Concrete Floors, Surface Preparation

Successfully coating concrete requires proper surface preparation. Coating concrete can be a challenge, especially if the surface is not prepared correctly.

Combine the increased use of concrete as a construction material with a general increased interest in color and decorative finishes, and it is likely that at some point, residential and commercial paint contractors will face the challenge of coating concrete. And coating concrete can be a challenge, especially if the surface is not prepared correctly. For the coating to bond properly, the concrete surface must be clean and sound and have a profiled surface rough enough for a good mechanical bond.

“Inadequate surface preparation causes 95 percent of all the problems associated with coating concrete,” says Lindy Ausburne, technical advisor of Euless, Texas-based Golden Look International. “Many contractors are familiar with the coatings but don’t fully understand the surface they are working on.”

Acid Etching Concrete
When treating exterior surfaces with an acid etch begin by pouring one part muriatic acid into five parts water. (Always wear suitable protective rubber gloves and eye wear when working with acid) Start by applying mixture with Hudson Sprayer to a 10’ by 10’ square. (Contractors often make the mistake of biting off too big of an area and not treating the complete surface adequately) If treated surface begins to turn yellow your acid mixture is being applied too heavily. It is best if your mixture produces more of a white foam as this will lead to the best surface profile. It is also important to scrub the treated surface with a suitable nylon brush prior to rinsing. Acid should be allowed to remain on the surface for 3- 5 minutes then rinse thoroughly. When treating the next 10’ by 10’ square be sure not to allow acid onto the treated areas. When pressure washing exterior surfaces use a minimum of 1500 psi (hot or cold), a 15o tip, cleaning a maximum distance of 3” to 6” from the surface with a 3” pattern and a 50% overlap. The recommended profile is the feel of 150 grit sand paper.

The basic procedure for acid etching is to mix the acid using a 30-70 dilution rate (30 percent muriatic, phosphoric, or other acid solution to 70 percent water), and to use a garden sprayer can to sprinkle it on the surface. A floor-scrubbing machine can work the solution into the concrete and break free what the acid is etching. After this process is completed, contractors should neutralize the acid and rinse the surface thoroughly. The most important part of etching is to apply the acid uniformly. Some experts recommend wetting the surface before applying the acid to create a damp but not wet, area. “The object of etching is to get floors or any other surface to a texture of 150-grit sandpaper,” says Peterson.

It’s best to work in small areas, normally 100 square feet, to allow the solution to be removed by vacuum before the acid dries and leaves a residue. “Never allow the acid solution to dry on the floor. That’s where you get residue and then you have to re-etch. If a residue remains, the coating bonds to it instead of the concrete,” says Peterson.

Important Safety Note: Remember to always pour the acid into the water and never the water into the acid.

Clay Peterson, product engineer for Commerce, Calif.-based Pearl Abrasives agrees. “If you don’t get down to a clean sound surface it doesn’t matter what you coat the concrete with it won’t adhere.”

Examine the Surface
All agree the first step in prepping concrete is to conduct a survey of the concrete surface.

“The surface should be checked for existing sealers, curing materials, grease, oil, efflorescence and any dirt that needs to be removed,” says A. Ray Anderson, owner, Thousand-Oaks, California-based, Only Under Pressure, a company that specializes in concrete surface prepping and coating systems.

Dirt and dust can be removed by air blasting, brushing with a stiff fiber or wire brush, hosing, or scrubbing. For extremely dirty surfaces, wet or dry sandblasting, water blasting, or steam cleaning may be needed.

Laitance and efflorescence, fine powder-like materials, can be removed with a steel scraping tool, scarifying, acid etching or shot blasting. Grease and oil should be removed with a 10 percent solution of caustic soda, trisodium phosphate, or with detergents that are specifically formulated for concrete. Steam cleaning is also an effective method for treating grease and oil on concrete surfaces. “A common and useful method of cleaning concrete is to use a rotary floor machine with an aggressive nylogrit scrubbing brush and an alkaline cleaner,” says Dean Owen, president of of Glendale, Arizona-based Arizona Polymer Flooring, Inc.

Contractors not sure which products previously coated the concrete can find out more by pouring alcohol on the concrete. If it softens the coating, the product used before was likely water-based, which could be coated with either solvent-based or water-based epoxy. If the alcohol has no effect but xylol softens the finish, an alkyd coating was probably used. In such a case, the surface must either be coated with a water-based epoxy, or will require a barrier coating between the existing coating and solvent-based epoxy.

Sprinkling water on the concrete surface to see if it beads up will determine if there are any clear sealers or waterproof coatings. If it does bead up, there’s either a sealer or oil on the concrete. “A good way to test for clear sealers is to pour a 25 percent muriatic acid on the surface. If the concrete does not react in the presence of the acid, a sealer is present and must be removed,” says Owen.

Measuring Moisture and Vapor Transmission
The next step is to measure the moisture content and moisture vapor transmission rate of the slab.

“When coating new concrete, failures may result because the alkaline moisture interferes with the bonding properties of the coating. Therefore, it is wise to test the moisture condition of the slab before coatings are applied,” says Jerry Colins, Concrete Engineer, Portland Cement Association.

The curing period for new concrete may vary with environmental conditions, location, type and thickness of the concrete, and generally, slabs on the ground require a longer curing time than suspended slabs.

