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Don't try to place an overlay on a slab that looks like this.

My sister-in-law called me recently to complain about the driveway she had installed a couple of years ago. Since I had encouraged her to use concrete and the surface was now spalling—probably from freeze thaw damage due to a lack of entrained air—I guess that made it partly my fault. I hate hearing about failed concrete, don't you?

Surface failures like this are one of the most common ailments with concrete, whether caused by freeze-thaw action, aggregate popouts, or corroded reinforcing steel. Many surface failures on concrete slabs are little more than cosmetic—the concrete remains functional but looks bad or has a chalky surface.

As with any repair, the first step is to figure out the cause of the failure. For example, if a slab has cracked due to poorly compacted subbase, an overlay will simply crack again. But if the subbase is well compacted, then the repair should be durable.

Concrete Walkways
Concrete Coatings of Illinois
Crest Hill, IL Concrete Walkways
Concrete Coatings of Illinois
Crest Hill, IL Thin overlays can resurrect damaged concrete. Concrete Coatings of Illinois

When faced with a surface repair, investigation will soon reveal how aggressive the repair needs to be:

  • Full-depth repair--cutting out the damaged concrete, replacing reinforcing steel, and replacing the concrete.
  • Partial depth repair—cutting around the edges of the damaged area (but not into the reinforcing steel), removing the damaged concrete, cleaning or replacing the reinforcing steel, then placing a repair mortar.
  • Overlays—effective to cover up surface problems—so long as good surface preparation is used.

Surface scaling from freeze-thaw action is all too common. Potomac Construction Industries

Spalled Concrete Videos

Understanding Spalled Concrete
Time: 05:56
Watch this easy-to-understand explanation of what causes concrete spalling, from expert Chris Sullivan.

Repair Spalled Concrete
Time: 05:24
Watch this easy-to-understand explanation on repairing spalled concrete, from concrete expert Chris Sullivan.

Which brings us to the topic of surface preparation. More repairs fail because of poor surface preparation than from any other cause. All unsound concrete must be removed, leaving sound, clean, and suitably roughened surfaces. For any overlay installation or surface repair, what constitutes "suitable roughness" should be specified using ICRI's Concrete Surface Profiles (CSP). This is a series of nine roughness levels ranging from acid etched to heavy scarification. The best way to know what each of these CSP levels looks like is to get a set of plastic chips from ICRI that simulate the surface profiles.

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Edco
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Edco
, Surface preparation machines can remove unsound concrete and leave the proper surface profile. Edco

There are a variety of ways to remove unsound concrete, including high-pressure water jets, sand blasting, grinding, scarifying, and, of course, old-fashioned impact methods, such as with scabblers or jack hammers. The greatest danger with impact methods is the bruising of the concrete surface—even though the surface may look like solid strong concrete, the impact methods create a zone of microcracking. Learn more about surface preparation.


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