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| Systems that dispense and retrieve only necessary amounts of water or cleaning solution will help to preventover saturation and an extended drying time. |
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Concrete Surface Preparation
It looks clean, but is it?
by Joan C. Stanus
Once you’ve got a handle on the slab’s character and condition, and have repaired and solved any problems, the next step is to clean and condition it thoroughly. To ensure a bond between the topping and the concrete, the slab must be absolutely free of dust, dirt, oil, grease, paint, curing components, coatings and all other contaminants.
To determine if a surface is clean, use the “water drop test” by applying a small amount of water onto the floor. If the water doesn’t penetrate into the pores, neither will a coating.
Sometimes cleaning a slab may be as simple as scrubbing the surface with an aggressive nylogrit brush and then applying a commercial cleaner. The surface can then be rinsed with a pressure washer and allowed to dry.
Pressure washing works well for chemicals and oils. Degreasers worked in with a floor scrubber/polishing machine can adequately remove dirt and grime. Detergents and cleansers can also be effective as long as the grease and contaminants are water soluble or emulsifiable by the detergent or cleanser. They work best for removing superficial grease, although more than one treatment is often necessary. High levels of contamination may require grinding.
The use of water in cleaning concrete should be kept to a minimum. Using excessive amounts of water will generally cause unacceptable time delays on a project because of the extended drying time needed before any coating application can begin.
Removing previous coatings and flooring products depends on the type and nature of the material to be removed. Sometimes a mix of different methods can be especially effective. For thin sealers, concrete grinders or brush blasting with a shot blaster are appropriate. For epoxy, mastic and thick coatings, concrete grinders with aggressive cup wheels or scarifiers can be used.
Machines with carbide-tipped slicers can quickly remove thick coatings. Hand-held or ride-on floor scarpers can effectively remove such materials as carpet, parquet flooring and other thick materials. If the surface has delaminated, shot blasting, scarifying or other chipping methods must be used.
Shot blasting works well as long as the material is not rubbery. If it’s too elastic, the shot will bounce instead of fracture the coating. A boon to using shot blasting to clean the slab is that it also provides a profile to receive the bond coat.
Muriatic acid also can be used to clean and profile concrete surfaces. When using acid, it’s safest to start with a minor dilution of about 20 parts water to one part acid, then work up to more concentrated solutions depending on the type of concrete surface you’re washing. Through advancements in technology muriatic acid is now offered in a gel. This gel keeps the acid on top of the slab where the profiling is obviously needed. Remember, when concrete is acid washed, it must be properly neutralized before putting a topping on. (See sidebar for tips)
By most standards, creating a proper “profile” is an essential ingredient in the job’s eventual outcome. It basically creates more surface area for the topping to adhere to.
“Profiling is real important in our book,” notes Leland of SureCrete. “It’s one of the keys to getting the cementitious topping to bond.”
To secure that bond, the concrete surface should be rough enough so that a “tooth,” or series of peaks and valleys, feels similar to a 120 grit piece of sandpaper.
A deeper profile than that, however, does not necessarily mean a better bond, cautions Al Slater, product specialist with Blastrac.
 “The bonding process can in some ways be considered a microscopic event,” he says. “A deep, coarse profile can contain micro-fractures that can weaken the integrity of a slab, depending on the preparation process used. Some deeper profiles also have the potential to bleed air as the overlay is being applied, causing fish eyes and pin holes that are ugly and difficult to fix.”
Although many in the industry are split between the benefits of roughing the surface mechanically or chemically, Casto, says each has its place.
Profiling the surface mechanically includes grinding, sandblasting, shot blasting and using other tools. Chemical profiling includes acid washing, chlorine washing and other chemical methods to remove surface imperfections.
The preferred method for large or difficult surfaces is mechanical profiling, however. Shot blasting is one of the most popular and cost-effective methods of creating a profile, and is most suitable for 1⁄8-inch or thicker toppings. Scarifying can resurface horizontal surfaces and smooth faulted joints, and cut deep groves into the concrete to create non-slip surfaces. Grinding creates less of a profile but is often used for edges.
Although a critical component of the job, concrete preparation can cost widely depending on the job type, size and geographic location. On average, however, when the expense of equipment is excluded, the cost in preparing concrete for cementitious toppings can cost anywhere from 5 cents to 30 cents per square foot.
As many in the field well know, in the long run, it’s well worth the money spent.
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