Concrete Floors, Epoxies, Epoxy Coatings
Epoxy coatings are hard working, resist impact, adhere to concrete and good performance in hostile environments. But the most common reason for floor-coating failures is that contractors do not follow instructions.
hard working, resistance to impact, excellent adhesion to concrete and good performance in hostile environments. Epoxies are easy to apply and as such the most frequently specified coating for concrete floors. Yet despite the widespread use of epoxy coatings, manufacturers agree the most common reason for floor-coating failures is that contractors do not follow instructions.
“Working with epoxy coatings requires that contractors pay particular attention to instructions. Mess up either mixing the resins and catalysts or on the drying times and the flooring system will fail,” says Dean Owen, president of Glendale, Arizona-based Arizona Polymer Flooring Inc.
All epoxies are two-component products-Part A is a resin and Part B is a catalyst or crosslinker. When the resin and catalyst are mixed an exothermic or heat-producing reaction occurs which causes the epoxy to begin hardening.
“The majority of product failures result from contractors not following the instructions at the mixing stage. Contractors must scrupulously follow mixing ratios for the two parts—and when it says to stir the product continuously for 15 minutes to homogenize the epoxy, it means the full 15 minutes,” says Owen.
Peter K. Fisher, senior technical advisor for Pleasant Prairie, Wisconsin-based Rust-O-leum agrees, “I have seen cases where contractors have applied the resin without mixing the curing agent. In some special epoxy applications where the instructions call for mixing ratios of 10 to one, contractors might leave out the curing agent or use too little of it and the epoxy is undercatalyzed.”
Time is an important consideration so once the epoxy is mixed start the clock. “It is critical that the contractor know the open time and the time they have to apply the epoxy before it starts to set up. A contractor must also be clear about the time required for the epoxy to cure before adding additional coats,” says Wes Clemmons, sales director for Garland, Texas-based Chem-Coat.
Drying times range from as little as an hour to eight hours or more, depending on the product and the ambient temperature. The benefit of fast-drying epoxy is speed. But slow-drying epoxies are generally less expensive, and there are times when fast-drying products may be too fast. At 70°F the speed may be ideal, but at 100°F, the pot life of the product will be significantly shortened and a slower product would work better.
The two epoxies specified in 99 percent of the flooring systems are high solid and water-based.
High-solid epoxy has solid contents between 80 and 100 percent and are available in high-build, self-leveling, non-slip and anti-skid formulations. High solids provide good chemical resistance and are usually applied at higher film thickness—up to 16 mils—and are particularly useful for filling holes and leveling irregularities in the concrete.
High-solid epoxy is designed to provide a balance of properties including impact and abrasion resistance, physical strength and chemical resistance. The material handles easily and, contrary to popular belief that they are stiff and unruly, high solid epoxies actually flow and level well when properly formulated.
Water-based epoxies are designed to work well on wet or damp concrete and have a quick drying time. These thin (2-3 mil) coatings are used as general-purpose coatings for concrete floors and as primers under epoxy, polyurethane and acrylic materials.
Before beginning a concrete floor coating job, Fisher and Owen agree on the importance for contractors to work with a specialized floor coating representative who is prepared to go to the site, look at the job and write a specification which diagnoses the situation properly.
“At any of these jobs, someone needs to look at the potential applications and make a decision about what problems need to be solved—the environment and flooring system that is best suited,” says Owen.
According to Fisher some questions that should be answered are: Will the coating be required to protect the concrete from chemical attack, or moisture penetration? Will the concrete surface be exposed to spillage of strong or weak acid, alkali or salts? Will the floor be subject to light, moderate or heavy foot traffic, rubber-wheeled traffic or steel-wheeled traffic?
The most important part of any floor-coating job is proper surface preparation. (See the Concrete Surface Prep article in this issue) “Surface preparation is extremely important for floor coatings. The surface must be clean with no grease present. A good surface profile created by acid etching or shot blasting is necessary. I can’t over-emphasize the importance of good surface preparation, because it’s the most time consuming and problematic part of the job,” says Owen.
During the surface preparation stage it is essential to determine the moisture-vapor emission rate of the concrete slab using a calcium chloride test (ASTM E1907).
“Vapor transmission is a leading cause of concrete coating failure. Moisture comes in many forms like condensation, sweating or puddling; and eventually leads to substrate deterioration, delaminating of the floor coating systems, and ultimately premature concrete floor coating failure,” says Owen.
Moisture-vapor emissions exceeding 15 pounds per 1,000 sq. ft./ 24 hours will limit, if not prohibit, the application of proper concrete floor coatings. Left uncorrected, moisture-vapor emissions can create many barriers that will inhibit the sound bonding of the floor coating to the concrete floor. Ultimately, this requires cleaning, stripping and applying new concrete floor coatings.
“Every contractor should include a warranty that addresses moisture-vapor emissions. It protects them if conditions change—a water pipe breaks or a new sprinkler system is installed and water finds it way under the concrete slab causing coatings to fail after-the-fact,” says Owen.
The most common floor coating systems are thin coat protective systems. These coating systems are used in a variety of applications, including light manufacturing and warehouse areas, automotive repair facilities, garage floors and driveways.
Thin film coatings usually involve an epoxy water-based primer and a finish coat; or an epoxy primer and polyurethane finish coat. Thicker applications are applied in three coats-epoxy primer, 100 percent solids epoxy build coat and polyurethane finish coat.
“For high traffic and heavy-wear systems, generally three coats are required. Sandwiched in between the coats, various materials including high silica quartz in assorted sizes and colors or aluminum oxides for ultra-abrasive wear patterns are broadcast,” says Lindy Ausburne, technical advisor for Euless, Texas-based Golden Look International.
“A primer coat is always necessary when coating concrete,” says Fisher. Usually a water-based epoxy is selected because they penetrate the surface and seal the pinholes and voids in the concrete. A common problem that occurs when using primers is that air becomes trapped in the voids and pinholes and if there is an ambient temperature change while putting down the coating, bubbling may occur.
“Little can be done to prevent bubbling in the primer coat. If you notice bubbling, allowing the epoxy to cure properly before the next coat is added will help to eliminate any adverse effects,” says Fisher.
High-solid epoxy is applied by pouring it out of a bucket and using a rubber squeegee to level, followed by backrolling it using a or inch nap roller. One gallon of 100 percent high-solid epoxy at 16 mils thickness will cover 100 square feet. With water-based epoxy, a 3-mil film thickness will cover 300 to 350 square feet, but may vary with less coverage on the primer coat because it penetrations into the concrete.
The clearest lesson is this. Before beginning any epoxy coating job, know-and know well-the surface to which it will be applied and the product you will be using. “A lot of calls come from contractors after they have done something wrong and it is too late to fix it,” says Fisher.