How to Patch Chipped and Pitted Concrete Sidewalks and Stairs (page 3 of 3)

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Choosing the Correct Mix

Finish the Patch to Match the Old Surface
Use the Correct Patching Mix
In a nutshell: Use a sand mix for repairs less than 2 in. deep. Use a concrete mix for repairs deeper than 2 in.

Concrete mix consists of sand, gravel and Portland cement. The gravel reduces shrinkage, so it won’t crack as readily when laid in thick. However it may not bond well in thin layers.

Replace 50 to 80 percent of the water with the acrylic fortifier in either mix to improve the patch’s bonding strength.

Sand mix (sometimes called topping mix) consists of sand and Portland cement. It’s easier to trowel in thinner layers (to about 1/4 in. minimum) for thinner patches or patches with thinner edges. However, if you apply it thicker than 2 in., its natural shrinkage might cause it to crack or break the bond with the old concrete.

Begin finish trowel work when the surface moisture starts to evaporate and the patch begins to harden. It may only be 10 minutes on a warm day or perhaps an hour on a cool day. One sign of evaporation is the loss of some of the surface gloss. When the patch appears to be getting stiff, test it with your thumb. Once the surface is about as stiff as the skin of a grapefruit, begin the finish work.

We used a sponge float to match the slightly rough texture of the old concrete. A sponge float has a rough rubber surface. If you want a smooth surface, use a steel trowel; for a lightly grooved appearance, drag a stiff-bristle broom across the surface.

Cover the Patch to Retain the Water
After using a float on the surface, cover the patch with plastic for two days. The plastic helps the concrete retain the moisture, which the concrete needs to cure and strengthen.

To Restore Broken Corners
Use the same basic patching technique for a broken corner as for spalled areas, but add three key steps.

Create a ledge for the patch to make a stronger bond with the old concrete. Don’t be afraid to make the cutout well beyond the edges of the original damaged area. Smaller patches simply won’t hold as well as larger ones.
Prop a wood form tightly against the vertical portion of the step to hold the concrete patch in place. Block the form against the step with bricks or other heavy objects.
When the concrete begins to set, use the edging tool to create a slightly rounded edge to match the step. Then remove the form board (carefully, because the concrete is still soft) and finish all exposed surfaces with a float or trowel to match the texture of the old concrete. Cover the patch with plastic and let it cure for at least two days before walking on it.

TIP: Coat the wood form with motor oil or WD-40 to keep the concrete from sticking to the wood.