Understanding cementitious decorative overlays

by Katie Daniel | December 10, 2015 2:41 pm

[1]

By Giovanni Natale
Overlays were born in the modern era out of necessity. (For simplicity’s sake, and for the purpose of this article, the term ‘overlays’ will be used instead of ‘polishable self-leveller’ or ‘wear-course self-levellers.’) When concrete needed to be refurbished and restored, but could not be elevated to the decorative concrete status, another material was needed to fill the void. Thanks to manufacturers’ research and development labs, formulations of engineered cements, commonly known as overlays, were developed. Overlays exhibit similar characteristics to concrete, but in a much thinner version—typically 9.5 to 50 mm (3/8 to 2 in.). Designers have come to expect overlays to look and perform similarly to concrete.

Many components within the decorative overlays are very similar to those of concrete—mainly the sands, portland cement, and the esthetic. These similarities cause concrete and overlays to be classified as the same family of building materials. Overlays are designed to give a concrete substrate its intended functionality and offer optimal performance.

Concrete has historically been used as a supporting material for structural applications, as well as a bed or substrate for embellished coverings. Only recently has the popular term ‘decorative concrete’ evolved and led the industry to think more in terms of the material as a finished product, rather than solely as a supporting structural building material. This is a new tool in the design professional’s toolbox that can complement certain décor styles. What does one need to know about these construction materials to use them in project design?

Any time concrete is to be transformed and used as a decorative final wear course, the concrete must be evaluated to ensure it is in suitable shape. Designers should check the uniformity of the existing concrete to ensure it is acceptable, or learn whether it needs to be repaired. However, patching and repairing concrete often leaves a sporadic, discoloured look that is not esthetically pleasing.

Concrete may be unusable in terms of decorative applications when:

• cracks needing repair cannot be camouflaged and will stand out because of their shapes and colour difference once repaired;
• spalls or rough concrete in need of repair shows visible colour variations and non-symmetrical shapes once repaired; and
• additions of new concrete slabs adjacent to old ones show colour differences as well as highlight the aggregates different concretes contain.

These are only a few reasons why the existing concrete may not be suitable for a decorative makeover. Whenever concrete is unable to be directly treated and refined into a decorative floor, the use of a cementitious overlay can be a solution.

[2]

Overlays are designed and manufactured to restore concrete to its original condition—similar to what it may have looked like the day after it was poured and finished. From that point, numerous other decorative options are possible. The most popular solutions are those requiring diamond polishing and clear resinous protective coatings. Both these systems can be pigmented and can contain integrated terrazzo strips or sawcuts to define shapes, colour schemes, and logos. They can also incorporate broadcast stone, glass, or metal. By using these ideas, the possibilities are almost limitless for the designer.

Surface preparation
The first step in the successful installation of these decorative overlays is to have a properly prepared substrate. All repairs or alterations to the concrete are usually done before surface preparation. In general, fast-curing, polymer-modified mortars that achieve high early strength are used for the repairs to accelerate the delivery of the project.

The non-profit industry association, International Concrete Restoration Institute (ICRI), provides a guideline to surface preparation for those dealing with concrete repair. The ICRI also provides a system of Concrete Surface Profile (CSP) chips numbered from CSP 1 to CSP 10, which give a clear visual indicator of the profile needed. Manufacturers recommend the appropriate CSP required for their products—simplifying the task for a design professional when writing specifications for tender.

A profile of CSP 3, produced by means of shotblasting, seems to be relatively common within the industry. Additionally, the use of a two-component, 100 per cent solids epoxy primer into which a 16-mesh clean silica quartz sand is broadcast to refusal can create a suitable profile for the overlay.

[3]

Overlay characteristics
Basic concrete is a composite made of portland cement, sand, aggregate, and water. While more complex concrete designs are readily available, however, this basic concrete recipe will easily yield concrete in the range of approximately 25 to 35 Mpa (3626 to 5076 psi) of compressive strength. In contrast, a typical cementitious overlay will provide anywhere from 30 to 50 MPa (4351 to 7251 psi) in compressive strength.

The engineering of an overlay is much more elaborate in its basic formulation, mainly because it is used in very thin applications—most commonly at 9.5 to 19 mm (3/8 to ¾ in.) thickness—far below the 50-mm (2-in.) minimum requirement for concrete. Therefore, when using thin construction materials such as cementitious overlays, these materials require superior adhesion, better cohesiveness of the cement matrix, higher compressive strength, and greater flexural strength to perform similarly to the concrete mass on which they are installed.

To meet these requirements, sophisticated formulations using special hydraulic cements are used to accelerate cure times and minimize shrinkage. Polymers are used in the formula to increase adhesion and flexural capabilities and to improve the cohesiveness of the cement matrix.

The rheology-modifying agents are used to minimize segregation and bleeding, in order to achieve high fluidity and long working time. High-efficiency super-plasticizers are incorporated in those formulas. The result is ‘engineered cements’ or overlays, and these formulations require extensive testing to offer high performance.

