The Washington PostDemocracy Dies in Darkness

The right way to repair and resurface eroded concrete

Q: I’ve got problems with some of the concrete on the outside of my home as well as in my garage. There are lots of places where the top finish and layer of the concrete has eroded away. I think it’s called spalling. It’s worse in some areas than others. My biggest question is if I should transform my concrete garage floor by installing a fancy epoxy coating. They look marvelous in my opinion. What are the pros and cons of this? How would you repair all the other wretched concrete outdoors?

— Steve H., Sylvania, Ohio

A: Unsightly concrete is an increasingly common problem, if my incoming email is any indication.

First, let’s discuss a few of the primary reasons spalling happens to some concrete but not to other concrete nearby. That should be your first clue that the problem probably is poor workmanship.

Don’t underestimate the difficulty of installing concrete

Concrete is simply artificial rock. Three of the 4 ingredients in basic concrete are or were rock at one point. The aggregate is made up of small stones. The sand is made from thousands of tiny pieces of rock in almost all cases. (Some sand is crushed seashells, but this is rare.) The Portland cement is powdered rock, primarily limestone and shale. The only ingredient that's not rock is water. You need the water to ignite the chemical reaction that transforms the powdered Portland cement back into solid rock again.

Here are just a few things that can go wrong with concrete that can cause spalling and cause the surface to flake off: The concrete can be mixed with too little Portland cement, or too much water can be added. Also, water added during the finishing process can dilute the amount of Portland cement in the top layer of the concrete. Finally, the concrete may not have been cured immediately after the finishing process ended.

There's a misconception that de-icing salts ruin concrete and cause it to spall. The evidence does not support this. You undoubtedly have seen lots of concrete sidewalks, floors and patios that have had de-icing salt spread on them for years, and yet there's no spalling. Why is that?

The reason that concrete survived the de-icing salts is that it was mixed, placed, finished and cured properly. De-icing salts can damage weak concrete just like the big bad wolf blew down the straw house made by the little pig.

Now, a word on epoxy coatings: Think about a new concrete garage floor for a minute. Imagine how good it looks. It's supposed to be maintenance-free for the most part, right? You just keep it clean with soap and water, and it will look amazing for years.

If you decide to apply one of the many magnificent epoxy coatings, you introduce a wild card into the equation. You hope the epoxy will hold and won’t peel in places. You hope you’ll do everything right during the installation process — a process you’ve never done before, I might add! You might create a new surface that requires more work to keep looking nice than your plain-vanilla concrete slab. Personally, I’d never install an epoxy finish because I don’t want more maintenance.

You can repair and resurface your spalled concrete in any number of ways. You can buy products that apply a very thin coating on the concrete. The larger premixed bagged concrete manufacturers make these, as do some other specialty companies. These products do work, but pay close attention to their limitations.

Deeply spalled concrete can be repaired with a concrete overlay. Think of this as just applying stucco to the concrete. You just mix up some sand, a little hydrated lime and Portland cement and trowel this onto the concrete. To get this stucco to bond permanently to the spalled concrete, the old concrete must be clean and solid, with all the loose concrete chipped away.

A very important step, taught to me by a very old and wise concrete mason decades ago, is to apply a very thin coat of cement paint to the old concrete before applying the stucco. Mix Portland cement with clean water to the consistency of regular paint. Spritz the old concrete with a little water to make it damp before applying the cement paint. It's important to immediately cover the wet cement paint with the stucco mix.

Concrete overlays can range in thickness from 1/8-inch to two inches. As the overlay gets thicker, you need to increase the size of the sand being used. Once the overlay approaches 3/4-inch in thickness, you can introduce pea gravel about the size of a pencil eraser.

Subscribe to Tim's free newsletter and listen to his new podcasts. Go to: www.AsktheBuilder.com.

Read more:

How to install a concrete driveway or sidewalk that will last

DIY tips on repairing a battered old garage

A do-it-yourself driveway facelift

Loading...