Harsh weather, inadequate subsurface preparation, or simple ageing can all result in problematic, water-collecting low spots on a concrete surface. In the past, the only practical way to repair a concrete pad which was damaged in this way was to take a jackhammer to it and replace the entire piece. Fortunately, simpler and less expensive options are now available. However, concrete does not naturally adhere well to concrete, so filling in a low spot requires careful preparation to prevent the patch from flaking away.
- Skill level:
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Things you need
- Trisodium phosphate (TSP)
- Power washer (optional)
- Polymer-modified concrete resurfacer, such as Quikrete Concrete Resurfacer or Ardex - 20-pound bag
- 5-gallon bucket
- Electric drill
- Mixing paddle
- Flat trowel (for small areas) or long-handled squeegee (for large ones)
- Push broom
- Clear, waterborne masonry sealer
Clean the concrete pad with TSP so that the patch material adheres properly to the pad. Read the manufacturer's mixing instructions, as they may vary. Typically, the dilution used for heavy-duty cleaning is 1 cup TSP to 3 quarts warm water. TSP can be applied with a sponge or brush or sprayed on. Rinse the cleaner away with plenty of clean water. A rented power washer can make both the TSP application and rinsing easy. Allow the cleaned surface to dry.
Pour 2.5 quarts of water into a clean 5 gallon bucket. Add a 20-pound bag of concrete resurfacer and mix continuously for 2 minutes with a heavy duty mixing paddle attached to an electric drill. Once the slurry is mixed, you typically have 30 minutes to spread it before it begins to set.
Fill the low spot on the concrete pad with the resurfacer mixture, spreading it with a flat steel trowel for small areas or a long-handled squeegee for large ones. Press down hard to force the slurry into every crevice.
Draw a push broom lightly over the wet surface to create a textured- slip-resistant surface. Allow the patch to dry. If the patch looks noticeably different from the rest of the concrete pad and you'd like to hide the repair, clean the entire surface of the pad and apply a thin slurry over the whole surface. After the surface has cured for 24 hours, apply a clear, waterborne masonry sealer to protect the new surface.
Tips and warnings
- For best results, do this repair on a low-humidity day with an air temperature of 21.1 to 23.8 degrees C.
- TSP is a strong base which can cause damage to unprotected eyes or skin. Wear appropriate clothing and eye protection when working with TSP.
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