Can You Fix Concrete That Has Hardened Before Being Finished?

Walls

• Pouring concrete foundation walls is a major task because the entire structure should be poured continuously to avoid cold joints that can weaken the foundation. If you don’t have enough workers to vibrate the concrete and smooth the top, the pour can get away from you. Fortunately, as long as you get the forms full, the problems will likely be limited to air voids in the finished walls and an uneven surface. You can use masonry cement to fill the voids and chisel away high spots on the tops of the walls, since this area will be covered with a sill plate.

Low-Traffic Slabs

• On a hot dry day, concrete can begin to set within 15 minutes, so you must plan ahead and work quickly. For porches, patios and other low-traffic slabs, as long as you were able to rough grade the top and the slope is correct, you can fix an ugly surface by pouring a cap. This requires building a form about 1/2-inch higher than the current surface and filling it with a vinyl patch masonry product.

Driveways

• Pouring a cap isn’t recommended for a driveway because heavy vehicles can crack it, causing it to crumble. If you were able to screed the surface level and the slope is correct, you’ll probably have to put up with cosmetic surface issues. If you end up with lumps and bumps, the only solution is to tear out the bad sections and repour. If part of the driveway is fine, use a concrete saw to cut a straight line separating the bad from the good and break out only the bad section.

During a Pour

• Sometimes, you’re in the middle of a pour when you realize the concrete is setting up too quickly. When this occurs, shift your focus immediately to salvaging as much of the pour as you can. For walls, settle the concrete by tapping rapidly and forcefully with a mallet or hammer on the inside and outside forms. For slabs, finish as much of it as you can, and let the rest go. This might mean quickly screeding a section, floating it and troweling it. If you’re able to get the concrete smooth, but it’s setting up before you can float it, spray it with water, trowel it and brush it with a stiff push broom. Even if you can fix the surface, your driveway's life may be reduced if you were unable to settle it and float it correctly.