Concrete Surface Preparation
It looks clean, but is it?
by Joan C. Stanus
Knowing what you’re working with — both internally and externally — is a key to successfully preparing a concrete surface for accepting cementitious toppings.
A slab may look clean and sound, but that doesn’t necessarily mean it is.
If unfamiliar with the original pour, contractors almost have to be concrete detectives before they can begin a job that involves applying toppings. Even if the surface looks good, what lurks beneath, if left alone, can break the bond between the concrete and toppings and, eventually, cause long-term headaches for contractors. The solution is to prepare well and eliminate surprises.
“Your system will only be as good as the surface underneath,” notes Dean Owen, president of Arizona Polymer Flooring. “As with all specialty coating applications, proper surface preparation is a crucial element of success.”
“It doesn’t matter how pretty the finished job is, if the topping doesn’t stick, who cares?” adds Charles Leland, vice president of SureCrete Design Products. “If you don’t do the best prep work you can, you risk losing the job or having lots of callbacks.”
Depending on its condition, every concrete surface requires some preparation to create a proper texture, or profile, for applying coatings. A visual evaluation is the first step in determining any problems or concerns with the existing slab and determining the profiling method to use.
“Usually a visual examination will tell you a great deal about the slab,” says Bill Glynn Jr. with Sawtec. “But the most important elements of inspecting a slab are common sense.”
“Walking the floor” to carefully note differences in surface cleanliness, grade and condition is a must. Look for any existing sealers, grease, oil, effervescence, curing materials and dirt that need to be removed. If you find excessive laitance or a weak layer of cement on the surface, the material must be removed down to solid concrete. Coatings and overlays will simply not bond properly to weak concrete.
A few easy tests can help gauge the slab’s structural integrity. Take a tool, such as a screwdriver or nail, and pry up test areas of the surface that appear loose or soft. Strike a ball peen hammer against the concrete surface to indicate hardness. Hollow spots can be determined by sweeping a surface with a metal chain or golf club and listening for pitch changes.
“They will perfectly mirror what’s below,” contends Leland.
“Invisible problems” below the concrete can be especially perplexing. Some indications do exist. “Cow patches,” black and white patterns in the slab, can be a signal that moisture problems are present.
Determining the moisture content and vapor transmission is critical to maintaining the structural integrity of the slab before any toppings or coatings can be applied.
“Moisture is a huge problem when it comes to surface preparation,” notes Matt Casto, vice president of technical services for Bomanite Corp. “It is extremely misunderstood and must be understood to ensure a long-term mechanical adhesion. Moisture can destroy that concrete stability.”
Moisture can basically do three things to concrete. It can hurt the concrete itself when the alkalinity reacts with the aggregate. It can destroy certain polymers, and it can expand to remove toppings from the surface.
Excessive vapor transmission occurs when a water source under the slab is combined with concrete that is unusually permeable. That source could be a high water table, broken water pipe, landscape sprinkler or rain. Overly permeable concrete usually results from a water-to-cement ratio that is too high. The excess water leaves the slab during the curing process and creates capillaries that serve as a pathway for water vapor to be drawn up through the slab.
This vapor collects condensation at the bond line, and that condensation attracts salt and dirt. Eventually, it leads to lessening of the bond.
Notes Casto: “Moisture really isn’t the problem. It’s the alkaline salts that it carries. They react adversely to coatings and toppings and can expand or blow them off, no matter how good the mechanical bond.”
The most cost-effective method of determining if moisture vapor is passing through the slab is the calcium chloride test. The test gives a measurement of the pounds of vapor passing through a 1,000 square foot area in 24 hours. Three or less is considered safe. Other methods exist, but they are often cost prohibitive.
In recent years, manufacturers have begun to introduce products onto the market that can help with solving moisture problems. Applied Concrete Technology Inc., for example, now sells its Protecrete Densifier that penetrates into the concrete to form a permanent gel-like barrier which can never be worn away. In one application, the product holds the hydrates in, thus waterproofing the concrete internally as well as externally.
“Basically it’s a do-all product,” notes David Johnson, president of Applied Concrete. “It’s essentially fixing the concrete.”
It’s a fact of the business: All concrete surfaces crack. Improper sub-base compaction, jointing practices, poor finishing and unsuitable curing can contribute to unsightly random cracks. It is important to repair cracks because, if left, they will spread and cause concrete to become unstable. Coatings, too, will fracture along joints or cracks that are not properly addressed.
The key in making repairs is to determine the kind of crack you’re dealing with. A cosmetic crack, which does not go all the way through the slab, will be addressed differently than a structural one.
Cosmetic cracks are usually easiest to handle. The contractor should “v” cut the crack and seal it with a hard filler material. Structural cracks are more difficult. Contractors often square cut the crack, and then apply a flexible epoxy resin or sealer. Because there are so many variables to a structural crack, these should be diagnosed on a case-by-case basis, as those in the industry recommend.
Most cracks that occur on slabs are not moving cracks, but some do move or vary in size because of seasonal differences in temperature, moisture levels, stress loads and other factors. Contractors should “honor” those moving cracks by designing their system around them.
“You should let the floor move,” says Sawtec’s Glynn. “Fighting a live crack, attempting to prohibit its movement or, worse, ignoring the fact that it will move, is a losing battle.”
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