Patching concrete floors

When a good floor goes bad, it can happen many ways. It can develop small cracks, large cracks, or pitting, or it can become uneven. The causes vary and usually have something to do with either the original mixture of concrete or a shifting surface below the concrete. But it’s the size of the crack, not the cause, that determines the cure.

Fix small cracks with a crack sealer, usually a ready-mix material that trowels in place. It comes both in a caulk like form and as a liquid. Use the liquid on floors and pads and the caulk on walls. Larger cracks require more work. Chisel away any weak spots and widen the bottom of the groove. Repair with a cement-and-sand mixture known as sand mix. (It’s stronger than concrete, which also contains gravel.) Use sand mix to fill pitted surfaces too.

If the crack resulted from shifting of the material under it, the floor may also have shifted, leaving one part higher than another. If you’re covering the floor with vinyl or laminate flooring, the floor will wear through at the crack. Start by using a self-leveling resurfacer, a mixture thin enough that it forms a smooth, level surface when you pour it on. This can be a lot of work, so find several helpers. If the patching compound requires mixing, add a little water and mix compound with a garden hoe (or a mason’s hoe, which has a couple of extra holes in it to help with the job). Add more water as needed.

MATERIALS: Concrete cleaner, sand, crack sealer, repair caulk, vinyl concrete patch or sand mix, bonding adhesive for deep cracks, self-leveling floor compound for uneven floors

TOOLS: Wire brush, caulking gun, putty knife, hammer drill and bit, trowel, wooden float, paintbrush, bucket, wheelbarrow or mixing bin, mixing hoe, goggles and rubber gloves (optional)

BIG CRACKS COULD MEAN BIG PROBLEMS - A big crack in a slab can mean that shifting and settling are going on below. Before you install a new floor over a big crack, find out what's causing it and make sure the shifting is no longer active, or the crack will reappear.

Repairing Small Cracks

1 CLEAN OUT THE CRACK WITH A WIRE BRUSH to remove dirt and loose stones. Wash the crack with a concrete cleaner. Get a commercially available nonacid concrete cleaner made by the same company that makes your patching compound. It's more convenient and safer than the large bottles of muratic acid masons use.

2 LET THE SURFACE DRY. If the crack is deep and it’s in a floor, fill it partially with sand, leaving an opening about 1/4 to 1/2 inch deep. Pour in the crack sealer until it forms a layer 1/4 inch deep.Let it dry overnight and then apply another layer. Repeat until the surface is flush with the floor. Don't overfill—apply just enough patch material to bring the surface flush with the floor. If using caulk, smooth with a putty knife. If you are using on acid-based cleaner, wear goggles and rubber gloves while you apply it.

Fixing Large Cracks

1 WITH A HAMMER DRILL, RESHAPE THE CRACK SO THE BOTTOM IS WIDER THAN THE TOP; this helps hold the patch in place. Chisel out any weak or crumbling spots too. If the crack is deeper than I/2 inch, paint it with a bonding adhesive made by the company that manufactures the patch you're using. Let glue dry thoroughly before applying the patch.

2 ON CRACKS UP TO ABOUT 1/2 INCH DEEP, TROWEL IN A VINYL CONCRETE PATCH. Mix according to the directions on the bag or pail, and trowel a 1/4-inch layer into the crack. If more layers are necessary, let the patch dry for several days before applying a new layer. Trowel the final layer flush with the surface and smooth.

3 ON DEEPER CRACKS, MIX SOME SAND MIX ACCORDING TO THE DIRECTIONS ON THE BAG. Trowel it into the crack, filling it flush with the surface. Initially, the patch on both deep and shallow cracks will have a watery sheen. When the sheen dries off, use a wooden float to smooth the surface and give it a texture that matches the rest of the concrete. If the existing concrete is very smooth, then smooth the surface with a metal finishing trowel.

Leveling Uneven Floors

1 CLEAN THE ENTIRE FLOOR and patch any holes following the directions above. Nail a strip of wood across door openings to keep the patching material from pouring into the next room. Be sure to work toward an exit when leveling an uneven floor.

2 MIX A THIN MIXTURE OF THE LEVELING COMPOUND and brush it on the floor as a primer. New concrete usually requires only one coat of primer; older concrete may require two coats. Follow the directions on the bag for both mixing and priming. Let the primer dry as specified by the manufacturer—usually for 1 to 2 hours.

3 MIX A THICKER CONSISTENCY OF COMPOUND, following the proportions on the bag. Starting in a corner on the narrow end of the room, pour the mixture on the floor. Pour the second batch next to it along the narrow wall. Repeat along the entire wall, and then work your way back across the room, pouring a new row of compound next to the old. Let the compound flow to level itself.

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