Coating failures are typically caused by inadequate surface preparation. For a coating to bond properly, the concrete surface must be sound, clean, free from surface defects and dry. The surface should also be properly roughened to establish a good mechanical bond. Failure to properly prepare the surface typically results in adhesion failures or the coating separating from the concrete wasting both time and money.
Before applying a decorative or protective coating to a vertical or horizontal concrete surface, prepare the surface by cleaning and roughening, repairing defects, inspecting and testing the surface for soundness and cleanness. Because surface preparation requirements vary from one job to the next and for different types of coatings, be sure to read and follow the recommendations provided by the coating manufacturer. Failure to follow the manufacturer's recommendations may void the manufacturer's warranty but, more importantly, may cause the coating to fail.
Perform surface survey
Before starting the job, examine and test the surface to be prepared for the presence of debris, dust, dirt, oil, grease, laitance, mortar splatter, efflorescence, curing agents, sealers and other contaminants. Any of these contaminants will prevent the coating from properly bonding to the concrete.
Test concrete for oil, grease, curing agents and sealers by performing a water absorption test. Place a few drops of water on the concrete. If the water beads up, then a surface contaminate exists. If the water absorption test is inclusive, place a few drops of diluted muriatic acid on the surface. If there is no visible reaction (foaming), then a surface contaminate exists. Next, try to determine the depth of the containment by progressively sanding or grinding the surface until the surface freely absorbs water or a reaction occurs with drops of acid. If a surface contaminate exists, it must be completely removed either chemically or mechanically before applying a coating.
For concrete floors, test the surface for laitance by scraping the surface with a knife. Laitance is a weak layer of cement and fines brought to the surface by bleedwater. If a powdery material can be scraped from the surface, excessive laitance is present and a coating won't bond properly.
If you anticipate using water to clean a floor surface, conduct a moisture test of the existing concrete to establish a baseline floor-moisture condition. Several common moisture tests are described later in this article.
If efflorescence exists on the surface, perform a moisture investigation because efflorescence may be evidence of a moisture problem that can cause future coating failures. Efflorescence is a deposit of salts, usually white, that forms on the surface of concrete related by moisture movements through the concrete. If efflorescence or a moisture problem exists, it must be addressed before applying a coating.
Know surface-condition requirements
Know and understand the project specifications for surface preparation and required roughness. Roughness is sometimes called anchor profile and is achieved by mechanically abrading or acid etching the concrete surface. Project specifications should specify how rough the surface must be before a coating can be applied. Specifications also may address surface uniformity, strength and dryness.
If the specifications give a minimum surface strength to ensure concrete soundness, use the field test procedure outlined in ASTM C1583-04e1. This test requires epoxying a 2-inch diameter steel or aluminum disk to the concrete and measuring the tensile force needed to pull the disk from the surface. The test also can be used to evaluate the need for and adequacy of surface preparation and relative differences in surface strength over the area to be coated. Typically, specifications require a minimum surface, or pull-off strength of 175 psi (Ref. 1) to 300 psi (Ref. 2). Expect variable strengths from this test method.
Read and understand the surface preparation directions and recommendations provided by the coating manufacturer. If the manufacturer's recommendations aren't compatible with the project specifications, discuss the discrepancies at the preconstruction meeting.
Schedule a preconstruction meeting
Meet with the project engineer, architect and owner before the job begins to discuss equipment selection, surface preparation and cleaning techniques, surface profile and coating requirements, and time schedules. Submit a work plan to the engineer for approval. Also, establish the means and frequency of testing and acceptable test results for surface soundness, cleanliness and roughness. If the preparation procedure requires water for cleaning, discuss the effect of drying time on the project schedule. Establish the means and frequency of testing for moisture content and values for acceptable test results.
For interior work, establish acceptable levels of noise, dust and fumes. For elevated slabs, ask about weight and vibration restrictions that might affect the choice of surface-preparation equipment or the work plan. Present a plan for wastewater and debris removal.
