The Pour-on Floor: Understanding advancements with polishable cementitious overlays

by Catherine Howlett | January 5, 2013 3:12 pm

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by Les Davis, Gary Vidal, and Steven H. Miller, CDT
All over the United States, a hard truth is being uncovered—underneath carpet and vinyl tile in buildings everywhere, there lurks ugly concrete. With acceptance of exposed concrete floors on the rise, and remodeling continuing to dominate the construction market, plans to polish existing concrete subfloors are numerous. However, some slabs are simply not up to the challenge.

A new alternative is to apply a self-leveling cementitious overlay on top of the problem slab. The overlay can be integrally colored, poured in a thin layer bonding to the subfloor, and then dyed and polished like concrete. Both the products and the process are new, but have taken off in popularity with design professionals and owners for applications ranging from large national retail chains to custom homes. The process offers the potential for affordable, clean, new floors, with bold color designs and the sustainable and durability benefits of exposed concrete.

Old or new floor?
Polished concrete has been steadily rising in popularity for two decades, but has experienced a recent spike after proving to be a recession- and eco-friendly flooring solution. As more remodeling projects call for polished concrete, subfloors not exposed for years are being uncovered. Some floors are in excellent shape and can be refinished to look almost like polished stone. However, the range of condition varies.

A few cracks can give a floor ‘character,’ and the occasional patched spall gives it ‘history.’ However, what happens when tile is removed to reveal a floor with a network of cracks and scars, several different-colored pours, and long-trench patches in un-matching colors? Some floors are not going to look beautiful, no matter how well they are colored and polished.

The poor concrete conditions in Figure 1 came from a home in Florida. Slabs that are cracked, cut, and patched from previous renovations, with crumbling surfaces and uneven elevations, are common not only in residential projects, but also in retail and commercial facilities. If polished, this concrete shows all its scars.

However, polishing contractors recently discovered a resolution to this dilemma. They applied a new class of self-leveling cementitious overlayment materials to unacceptable concrete and then ground and polished it like concrete. The materials could be integrally colored before they were poured on, and dyed after they hardened, offering a fast way to create an even, new, colored, polished floor.

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With a little ingenuity, ways of completing multi-colored pours were discovered. These pioneering efforts suggest polishable overlay is not just a last resort for a damaged floor, but also an aesthetic solution with unique properties. Design professionals quickly picked up on the proof of concept provided by the applicators, and are starting to integrate this technology into their designs.

How it’s done
Self-leveling overlays were originally developed as a fast way to repair surface damage and level elevation differences before applying a floorcovering to concrete substrates. Recently, overlays that cure hard enough to polish in 12 to 24 hours (unlike the 28-day wait for conventional concrete) have come on to the market.

There are several different products available, with varying chemistry. Some are based largely on portland cement, while others are not. Most options are polymer-modified, but all tend to be fast-curing and hard. Materials are available that reach compressive strengths of 21 MPa (3000 psi) in just four hours, 35 MPa (5000 psi) in 24 hours, and 45 MPa (6500 psi) in 28 days. By contrast, ordinary portland cement (OPC) concrete designed for a single-family residential slab in a mild climate is often 17 MPa (2500 psi) at 28 days.

The dry overlay materials come premixed and include fine aggregate; there is generally no coarse aggregate. When cured, the overlay presents a fine-grained surface and, once ground and polished, only sand-like aggregate is exposed, so the scale of detail in the floor remains very fine. The raw matrix color of these materials tends to be light gray or cream, lighter than OPC.

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The floor is prepared by grinding off old mastic, underlayment, or any loose or crumbling concrete. Holes and cracks should be patched, with shallow problems often repaired using the same overlay material as for the floor finish. The slab should be tested for excessive water vapor emission. Depending on the manufacturer’s recommendation for the specific overlay being used, slabs with high moisture content may require special preparation, such as the use of an extra primer application of a moisture-insensitive epoxy before the regular sanded primer.

