When coatings applied to concrete surfaces fail, the cause usually is inadequate surface preparation. For a coating to bond properly, the concrete surface must be sound, clean, free from surface defects, and dry.
The surface also should be rough enough to establish a good mechanical bond. If the surface is not properly prepared, the coating is likely to separate from the concrete, wasting both time and money.
Before applying a decorative or protective coating to a vertical or horizontal concrete surface, review the following recommendations to avoid coating failures caused by inadequate surface preparation.
The checklist addresses cleaning and roughening of concrete surfaces, repairing surface defects, and inspecting and testing surfaces before the coating is applied. Because surface preparation methods can vary from one job to the next, follow the recommendations that best apply to your project.
Conduct a Preliminary Survey
- Examine the surface to be prepared for the presence of debris, dust, dirt, oil, grease, laitance, mortar splatter, efflorescence, curing agents, sealers, and other contaminants.
- Test slabs for laitance by scraping the surface with a knife. Laitance is a weak layer of cement and fines brought to the surface by bleed water. If a powdery material can be scraped from the surface, excessive laitance is present and the coating won't bond properly.
- If you anticipate using water to clean the surface, conduct a moisture test of the existing surface to establish a baseline floor-moisture condition. Several common moisture tests are included later in this checklist.
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 say how clean and rough the surface must be before you apply a coating. Specifications also may address surface uniformly, strength, and dryness.
- If the specifications give a minimum surface strength to endure concrete soundness, use the field test procedure outlined in Appendix A of ACI 503R-93. This test requires e-poxying a standard 1½-inch-diameter pipe cap to the concrete and measuring the force needed to pull the cap from the surface. You also can use the test 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 to 300 psi (ASTM C 811-96).
- Read and understand the surface preparation directions and recommendations the coating manufacturer provides. If the manufacturer's recommendations are not compatible with the project specifications, discuss this at the preconstruction meeting.
Arrange a Preconstruction Meeting
- Meet with the project engineer, architect, and owner before the job begins to discuss equipment selection, surface preparation techniques, surface profile and coating requirements, and time schedules. Submit a work plan to the engineer for approval.
- Discuss the means and frequency of testing and acceptable test results for surface soundness, cleanliness, and roughness.
- If you use water to clean the surface, discuss the effect of drying time on the project schedule. Establish the means and frequency of testing for moisture content.
- For inside 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. Plan for waste-water 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 vacuuming, 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 two feet 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.
- Scrape off heavy deposits of oil, grease, or other loosely attached contaminants. Then, use detergent or a nonsolvent 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 four ounces per gallon of water.
- To remove animal fat, use a 10% solution of caustic soda. Be sure to flush treated areas with plenty of fresh water until you remove all residue of the cleaning solution and the pH of the rinse water meets acceptable levels.
- If necessary, remove heavy deposits of oil and grease by steam cleaning. Use detergent or a nonsolvent emulsifier for extra cleaning power.
Clean and Roughen the Surface
- When removing elastomeric coatings, use special coating removal machines to shave off these thick, flexible materials. Then continue with standard surface preparation techniques.
- Remove thick overlays of dirt, laitance, efflorescence, mortar splatters, incompatible curing compounds and form release agents, the 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 and level slabs, or grinders to reduce or remove slight surface irregularities. Thoroughly remove all debris and dust.
- For horizontal surfaces, use shot-blasting to clean and roughen in one step. Brush blasting or light etching creates a 4- to 5-mil profile height. Standard blasting can create profile heights exceeding 20 mil. Be sure to compare the coating thickness to the surface profile height. If you applyl a thin film coating (7 to 10 mils) 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.
- If preparing surfaces by shotblasting or scarifying, take precautions before applying a nonbreathable coating. Cutting away the top surface of the concrete opens pores in the concrete, allowing water vapor to escape more readily. To avoid 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 because air and concrete temperatures are similar.
- 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 one 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. You can apply the acid solution using a low-pressure sprayer or a sprinkling can. After the foaming subsides, flush the area with water, and scrub the surface with a stiff-bristle brush. An area not showing any foaming action indicates a contaminant is blocking the acid from the concrete. Etching should create a profile similar to the texture of fine sandpaper.
- Where floor hardeners have been used, you must use special methods to prepare the surface. Ask the coating manufacturer for suggestions.
Repair Surface Defects
- Chip, grind, or bushhammer fins, mortar splatters, or other protrusions from concrete 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 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 sawcut the edges, forming a slight undercut.
- Obtain recommendations from the coating and repair material manufacturers about surface prep procedures and material compatibility. For example, determine the cure time for the repair material and whether the patch should have a roughened or steel-troweled finish. Don't coat patches before they have cured unless the manufacturer states otherwise.
Inspect and Test the Surface Before Coating
- 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 you must re-clean it.
- Sprinkle 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 for coating application.
- 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:
- Tape tightly to the concrete surface an 18x18-inch sheet of clear polyethylene film, about 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 (see box). If moisture collects under the plastic, the moisture content of the concrete is too high and the coating will fail.
- To measure the rate of moisture-vapor emission, place a measured amount of calcium chloride in a dish inside a transparent plastic cover that's sealed to the floor. 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 1000 square feet, per 24-hour period. Compare the measured rate of moisture-vapor emission to acceptable values the coating manufacturer provides.
- Use a surface moisture meter and compare the measured value to the coating manufacturer's recommendations.
- Measure the concrete surface temperature before applying a coating, and compare the result to the manufacturer's recommendations. If the surface is too cold or hot, it may adversely affect the coating bond and quality of the job.
- Prepare a trial area to check 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 actual coating application. After the coating cures, check the bond. Unfortunately, there are no standard test methods for evaluating bond strengths of coatings. For the recommended test methods, ask the coating manufacturer. Also, use test patches to evaluate different surface prep methods to determine what method works best.
Kim Basham PhD, PS Senior Structural Engineer, has written several articles on concrete and concrete repair. He is a regular speaker at the World of Concrete seminars. This was originally published in Concrete Construction magazine.