Although painting concrete is the term most often used, it's not correct. Coating is the proper term for products that provide decoration and protection for exterior concrete walls; sealers or coatings are the products used on floors. Lorella Angelini, marketing manager for wall coatings and concrete repair for Degussa, Shakopee, Minn., says that the thickness of paint finishes is only 2 to 4 mils, while coatings build up to 10 to 16 mils.
David White, a technical director for Sika Corporation, Lyndhurst, N.J., says that acrylic resins are extensively used now as the primary bonding ingredient in coatings for concrete because they have good water-vapor transmission rates, good adhesion characteristics, and can be pigmented.
But the acrylic formulations for concrete coatings are special.
Elastomeric coatings are also part of the same family. Leo Chippy, a marketing manager for Pratt & Lambert Paints, Cleveland, says these coatings are widely used because they can handle a certain amount of building movement without developing cracks or losing bond, and they can bridge small cracks in concrete surfaces.
Here's what you should expect from a good coating:
- It should be breathable. Water vapor should be able to move from the concrete through the coating to the air at the same time that it repels liquid water.
- It should have good resistance to wind-driven moisture.
- It should perform well on moderately alkaline surfaces.
- It should be UV-resistant and protect concrete from carbonation and chloride penetration.
- It must be able to withstand movement resulting from freeze/thaw activity.
- It must have enough viscosity to build up the required thickness, close pores, and not run.
- It can bridge dynamically moving cracks.
Both solvent- and water-based coatings are available, but the industry is moving solidly in the direction of water-based materials. White says that water-based products emit fewer volatile organic compounds (VOCs) than solvent-based products, making them safer to work with and enabling them to satisfy even the most stringent environmental regulations.
Concrete poses special problems
Compared with other building materials, concrete has higher levels of both moisture and alkalinity. Coatings must therefore be formulated to be compatible with the pH level and moisture content of concrete.
The pH of concrete is related to the portland cement and pozzolans used in the mix. On the surface, the pH is highest shortly after it's placed and decreases with time as the surface carbonates. The pH is also related to the moisture content of a slab—concrete with higher moisture levels tends to have a higher surface pH. Cured concrete can go from neutral (pH 7) to highly alkaline (pH 12 or 13). The pH that coating products can handle varies. Sherman Balch, Balch Enterprises, Hayward, Calif., a tilt-up contractor, says his company tries to install finish coatings for the outside faces of its panels when the pH is close to neutral. “When the pH is over 9 we install a primer first and then apply decorative coatings,” he adds. Depending on the product, primers can be applied to concrete with pH numbers as high as 13.
The moisture content in concrete is more important for floor installations than for walls. You should measure the relative humidity of floors before installing coatings, and it's a good idea to test walls, too. Balch's tilt-up business is in an arid part of California, and he typically waits 30 days before coating walls. He also warns owners not to paint the inside surfaces of his panels for a year afterwards so that moisture doesn't get trapped.
The devil is in the details, and the details are all in the preparation of the concrete. The challenge is to provide a surface free of bond breakers, dirt, and laitance. Coatings must absorb a little bit into concrete surfaces in order to achieve the proper bond. Here are some recommended steps:
- Patch and repair cracks, spall marks, and unsightly surface areas.
- White recommends following the International Concrete Repair Institute's “Selecting and Specifying Concrete Surface Preparations for Sealers, Coatings, and Polymer Overlays” guidelines. The CSP 3 profile (shown, p. 50) is the correct one to use. The profile for floors should be cut by light shotblasting. On walls, contractors frequently use pressure washers with the tip held close to the work.
- Perform a water absorption test to ensure that all bond breakers and contaminants are gone. Put water on the surface and watch to see if it's absorbed quickly into the concrete surface.
- Perform a pH test. This can be done using a pH pencil. If the reading is too high for the coating being used, allow further curing or apply a primer designed for high pH conditions.
- Perform a relative humidity check to measure the moisture content of the concrete. For walls, the level should be approximately 85% or lower. The manufacturer should state the product's requirement.
- Temperature is important when coatings are applied. Materials should be between 60° F and 75° F in order to spread properly. Ambient temperatures of 45° F to 85° F should be observed during the application.
- At least two applications are advised. White says the second coat fills any pinholes that are left by the first one.
Where they are used
We tend to think of decorative concrete as stamped, chemically stained, or stenciled. But coatings for precast and tilt-up building construction are huge markets for decorative coatings. It's a primary way to add decoration to outside wall surfaces and provide protection for concrete from the weather.
To receive information about guidelines for preparing concrete to receive sealers, coatings, and overlay, contact the International Concrete Repair Institute at 847-827-0830 or visit its Web site at www.icri.org.