Concrete Floors: Are ‘sealed’ and ‘polished’ synonymous?

October 6, 2015

All images courtesy Adaptive Concrete Innovations LLC

By Andy Bowman
The continued rise in popularity of polished concrete floors, combined with a spate of new ‘polishing’ products, has created a perfect storm for construction specifiers. The industry has responded by releasing a new standard to measure and evaluate these surfaces.

Polished concrete has become a popular flooring choice in contemporary design. It is economical and durable—making it a good fit for warehouse and retail applications. However, economy alone cannot account for the material’s popularity. Polished concrete’s allure stems from its artisanal properties as rustic, yet refined. The combination of earthy material with industrial aesthetic has caused polished concrete floors to appear everywhere from upscale commercial environments to residential design.

The rapid increase in polished concrete installations has made it difficult for scientific methods and standards to keep pace. Today’s concrete polishing industry uses a wide range of systems and equipment, and various specially formulated chemicals and abrasives. This means the specification of a polished floor can result in a myriad of actual finished surfaces.

In particular, many coatings are now being marketed to accomplish the sealing of concrete floors. However, the origin of the gloss and shine has become obscured. In traditional polishing operations, the concrete itself creates the characteristic glossiness and shine. A polished concrete surface is achieved when abrasive tools, with progressively finer grit, have been used to refine the concrete material.

While small-scale pits or imperfections refract light, highly polished concrete reflects it. Ideally, polished concrete should be scratch-free and have a surface texture without abrasion points. Therefore, these properly polished floors would not rely upon a sealant to produce its gloss.

Historically, the gloss meter has been used to provide a quantitative standard for finished concrete surfaces. While this tool records the light reflected from a surface, it does not measure the surface texture. The use of a gloss meter alone to assess a surface can mask its imperfections by measuring the reflection produced by topical concrete polishing sealers that are actually covering flaws and scratches.

With sustainability and lifecycle issues coming increasingly to the fore, the polishing industry would benefit from deemphasizing the mere appearance of glossiness and taking a more holistic approach. To accomplish this, there must be a shift in focus toward performance-based standards and encouragement to use a good concrete mix in the initial pour. When this happens, gloss will become a natural byproduct of the polishing operation.

As shown above, a high-quality concrete surface, polished to be scratch-free and without abrasion points, is naturally glossy.

The ideal concrete finishing process ends up being a multidisciplinary activity resulting in tailored properties improving not only the appearance, but also the function and serviceability of a surface. Delivering a high-quality concrete surface will reduce maintenance costs currently borne by end users, as many are currently spending more than expected on maintenance to maintain their floor’s appearance.

Defining a more specific process
Concrete surface refinement involves a series of progressive steps that eliminate scratches and unevenness. Various finishing techniques and tools can mean a high-quality result. Therefore, it makes sense not to prescribe a narrow range of accepted procedures, but rather to adopt a measurement standard that sets a high bar for surface measurement and establishes specific objectives for small-scale refinement qualities.

In the fall of 2013, the Concrete Sawing and Drilling Association (CSDA)—a group representing North American contractors in those industries—adopted a new standard, known as Measuring Concrete Micro Surface Texture (ST115). The association says the document was produced in response to concerns on the part of architects and engineers regarding quality issues with polished concrete.

The standard has two main components[3]. It identifies approved instrumentation for measuring surface texture, and provides a benchmark measurement procedure for contractors and specifiers to follow. It also includes directions for documenting the texture readings and contains a resource section with relevant code information and glossary terms.

One can specify the desired finish from this surface texture chart.
This chart shows the comparative analysis between surface texture grades, their glass readings, distinctness-of-image (DOI), and produced finish.

CSDA ST115 is the concrete industry’s first standard for quantifying a concrete surface by accurately reading its texture and assigning the proper designation code or surface texture grade (STG). It allows the design community and end-users to have a more structured conversation about the quality and sustainability of a polished concrete surface.