Clean the surface
Remove all loose dust, dirt and other debris by sweeping, vacuuming, air blasting or water hosing. When sweeping, use a clean stiff-bristled industrial broom. If vacuum cleaning, use a heavy-duty industrial vacuum that will remove all dust from the surface. For air blasting, use a compressed air stream at 80 to 100 psi, delivered through a nozzle held about 2-ft from the surface. Be sure the air stream is free of oil. If water cleaning, use clean, potable water at a pressure high enough to remove dust, dirt and water-soluble surface contaminants. It may also be necessary to hand or power-scrub the surface with a stiff-bristled brush. These methods are not intended to alter the surface profile, only to clean the surface.
Scrape or steam off heavy deposits of oil, grease or other loosely attached contaminants. Then use detergent or a non-solvent emulsifier and a stiff-bristled brush to remove any remaining contaminants. Don't use solvents to remove oil or grease because they can spread contaminants over a larger area or wash them deeper into the concrete. A good cleaning solution is trisodium phosphate mixed with hot water. Use at least 4 ounces of trisodium phosphate per gallon of water. To remove animal fat, use a 10% solution of caustic soda (sodium hydroxide). Be sure to flush treated areas with plenty of fresh water until all residue of the cleaning solution is removed and the pH of the rinse water meets acceptable levels as recommended by the coating manufacturer.
Clean and roughen the surface
When removing elastomeric coatings, use special coating-removal machines designed to shave off these thick, flexible materials, and then continue with standard surface-preparation techniques.
Remove thick overlays of dirt, laitance, efflorescence, mortar splatters, incompatible curing compounds and form-release agents, old coatings and unsound concrete by mechanically abrading the concrete surface. Abrading the concrete surface will alter the surface profile of the concrete, creating a roughened and textured surface for better coating adhesion.
Use scarifiers to remove high spots or grinders to reduce or remove slight surface irregularities. Be sure to thoroughly remove all debris and dust.
For horizontal surfaces, use shotblasting to clean and roughen in one step. Light shotblasting creates a 4 to 10-mil profile height or up to a Concrete Surface Profile (CPS) 3; medium to heavy shotblasting can create profile heights from about 40 mils to over 1/8-inch or from CPS 3 to CPS 8 (Ref. 3). Be sure to compare the coating thickness to the surface profile height. If a thin-film coating (7 to 10-mils) is applied over a 20-mil profile, the coated surface will be rough and uneven. It may be necessary to apply one or more primer coats to smooth the surface. Be sure to review the coating manufacturer's recommendations to establish the required surface profile.
Use acid etching only for areas where no alternative cleaning methods are possible. Etch with a 10-90 to 20-80 dilution ratio of commercial-grade hydrochloric (muriatic) acid in water, applied at a rate of 1 quart per square yard. If chlorides are not allowed, use solutions of citric acid (20%) or phosphoric acid (15%). Citric and phosphoric acids do not contain chlorides and will not corrode embedded reinforcing steel. Apply the acid solution using a low-pressure sprayer or a sprinkling can. After the foaming action subsides, flush the area with water and scrub the surface with a stiff-bristle brush. An area not showing a foaming action indicates a contaminant is blocking the acid from the concrete. Etching should create a profile similar to the texture of fine sandpaper or CSP 1 to CSP 3.
Repair surface defects
Chip, grind or bushhammer fins, mortar splatters or other protrusions from concrete wall surfaces. Remove all projections greater than 1/16 inch.
Remove unsound concrete and clean and fill holes, spalls, cracks, honeycombed areas and other surface defects with a commercial Portland-cement-based grout, epoxy compound or other proprietary patching material. Repair all defects wider than about 1/8-inch. When chipping along edges of repair areas, don't create a featheredge. Chip the edges so they are perpendicular to the surface or preferably sawcut the edges, forming a slight undercut.
Obtain specific recommendations from the repair-material and coating manufacturers about material compatibility and surface-preparation procedures. Don't coat patches before they have cured unless the manufacturer states otherwise.
Inspect and test the surface
Test for dust by wiping the surface with a dark cloth. If a powder is on the cloth, then the area is too dusty and must be recleaned. Sprinkle drops of water on the dried surface. If the surface is free of oil and dust, the water will spread out immediately instead of forming droplets. If droplets form, then the surface is too oily or dusty.
If acid or other chemical solutions were used to clean the surface, use pH paper to determine the acidity at the concrete surface. Repeat the flushing operation if the pH level is less than 4.