Guidelines established by the American Concrete Institute suggest poured concrete be cured for 28 days at a temperature of 70F to achieve maximum hardness and strength. But environmental factors such as high humidity can effect the drying times. (It is well advised that contractors have the client sign a waiver if they insist on having the concrete coated if it has cured fewer than 28 days.)

The best way to determine if new concrete is dry enough to coat is to test the moisture conditions. However, choosing the proper placement of a test is important. Dark spots on concrete surfaces can indicate areas that are high in moisture content. Performing Ph tests on these areas, although important for determining potential problems, generally don’t provide accurate overall Ph levels. Before using a test, the coating manufacturer should be consulted as to which test is required for a particular product.

A standard test described in ASTM D 4263-83 uses an 18 x 18-inch polyethylene sheet approximately 4 mils thick. The sheet is taped tightly to the concrete and remains in place for 16 hours. If no moisture accumulates on the underside of the plastic film the slab is considered dry enough for most coating applications.

Moisture meters are also available to measure relative humidity or moisture content. The meter uses electrical resistivity to determine moisture content of the concrete at or below the surface. To use a moisture meter, two -inch-diameter holes are drilled three-quarters of an inch apart into the concrete to a depth of which the moisture level is to be checked. Nails are then driven into the holes and the two pins of the electrodes are attached. The subsurface moisture content is read from the meter.

Moisture content and Ph levels can also be determined by using a pH pencil. This common method requires wetting the surface with distilled water and marking the surface with the pencil. After 10 minutes the color of the pH pencil mark will change and by comparing the color to a chart the pH value of the substrate can is determined. Typically higher greens and blue colors indicate higher pH values.

Excess moisture-vapor transmission through the slab is one of the biggest problems. “Vapor transmission is a leading cause of concrete coating failure. Moisture comes in many forms like condensation, sweating or from puddling under the slab and eventually leads to substrate deterioration, delaminating of the floor coating systems, and ultimately premature concrete floor coating failure,” says Owen. “Moisture-vapor transmission exceeding 15 percent will limit, if not prohibit, the application of proper concrete floor coatings.”

To measure the rate of moisture-vapor emission, a measured amount of calcium chloride can be placed in a dish inside a transparent plastic cover that is sealed to the floor. After 60- 72 hours, the calcium chloride is weighed to determine the amount of water absorbed. The moisture-vapor emission rate is calculated in pounds of water per 1000 feet per 24-hour period. Measured moisture-vapor emission can be compared to acceptable values provided by the coating manufacturer.

Repair surface defects
Structurally the concrete should be solid and any cracks and holes repaired.

Unsound concrete should be removed and holes, spalls, cracks and other surface defects filled with a commercial Portland-cement-based grout, epoxy compound or other proprietary patching materials. Coating manufacturers should be contacted regarding the compatibility of any patching material with the floor coating system.

“Chip or grind mortar splatters or other protrusions from the concrete surfaces and remove all protrusions greater than 1/16 of an inch,” says Anderson.

Profiling the surface
Next, it is necessary to create a profile on the concrete surface-that is, roughing up the surface to mechanically bond with the coating. The most popular choices for this task are shot blasting, acid etching, sandblasting, scarifying and grinding.

Shot blasting hurls tiny steel particles at the surface, while sand blasting sprays sand particles at the concrete. It hits the surface at a high velocity and the shot or sand and dust bounce off the surface and into a recovery chamber. Because blasting produces little dust, it takes little time and is a dry process that requires little clean up and is a popular choice for surface profiling.

But despite the advantages, shot blasting and sand blasting are not without problems. To create a uniform surface across the floor requires great care be taken. “Make sure the machine travels at a uniform speed to create a uniform profile, which isn’t a problem on power driven machines. It’s the manual machines where problems are likely to occur since they may dig out more concrete than intended,” says Peterson.

Done properly, shot and sand blasting can remove some contaminants and profile the surface in one process. Note, however, that heavy contaminants like grease and gum will still need to be removed prior to shot blasting.

Most contractors recommend a profile test patch to determine how the concrete surface will react to the profiling process. And after the coating is applied to the test area, the contractor can get a preview of the finished product before proceeding with the entire job.

Brush blasting or grinding can create a 4-5 mil profile height, while standard shot or sandblasting can create profile heights exceeding 20 mils. Be sure to gauge the coating thickness so it’s in proportion to the surface profile height. If a thin-film coating (6- 10 mils) is applied over a 15-mil profile, the coating surface will be rough and uneven.

Scarifying mechanically scrapes the concrete. The scarifier turns in circles and scars or scratches the concrete. As with blasting, the difficulty is to maintain a constant depth of profile when using larger scarifying machines.

Whether using acid etching, (See sidebar) shot blasting, sand blasting or scarifying, contractors should take precautions before applying a non-breathable coating. According to Anderson, profiled surfaces should be allowed to dry four to five days before surfaces are coated. “Cutting away the top surface of the concrete opens pores in the concrete, allowing water vapor to escape more readily. To avoid the formation of bubbles and craters, consider applying multiple primer coats or apply the coating at night when less water vapor tends to be drawn out of the concrete,” says Anderson.

It is important to note that some stains and coatings are not recommended for use after an acid etch. Therefore it is always important to carefully consider the best and most effective means for concrete surface preparation. Knowing the characteristics of concrete and closely following the coating manufacturer’s recommendations for surface preparation, contractors can help eliminate costly call backs which leads to greater customer satisfaction.

ADVERTISERS