A guide to decorative concrete treatments and coatings.

Published: Monday, Oct 26th 2009

Journal of Architectural Coatings Jun-Jul 2009 · Vol. 4, No. 6: 20-29 CONCRETE FLOOR COATINGS AND TREATMENTS: A GUIDE

Gray? No way!

In This Article:

Fundamental lesson No. 1: The nature and chemistry of the concrete medium
Preparation methods
Coatings and sealers
Concrete stains
Other coloring methods and materials
Polished concrete
Polymer overlays

A varied palette of color and design options creates a host of choices for the concrete floor Joe Maty Editor, JAC.

Concrete, in a chromatic variation of the one-note theme of Henry Ford’s uniformly black Model T, may have once given the impression of being available in any color the customer wanted—as long as it was gray.

But unlike Mr. Ford’s everyman conveyance, concrete in the 21st century is anything but uniform in color and appearance, and nowhere is this bold new reality more dazzingly apparent than in the contemporary concrete floor.

Simply put, it’s amazing what they can do with concrete floors these days, thanks to advances in coatings, stains, concrete polishing, and other decorative methods, materials, and artistry.

Here, in this “Concrete Floors 101” review of coatings and treatments, we present a survey of these technologies, gleaned primarily from the extensive coverage given this subject area in JAC over the past several years. For this, we can acknowledge the contributions of an authoritative roster of industry experts.

Reactive stains were used to create color and design elements on the concrete floors of a restaurant (left) and a retail store (right). Photos courtesy of Increte Systems.
Fundamental lesson No. 1: The nature and chemistry of the concrete medium

Before we explore the various types of concrete-floor coatings and treatments currently offered in this highly diverse portfolio, it may be useful to gain a basic understanding of the nature of the concrete floor or slab, and the keys to successful application of these materials.

The horizontal concrete surface may appear to be a relatively monolithic, static mass, but this impression can be highly deceptive thanks to the dynamic forces exerted by moisture, surface profile, and surface chemistry1. Thus, applying coatings or other floor treatments and coverings can present serious challenges to long-term performance of the given treatment and the concrete substrate itself.

Typically, newly placed concrete should cure at least 30 days to allow the substrate moisture level to fall sufficiently. In addition, external sources of water in the surrounding environment can adversely affect the substrate and applied coatings and treatments in the short and long term. Sources of moisture can include groundwater, poor drainage around a structure, and joints that are inadequately sealed and allow moisture to penetrate.

A high level of alkalinity, or pH, in new concrete can also be problematic, although pH will decrease over time as the concrete cures and the moisture level abates.

The moisture level in horizontal concrete can be measured with a number of different test procedures, such as the plastic-sheet method, the calcium-chloride test, and with moisture-meter instruments1. These methods are detailed in ASTM standard test methods, including ASTM D4263 and ASTM F1869.

If excessive moisture is found to be a problem, the situation can be addressed in many cases by allowing additional time for concrete cure. In situations where moisture presents an ongoing issue, more aggressive mitigation action may be required, such as installing drainage systems to remove excess moisture and prevent moisture entry.
Preparation methods

Once the question of moisture is managed, the matter of needed repair and surface-preparation procedures takes center stage. Degradation of the substrate in the form of cracks, spalls, and voids should be remedied with appropriate repair materials to provide a surface that is sound and intact. These repair materials should be specifically formulated for use on concrete and for the given service environment. Products of the acrylic chemistry type should generally be based on 100% acrylic resins, and not on acrylic-vinyl or polyvinyl acetate resins. Materials containing polyvinyl acetate may degrade in moist, alkaline environments, a common condition in floor slabs, particularly those situated below grade1.

With specific regard to the matter of concrete surface preparation, any formulation of a successful program begins with an assessment of the degree of preparation needed for the specific coating or treatment. In any case, the surface must be cleaned of any contaminants using agents consisting of detergents recommended for use on concrete. Common cleaning and preparation methods include pressurized washing with water and mechanical abrasion to facilitate coating adhesion. Preparation methods for application of stains, however, differ from processes employed for application of conventional coatings.

Acid etching is also sometimes employed on the surface to create surface roughness as a way to facilitate coating adhesion1. A thorough rinsing is needed following acid application to remove excess acid and reaction products, and scrubbing of the surface may be required. The use of acid etching, however, may be avoided due to concerns about the surrounding environment or spent-water disposal.

Pressurized water cleaning can serve as an adequate surface-treatment method in situations where coatings of other treatments do not require a great deal of surface roughness. Water cleaning is effective in removing water-soluble contaminants, weak concrete, and degraded existing surface coatings.
Coatings and sealers

If the primary objective of applying a coating or treatment to the horizontal concrete surface is protective in nature, sealers and film-forming coatings are often recommended as a way to significantly extend the service life of the substrate. These materials can prevent water, soluble salts, and other contaminants from penetrating the inherently porous concrete substrate, and can enhance chemical resistance and protection against damage from freeze-thaw cycles2.

