Q: On several jobs we’ve been on lately, we have noticed big differences in sealer performance. With so many concrete sealing products on the market today, how do I know which one to use for which job and for different applications?
A: Concrete sealers are designed to provide long-term color enhancement and protection from moisture, ultraviolet light, yellowing, efflorescence, abrasion, and chemicals to concrete surfaces after they have cured—ideally 28 days. Good sealers should last for at least a year or more before reapplication is needed.
However, contractors must understand that no single concrete sealer is appropriate for all projects. Although they may look similar, they are chemically different and are designed for very different purposes. Performance problems usually involve not knowing what type of sealer is needed for specific applications, not properly preparing the surface, and not considering the recommended application conditions.
What follows is a review of the different types of concrete sealers, followed by a brief discussion of surface preparation and application considerations.
There are two broad categories of concrete sealers: penetrating and film-forming.
Siloxanes, silanes, and silicones react chemically with concrete to form a water-repellent layer within the concrete at the surface. By reducing water intrusion into the concrete, it becomes less susceptible to freeze/thaw cycling issues, such as cracking and spalling, and corrosion of the underlying rebar. Essentially invisible, penetrating sealers are good for nondecorative exterior surfaces.
These are typically used for decorative concrete because they often impart a sheen to highlight surface colors and features while forming a protective film across the surface. Like penetrating sealers, film-forming sealers also reduce water penetration into the concrete. But because they form a film on the surface, they allow for easier removal of stains versus uncoated concrete and concrete coated with penetrating sealers.
Film-forming sealers can be broken down into three types, each available in either water- or solvent-based formulations. Each has its advantages and disadvantages.
Acrylics. These offer good protection against sunlight, water, and chloride intrusion while enhancing the appearance of colored, stamped, and exposed aggregate concrete. Generally, 100% acrylic sealers have better ultraviolet light resistance than styrene/acrylic sealers. Acrylic sealers typically are very cost-effective choices. For indoor surfaces, acrylic sealers need regular maintenance to prevent wear and black heel marks.
Epoxies. These sealers produce a tough, long-lasting finish for high-traffic flooring areas that require good abrasion resistance, as well as for concrete countertops. They normally are used for indoor applications because they often are unstable when exposed to ultraviolet light. Although they provide excellent water resistance, many epoxy-based sealers have low impermeability, so they can trap moisture in the concrete. This can cause blistering or delamination of the epoxy sealer.
Polyurethanes. Widely used for exterior and interior concrete floors in high-traffic areas, polyurethane sealers provide very good stain, scuff, and abrasion resistance. They enhance the appearance of most colored, stamped, or exposed aggregate concrete surfaces. Generally the most expensive topical clearcoat sealers, they also require harsh chemicals or sandblasting to remove.
No sealer will function properly if the surface is not prepared according to manufacturer specifications.
The first step is ensuring a clean surface. Remove leaves, loose dirt, dust, and other materials that could compromise sealer performance. Sweeping and power washing usually will suffice, but removing residual curing membranes, stains, and efflorescence may require using solvents, sandblasting, or acid etching. If acid etching is used, the surface should be thoroughly rinsed afterward to neutralize the pH before applying the sealer.
The other surface consideration is proper porosity, which affects the sealer’s ability to wet out, or perfectly cover, and adhere to the surface. Hand-troweled concrete surfaces are generally porous enough, but machine-troweled surfaces may require light sanding or acid etching to make a dense concrete surface sufficiently porous. Again, be sure to follow manufacturer recommendations.
Once the surface has been properly prepared, you may apply the sealer. It is important to consider the environmental conditions for which the sealer has been designed. Some respond poorly to moisture and high pH, which is why they should not be applied before concrete has cured for about 28 days. Poor adhesion or appearance may result if you apply the sealer too early.
The best way to avoid such problems is to be aware of how long the concrete has had to cure. If 28 days have passed and ambient conditions have been dry with low humidity, problems are less likely to develop. If this is not possible, try using sealers designed for “cure and seal” applications, as they have been designed to be applied across concrete less than 28 days old.
As for temperature, follow instructions. Many water-based sealers specify an application range between 50° F and 90° F. Colder temperatures slow drying/curing. Therefore, it often will take a longer time to develop hardness at colder temperatures. In the case of water-based sealers, applying below the “minimum film formation temperature” ultimately can lead to cracking of the sealer, or even freezing of the water if the sealer if applied below 32° F.
If applied at higher temperatures, the drying and curing of the sealer proceed more quickly. For a two-part sealer, a very warm environment can lead to reduced working time, or pot life, of the sealer because the mixture begins to thicken prematurely. Under these conditions, spider webs may come off the roller or spray tip during application. Fast drying under very warm conditions also may lead to skinning and subsequent bubbling/blistering of the sealer.
A basic understanding of the different types of concrete sealers and their applications is the first step toward successfully using these versatile materials. Proper surface preparation and application procedures of an appropriately selected concrete sealer will almost always result in trouble-free performance. When in doubt, always follow the manufacturer’s recommendations.
Visit www.dowconstructionchemicals.com for more information.