Poor Drainage Is Causing Sidewalk Discoloration

Question:
I am coordinating property improvements for a homeowner in the Los Angeles area. A concrete walkway poured by a licensed contractor less than a year ago is showing efflorescence and strange color variances. The sidewalk is tinted a Santa Fe color and is 4 inches thick, with a brushed finish. I was told by the contractor that it would take awhile to cure and to expect it to do all sorts of "weird stuff" while it was curing. But now the walkway is mottled and looks like a pinto pony. The contractor said the uneven color was because overhead sprinklers on a hill above the walkway were keeping it too wet. Also, the wetting was uneven across the length of the project. I revamped the sprinklers so they have almost no contact with the walkway, and waited for it to dry out. Some of the pinto effect did fade, but the concrete still has dark areas. The contractor said he is thinking about doing a light vinegar wash to help with the problem.

Chris Sullivan
Located at the foot of a hillside, this sidewalk is showing mottling and efflorescence due to poor drainage and over-irrigation.		 Chris Sullivan
A close-up of the pitting, showing the brownish ooze seeping up from the holes.

Recently, I noticed an even bigger problem: pitting! An amber-colored goo is coming up from below the surface through round areas about the size of a quarter, where the concrete has broken off. On closer inspection, you can see small rocks in the middle of some of these pits, while other pits have nothing in them but are very unsightly. The area also has a few star-shaped popouts. This pitting problem is sporadic along the entire 125-foot length of the sidewalk, but it is more pronounced in some areas. The areas that got less or no overspray from the sprinklers have less pitting. Are the discoloration and pitting problems caused by the sprinklers, or is there something more to it? Please tell me what can be done with this mess.

Answer:
You have multiple issues going on, so I will address each individually. First, the cause of the discoloration is pretty straightforward. As your contractor said, it is due to moisture. Because the sidewalk sits at the bottom of a pretty steep hill that is irrigated year-round, moisture is migrating downhill and soaking into the concrete. Even if you stopped watering altogether, you would still have ambient moisture in the soil moving down and affecting the concrete. Magnifying the problem is the fact that the concrete sits at the bottom of the hill, where the water concentrates. Making matters even worse is that you have irrigation and plants on the house side of the concrete as well, so it gets wet from both sides. How much sun does the concrete get? It looks pretty shaded, which does not help the situation. You stated that you changed the irrigation system, with some improvement. But the only way to get rid of the discoloration is to stop the flow of water. Since it is unrealistic to stop irrigating, you need to install some type of moisture block (such as a plastic barrier with a French drain) to keep the water from touching the concrete. A less-expensive option is to apply a special waterproofing sealer to the concrete. That will help, but it won't completely stop the water migration.

The efflorescence problem is also caused by moisture. Stop the water, and you stop the white haze of efflorescence from forming. You could remove the efflorescence by using the mild acid cleaner, or vinegar wash, as your contractor recommended, but unless the water is stopped, the discoloration and white haze will return. See this guide to efflorescence and the measures you can take to prevent it.

Chris Sullivan
Recommended alterations to the landscape to improve drainage around the sidewalk.

Finally, the pitting appears to be caused by alkali-silica reaction, or ASR. In a nutshell, ASR is a complex chemical reaction between water and certain silica aggregates in concrete that causes expansion forces exceeding that of the concrete matrix. ASR or varying forms of it can show up as random cracking or popping and pitting of the concrete surface. This link to information from the Portland Cement Association will tell you more about ASR and its causes. This article from Concrete International magazine is also useful.

Here's the bottom line: The location of the sidewalk, the water migration, and the makeup of the concrete are all converging in a perfect storm to cause these problems. My recommendation is to use a lithium-based penetrating concrete sealer, along with some landscape modifications. The sealer will help fill the pores of the concrete to slow or stop the flow of water from below the sidewalk to the surface. This will help to prevent the white efflorescence and even out the color. First, a good cleaning will be needed to prepare the concrete to take the sealer. Refer to the recommendations provided by the sealer manufacturer. I would also cut back the hillside and position the retaining wall blocks on the soil, rather than on the concrete (see drawing). Re-grade the ground so that the blocks sit lower than the concrete, and that area becomes the low point. Then install a French drain in the new low spot to help move water away.

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