Q: We were pouring a slab for an outside loading area and had a sudden heavy rain on the last day of a two-day pour. We tried to refinish the surface with a grout but it's not holding. The rest of the slab has a smooth troweled finish, and the owner wants the whole slab to look the same. Short of replacing the slab, is there anything I can do?

A.: If you had adequate air entrainment in the concrete for durability, yes there are some possibilities. One of our most impressive exposures to this problem occurred some years ago on a small street paving project. To complete the job, a small fill-in section had to be placed around a curb radius in each of two intersections a block apart. Just after placing, both sections were rained on, but the finishers had time to refinish only one of them before the concrete had set. After one winter, the refinished surface was 100 percent scaled. The surface that had not been refinished had not scaled. It still had the same grainy surface texture that the rain had caused by washing away the surface mortar the year before.

There are two lessons here:

  1. As far as surface durability is concerned, refinishing a wet rained-on surface, unless done with great care, is likely to be worse than doing nothing at all.
  2. If refinishing proves unsuccessful, removing the weak surface material from the hardened concrete should leave an exposed surface that is durable. That assumes, of course, concrete of adequate quality was placed.

A light grinding or scarifying of the surface may work well and be the most economical way to achieve a uniform surface. There is a variety of equipment available, large machines for highway work and smaller walk-behind equipment that would be suitable for your purpose. To achieve uniformity, scarify the surface of the entire slab, both the troweled and the rained-on areas, using a walk-behind scarifier with lightweight cutters. The unsound material will be removed from the rained-on area and the entire slab will have a surface texture similar to a broomed or burlap-drag finish. It should be a reasonably uniform non-skid surface that might be preferred over a smoothly troweled surface that can be slippery when wet. You should be able to match surface textures closely enough that minimum wear and weathering will give you satisfactory uniformity.

Another possibility is grinding only the rained-on surface with a smooth cutter, producing a surface similar to terrazzo. It is as close as you can get to the smooth troweled finish, but it is not likely to be a satisfactory match.

In either case the cost should be well below the cost of replacing the slab. The equipment needed is manufactured by several companies and is available through equipment rental outlets.