To create the standard, representatives came together from five major industry associations across five continents:

Committee members were experts in the fields of surface metrology (the study of surface geometry), concrete finishing, concrete repair, and mix designs. They considered a wide array of information
on surface measurement techniques and surface parameters in practical use.

While ST115 is the first standard of its kind for the concrete industry, there are equivalent standards in other industries. The metal and plastic finishing industries, for example, both adopted similar standards more than 50 years ago because they experienced the same challenges with interpretation and processing techniques. Further, the standard promotes using profilometers to measure surface roughness; these instruments have been used in the wider concrete industry for years. Profilometers meet the standard’s specific requirement for using “contact, skidded instruments.” (A skid is a metal rest that is attached to the probe on a profilometer; it moves in conjunction with the stylus). It is the use of these devices that enable the assignment of a texture grading code to the concrete finish.

Schematic Diagram of Surface Characteristics[6]
Profilometer readings are separate from—but compatible with—readings from gloss meters and distinctness-of-image (DOI) meters.

Taking a closer look
CSDA ST115 is concerned specifically with numerically quantifying the geometric, finely spaced irregularities of a concrete surface. It is not intended to address waviness, which is defined as the more widely spaced component of surface texture. Since profilometers measure both roughness and waviness, filters have been defined within the standard. These filters provide long- and short-wavelength cutoff values to eliminate data that does not significantly contribute to the roughness measurement.

Collecting this finely tuned range of data is possible because of the sensitivity of modern surface texture measuring profilometers. Their electronic recording systems capture minor fluctuations of stylus movement over the concrete’s evaluation length. ‘Ra,’ or average roughness, is the parameter used as the basis for CSDA ST115. The intent of the measurement method is to examine the ways in which finishing processes influence surface textures and how the surface texture, in turn, influences gloss, friction, and sustainability. The recommended profilometers can be used in conjunction with gloss meters and distinctness-of-image (DOI) meters.

The CSDA standard applies to any surface produced by processes of abrading, cutting, grinding, milling, honing, or polishing. It allows for the possibility of encountering high amounts of flaw, roll out, or voids and provides solutions for how to properly accommodate these flaws and get a reliable reading. The quantitative surface refinement number does not vary depending on the type of polish guard or sealer used.

Test results meeting the standard will indicate if steps in the polishing process have been skipped or are incomplete. They also enable the processor of a polished floor system to monitor the refinement capability of a specific tool in a multiple tooling system by measuring the surface of the concrete after each individual tool used. By knowing the quality of abrasives being currently used, a processer can then make an informed comparison to similar abrasives offered by competitive manufacturers. He or she can keep track of each step’s finish grade and potentially assemble the best tool selection procedure for the particular equipment or process.

CSDA ST115 is just the first step in moving the industry toward more systematic and responsible polishing practices. The standards committee assembled by the association also developed a companion document, a best practices guide known as CSDA BP-015, Green Polishing and Grinding Practices. This guide helps concrete polishing and grinding contractors perform their work in a more environmentally-responsible manner. Its recommendations cover work performed from office to jobsite. Additionally, a standard based on CSDA ST115 is likely to be adopted
by ASTM International. A draft of that document is currently in subcommittee review.

Implementing ST115 will result in a very detailed map of a concrete surface; it gives contractors, designers, and owners a more complete understanding of their finished product. Once a surface meets the requirements outlined in ST115, it will automatically produce a high gloss, since gloss is a by-product of surface refinement. The result will be improved operations and maintenance workflows, reduced costs, environmental benefits, and a better material in service for the project’s end users.

Andy Bowman is a surface metrology specialist and principal of Adaptive Concrete Innovations, LLC. He has served as chair of a committee comprised of 16 experts in the fields of surface metrology, concrete finishing, concrete repair, and concrete mix designs. This committee, which authored the ST115 Concrete Sawing and Drilling Association (CSDA) standard described in this article, represented five major industry associations and five continents. Bowman can be reached at[7].

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