Epoxy primer is applied, and sand is broadcast onto it to provide a key for the mechanical bonding of the overlay. After it cures, excess sand is removed and the floor is ready for the topping.

The overlay can be used in its raw or natural state, or integrally colored. Color dosage can be varied to a degree, for more or less intense hues. Since overlay base color tends to be light, integral colors may be brighter and more saturated than normally seen in colored concrete. Some colorants are packaged in pre-measured quantities corresponding to a one percent dose in a single bag of overlay mix. One or two packages can be used per bag, depending on the floor’s desired look.

The ability to dose accurately, combined with general bag-to-bag consistency of premixed products, makes it possible to reasonably replicate a color from one location to the next. This is valuable when designing for consistent branding in multiple locations, such as a national store chain or franchise.

Integral color is usually made from mineral oxide pigments. These types of materials have a long track record, and are considered colorfast. They are compatible with both interior and exterior uses.

If a new concrete slab is being designed with integral color, substitution of a colored top layer may be more economical than pigmenting the slab’s entire depth. The choice would also depend on whether there is a desire for exposed coarse aggregate, a component of concrete generally absent in overlays. Special decorative aggregate can be seeded into the surface of either concrete or overlay, and ground off for interesting effects.

Pour on the floor
Once the material is mixed, it is poured onto the floor and spread. Thin layers can bond effectively and are durable. Some products can be applied as thin as 3 mm (1/8-in.) over the slab’s highest point.

It is also possible to pour multiple colors on a single floor. Areas are dammed off with adhesive-backed foam weatherstripping applied to the floor. Overlay is flowed up to the stripping. After the topping cures, the foam is removed, and the second pour can be butted-up against the first. This technique allows for smooth curving lines of contrast and elaborately colored designs.

The curing time of overlays varies. Some are based on fast-setting cements such as calcium sulfo-aluminates, and can cure to polishing strength in 24 hours or less. Others may cure hard enough to walk on—17 MPa (2500 psi) within 24 hours—but not be hard enough to polish for 28 days.

Certain overlays form a polymer cap on the surface, designed to aid curing. Before the regular grinding and polishing sequence can begin, this tough skin must be ground off the cured overlay with aggressive abrasives.

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Not the same as concrete
The chemistry of self-leveling overlays is different from concrete. There are varying degrees of polymer modification. Products based on non-portland cements may produce little or no calcium hydroxide (i.e. lime) by-product—an important factor in the chemical densifying treatment associated with polishing.

Due to these differences, materials formulated for working with concrete may not perform as expected with overlays. Colorants have been observed to shift hue because of the difference in chemistry. Conventional densifiers may not properly react.

However, materials specifically formulated for compatibility with overlays are emerging. Dyes are the colorant of choice for polished concrete, and can be used the same way on an overlay, or in addition to integral color in the overlay. A dense overlay may not accept conventional acetone-based concrete dyes. Specialty dyes have been developed using an alternative, non-flammable, low-volatile organic compound (VOC) solvent that penetrates more effectively and deeply. For OPC, dye is usually applied after the 200-grit grind before densifying, but with the speciality dye, due to its unusual depth of penetration, the standard sequence of operations should be altered and the surface densified before dyeing.

Densifiers are surface-applied as a liquid. They react with lime in the concrete and grow additional cementitious materials that fill in the pore structure and harden the surface, improving the ability to take a polish. Densifiers formulated for concrete may not react effectively with some overlays. To ensure proper reaction between materials, and promote more predictable results in coloration, it is wise to specify colorants (integral color and dye) and densifiers tested for compatibility by the overlay manufacturer.

Similarly, although concrete polishing is the model for overlay polishing, following a similar sequence of ever-finer diamond abrasives, the operator techniques may be different. An overlay-experienced applicator is recommended.

