Concrete parking surfaces often require protective coatings to extend their service life. In contrast to other building elements of urban commercial and residential infrastructure, parking structures present maintenance challenges due to their unique exposure conditions. Additionally, more than most buildings, parking garages rely on concrete as both a structural element and finished surface.
Whether they're inside of residential or commercial buildings, or part of an independent structure, traffic-bearing surfaces of concrete parking decks can be exposed to conditions more akin to pavements and highway bridge decks than to most concrete floor slabs.
Parking structures need coatings for the same reasons other types of buildings do—to protect the walls from moisture penetration. Built-up membranes of bituminous material are typically used during construction to waterproof below-grade exterior walls. Spray-applied liquid rubber with elastomeric properties has become available over the past 10 years. While the rubber systems can cost as much as four times that of the bituminous systems, their ability to bridge cracks can be invaluable.
Above-grade walls and columns
Although walls and columns are often left exposed, owners may choose to apply coatings to improve appearance, increase durability, and enhance light reflectance. Any decorative coating should be alkali-resistant and designated specifically for use on concrete.
Where the potential for graffiti is a concern, owners may opt to apply one of several types of anti-graffiti coatings to walls, columns, and bulkheads. These clear coatings are designed to prevent spray paints, markers, grease pencils, or other media from penetrating the concrete surface. Maintenance employees can then remove graffiti by following the coating manufacturer's instructions (often using a designated cleaning agent).
In our fieldwork we have noticed that the specifier calls for three main classes of graffiti-resistant coatings: solvent acrylics, epoxies, and polyurethanes.
Acrylics offer relatively low material cost and are easy to apply, reapply, or repair after cleaning graffiti. Disadvantages of acrylics are their lesser stain resistance and durability as compared to epoxies and urethanes.
Epoxies form a tough, hard film with excellent resistance to solvents and abrasion. However, they discolor when exposed to sunlight and can be difficult with remove or recoat once cured.
Polyurethanes also form a tough, hard film that is highly resistant to solvents and abrasion. They are available in clear or pigmented formulations that offer good gloss and color retention. Among their drawbacks, the application and curing process is sensitive to temperature and humidity conditions. Polyurethanes can also be difficult to remove or recoat once cured.
It is the parking structure's traffic-bearing surfaces that pose the greatest challenge and investment for owners and facility managers. Unlike other elements of buildings, a parking structure deck in temperate continental and marine climates may be exposed to environmental and operational conditions that impair its life prematurely.
The direct use of de-icer salts in an unheated, open structure or salt spray dripping from vehicles promotes corrosion of reinforcement. Further, garages in northern climates are prone to internally-initiated degradation of concrete, which freezes and thaws while it is wet. Therefore, the designer, contractor, and owner of a parking structure must anticipate and plan operations and maintenance to successfully resist service conditions nearly as severe in northern climates. These plans often resemble the maintenance plans established for a highway bridge deck.
A parking structure's maintenance and rehabilitation strategy varies generally with the age of a structure; coatings and sealers are often an important element in this strategy in addition to other available techniques. Quality products, properly applied, can resist or substantially reduce the penetration of moisture and de-icer salts into concrete. The service life of a parking structure can be greatly improved through judicious use and maintenance. The range of available coatings and sealers is diverse, and their performance, service life, and maintenance needs are well understood, based on decades of experience gained by concrete contractors and facility owners nationwide. Generally speaking, they can be organized into the following classes, presented in order of increasing application cost and material service life:
- Surface sealers
- Penetrating sealers
- Thin bonded overlays
- Traffic-bearing membranes
Surface sealers, such as epoxy and polyurethane floor sealers, produce coatings or films on the concrete surface. Suitable only for pedestrian and low-volume vehicular traffic areas, these sealers have low resistance to abrasion. Their material cost is relatively low (about 50¢ per square foot) but they need to be reapplied frequently to remain effective. Caution is urged when considering them for traffic-bearing surfaces.
Penetrating sealers comprise materials such as silanes and siloxanes. Silanes and siloxanes are part of a broad group of silicone-based water repellants. These compounds penetrate measurably into the concrete surface and react with the silica in the concrete to create a layer that repels water and is a barrier to chlorides. Penetrating sealers do not create a vapor barrier. Penetrating sealers are relatively inexpensive ($1 per square foot), but they do require reapplication since their effectiveness diminishes with time. These materials have been known to last three to five years between applications.
Thin bonded overlays are 1/8- to 3/8-inch-thick applications of epoxy, polyester, or methyl methacrylate polymers mixed with graded aggregates. They form an extremely hard surface that is impervious to water penetration but will not bridge cracks that are active or form after application. Because polyesters and methyl methacrylates cure fast even at low temperatures, they are often used during cold weather or when downtime must be minimized. These coatings cost $4 to $6 per square foot installed, and have a service life from five to ten years.
Traffic-bearing membranes are liquid-applied in layers of epoxy and/or urethane polymers. These systems are relatively thick (20 to 100+ dry mils) and extremely strong, with high surface hardness, bond, tensile capacity, and tear resistance. They have the ability to bridge moving cracks and can be loaded with aggregates to build thickness and improve surface traction, abrasion resistance and durability. Installed at a typical cost of $5 to $7 per square foot, these membranes are typically warranted for five years and can remain serviceable for much longer, when properly maintained.
Cyler Hayes is a CTLGroup senior chemist specializing in evaluation and testing of sealer, coating, and admixture systems. He has a professional background of nearly 15 years in manufacture and troubleshooting for sealers, cure and seals, curing compounds, and textured coatings. Contact him at firstname.lastname@example.org.
Scott Tarr, P.E., CTLGroup principal engineer has been involved in problem-solving distressed industrial slabs on grade, parking decks, pavements, and concrete slab moisture issues. He has evaluated more than 25 million square feet of concrete floors, and is a frequent seminar speaker at World of Concrete events. Contact him at email@example.com