For the purposes of this review, the term sealers refers to penetrating or thin-film water repellants, while the term coatings refers to thicker film-forming products that provide barrier protection to concrete.

As mentioned earlier, surface cleaning and preparation are critical to successful application and performance of coatings, sealers, and other treatments. These processes should include the removal of efflorescence—deposits of soluble salts from the substrate on the surface of the concrete—and laitance, a thin layer of weak or unreacted cement that migrates to the surface during installation or finishing. Other contaminants such as salts, oil, grease, or other hydrocarbon substances can also interfere with successful coating application, and must be removed from the surface.

Sealers are generally intended for use in above-grade applications where concrete is frequently exposed to moisture, and where a thicker, film-forming coating is not considered a desirable option. Sealers are designed to resist penetration of water, but are formulated to be permeable to transmission of water vapor from the underlying substrate. This water-vapor permeability, or “breathability,” is important in preventing moisture from being trapped in the substrate, where it can cause defects or other problems. Sealers penetrate the pores of the concrete, leaving little measurable film on the surface.

Silicon-based products—silanes, siloxanes, and silicones—represent a major class of materials defined as sealers or water repellants. Other sealers are based on epoxies and linseed-oil solutions.

A variety of film-forming coatings are used for floor surfaces, including acrylics, vinyls, epoxies, polyurethanes, and specialized elastomeric linings. Alkyds and other oil-based coatings are generally not recommended, as the resin may chemically react with alkaline compounds and moisture in the concrete, degrading the coating.

Application of penetrating and thin-film sealers does not require mechanical abrasion or etching of the concrete surface. Such processes are recommended, however, to facilitate adhesion of film-forming coatings.
Concrete stains

Stains used to color and decorate concrete floors include chemically reactive products, non-chemically reactive stains and dyes, and opaque, non-penetrating stains.

Chemically reactive stains, also called acid stains, are water-based acidic solutions that contain metallic salts. These solutions react with calcium hydroxide that is formed in the concrete during hydration of the concrete mix3. The greater the calcium hydroxide in the concrete, the more potential there is for coloring with chemically reactive stains. The reacted color compounds become a permanent component of the top layer of the concrete surface.

Coloring of concrete using chemically reactive stains can produce varied results, depending on a number of factors. These factors include cement properties and the content of the concrete mixture; aggregate type; the presence of chemical admixtures; finishing and curing methods; and moisture content during stain application.

In cases where a hard-troweled concrete surface is installed, successful application of chemically reactive stains may require additional abrasion or stronger acid solutions to “open” the surface to stain penetration.

Non-reactive stains and dyes encompass a range of translucent, mottled color materials for concrete, and are based on color concentrates mixed with water or solvent4. They do not produce color by means of a chemical reaction with the concrete; rather, the color concentrate’s ultrafine pigment particles are transported into the concrete surface by the water or solvent carrier.

Surface cleaning and preparation are crucial to successful application of non-reactive stains and dyes, as the material must penetrate the concrete surface rather than dry on the surface like a paint or coating. These non-reactive stains are recommended for use on concrete in situations where chemically reactive stains cannot produce the desired color due to a limited palette, or when the composition or condition of the concrete does not lend itself to the use of chemically reactive stains. Also, custom colors are more easily obtained with non-reactive stains.

Non-reactive stains may also be preferred in situations where concerns about the safety or environmental effects of acid-based materials are a factor.

Not all non-reactive stains and dyes are UV stable, and thus may be unsuitable for outdoor use. Application of a sealer is recommended with the use of both chemically reactive and non-reactive stains.

Opaque stains are offered in a broader range of colors than is the case with chemically reactive and non-reactive stains, with earth tones and bright and vibrant colors available4. The color produced is also more uniform than the other types of stains, and it hides or masks the underlying concrete surface. These stains are mixed with water, and produce a low-gloss, abrasion-resistant surface. It may not be necessary to apply a sealer following use of opaque stains.

Opaque stains can be used for coloring new (cured 28 days) and existing concrete surfaces, and are well suited for recoloring previously colored concrete or renovating weathered or discolored concrete surfaces.

As is the case with application of any concrete treatment, cleaning and surface preparation are crucial in ensuring successful application of stains and dyes. Scrubbing and, in some cases, mechanical abrasion is required to remove dirt, contaminants, and existing coatings or sealers that would impede penetration or adhesion of stains on existing surfaces. Newly placed concrete should be allowed to cure properly, and then pressure washed or scrubbed and rinsed. Also recommended is testing of water penetration to determine whether the stain will penetrate properly.

Application techniques for the different types of stain are somewhat varied, and can include spray, roller, and brush methods. Maintenance programs also are important in ensuring long-term appearance qualities of concrete surfaces colored with stains4.
Other coloring methods and materials

In addition to the stain materials described previously, concrete floors can be colored and decorated with integral colors and dry-shakes. Texturing and color combinations are also employed to add decorative flair to concrete surfaces3.

