Q.: I had a concrete driveway poured 7 days ago, and noticed a couple days ago small pop marks throughout the concrete. The concrete finisher spoke with the ready-mix producer, and the salesperson said it could be lignite. Do you have any suggestions on what should be done with my concrete? Will it continue to pop? Is there some type of sealer that needs to be put on it? Should I have it torn out and redone? I live in Kansas City, Mo., and the concrete was poured early on a summer morning, before the temperature hit 100º. For the next couple of days a water sprinkler was on it to keep the concrete cool.
A.: The small pop outs that are occurring on your new concrete driveway are sure to be an irritation. Nobody wants or expects that kind of thing when they have a new driveway installed. From what you describe, and if there was indeed lignite in the concrete, it sounds like the problem stems from the fine aggregate (sand) used to make the concrete.
Lignite is sometimes found in natural sand. The amount varies, depending on the quarry and the particular deposit. When sand containing lignite is used in making concrete, lignite particles near the surface can expand and cause the pop outs you describe. Although it sounds as if the placement and curing methods were appropriate, working the concrete can move the lignite toward the top. At first you’ll be able to see the small, dark lignite remaining at the bottom of the pop outs. But because it is relatively soft, it will soon wash away.
There is a standard limiting the amount of acceptable lignite in fine aggregate, ASTM C33, “Standard Specification for Concrete Aggregates.” It addresses both strength concerns and the concrete’s fitness for the purpose, which includes appearance issues. Although the standard allows up to 1/2 percent “coal and lignite” (for pavements), it permits a higher percentage if the strength is not adversely affected and some other non-expansive criteria are met.
How you deal with this problem depends on how unsightly you find the finished product. If you have one pop out per square foot, it might not be worth pressing the issue. But if you’re averaging a dozen pop outs per square foot, it might be worth the aggravation you’re sure to encounter if you try to have the contractor or producer remove and replace the concrete. If you pursue that, you might want to find a consulting engineer who specializes in concrete and will probably end up taking core samples for analysis.
The strength of the slab should not be adversely affected by the lignite content. Applying a sealer may help keep additional water from getting in and expanding any more of the lignite. If you choose to try that, choose an acrylic sealer that has good moisture vapor transmission qualities.