Ask the Builder: A repair for cracked concrete that almost looks perfect

Q. Tim, I need to replace a cracked-off section of my concrete driveway. My primary concern is I want the color and texture of the new patch to match as closely as possible to the existing driveway. It doesn’t need to be perfect, just close. How does one accomplish this? I’ve read your past concrete repair columns at askthebuilder.com, so I understand how to mix, place and finish the concrete. Also, what do contractors use to paint the concrete to make it white? – Donn R., Clemson, South Carolina

A. You might be facing the same conundrum as Donn. Maybe you’re trying to match a wall stucco patch. Or perhaps you want to repair the mortar between brick on your home.

I’m sure you’ve seen garish repairs where the new patching material stands out like a red wine stain on a white blouse. While it’s possible to get close when doing these types of masonry repairs, it’s next to impossible to get a perfect match. If you have patience – and many don’t – you can really come close to achieving perfection.

My advice is to do a thorough cleaning of the concrete before you start. You can accomplish this using a pressure washer equipped with a 15-degree tip at the end of the wand. Cleaning the concrete allows you to see exactly what you’ll be matching.

After your clean concrete is dry, get on your knees and really look at it. Odds are you’ll be amazed. You’re going to see the individual pieces of sand and tiny stones used to make the artificial rock. Your first task is to go to local gravel pits and find sand that looks just like what you see.

Pay attention to the different colors of the sand as well as the grain size. You may have to mix sand from different sources and blend them to get a close match. You have to get the sand right.

Now it’s time to look at the cement paste that’s in between the grains of sand in the concrete. What color is it: gray, buff, off-white? It’s likely to be some shade of gray. Gray is good, and it’s the easiest to match.

Remember when I talked about patience? You can only achieve perfection or get close to it by practicing. You’re going to mix up a test batch of concrete yourself. Don’t buy the prebagged concrete from the home center or local hardware store. Visit a local building supply store and get a bag of pure Portland cement.

You’ll need clean gravel with stones ranging in size from grapes to green peas. Mix three parts of the gravel with two parts of the sand and 1.5 parts cement. Blend all this dry and add water until the concrete is the consistency of stiff applesauce.

You can watch a video at askthebuilder.com titled “Mixing Concrete” in which I show the perfect consistency.

Pour this mixture in a small 1-foot by 1-foot form and finish it where the top texture matches the existing concrete as close as possible. Cover the test patch with a new plastic garbage bag and secure it so it doesn’t blow off. Wait for two weeks.

At the end of the waiting period, mix 1 part of muriatic acid to 10 parts water. Read the warnings on the label of this powerful acid and follow all instructions.

Remove the plastic from the test patch, spritz the concrete with a small amount of water, and brush on the acid solution. It should immediately start to fizz. Brush on more acid solution and let it work for about 10 minutes, never allowing it to evaporate. Use an old scrub brush to brush the concrete patch, rinse with plenty of clear water, and allow it to dry.

The acid will wash off the gray cement paste from the sand on your new patch, allowing you to see the color of the sand grains. If you don’t do this step, you’ll have a monolithic color gray for years on end. You’re just doing in minutes what Mother Nature has done since the day the driveway was poured.

There’s a great chance you’ll have a close match. If so, celebrate and proceed to duplicating your efforts on the actual driveway.

Are you like a few of my friends and lack this patience? Clean your entire driveway and just patch the area as I state in the columns at askthebuilder.com. A month later, buy penetrating colored masonry stain and colorize your entire driveway. I’ve got a video at askthebuilder.com that shows how to colorize Mexican tile. It’s the same process for your concrete driveway.

The white paint that Donn asked about might be a traditional hydrated lime whitewash. It’s a great way to make your driveway one color. You can make whitewash any color using dry pigments. Do you want to know how to do that? Well, just go read all my past whitewash columns.