Integral colors or pigments are blended with the concrete mix to produce color throughout the newly placed concrete surface. These pigments are finely ground minerals available in a broad range of colors, and can be blended to produce custom colors.

Combinations of stains and other decorative effects can be used in interior and exterior settings. Here, the design pattern and color were the result of concrete stamping and application of stain.

Dry-shakes are typically a combination of pigments, cement, fine aggregrate, and proprietary ingredients. These products are broadcast over the freshly finished concrete surface, usually in two or three applications, and are then floated into the surface to mix with the cement paste and densify it, acting to color and harden the topmost concrete layer at the same time.

Textured concrete-floor surfaces can be floated, broomed, or troweled. Floated and broomed surfaces are generally recommended for exterior settings, due to the slip resistance properties they provide, even when wet. Troweled surfaces are more suited for interiors that will be primarily dry.

Texturing techniques also include stamping patterns to give the impression of natural stone or brick pavers, and grooves or sawed joints can be used to add definition of shapes and create patterns. A variety of effects can be created on concrete with combinations of decorative approaches that include the use of exposed aggregate, integral color, shake hardeners, stains, and dyes3.
Polished concrete

Concrete polishing involves a sequence of steps that includes grinding, application of hardeners/densifiers, and polishing with machines employing diamond grit-impregnated discs. The technology produces a surface that is durable, attractive, and highly reflective.

The polishing process encompasses a five- to 10-step procedure that begins with initial grinding of the surface with large-grit diamond-impregnated discs. The process employed can be of the dry or wet type, with different methods employed to remove the concrete dust and particles generated during the grinding process.

Following the initial grinding stage, a specialized hardener/densifier product is applied to the concrete surface. The densifier chemically converts the weak calcium hydroxide and calcium carbonate compounds in the concrete to calcium silicate hydrate, or CSH5. This CSH is insoluble in water and resistant to water, acids, and other chemicals. The result is a hard, dense, sealed, and abrasion-resistant surface.

Densifier products used in the polished-concrete process include lithium silicate, sodium silicate, and potassium silicate chemistries.

The polished-concrete surface process is completed with subsequent polishing with finer-grit diamond discs, and optional application of a topical or penetrating agent to immediately seal the surface until the densification/hardening reaction is complete.

This walkway design was created with concrete stamping, integrally colored concrete, and color-release materials that contributed to the look of stone and brick.

Polished concrete has received positive reviews as a “green” and sustainable design and construction technology, due in part to the relative permanence of the densified and polished concrete surface. Also earning “green” points for the technology is the resulting concrete surface’s durability, ease of maintenance, and “structure as finish” condition, with no overlay, topping, coating, or additional floor covering required6. The process also generates zero or near-zero VOC emissions, a plus for indoor air quality.

Polished concrete also results in heightened reflectivity, offering the potential for the use of less artificial lighting, and the thermal mass of the concrete enhances passive solar heating and night “flushing” for cooling effect.
Polymer overlays

Cement-based polymer overlays—a blend of polymer resins with cement and aggregate—are applied over concrete in layers that can range in thickness from several mils to several inches. Decorative overlays can be based on acrylic or epoxy resins, which are often blended in proprietary formulas to produce specific characteristics. Overlays can be used to cover deficiencies in existing concrete surfaces and transform plain-looking concrete floors or slabs into visually appealing floors, walkways, patios, and plazas.

The use of polymer resins in these overlay materials enhances the performance characteristics and versatility of cement, facilitating application in relatively thin layers. Polymer cement overlays also exhibit enhanced resistance to chlorides, petrochemicals, UV exposure, weathering, and traffic wear.

Polymer overlays in interior and exterior settings are used to resurface damaged, pitted, flaking, and stained concrete; to repair and level concrete surfaces that have settled; and to restore and redecorate existing concrete substrates. They can be used to produce various visual effects by means of coloring, texturing, stamping, and combinations of techniques.—JAC

    1.  “Some hard truths on concrete coatings and coverings,” Jayson L. Helsel, P.E., JAC, Jan.-Feb. 2007, p. 70.
    2.  “The cold hard truth on concrete sealers and coatings,” Jayson L. Helsel, P.E., JAC, Jan. 2006, p. 56.
    3.  “Coloring of concrete: Transforming a gray area,” Jamie Farny and Terry Collins, JAC, June-July 2008, p 22.
    4.  “Concrete stains and dyes: Surveying the color spectrum,” Howard Jancy, Butterfield Color, JAC March-April 2009, p. 18.
    5.  “Getting up to speed on concrete polishing,” Mark B. Vogel, JAC, Dec. 2008, p. 16.
    6.  “Going green, whatever that means,” Joe Maty, JAC, Oct-Nov. 2008, p. 12.

Journal of Architectural Coatings ©2009 Technology Publishing Company