Installation of overlays
Once the primer has set and the un-bonded excess sand has been removed, the overlay is ready to be installed. Experienced crews versed in overlay placement understand the importance of properly staging workstations, strategically planning the size and direction of the pour, and having the correct number of skilled workers on hand. The use of such specialized crews is highly recommended as final results rely greatly on correct installation of the overlays.

[4]

At this stage, the floor may have been designed with decorative separator strips, metal inserts, or simply formed some other way to make shapes and colour patterns. In either case, the options are almost inexhaustible, as the design professional can work with colour pigments, forms and shapes. Decorative glass, stone, sand, or metals can also be incorporated into the finish, which yield interesting effects when exposed by diamond polishing.

Once the overlay is installed, it is comparable to an artist’s canvas. Colour pigments can be integrated within or installed over top of the overlay with products such as stains or dyes once it has set—creating even more possibilities.

Overlays can be advantageous because they:

• are more economical than replacing concrete;
• can be laid down as thin as 9.5 mm (3/8 in) and as thick as 50 mm (2 in.);
• lend well to designs, including;
• colouring or addition of aggregates and decorative elements; and
• offer a unique look and feel.

However, overlays have disadvantages because they:

• are cement-based, which can result in cracks;
• require skilled craftsmen; and
• are a manmade product, and can potentially have a mottled variable look.

Clear protective resin coatings
One option for finishing the overlays is to use clear protective coatings such as epoxy, poly-urethane, or poly-aspartic coatings or specialized waxes. This final step ensures the stain protection needed for a durable long-lasting floor.

Advantages of these coatings include:

• relatively fast execution;
• ability to re-coat years later if needed;
• being an economic option;
• provision of a good barrier against staining; and
• salt-resistance.

Some of the disadvantages include:

• a synthetic, plastic look—which may be undesirable;
• subject to scratching (especially epoxies);
• the tendency of certain products to darken the colour of the floor; and
• the production of outgassing traces, in the form of pinholes, moon, or fish eyes on the surface due to displacement of air/moisture from the substrate through the coating before it has a chance to cure.

[5]

Diamond polishing
Diamond polishing of decorative overlays, another technique for finishing decorative concrete, is similar to polishing marble, granite, terrazzo, or concrete. The goal is to bring out the natural beauty of the sand and tiny aggregates, giving a lustre and depth to the product while offering light reflectivity as a bonus.

Once the material is cured according to manufacturer’s recommendations, the grinding, honing, densifying, polishing, and protective waxing process begins. Specialized craftsmen, using planetary grinding equipment, diamond tooling, and high-efficiency-particulate-arrestance (HEPA)-certified dust extractors, are capable of attaining a honed or highly polished surface. Though the surfaces are refined through polishing, they still are capable of providing slip resistance according to building codes. There are a growing number of overlays on the market that cater to this style of flooring.

A distinct advantage of overlays is their lack of large aggregates concrete contains. Diamond-polished overlays offer a true salt and pepper look, which many find very desirable. Canadian design professionals have access to non-profit associations, such as the Concrete Polishing Association of America (CPAA), which offers aid with written specification guides for design professionals wishing to accurately specify required slip resistance, level of shine, distinction of image, clarity, and the type of polish required—whether it is abrasive or topical. These specifications greatly reduce margins for misinterpretation between contractor and specifier.

[6]

Advantages of overlay polish are:

• better abrasion resistance than resinous coatings;
• increased light reflectivity;
• a natural look and feel;
• easy, low-cost maintenance;
• the ability to be re-waxed and burnished periodically; and
• the ability to be re-honed and polished.

Nevertheless, overlay polish can be disadvantageous because it can:

• be relatively costly;
• have poor salt resistance; and
• be more porous, resulting in the need to address spills immediately.

Conclusion
Decorative overlays offer a solution for a concrete look and feel when the existing concrete is unable to attain this due to blemishes, cracks, pitting, and necessary repairs. It is a tool for the design professional that can be fashioned in many ways to attain specified requirements. However, it is important to refer to qualified consultants, manufacturers, and craftsmen because overlays are prone to limitations similar to concrete.

Installation requires precision and must be properly planned and staged. There is only one shot at making overlays work perfectly, so any naturally or unnaturally occurring incidents during placement may have an effect on the final product. For these reasons, it is recommended to rely on industry experts experienced with these overlays and types of projects to ensure successful delivery.

Ultimately, the look of concrete or overlays will not satisfy everyone; and whether or not overlays are the correct building material for a given project remains in the domain of personal taste. However, those who work with decorative overlays are often repeat users who enjoy working with these products because they offer an opportunity to create a look and feel totally unique and customizable.

[7]Giovanni Natale is a second-generation cement finisher/contractor with 35 years of industry experience. In his career, he has been responsible for the installation, supervision, and coaching of crews working with concrete repair systems, flatwork, decorative concrete, floor and wall coatings, and carbon-fibre-reinforced polymer (CFRP) systems. More recently, his work with MAPEI has brought him to assist professionals with concrete restoration solutions. He can be reached via e-mail at gnatale@mapei.com[8].

Source URL: https://www.constructioncanada.net/understanding-cementitious-decorative-overlays/

---