Question: We have several large spalls in a concrete retaining wall behind our building. They're near the top and don't appear to have affected the structural integrity of the wall, but they don't look good. How do I fix them?
Answer: Numerous patching and repair materials are available that should suit your needs. You should consider several questions before beginning. What is the root cause of the damage? Where is the rebar and is it corroded? How deep a patch will you need and how large must it be? Are there particular surface preparation requirements for the repair material you're considering? What are the weather conditions likely to be when you do the repair? How quickly must it be finished? The answers to those questions will influence your selection of repair materials.
When you begin the hands-on part of the repair, sound the concrete to determine the extent of the problem. This can be done with a hammer, or on horizontal surfaces, with a chain. Remove the unsound concrete and be sure to repair the edges of the area you're repairing. Sawcutting can be good for that. Remember that if possible you don't want to have feather edges on your repair. Also, avoid abrupt changes in width or depth of the repair area.
Good surface preparation is a must in any concrete repair. Be sure the surface is free of loose material and contaminants. For example, an oil spill of any kind under the repair material will lead to problems down the road. Clean it or remove it.
Read and follow the manufacturers' instructions on mixing the material and how to apply it. Finally, be sure to properly cure cement-based products.