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Local color
Dye can be spray-applied to both natural and integrally colored overlays; it is then allowed to soak into the surface. If added over integral color, dye can deepen existing color, add darker accents, or subtly alter its tint.

In a recent residential project in Florida, for example, the color sample was a piece of steel-gray stained glass. The manufacturer was able to match it with a combination of a one percent dose of integral black mixed into the overlay, and a blue dye application during polishing.

Dyes can also be selectively applied using adhesive-backed stencils. Intricate designs are possible with multi-layer, registered stencils, including graphics and detailed pictorial effects. (This technology’s full possibilities are beyond the scope of this article.)

Dyes may be degraded by long-term ultraviolet (UV) exposure, and are not considered suitable for exterior applications.

After densifying and dyeing, the polishing process is continued to very fine abrasives, often up to 1500 grit. When properly performed, a floor can be brought to high gloss with mirror-like reflections.

Specifying protection
One property common to exposed cementitious floors—polished concrete and overlay—is the need for stain protection. This class of product is a breathable sealer applied at the end of the polishing process. It deposits a thin polymer layer on the surface that resists liquid penetration, but allows water vapor to exhale from the slab. It tends to slightly deepen colors and enhance a wet look in the polish.

Stain protector’s liquid resistance minimizes staining. It can also temporarily shield against spills of acidic substances—pickle juice on a supermarket floor is the classic example—that can chemically etch the surface, spoiling the polish and potentially altering colors. However, a stain protector is not a total shield; it is only designed to provide a window of opportunity to remove spills. Manufacturers typically recommend a maximum of 30 minutes before cleanup.

Stain protector also helps keep dye color in the slab. If the protector wears out and is not renewed in a timely manner, dyes can be gradually washed out of place, lowering the saturation of color in the surface. Often mistaken for dye fading, this color loss has been observed in concrete floors, and may also occur in polishable overlays. However, these materials are still too new for most original stain protection applications to have worn off.

Prudence would suggest using experience with concrete as a guide. For concrete, manufacturers typically recommend re-application every two years, and more frequently in high traffic areas; it is reasonable to expect the same performance
on overlays.

Owners need to be informed that stain protection should be inspected yearly, either by the original applicator or other qualified professionals. Maintaining stain protection is the single most important factor in keeping a colored, polished cementitious floor looking and performing as designed.

Conclusion
Polishable overlays offer a cost-effective solution for re-finishing damaged concrete floors into new, decorative, exposed concrete surfaces. They are sufficiently ‘concrete-like’ to provide similar performance benefits, but have a range of subtly different looks. Due to the ease of pigmenting them before application, overlays bring the special qualities of integral color to existing slabs, an option previously unavailable. Combined with dyeing and polishing options, this system has quickly evolved into a durable way to make any floor into a designable surface.

One of the most intriguing aspects of the recent development of polishable overlays is the speed with which new techniques are being discovered and explored. Three years ago, these products were used as substrates. Now, they are a medium for creating permanent graphic art on floors. Like a great deal of the advances in decorative concrete, this is being driven by the imagination of applicators. It is a field worth watching, both for the new techniques being pioneered and materials being developed to enable creativity.

Les Davis is president of the AmeriPolish Inc. (formerly American Decorative Concrete Supply Company), manufacturers of products for coloring and polishing cementitious overlays and polished concrete. He can be contacted via e-mail at les@ameripolish.com[6].

Gary Vidal is inside sales manager for AmeriPolish Inc. He is a veteran concrete polisher, and was the first contractor to field test the first acetone-based concrete dye before it went on the market. Vidal can be contacted at gvidal@ameripolish.com[7]. 

Steven H. Miller, CDT, is an award-winning writer and photographer, and a marketing consultant. He specializes in issues of the construction industry. Miller can be reached at steve@metaphorce.com[8].

Source URL: https://www.constructionspecifier.com/the-pour-on-floor-understanding-advancements-with-polishable-cementitious-overlays/

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