Test for moisture using one or more of the following methods:
1. Tape tightly to the concrete surface an 18 x18-inch sheet of clear polyethylene film, approximately 4 mils thick, and allow it to remain in place for at least 16 hours. This is a standard test described in ASTM D 4263-83. If moisture collects under the plastic, most likely the moisture content of the concrete is too high and the coating will fail.
2. To measure the rate of moisture-vapor emission, place a measured amount of anhydrous calcium chloride in a dish inside a transparent plastic cover that's sealed to the floor described in ASTM F1869-11. After 60 to 72 hours, weigh the calcium chloride to determine the amount of water absorbed.
The moisture-vapor emission rate is calculated in pounds of water per 1,000 square feet per 24-hour period. Compare the measured rate of moisture-vapor emission to acceptable values provided by the coating manufacturer.
3. Use a surface moisture meter and compare the measured value to the coating manufacturer's recommendations.
Prepare a trial area to check the adequacy of the surface preparation. Apply the coating to a typical prepared area under the same conditions of surface moisture and ambient temperature that will exist during the actual coating application. After the coating has cured, check the bond. For recommended test methods and acceptable test values, ask the coating manufacturer. Also use test patches to evaluate different surface-preparation methods to determine what method works best.
Standards and References
The following ASTM standards can serve as useful guidelines when you are preparing concrete surfaces. To obtain copies of these standards, visit the ASTM Web site www.astm.org.
Standard Test Method for Measuring Moisture Vapor Emission Rate of Concrete Subfloor Using Anhydrous Calcium Chloride (ASTM F1869-11)
Standard Practice for Surface Cleaning Concrete for Coating (D 4258-05)
Standard Practice for Abrading Concrete (D 4259-88)
Standard Practice for Liquid and Gelled Acid Etching of Concrete (D 4260-05)
Test Method for pH of Chemically Cleaned or Etched Concrete Surfaces (D 4262-05)
Test Method for Indicating Moisture in Concrete by the Plastic Sheet Method (D 4263-83(2005))
Test Method for Indicating Oil or Water in Compressed Air (D 4285-06)
Standard Test Method for Tensile Strength of Concrete Surfaces and the Bond Strength or Tensile Strength of Concrete Repair and Overlay Materials by Direct Tension - Pull-off Method (ASTM C1583 -04e1)
1. ACI 503R-93, Use of Epoxy Compounds with Concrete, Reapproved 1998, American Concrete Institute, www.concrete.org
2. ASTM C811-98(2008) Standard Practice for Surface Preparation of Concrete for Application of Chemical-Resistant Resin Monolithic Surfacings, ASTM International, www.astm.org
3. Selecting and Specifying Concrete Surface Preparation for Sealers, Coatings and Polymer Overlays, Guideline No. 03732, 1997, International Concrete Repair Institute, www.icri.org
Registered Professional Engineer: Wyoming, Colorado, Arizona, Texas, Rhode Island, Utah, Washington, Massachusetts
B.S. & M.S. in Structural Engineering from Virginia Polytechnic Institute & State University
Ph.D. in Civil Engineering from the University of Wyoming.
Activities and Experience
Dr. Basham is a member of the Cold Weather Concreting Committee, ACI 306, American Concrete Institute; American Society of Civil Engineers; The Concrete Society, American Society of Concrete Contractors; and National Society of Professional Engineers.
He is a certified Trainer and Examiner for the Concrete Field Testing Technician and Concrete Flatwork Technician & Flatwork Finisher programs sponsored by ACI.
Dr. Basham has taught structural analysis, concrete and masonry design, and concrete material courses at Virginia Military Institute, the University of Wyoming and the University of Colorado at Denver. He has been an instructor in many concrete seminars and short courses directed towards architects, engineers, and contractors across the United States and abroad.
He has published over 100 articles and coauthored Excavation Safety published by The Aberdeen Group. Dr. Basham has also served as an expert witness in numerous litigation and arbitration cases concerning concrete construction and engineering.
Dr. Basham specializes in structural analysis, concrete and masonry design, formwork and shoring, temporary structures, concrete forensic investigations, structural evaluations, nondestructive testing, concrete repair, concrete construction techniques and concrete technology.
Kim D. Basham can be reached by phone at 307-635-7240
© 2012 L&M Construction Chemicals, Inc. | ConcreteNews Summer 2012.