problem with using rebar in thinner slabs is that, say you place your rebar grid in the middle of a 4 inch thick slab. You've only got 2 inches of concrete on top of the rebar. People with extensive experience in pouring slabs report frequent hairline cracking above rebar in slabs less than about 6 inches thick.

This observation is in line with recommendations of the American Concrete Institute, which says that inadequate concrete cover on steel rebar - particularly thicker rebar - can result in settlement cracking over the rebar. Concrete cover should be at least 2 inches over rebar. If you really want to use rebar in a slab less than 6 inches thick, then use the smaller size (#3), and use low slump concrete if possible, which is concrete that does not "flow" much. Low slump concrete is more difficult to work with, however.

So, if your slab is 6 inches or thicker, then #3 rebar reinforcement is in order. With #3 rebar, the spacing should be no greater than 18 inches center to center, and lap should be no less than 30 x the bar diameter, which, for 3/8 inch rebar would be 11.25 inches.

If your slab is thinner than that, you should consider using wire mesh instead of rebar to prevent hairline cracks over the rebar grid.

Build well,

Alan
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