Despite strenuous efforts to give concrete a good finish, sometimes the finished product has significant imperfections. Surely neither a carpet placed on it nor a vinyl floor will cover all imperfections, so making a repair that corrects the problem is the best option for both the builder and the homeowner.
Calculate the scope of the problem. Possible problems may include:
- Ridges or mounds left by builders.
- Cracks due to expansion or contraction. The properly installed concrete must have joints that prevent these unpleasant cracks. In the case of a 4 inch thick slab, the joints should have a separation of 8 to 12 feet. If the joint is not enough or is placed incorrectly, cracks may occur.
- Waste like pieces of wood sticking out of the concrete while it is still malleable.
- Damage caused by heavy objects that have fallen on the block before it has finished setting.
- Elevation of the surface due to improper finishing or freezing temperatures during the curing process. Elevation occurs when parts of the concrete surface come off.
Use a measuring tape to determine the length and width of the area to be repaired if it is not self-evident. This can include mounds, depressions or ridges. You can disguise the small surface areas with shallow irregularities with a “floor leveling compound”, instead of lifting and replacing the existing concrete.
Chop the surface of the concrete to remove all “lift” in the material, and to create a rough surface so that the cement mix will bond well. If the work is small, a chisel hammer will suffice, but for larger repairs it will be better to use an electric hammer or even a jackhammer.
Clean dust and debris from the area you have chopped. This way the new material will have a solid surface to adhere to.
Mix the cement and sand you will use to fill the area you have chopped. Mix the dry ingredients first, using a ratio of 1 part of cement (type I or II) to 2 ½ parts of “clean” masonry sand.
Add a liquid latex binder or polymers to dry the cement / sand mixture, using enough to wet the material and bring it to a consistent but malleable state. Note that some binding agents are applied directly on the patching area, such as glue, so follow the manufacturer’s instructions.
Wet the area where you will apply the new material with fresh and clean water. Do not flood the area, but make sure that all surfaces are wet, as this helps in the adhesion of the new cement mixture, as well as preventing it from drying too fast. Dry surfaces simply “remove” moisture from the cement mix too quickly, which will cause it to contract, and eventually break.
Take a “spoonful” of cement mix from the mixer with a “tipped mason’s spoon” (or some other appropriate tool). Place the mixture inside, or over the patch, and push it hard to remove all air bubbles.
Level the wet surface with the mason’s spoon, leaving it slightly higher than the adjacent edges, to allow it to settle and contract. Allow to dry and harden for an hour or so, depending on humidity and room temperature.
Smooth the area with a steel finishing spoon when the patch material is consistent. This will flatten and soften the surface and cause the cement paste or cream to “rise” to the surface. If the surface is very large and deep, you can smooth it with a magnesium paste to get more cement cream up. This paste or cream is the material that forms the surface of the finished block.
Let the concrete “settle” or harden for an hour or two more and then finish smoothing it. At this point, you may need to splash a little water on the surface to delay drying, in addition to making the finishing process easier. Use the edge of the spoon to “shave” or scrape the splashes of mixture close to the finished surface. Another good choice of materials to repair are hydraulic cements. They usually settle in less than 30 minutes.
Clean the tools and remove all excess material.
- Mix enough cement and sand to finish the entire patch in one go.
- For smaller areas, you can buy fortified polymer material or dry pre-mix to patch concrete.
- To fix the high points on a concrete floor that will be covered by another floor, you can use a grinder equipped with a masonry grinding wheel.
- Once the repair is done, keep it wet for a few days, if possible. Use a lot of water.
- Protects the repair of direct sunlight. A cardboard box will suffice.
- High lime cement is more flexible and may be more appropriate for repairing concrete.