Safety First
Wet concrete is highly alkaline and can cause severe burns to bare skin. Immediately wash off any that gets on your skin with cool water.
The Recipe for a Strong Patching Mix
For your patching mix, use either a prepackaged sand mix or concrete mix, depending on the depth of the patch. (See “Use the
Correct Patching Mix”) Each 60-lb. bag makes about 1/2 cu. ft. of concrete, enough for a 2-in. thick patch about 1 ft. wide by 3 ft. long. Estimate the volume of patching material by multiplying approximate length, width and thickness (in feet) to arrive at cubic feet, and buy a bit more than you think you’ll need. At only $2.50 per bag, it’s cheap enough to
buy extra. Better to have too much than to fall a few scoops short!
Mix it with water and acrylic fortifier, following the mixing directions on the package. The fortifier strengthens the new concrete and helps it bond better to the old concrete. The mix should be just wet enough to hold together when it’s troweled into the repair. Don’t add too much water. It’ll result in weaker concrete.
We used a normal sand mix, which sets hard enough to walk on in about a day or two. It continues to cure and harden for weeks. If you have to use the area right away, you can buy a special fast-setting concrete, which hardens in about an hour. It costs about twice as much and you won’t have as long to spread and smooth it, so stick to the regular mix when possible. Most building supply dealers that carry regular concrete also carry the fast-setting type.
Pack the Repair Mix Firmly
After mixing the concrete, pack it firmly into the repair area using a wood float (Photo 4), the tighter the better. Mound the mix so it’s slightly higher than the old concrete. Then immediately level it even with the old concrete using a straight board (Photo 5).
Finish the Patch to Match the Old Surface
Use the Correct Patching Mix
Click image to enlarge.
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 (Photo 6). Once the surface is about as stiff as the skin of a grapefruit, begin the finish work.
We used a sponge float (Photo 7) 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 (Photo 8).
