Q: The corner of one of the concrete steps leading to the front entrance of my building has broken off. It’s the top step of four. Is it possible to repair it without replacing the whole staircase?
A: Yes, you can repair one step, or even several, assuming the concrete below the surface is still firm and intact. If the concrete has so many cracks that it’s literally crumbling, a repair wouldn’t hold and it would be better to rebuild; that’s what you would eventually need to do anyway.
If the damage really is just to the front and top of one step, you’d need to start by building a form — a support for the vertical part, or riser — so the patch material doesn’t slump while it is stiffening. Home Depot has a tutorial on its website showing the type of support you need. Screw together a couple of L-shape brackets made of scrap wood, placing the long part of the L flush with the top of the step and the foot of the L reaching down most — but not quite all — of the way to the bottom of the riser. Then nail a flat board at least as wide as the part of the riser that needs patching to the foot of each bracket. Align this board so the brackets are on either side of the patch. Or, where a patch butts into masonry, attach the board so both brackets are on one side of the patch. Spray or brush cooking oil onto the board so the concrete patch won’t stick to it.
Before repairing the step, brush any loose material from the broken edges of the concrete. The concrete needs to be clean for the patch material to bond well, so scrub away any moss, algae or dirt, then let the material dry. Duct tape is the easiest way to attach the long ends of the brackets to the top of the step you’ll be repairing, so it also helps if the concrete is dry when you attach the form. Tape the brackets in place and consider placing a few bricks along the bottom of the board to help keep everything in place.
For the patch, use a mixture that contains Portland cement. Concrete, whether in the original mixture used to build your steps or in a patch material, consists of the cement, which is the glue component, plus aggregate, the filler. Aggregate can be sand, gravel or a mixture of the two. Concrete mixes, especially those formulated for patching, may also contain polymer, which makes the concrete less porous and therefore stronger. A patch that contains only sand isn’t as strong as a patch that contains gravel. But patches that contain gravel are likely to break off if the biggest particles exceed one-fifth of the thickness of the patch.
In your case, the picture you sent shows that the patch might need to be 3 or so inches deep in part of the area. If that’s the case, a customer-service representative for Quikrete, which makes an array of concrete mixes and patch materials, said the most cost-effective patch would be regular concrete. Quikrete High Strength Concrete Mix is sold in bags as small as 10 pounds, which is probably all you need. (It’s $3.77 at Lowe’s.) A high-strength concrete mix like this has more cement than a mix made for utilitarian purposes such as setting fence posts, plus it sets up faster. To help the patch bond, brush concrete bonding adhesive ($12.99 for a quart of Quikrete Concrete Bonding Adhesive at Ace Hardware) on the existing concrete and let it dry for an hour or two before patching.
If the patch needs to be only 2 inches deep at most, get a patch material such as Quikrete Quick-Setting Concrete, which is sold in 10-pound pails ($7.63), or Quikrete FastSet Repair Mortar, which is available in containers as small as a 3-pound bag ($6.97). The small bags cost more per volume, but they are designed so you can pour in the water and mix by squeezing the bag. The convenience of buying one or two bags and not having to deal with cleaning up a mixing container or storing leftovers may make this a good option.
You also have the option of buying a patch designed for holes no deeper than 2 inches, then adding a few handfuls of gravel to use for patching the deeper areas. But don’t use a material intended for skim-coating — resurfacing concrete with a layer 1/16- to 1/8-inch deep — even if you add gravel. The sand in topping mixes is too fine for patching steps.
When you’re ready to patch, put on thick rubber gloves, eye protection and a disposable mask. If you aren’t using the mix-in-bag product, consider a wheelbarrow or an old plastic dish pan. Use a trowel or just your gloved hands, rather than a shovel, to scoop out a small amount. If any concrete mix touches your skin, rinse it off immediately; Portland cement is very alkaline.
Add the dry ingredients, stirring them if they’ve settled, then add water. For a 10-pound bag, which will make much more than you probably need, start with just 1½ cups of water; adding too much makes for weak concrete. Work out all the dry lumps. If the mix seems too stiff to work into the form, add a little more water, but do not go over 2¼ cups for 10 pounds of mix. Pack the mix into the bottom of the form and work your way up, adding more as you go. Remove pieces of large gravel when patching thin areas, but leave them where the patch needs to be deep. When the patch is full to the top, press down repeatedly to work out air bubbles. You will see a cream-like mixture of fine sand and cement rise to the surface. Let that set a bit, then trowel smooth. For a small patch like this, you can use a wide plastic putty knife if you don’t have a trowel.
When the patch stiffens, which might take as little as 10 minutes, depending on the weather and the patch material, carefully remove the form. Slice down with the trowel or putty knife to separate the board from the riser. If you see gaps, which is likely, use the trowel or putty knife to smooth the surface and fill the holes. Add small amounts of leftover sand and cement, if needed. Also round over the top edge to match the existing step.
Concrete stiffens quickly but it needs days to fully harden. However, the hardening process continues only as long as the concrete stays damp. (Once it dries out, you can’t restart the curing process by adding moisture.) If the weather is warm and dry, consider covering the patch with burlap (a nearby coffee roaster may have bags to give away) and dampen the fabric frequently. To keep dye from transferring from a coffee bag onto the concrete, hose down the bag first.
Have a problem in your home? Send questions to localliving@washpost.com. Put “How To” in the subject line, tell us where you live and try to include a photo.
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